RF & Microwave Fiberoptic Design Guide

RF & Microwave Fiberoptic Design Guide
Design Guide
RF and Microwave Fiber-Optics
MICROWAVE
is switched on and off to send digitally coded information
Introduction
through a fiber to a photodiode receiver.
In 1984 Ortel Corporation began developing and producing lasers and detectors for linear fiberoptic links. Since
By comparison, in linear fiber-optic systems developed by
that time Ortel, now Emcore has continually refined these
Lucent, the light sent through the fiber has an intensity
optoelectronic components have been continually refined
directly related to the input electrical current. While this
for integration into a variety of systems that require high
places extra requirements on the quality of the lasers and
fidelity, high frequency, or long-distance transportation of
photodiodes, it has been essential in many applications to
analog and digital signals. As a result of this widespread
transmit arbitrary RF and microwave signals. As a result,
use and development, by the late 1980s, these link products
tens of thousands of Ortel’s transmitters are currently in use.
were routinely being treated as standard RF and microwave
components in many different applications.
The information offered here examines the basic link
components, provides an overview of design calculations
There are several notable advantages of fiber optics that
related to gain, bandwidth, noise, and dynamic range and
have led to its increasing use. The most immediate benefit
distortion. A section on fiberoptic components discusses a
of fiber optics is its low loss. With less than 0.4 dB/km of
number of key parameters, among them wavelength and
optical attenuation, fiber-optic links send signals tens of ki-
loss, dispersion, reflections, and polarization and
lometers and still maintain nearly the original quality of the
attenuation. Additional information evaluates optical
input. This low fiber loss is also independent of frequency
isolators, distributed-feedback lasers and Fabry-Perot lasers,
for most practical systems. With laser and detector speeds
predistortion, and short- vs. long-wavelength transmission.
up to 18 GHz, links can send high-frequency signals in their
original form without the need to down-convert or digitize
them for the transmission portion of a system. As a result,
signal conversion equipment can be placed in convenient locations or even eliminated altogether, which often leads to
significant cost and maintenance savings.
Savings are also realized due to the mechanical flexibility
and lightweight fiber-optic cable, approximately 1/25 the
weight of waveguide and 1/10 that of coax. Many
transmission lines can be fed through small conduits,
allowing for high signal rates without investing in expensive architectural supports. The placement of fiber cable is
further simplified by the natural immunity of optical fiber
to electromagnetic interference (EMI). Not only can large
numbers of fibers be tightly bundled with power cables,
they also provide a uniquely secure and electrically isolated
transmission path. The general advantages of fiber-optics
first led to their widespread use in long-haul digital telecommunications. In the most basic form of fiber-optic
communications, light from a semiconductor laser or LED
1
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Table of Contents
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Contents
Figure
Basic Link Applications and Components...............................
Typical Linear Link Applications ........................................
Typical Linear Link Components .....................................
Optical Transmitter ......................................................
Optical Receiver ...........................................................
Fiber-Optic Cable .........................................................
Link Design Calculations ........................................................
Gain ..................................................................................
Doubling the Optical Loss Term ..................................
Resistively Matched Components ...............................
Reactively Matched Link .............................................
Bandwidth .......................................................................
Noise................................................................................
Laser Noise ...................................................................
Shot Noise ...................................................................
Receiver Noise .............................................................
Total Link Noise ..........................................................
Cascading Noise Figures .............................................
Unconverted Noise and SNR ......................................
Noise-Equivalent Bandwidth ......................................
Dynamic Range and Distortion ......................................
1 dB Compression Point .................................................
Third-Order Intercept and Spur-Free Dynamic
Range ...........................................................................
Large Number of Carriers ...........................................
Placement of Amplifiers .............................................
Example ...........................................................................
Transmitter and Receiver Choice ................................
Gain .............................................................................
Noise ...........................................................................
Dynamic Range............................................................
Distortion ....................................................................
Selection of Optical Fiber Components ................................
Wavelength and Loss .....................................................
Dispersion .......................................................................
Ruggedization.................................................................
Connecting Fibers ...........................................................
Fusion Splice.................................................................
Mechanical Splices.......................................................
Connectors ..................................................................
Reflections and Laser Noise............................................
Reflections and Interferometric Noise...........................
Polarization Mode Dispersion .......................................
Optical Attenuators .......................................................
Additional Transmitter Considerations ................................
Optical Isolators ..............................................................
Distributed-Feedback (DFB) vs. Fabry-Perot (F-P) Lasers....
Predistortion....................................................................
1310 nm vs. 1550 nm Wavelengths ...............................
Summary ................................................................................
Appendix.................................................................................
Performance Characteristics...........................................
Gain .............................................................................
Total Link Noise ..........................................................
Glossary .................................................................................
References .............................................................................
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Figure 1. Block Diagram Depicting a Basic Fiber-Optic Link of
Transmitter, Receiver, and Optical Fiber ................
Figure 2. Input Current vs. Output Power.............................
Figure 3. A1612P DFB Laser Module .....................................
Figure 4. Transmitter Package Styles: Flange-Mount an
Plug-In.....................................................................
Figure 5. Photodiode Linear Responsivity Curve .................
Figure 6. Effects of Optical Loss and Transmitter RF
Efficiency .................................................................
Figure 7. Resistively Matched Link ........................................
Figure 8. Laser Frequency Response as a Function of
Increasing Bias Current ..........................................
Figure 9. Cumulative Loss Effects of Laser Noise, Photodiode
Shot Noise, and Receiver Thermal Noise on Total
Link Performance.....................................................
Figure 10. Reactive Matching at the Transmitter and
Receiver Imparts Improved Noise Performance...
Figure 11. Effects of Unconverted Low-Frequency Noise on
SNR .......................................................................
Figure 12. Third-Order Intermodulation Distortion
Spectrum ............................................................
Figure 13. Third-Order Intercept and Spur-Free Dynamic
Range......................................................................................
Figure 14. Precise Indices of Refractions Enables Total
Internal Reflection in Optical Fiber ....................
Figure 15. Scattering and Absorption Losses vs.
Wavelength ..........................................................
Figure 16. Wavelength Dependency of Dispersion for
Standard and Dispersion-Shifted FIber ..............
Figure 17. Cross Section of Typical Ruggedized, Simplex
Cable......................................................................
Figure 18. Periodic Spikes Can Degrade Performance in
Analog Fiber-Optic Links ....................................
Figure 19. Low-Reflection Components (Terminators and
Optical Noise Splitters) Help To Avoid
Interfermetric Noise............................................
Figure 20. Comparative Noise Performance for DFB and F-P
Lasers...................................................................
Figure 21. Gain Curve 1, Resistively Matched Photodiode
for 50 Ω ...............................................................
Figure 22. Gain Curve 2, Unmatched Photodiode ..............
Figure 23. Gain Curve 3, Unmatched Photodiode ...............
Figure 24. Noise Curve 1, Equivalent Noise Input vs. Optical
Loss........................................................................
Figure 25. Noise Curve 2, Equivalent Input Noise vs. Optical
Losses......................................................................
Figure 26. Noise Curve 3, Equivalent Input Noise vs. Optical
Losses......................................................................
Figure 27. Noise Curve 4, Equivalent Input Noise vs. Optical
Losses.....................................................................
Figure 28. Noise Curve 5, Equivalent Input Noise vs. Optical
Losses......................................................................
Figure 29. Noise Curve 6, Equivalent Input Noise vs. Optical
Losses.....................................................................
Figure 30. Noise Curve 7, Equivalent Input Noise vs. Optical
Losses.....................................................................
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Basic Link Applications and
Components
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Typical Linear Link Applications
Typical Linear Link Components
One of the primary uses of linear fiber-optic links is
In each of these applications, as well as many others, Ortel’s
sending RF and microwave signals between transmit or
transmitters and receivers comprising the links are similar
receive electronics and remotely located antennas.
and can be treated as standard microwave components.
Focusing on these common elements, this design guide
Due to the flexibility of fiber-optic links, the antennas may
describes the general technical considerations and equations
be designed for analog or digital signals from any of a
necessary for engineers to choose the most appropriate
number of sources, including military and commercial
Ortel components for their systems. These equations also
communication satellites, global positioning satellites,
have been incorporated into various programs, which an
telemetry/tracking beacons, or wireless cellular networks.
Ortel applications engineer can use to provide an analysis
for a specific link application.
Another type of link is the fiber-optic delay line, which
combines a transmitter, a receiver, and a long length of
Figure 1 shows the three primary components in a
fiber in a single package to provide a unique combination of
fiber-optic link: an optical transmitter, a fiber-optic cable,
long delay times, high bandwidths, and low weight. These
and an optical receiver. In the transmitter, the input signal
higher-frequency RF and microwave products have
modulates the light output from a semiconductor laser
benefitted indirectly from another application, the
diode, which is then focussed into a fiberoptic cable. This
overwhelming use of linear fiber optics in cable television.
fiber carries the modulated optical signal to the receiver,
Here, fiber extends the transmission distance of TV signals,
which then reconverts the optical signal back to the original
improves their quality and system reliability, and even
electrical RF signal.
reduces costs when compared with systems employing only
coax cables.
OPTICAL
TRANSMITTER
OPTICAL
RECEIVER
MODULE
MODULE
dc BIAS
RF
IN
Z
MATCH
dc BIAS
ISOLATOR
OPTICAL
FIBER
LIGHT
Z
MATCH
AMP
AMP
LASER
DIODE
RF
OUT
PHOTODIODE
POWER
MONITOR
MONITORS
AND ALARMS
TEMP.
CONTROL
1-1215F
Figure1.BlockDiagramDepictingaBasicFiber-OpticLinkofTransmitter,Receiver,andOpticalFiber
3
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Basic Link Applications and
Components (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
package contains the laser chip, optical fiber, and imped-
Optical Transmitter
ance-matched electrical connections in a hermetically sealed
For RF systems, distributed feedback (DFB) lasers are used for
low-noise, high-dynamic range applications, and Fabry-Perot
lasers for less demanding applications. The wavelength of
these lasers is either 1310 nm or 1550 nm.
container such as the one shown in Figure 3. Modules also
may contain a photodiode for monitoring the laser power,
a thermistor and a thermoelectric (TE) cooler for monitoring
and controlling the laser temperature, and an optical
The intensity of the laser light is described by the simplified
isolator for reducing the amount of light reflected back to
light-current (L-I) curve in Figure 2. When the laser diode is
the laser from the fiber.
biased with a current larger than the threshold current, ITH,
the optical output power increases linearly with increasing
input current. Analog links take advantage of this behavior
by setting the dc operating point of the laser in the middle
of this linear region. Typically, this bias current for Ortel
transmitters is set somewhere between 40 mA to 90 mA. The
threshold current ranges from 10 mA to 30 mA.
Figure 3. A1612P DFB Laser Module
The basic laser module, although available as a
subcomponent, is usually integrated into a complete
transmitter housing such as the flange-mount and plug-in
packages shown in Figure 4. These transmitters also may
include dc electronics to control the laser temperature and
bias current, amplifiers and other circuitry to precondition
the RF signal, and various indicators for monitoring the
overall transmitter performance. Because analog fiber-optic
Figure2.InputCurrentvs.OutputPower
transmitters are used in a variety of applications, the exact
The efficiency with which the laser converts current to
implementation of these product features varies as well.
usable light is given by the slope of the L-I curve and is called
the modulation gain. For typical Ortel lasers, this dc
modulation gain ranges from 0.02 W/A to 0.3 W/A,
depending on the model chosen. The wide variation is
largely due to differing methods of coupling the light into
the optical fiber. The modulation gain also varies somewhat
with frequency, so it must be specified whether a particular
value is a dc or higher-frequency gain. In addition to the
laser diode, transmitters also contain a variety of other
components, depending on the specific application or level
of integration desired. The most basic laser module
Figure4.TransmitterPackageStyles:Flange-Mount(left)andPlug-In(right) 4
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Basic Link Applications and
Components (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
These photodiode modules are often integrated into more
Optical Receiver
At the other end of the fiber-optic link, the light is detected
by the receiver PIN photodiode, which converts the light
back into an electrical current. The behavior of the
photodiode is given by the responsivity curve shown in
Figure 5. Once again, note that the response is very linear.
The slope of this curve is the responsivity, which typically is
greater than 0.75 mA/mW for a photodiode chip without
complete receiver packages similar to the flangemount and
plug-in varieties of the transmitters. In these receivers,
circuitry reverse biases the diodes to increase the response
speed. Receivers also contain monitor and alarm outputs.
Some receivers may include a post amplifier, broadband
current transformers, and/or impedance matching networks
to improve the link gain. Due to such circuitry, the efficiency
of a receiver generally will differ from the responsivity of
any impedance matching.
the photodiode chip alone.
Similar to Lucent’s laser diodes, photodiodes are packaged in
a hermetic module containing an impedance matching
network and electrical lines to provide dc bias and RF
output. However, unlike the laser, the photodiode is
relatively insensitive to temperature so a thermoelectric
cooler (TEC) is not required. Special precautions also are
made to minimize optical reflections from returning back
through the fiber, which otherwise could degrade a link’s
performance.
Fiber-Optic Cable
A fiber-optic cable is the third primary component in a linear
optical link. Single-mode fiber, as opposed to multimode
fiber, is always used with Ortel links because of its low
dispersion and low loss. At a wavelength of 1310 nm, the
fiber attenuates the optical signal by less than 0.4 dB/km; at
1550 nm, less than 0.25 dB/km. Typically, the fiber is cabled
in rugged yet flexible 3 mm diameter tubing and connected
to the transmitter and receiver with reusable optical
connectors. The modular nature of the cable simplifies the
design of the physical architecture of the system and enables
a wide range of configuration possibilities. Although
Ortel does not supply optical fiber, several important
considerations should be followed in selecting these
components. The section entitled Selection of Optical Fiber
Components, page 19, describes these issues in detail.
Link Design Calculations
When selecting the proper components for a fiber-optic link,
Figure5.PhotodiodeLinearResponsivityCurve
there are several critical quantities that must be defined and
calculated prior to its implementation, just as would be done
with any RF or microwave communication link. Topics of
discussion in this section include link gain, bandwidth, noise,
dynamic range, and distortion, and the use of that
information as an example for a typical link. The detailed
equations in this section also have been incorporated into
various design programs, which an Ortel applications
engineer can use to provide the predicted performance of a
link in a specific application.
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Link Design Calculations (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Gain
Substituting equation 2 into equation 1 then gives the total
The RF loss (or gain) of an optical link is a function of four
variables, including transmitter efficiency, fiber loss, receiver
gain of a link:
equation 4,
LINK, RATI O =
efficiency, and the ratio of the output to input impedances.
(
TX, RF)
( RX, RF ) C 2
R OUT
- ) --------------(R )
(-----------------------------L ,
O RATIO
IN
GLINK, dB = 20 log( Tx, RF)( Rx, RF) – 2 LO
+ 10 log(ROUT/RIN)
In its most basic form, the power gain of the link can be
written in terms of the input and output currents as
The factors of ηTx, RF and ηRx, RF are sometimes converted
equation 1,
I OUT
G LINK = -----------I IN
(
2
) (R-------------R )
OUT
IN
where ROUT is the load resistance at the receiver output
to a form more similar to traditional RF gains by taking
20log, so that equation 4 can be simplified to:
equation 5,
GLINK, dB = TG + RG – 2Lo + 10 log(ROUT/RIN)
and RIN is the input resistance of the laser transmitter. The
(IOUT/IIN) term can be expanded in terms of the link
GLINK, dB = TG + RG – 2Lo + 10 log(ROUT/RIN)
characteristics as:
where TG is the transmitter gain in dB2W/A and RG is the
equation 2
receiver gain in dB2A/W. TG and RG are related to the unit’s
I OUT
( TX, RF ( RX, RF ) )
------------ = ----------------------------------------------I IN
L OPT
total RF efficiency expressed in W/A or A/W as follows:
equation 6,
where ηTx, RF is the efficiency of the total transmitter, in-
TG = 20log (Tx, RF)
cluding any amplifiers and matching networks, in converting
input RF currents into optical power
equation 7,
modulations. ηRx, RF is the efficiency of the total receiver in
RG = 20log (Rx, RF)
converting optical power modulations into RF output
current. (This RF value is not the same as the dc photodiode
responsivity, as described in the section on Bandwidth, page
8.) The units for ηTx, RF and ηRx, RF are W/A and A/W,
respectively. LO is the optical loss of the fiber portion of the
link measured as:
dB2W/A, a 75 Ω receiver with an RF of +20 dB2A/W and a 12
dB optical loss, would give an RF gain for the link of:
G = –1 dB²W/A + 20 dB²A/W – 2 (12 dB) +10log(75/75)
= –5 dB.
Figure 6 shows the effects of optical loss and transmitter RF
equation 3
L O, RATIO
For example, combining a 75 Ω transmitter with a TG of –1
=
Optical Power at Transmitter
---------------------------------------------------------------------------Optical Power at Receiver
efficiency for a receiver with an efficiency of 0.375 mA/mW
(RG of –8.51dB2A/W), as calculated with equation 4. The Appendix, page 28, contains additional sets of curves for other
Lo = 10 log LO, RATIO
typical transmitter and receiver efficiencies.
Figure6.EffectsofOpticalLossandTransmitterRFEfficiency
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Link Design Calculations (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
equation 8,
Doubling the Optical Loss Term
Rx, RF
An interesting and often overlooked aspect of equation 4
R PD
= --------------------------------- • R PD
R PD + R OUT
For a 50 Ω matched system, RPD and ROUT each would be
is the 2 LO term. As this indicates, for each additional dB of
approximately 50 Ω, therefore, the receiver RF efficiency
optical loss, there is an additional 2 dB of RF loss. This oddity
would be half that of the photodiode chip on its own. This
occurs as a result of converting optical power to RF energy.
decreases the overall link gain by 6 dB. For a
Here, the RF current is directly proportional to the optical
resistively-matched photodiode receiver:
power, but the RF power equals the square of the RF
equation 9,
current. When taking the log, this squared term turns into a
factor of 2 in front of the optical loss. For example, a
ηRx, RF = RPD/2
transmitter and receiver pair that have a –35 dB RF gain
The transmitter RF efficiency, on the other hand, does not
when they are directly connected with 0 dB of optical loss,
experience such a drop of 6 dB due to the fact that the
would have a –39 dB RF gain when connected with a 2 dB
matching resistor is placed in series rather than in parallel.
loss fiber.
Therefore, within the bandwidth of a transmitter, its RF
Resistively Matched Components
efficiency is approximately equal to the dc modulation gain
of the laser diode. As an example, consider a transmitter
To calculate the total insertion loss for a specific link,
with a dc modulation gain of 0.1 W/A, a resistively matched
consider the broadband resistively matched link shown in
receiver with a dc responsivity of 0.75 and a fiber with an
Figure 7. In this case, the laser transmitter includes the laser
optical loss of 3 dB. To the first order, RF efficiency of the
diode, with a typical impedance of 5 Ω, and a resistor to
transmitter will be 0.1 W/A and the RF efficiency of the
raise the total input impedance, RIN, up to the impedance of
receiver will be 0.375 A/W. If both the transmitter and
the external signal source. The photodiode module includes
receiver are matched to 50 Ω, then the impedance match-
the photodiode, with a typical impedance of several kΩ, and
ing term of equation 4 drops out, leaving an RF link gain of
a resistor RPD to match to the output impedance RL. Such
approximately:
matching resistors substantially improve the VSWR of the
equation 10,
link over that of an unmatched link. Due to this extra
photodiode resistor, the current output from the receiver,
GLINK, dB = 20 log(
IOUT, will be less than the total current produced by the
Tx, RF( Rx, RF))
R OUT
+10 log --------------R IN
(
photodiode chip, IPD. The RF efficiency of the receiver, ηRx,
RF, is therefore correspondingly smaller than the responsivity
– 2 LOPT, dB
)
GLINK (20 log [(0.1 mW/mA) (0.375 mA/mW)]
– 2 x 3 dB + 0
of the photodiode chip alone, RPD:
GLINK = –35 dB
Figure7.ResistivelyMatchedLink
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Link Design Calculations (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Reactively Matched Link
response of the basic laser diode chip shown in Figure 8 is
To overcome such a loss, many links incorporate additional
of prime interest. As can be seen, the frequency response
amplifiers, which are described more fully in the sections on
of the chip varies with the bias current, thus an optimum
Receiver Noise, page 10; Placement of Amplifiers, page 15;
point is chosen to balance this frequency response and other
and in the section entitled Example, page 15. As an
current-sensitive parameters such as noise, linearity, and life
alternative for narrowband systems, the link gain can be
of the device. When integrating the laser chip into a
improved by impedance matching so that the laser diode
complete transmitter, other components such as amplifiers
and photodiode see an effective ROUT/RIN > 1. The match-
or matching networks also can limit the response. The
ing electronics used in such links are carefully designed to
bandwidth of receivers is limited by the capacitance and
produce this extra gain without creating reflections or poor
carrier transient times of the photodiode chip or by
VSWR.
additional electrical components such as amplifiers and
matching networks. In most cases, the speeds of these
Bandwidth
receivers are faster than that of the other components in
The range of frequencies over which a fiber-optic link can
transmit is limited by the bandwidth of the transmitter and
receiver and by the dispersion of the optical fiber. The
bandwidth limit of a link generally is defined as the
frequency at which the microwave modulation response
decreases by 3 dB. In most cases, the bandwidth of the laser
transmitter limits a link’s frequency response, therefore, the
the link. In certain situations, the fiber itself may smear out
rapidly varying signals due to the fact that different
wavelengths travel at different speeds along a fiber. To
avoid this chromatic dispersion, lasers with narrow optical
bandwidths, such as Lucent’s DFB lasers, are used with fiber
that has low dispersion. Using a DFB at 1310 nm, where
fibers have a natural minimum in their dispersion, bandwidths in excess of 15 GHz can be achieved over fibers as
long as tens of kilometers. The section on Dispersion, page
20, describes these fiber effects more fully.
Figure8.LaserFrequencyResponseasaFunctionofIncreasingBiasCurrent
8
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Link Design Calculations (continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Noise
3. Shot noise at the photodiode
Within the bandwidth of the link, the contribution of noise
4. Noise from amplifiers and/or matching components at the
from the various components must also be considered.
receiver
When specifying the noise of a link, the tendency is to use
Since fiber-optic links typically exhibit a significant amount
equivalent input noise (EIN). EIN is defined as the amount
of signal loss, the noise from the transmitter amplifiers are
of RF noise at the input of a link that would be needed to
generally much less than that from the other components,
produce the amount of noise observed at the output of the
and will be neglected. The calculations discussed in this
link if the total link itself were noiseless. Its units can be mW/
section are also summarized in the performance curves in
Hz or dBm/Hz:
the Appendix, page 28.
equation 11,
Laser Noise
EINLINK, mW = NoiseOUTPUT, mW/GRATIO
Laser noise arises from random fluctuations in the intensity
EINLINK, dBm = NoiseOUTPUT, dBm – GdB
of the optical signal. There are two main contributions to
An alternate measure of noise is the noise figure (NF), which
this effect. The first is the actual fluctuations in the intensity
is the ratio in dB of the actual noise power to the amount
of the light as it is generated at the laser diode. The second
that would be produced by a similar device with perfect
is fluctuations in the frequency of the light, which can
noise performance. It is further defined that the inputs of
degrade the signal if the fiber is dispersive. This second set
this ideal device are terminated by a passive load at the
of effects will be discussed more fully in the sections on
standard temperature of 290 K(TO). Since the available noise
Reflections and Interferometric Noise, page 24; Polarization
power from such a load is:
Mode Dispersion, page 25; and Distributed-Feedback (DFB)
KTo = (1.38 x
10–20
mW/(k – Hz)) (290 K)
The laser noise measured directly at the transmitter is often
= 4.0 x 10–18 mW/Hz
referred to as relative intensity noise (RIN), so named
= –174 dBm/Hz
because it is the ratio of the mean square amplitude of the
EIN is related to NF by:
noise fluctuations per unit bandwidth, <P2>, to the square
equation 12,
of the dc optical power, Po:
NF = 10 log (EINmW/Hz/KTo)
RINRATIO = <P2>/PO2
NF = EINdBm/Hz + 174 dBm/Hz
The noise also can be specified in terms of the equivalent
input temperature, T, which is given by:
equation 13,
vs. Fabry-Perot (FP) Lasers, page 25.
This value is related to EIN by: equation 14,
EIN LASER, mW/Hz =
2
M dc
RIN RATIO ( ( Idc – I TH) • R IN ------------------Tx, RF
IN LASER, mW/Hz
EINmW/Hz = KTo + KT
10 [ ( EIN, dBm/Hz ) /10]
T = -------------------------------------------------------K – TO
2
1000
=
2
Mdc
IN d B + 10 log ( Idc – I TH ) ( R IN) -------------------
2
Tx, RF
( – 30 )
where Idc is the dc bias current in mA applied to the laser
For example, a link that has an output noise of –85 dBm/
diode, ITH is the laser threshold current, RIN is the laser
Hz and a gain of –40 dB would have an EIN of –125 dBm/Hz,
input dc impedance, Mdc is the dc modulation gain of the
an NF of 49 dB, and a T of 2.3 x 107 K. Equations 11 and 12
laser diode, and ηTx, RF is the RF efficiency of the transmitter
require that EIN be expressed over a 1 Hz bandwidth. With
at the frequency of interest. As an example, a laser biased 60
these terms defined, the four primary noise sources in a
mA above threshold with an RIN of –153 dB/Hz, an input
fiber-optic link can be defined:
impedance of 50 Ω, and a modulation gain ratio Mdc/ηTx,
1. Noise from amplifiers in the transmitter
RF of 1 would have an EINLASER of –130 dBm/Hz. In general,
2. Noise from the laser diode
both RIN and EIN vary with the bias current and frequency.
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MICROWAVE
Shot Noise
Receiver Noise
The second main contributor to link noise is from a subtle
The receiver also will add noise from any amplifiers or
effect called shot noise. Shot noise occurs because light is
resistive matching elements incorporated in or immediately
composed of discrete packets of energy called photons,
after the receiver. Since it is necessary to have such
which convey a signal not as a smooth flow of energy but
amplifiers in most situations, it is important to carefully
instead as a stream of infinitesimal quanta of energy. The
consider its contribution as well. The receiver noise is
randomness of the arrival time of each photon generates a
referred back to the input just as in equation 10 on page 7.
random noisiness in the current at the output of the
EINRx, dBm = NoiseRx, OUTPUT, dBm – GLINK, dB
photodiode:
Receiver noise often is specified by the equivalent noise
equation 15,
iSHOT = (2 • e • Idc • BW)1/2
current. Similar to EIN, the equivalent noise current is the
where iSHOT is the rms value of the shot noise at the
theoretical amount of rms current at the photodiode that
photodiode chip; e is 1.6 x 10–19 Coulombs; Idc is the dc
would be required to create the actual amount observed
electrical current through the photodiode, and BW is the
noise leaving the photodiode, imagining that all other
channel or resolution bandwidth of the measurement. The
components in the receiver had nonoise.
amount of shot noise that leaves the receiver, iSHOT, OUT
equation 18,
will depend on the ratio of the photodiode responsivity and
ENC
the efficiency of the total receiver:
(
Rx, RF
i SHOT, OUT = i SHOT • ------------------R PD
)
(
Rx,RF
)
where iENC is the rms equivalent noise current at the
photodiode and iN,OUTPUT is the actual rms noise out of
The contribution from shot noise per unit bandwidth can be
the receiver. As an example, consider the 50 Ω
referred back to the input of the total link to give:
resistively-matched receiver in Figure 7 on page 7. The
equation 16,
EIN SHOT, mW
R PD
= i N, OUTPUT ------------------
thermal noise of a resistor, here RPD, can be modeled as a
P LASER • R IN
= 2e -------------------------------------------- • L OPT, RATIO
2
( Tx, RF) ( R PD )
current source in parallel with the resistor with an rms
current of:
i RESISTOR
where PLASER is the optical power launched into the fiber
immediately after the laser. For example, if PLASER is 4
mW, R is 50 Ω, ηTx, RF is 0.1 W/A, RPD is 0.75 A/W, and
LOPT,RATIO is 2 (i.e.,3 dB) then:
4 KT ( BW )
= ---------------------------R
1/2
iRESISTOR NOISE = 18 pA/Hz
In Figure 7, half of this 18 pA/Hz will pass through the
matching resistor, RPD, and half will pass through the load,
equation 17,
thus the output noise current is 9 pA/Hz (or in power is –174
dBm/Hz). Using equation 17 and remembering from sec-
EIN SHOT, mW =
2 ( 1.6 × 10
NOISE
– 19
( 4 mW ) ( 50 )
C) ---------------------------------------------------------- ( 2)
2
( 0.1 W/A) ( 0.75A/W)
EINSHOT, mW = 1.71 x
10-14
mW/Hz
tion on Resistively Matched Components, page 7, that the
ratio of RPD/ηRx, RF = 2 for a resistively-matched receiver,
the equivalent noise current for a resistively-matched 50 Ω
receiver is simply 18 pA/Hz.
To convert this equivalent noise current into EIN of the link
EINSHOT, dBm = –137 dBm/Hz.
use the following:
EINLINK, mW/Hz = EINLASER, mW/Hz + EINSHOT, mW/Hz +
EINTH, mW/Hz
10
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Figure 10 shows the benefits that can be achieved when
Total Link Noise
When the laser noise and photodiode shot noise are added
together with receiver thermal noise, as in Figure 9, each
noise source obeys a different law with respect to the
amount of optical loss in a given system. Specifically, the
laser EIN stays constant, the photodiode EIN grows
proportional to the optical loss, and the receiver thermal
EIN grows proportional to the square of the optical loss (Of
course, the actual thermal noise is constant, but since it is
referred back to the link input its contribution relative to
the input signal grows as the losses of the link increase.)
reactive matching is used. Reactive matching at the
transmitter decreases the level of all three components of
the link noise, which means that lower gain pre-amps are
needed to achieve the same signal to noise ratio. Reactive
matching at the receiver further reduces amplification
requirements by increasing the link gain, but it also
specifically lowers the relative contribution of receiver
thermal noise, which is often the limiting factor in longer
links.The Appendix, page 28 includes a more complete set
of EIN curves. Highpower photodiodes also can improve the
noise performance of a link because generally they do not
equation 19,
require extra optical attenuation for protection, thus the
EINLINK, mW/Hz = EINLASER, mW/Hz + EINSHOT, mW/Hz +
EINTHERMAL, mW/Hz
optical losses of link can be lower.
Figure9.CumulativeLossEffectsofLaserNoise,PhotodiodeShotNoise,andReceiverThermal
Noise on Total Link Performance
Figure10.ReactiveMatchingattheTransmitterandReceiverImpartsImprovedNoisePerformance
11
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Cascading Noise Figures
Unconverted Noise and SNR
Once the noise contributions of an optical link are calculated
The noise phenomena described in the previous section all
and reduced to a single quantity for the EINLINK, this noise
occur independent of the presence of an RF signal; how-
can be cascaded with other microwave components in the
ever, there is another class of noise phenomena that occur
system. Friis’ formula states that if a single component with
only when a signal is present. Such noise is present at low
a noise figure of NF1 and gain of G1 is followed by another
frequencies even without a signal, but is translated to the
component with a noise figure of NF2, then the total noise
neighborhood of the signal when the light is modulated, as
figure will be:
shown in Figure 11. This upconverted noise may reduce the
equation 20,
signal to noise ratio (SNR, C/N, or CNR) below what would be
NF , RATIO – 1
NF TOTAL, RATIO = N F 1, RATIO ---------------------------------G 1, RATIO
(
)
calculated if only the EIN was considered. Fabry-Perot lasers
are especially susceptible to this noise and this is reflected in
where both the NF and G must be expressed as ratios when
their SNR specification. For a Fabry-Perot laser, this
computing the algebra. Friis’ formula also can be converted
low-frequency noise results largely from mode partition
to EIN and T by using the definitions in equation 12 and
noise, which increases with fiber length and modulation
equation 13 on page 9:
frequency. For DFB lasers there is only one optical mode (or-
equation 21,
wavelength) so these effects are absent. The remaining
low-frequency noise for DFBs primarily results from Ray-
EINTOTAL, mW/Hz =
EIN 1 ,
mW/Hz +
– KT
------------------------------------------------)
(EIN
G
2 mW/Hz
O
1, RATIO
leigh scattering in the fiber, which only becomes apparent
for links on the order of 20 km or more and for high SNRs.
These upconverted noise sources are described in more de-
equation 22,
tail in the sections on Reflections and Interferometric Noise,
T TOTAL
T1 + T2
= ------------------------G 1, RATIO
page 24, and Distributed-Feedback (DFB) vs. Fabry-Perot (FP)
Lasers, page 25.
Figure11.EffectsofUnconvertedLow-FrequencyNoiseonSNR
12
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Noise-Equivalent Bandwidth
1 dB Compression Point
The SNR of the link depends not only on the RF signal level
The most straightforward limitation on the power of an
outlined above, but also on the noise-equivalent bandwidth.
input signal is the 1 dB compression point, P1dB. At this RF
Wider channel widths will include more noise power and
input power, the output signal is 1 dB less than what would
thus reduce the SNR. Although the link may be passing many
be predicted by the small signal gain of the link. Returning
channels over a wide band, the receiver can be tuned to a
to the L-I curve of the transmitter, Figure 2 on page 4, it can
single channel within this band. This single-channel
be seen that once the magnitude of the signal approaches
bandwidth is important in determining the SNR and, in the
that of the bias current, the signal will clip at the lower
next section, the dynamic range.
level of the curve. This limit defines the 1 dB dynamic range,
equation 23,
DR1dB, as follows:
NoiseCHANNEL = EINdBm/Hz + BWdB, Hz
equation 24,
DR1dB = P1dB – NoiseCHANNEL
DR1dB = P1dB – EINdBm/Hz – 10 log (BWHz)
Dynamic Range and Distortion
While the noise floor determines the minimum RF signal
detectable for a given link, non-linearizes in the laser and
Third-Order Intercept and Spur-Free Dynamic
Range
amplifiers tend to limit the maximum RF signal that can be
A more precise treatment for a large number of carriers uses
transmitted. For links transmitting a single tone where there
the third-order intercept. Even in the middle of the linear
is little concern for it interfering with other signals, the 1 dB
portion of a laser’s L-I curve, non-idealities distort the output
compression point is generally used to specify the dynamic
and cause higher order, intermodulation signals. In
range. For links transmitting a larger number of signals, the
particular, if two equilevel sinusoidal tones at f1 and f2
third-order intercept point is frequently used to calculate
modulate the fiber-optic link, third-order distortion products
the spurfree dynamic range. Both definitions are discussed
are generated at 2f1– f2 and at 2f2 – f1, as shown in Figure
below.
12. The magnitude of these distortion products expressed in
dBm has a slope of three when plotted against the input or
output power level, as shown in Figure 13 on page 14.
Figure12.Third-OrderIntermodulationDistortionSpectrum
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MICROWAVE
Figure13.Third-OrderInterceptandSpur-FreeDynamicRange
To quantify this effect, the slopes of the output signal and
follow the traditional relationship observed in amplifiers
distortion terms are extrapolated to higher power until they
because it is roughly 10 dB above the 1 dB compression
intercept. The input power corresponding to this
point. For lasers, the difference between these two powers
intersection is defined as the input third-order intercept
is very dependent on both the frequency and the dc bias
point (IIP3 or input TOI), which can be calculated by:
current. Additionally, some transmitters include predistort-
equation 25,
ers, which specifically improve the IIP3 without necessarily
C
IIP3 dBm = S IN, dBm + --------------------------- /2
I 2 TONE, dB
(
)
affecting the 1 dB compression point. Once the IIP3 is determined, the spur-free dynamic range (SFDR) can be calculat-
SIN, dBm is the input power of one of the carriers and C/IdB
ed. The SFDR corresponds to the case of a link transmitting
is the ratio of the output power of the carrier to that of one
two input signals of equal power. The SFDR is defined as the
of the intermodulation distortion signals. (For Ortel
range of the two input signals in which the signals are above
components, the input TOI rather than the output TOI is
the noise floor and the third-order
usually specified because the input TOI will be independent
products are below the noise floor. Graphically, this is shown
of the link gain. To find the output TOI, simply add the input
in Figure 13. If the noise floor is lowered either by using
TOI to the link gain in dB.) Equation 25 also can be used to
a quieter laser or by operating over a narrower frequency
approximate the worst-case distortion terms for a given
band then the SFDR will increase at a 2/3 rate, which is the
input power. For example, if the IIP3 of a transmitter is 35
difference between the slope of the output signal and
dBm and a pair of signals is input with –5 dBm in each, then
distortion curves. In dB this gives:
the third-order terms will be 80 dB below these at –85 dBm.
equation 27,
SFDR = 2/3 (IIP3dBm – NoiseCHANNEL) (dB – BW)2/3
equation 26,
C/I2 TONE, dB = 2 (IIP3dBm –SIN, dBm)
C/I2 TONE, dB = 2 (35 dBm – (–5 dBm))
C/I2 TONE, dB = 80 dB
SFDR = 2/3 (IIP3dBm – EINdBm/Hz – 10 log BW) (dBHz2/3)
For example, a link with an IIP3 of 35 dBm and an EIN of
–130 dBm/Hz would have a SFDR of 110 dB-Hz2/3 over a 1 Hz
An important note to make is that this IIP3 power level is
never measured directly because it is strictly a small signal
linearity measurement. The IIP3 of a laser also does not
bandwidth. If the same link had a bandwidth of 1 kHz, then
its SFDR would be 90 dB-kHz2/3. The SFDR value that results
from these calculations can be applied to either the input or
output.
14
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MICROWAVE
Large Number of Carriers
are placed. Specifically, a trade-off must be made between
If a large number of carriers (or channels) are transmitted
the noise and distortion performance of the link. Placing
through a link, then, in certain situations, the distortion
an amplifier before the transmitter raises the signal above
products can be higher than those predicted by the
the noise floor and, therefore, lessens the noise figure of
two-tone IIP3 and SFDR. In particular, when the input
the link. However, if the amplification is too large, then
channels are evenly spaced, several different
the transmitter or the amplifier itself may begin to distort
intermodulationtones may add together at the same
the signal. To avoid such distortion some intermediate level
frequency, creating a stronger third-order term than what
pre-amp is chosen appropriate to the given application. If
would be produced by only two carriers. To approximate the
necessary, another amp after the receiver can then be used
increase in the C/I, the following equation is commonly used:
to provide any additional gain.
equation 28,
EXAMPLE:
Consider an X-band antenna that needs to be remotely
C/ITOTAL, dB = C/I2 TONE, dB – [6dB + 10 log (x)]
operated 5 km from the receiver electronics and has the fol-
where x is a counting term that accounts for the overlap of
lowing RF requirements:
intermods, and the 6 dB term normalizes the result to the
Frequency range: 7.9 GHz to 8.4 GHz,
two-tone case. This equation assumes that all of the input
Five channels,
carriers are equally spaced, add in power (not voltage), and
have equal powers. If they are not equally spaced, then the
two-tone calculations of the section on Third-Order
Intercept and Spur-Free Dynamic Range, page 13, will be
more appropriate.
Channel width = 35 MHz,
SCHANNEL, RF(MIN) = –60 dBm,
SCHANNEL, RF(MAX) = –35 dBm,
SNR = 12 dB,
Total link gain = 0 dB,
Input and output impedances = 50 .
Table1.CarrierandCountingTermCalculations
Transmitter and Receiver Choice
Carriers
X
6 dB + 10 log (x)
2
0.25
0 dB
Several transmitter/receiver pairs cover the range of interest.
3
1
6 dB
For this example, the minimum data sheet specifications for
4
2.3
9.6 dB
5
4.5
12.5 dB
6
7.5
14.8 dB
7
11.5
16.6 dB
8
15.5
17.9 dB
a typical DFB transmitter are used. (Because performance
varies for different products, other values can be
appropriate, depending on the specific transmitter chosen.)
EINLASER < –120 dBm/Hz
PLASER > 2.4 mW
IIP3 > +25 dBm
P1 dB > +13 dBm
dc modulation gain > 0.06 mW/mA
9
20
19.0 dB
10
26
20.1 dB
11
33
21.2 dB
12
40
22.0 dB
13
48
22.8 dB
14
57
23.6 dB
50 Ω and has no built-in amplifiers:
15
67
24.3 dB
RPD > 0.75 mA/mW
16
77
24.9 dB
n > 16
~ (3/8) n2
6 dB + 10 log ((3/8) n2)
For the receiver, consider one that is resistively matched to
Rx, RF
> 0.375 mA/mW
Throughout the example, these minimum specification
Placement of Amplifiers
values will be used with the understanding that the actual
For optical links that incorporate amplifiers, the amount of
link would be expected to perform better than the final
distortion produced will be affected by where the amplifiers
answers.
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MICROWAVE
Gain
Converting this to a noise figure with equation 12 on page
The first thing to consider is the gain. Since the transmitter is
9 gives:
resistively matched for broadband operation, its RF
NFTOTAL LINK = EIN + 174 dBm/Hz
efficiency will be approximately equal to the modulation
NFTOTAL LINK < –147 dBm/Hz + 174 dBm/Hz
gain of 0.06 mW/mA. The optical losses can be determined
NFTOTAL LINK < 27 dB.
with:
Now, the actual performance of the optical link can
equation 29,
computed. The performance curves resented in the appendix
LOPT, dB = (fiber length) • (fiber attenuation) +
(# connectors) • (connector loss)
0.4 dB (OPT)
L OPT, dB = ( 5 k m) • --------------------------------- + ( 2) • ( 0.5 dB max)
km
(
)
(or equations 15, 17, and 18 on page 10) predict that the
above specified transmitter and receiver pair, with an optical
loss of 3 dB, will have a NF of better than approximately:
NFOPTICAL LINK = 54 dB.
LOPT, dB = 3 dB.
Substituting these values into equation 4 on page 6 gives for
The preamplifier and postamplifier also will affect the total
the gain of the optical transmitter, receiver, and fiber:
link NF. If, for example, the selected components are a
equation 30,
preamp with a gain of 30 dB and an NF of 5 dB, and a
GOPTICAL LINK = 20 log( Tx, RF( Rx, RF) – 2 LOPT, dB +
10 log(ROUT/RIN)
postamp with a gain of 9 dB and an NF of 6 dB, then Friis’
GOPTICAL LINK = 20 log [(0.06 mW/mA)
(0.375 mA/mW)] – 2 x 3 dB + 0
the noise figures. First, the optical link and the postamp
GOPTICAL LINK = –39 dB.
equation 32,
formula (equation 19 on page 11) can be used to cascade
should be cascaded as follows:
Alternatively, this value could have been read directly off of
NF OPTICAL LINK AND POSTAMP = ""
the gain curves in the Appendix. To offset this loss,
( NF POSTAMP, RATIO – 1 )
NF OPTICALLINK, RATIO + ------------------------------------------------------------------G OPTICAL LINK, RATIO
NF OPTICAL LINK AND POSTAMP =
amplifiers must be added prior to the transmitter and/or
after the receiver. The placement of the amps will affect
both the noise performance and distortion, as described
(
0
54 dB
---------------10
below.
Noise
– 39 dB
------------------10
Next, the result can be cascaded with the NF of the preamp:
Due to the SNR requirement of 12 dB, the minimum
detectable signal will need to be –72 dBm (i.e., –60 dBm – 12
dB). Therefore, the total noise in any given channel
bandwidth must be less than –72 dBm. A rearranged
NF TOTAL
LINK
= ""
( NF OPTICAL LINK AND POSTAMP – 1
NF PREAMP, RATIO + ----------------------------------------------------------------------------------------G PREAMP
equation 22 on page 12 can then be used to determine the
NF TOTAL
maximum acceptable EIN:
6 dB
------------10
NFOPTICAL LINK AND POSTAMP = 275,000 = 54.4 dB.
Before calculating the noise of the optical link, the overall
link requirements should be converted into a noise figure.
( )– 1 )
)+ -----------------------------------( 10
( )
10
LINK
5 dB
------------10
( )+
= 10
equation 31,
EINTOTAL LINK = NOISECHANNEL – BWdB, Hz
275, 000 – 1
--------------------------------
(
)
(
)
10
30dB
-------------10
F OPTICAL LINK = 278 = 24.4 dB.
EINTOTAL LINK < –72 dBm – 10 log (35 x 106)
Thus, the link satisfies the 27 dB NF requirement.
EINTOTAL LINK < –147 dBm/Hz.
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MICROWAVE
Dynamic Range
Distortion
Next, it must be ensured that the maximum signals do not
For a multitone input, the third-order intermodulation terms
saturate the transmitter. With the 30 dB preamp chosen, the
must not interfere with other signals. As a quick check of
maximum input per channel to the optical transmitter would
such distortion, the spur-free dynamic range can be
be:
calculated:
equation 34,
SCHANNEL, Tx (MAX) = –35 dBm + 30 dB
SFDR = 2/3 (IIP3dBm – EINdBm/Hz – 10 log BW)
SCHANNEL, Tx (MAX) = –5 dBm
SFDR = 2/3 (+ 25 dBm – (–120 dBm/Hz) –
10log(35 x 106 Hz))
Since there are five channels, the total RF power into the
SFDR = 47 dB
transmitter is:
which again indicates that with the correct choice of a
STOTAL, Tx (MAX) = –5 dBm + 10 log(5)
preamp, the optical link can satisfy the 37 dB dynamic range
STOTAL, Tx (MAX) = 2 dBm
calculated above.
The 1 dB compression of the laser is greater than 13 dBm,
To verify that the amplifier chosen is correct, the actual
therefore, the optical link has at least another 11 dB of
power of the intermodulation terms can be calculated using
dynamic range before the laser transmitter begins to clip
equation 25 on page 14:
significantly. (Alternatively, a rough check of the dynamic
C/I2 TONE = 2 (IIP3 – SCHANNEL, Tx (MAX))
range of only the optical portion of the link without any
C/I2 TONE > 2 (25 dBm – (–5 dBm))
amplifiers can be calculated quickly with equation 24 on
C/I2 TONE > 60 dB
page 13.)
This corresponds to an intermodulation signal level of:
The dynamic range requirements of the entire application
I2 TONE <= SCHANNEL, Tx(MAX) – C/I2 TONE
are:
I2 TONE < < –5 dBm – 60 dB
equation 33,
I2 TONE < –65 dBm
DR1dB = P1dB – EINdBm/Hz – BWdB, Hz
The distortion can then be compared with the lowest power
DR1dB = +13 dBm –(–120 dBm/Hz) –10log (35 x 106)
signal at the transmitter input. Although the sensitivity of
DR1dB = 57 dB
the total link needs to be better than –72 dBm, at the
transmitter the minimum input signal will be 30 dB higher
This quick calculation indicates that, by adding a
due to the preamp:
preamplifier with an appropriate gain and reasonable noise,
the optical link can satisfy the dynamic range requirements
SCHANNEL, Tx (MIN) = –72 dBm + 30 dB
of the application for a single channel input.
SCHANNEL, Tx (MIN) = –42 dBm
DRAPPLICATION = [SCHANNEL, RF (MAX) –
SCHANNEL, RF (MIN)] + SNR
Since I2 TONE is well below this lowest input signal of –42
dBm, two tones of –5 dBm in each can be input into the
DRAPPLICATION = [–35 dBm – (–60 dBm)] + 12 dB
DRAPPLICATION = 37 dB
transmitter without creating third-order distortion terms
above the noise floor.
17
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MICROWAVE
Distortion (continued)
The result is still better than the minimum detectable signal
Alternatively, if the input signal of all five channels wer-
of –45 dBm. (In fact, for this case, the –45 dBm is more
raised equally to the maximum, and if they were equally
demanding than necessary. Since it was assumed that all the
spaced in frequency, then their intermodulation products
channels were raised to the same power, the lowest power
could accumulate, as described in the section on Large
signals that would need to be detected would be within
Number of Carriers, page 15.
the SNR of the carrier power level. Since the SNR is only 12
dB, the C/I5 CHANNELS of 47.5 dB far exceed the linearity
Using the counting factor of equation 28 on page 15, the C/I
requirements of this application.) In summary, the above
would decrease by 12.5 dB:
described link should work well in remoting an X-band
C/I5 CHANNELS = C/I2 TONE – [6 dB + 10 log (x)]
antenna.
C/I5 CHANNELS = 60 dB – 12.5 dB
C/I5 CHANNELs = 47.5 dB
Similarly, the intermodulation term would increase by 12.5
dB to:
I5 CHANNELS = –5 dBm – 47.5 dB
I5 CHANNELS = –52.5 dBm
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Selection of Optical Fiber Components
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Although Ortel does not manufacture optical fiber cable,
of the fiber. Total internal reflection occurs because the
the performance of the transmitters and receivers in linear
inner core is composed of a glass with a slightly higher index
fiber-optic links depends on the characteristics of the
of refraction than that of the outer cladding. While
optical fiber being used. The chief parameters for the cable
propagating down the fiber, some of the light is lost due to
are wavelength, loss, dispersion, ruggedization, and
optical absorption from impurities in the glass, scattering
connectorization.
from non-uniformities in the material, and bend loss (when
the fiber bend radius is smaller than roughly 1 inch). The
Wavelength and Loss
unavoidable scattering and absorption losses depend on the
The fundamental part of an optical fiber consists of an inner
wavelength of the light, as shown in Figure 15. Due to the
core and an outer cladding of glass, as shown in Figure 14.
two minima of this curve, the most common fiber
When light is launched in this inner core at a low enough
wavelengths used today are 1310 nm and 1550 nm,
angle, it will remain trapped by the effect of total internal
although some 840 nm fiber is still used for some less
reflection and propagate down the length
demanding applications. Typically, the loss in single-mode fiber at 1310 nm fiber is less than 0.4 dB/km; and at 1550 nm,
0.25 dB/km.
CLADDING
CORE
1-1225F
Figure14.PreciseIndicesofRefractionsEnablesTotalInternalReflectioninOpticalFiber
OPTICAL LOSS, dB/km
10.0
0.1
0.1
800
1000
1200
1400
1800
WAVELENGTH, nm
Figure15.ScatteringandAbsorptionLossesvs.Wavelength
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
dispersion, albeit much smaller than modal dispersion due to
Dispersion
the fact that light of different wavelengths travels at
In addition to loss, a given fiber will have a characteristic
different speeds in a fiber. Figure 16 shows the wavelength
dispersion, which also depends on the wavelength of the
dependency of this dispersion for standard
optical signal. If a square pulse of light is launched into
telecommunication fiber and for special dispersion shifted
a fiber, it will emerge at the end somewhat rounded and
fiber. Roughly 90% of fiber presently installed is the stan-
broadened due to a combination of chromatic dispersion
dard type centered at 1310 nm. The bandwidth limit due to
(wavelength spreading) and modal dispersion. This modal
dispersion is given approximately by:
dispersion occurs in multimode fibers, such as the one shown
equation 35,
in Figure 14 on page 19. These fibers have cores with
BW = λ 1/(2s·η(L))
diameters 50 μm or larger, which allow light to follow a
number of different paths, each with differing lengths and
where s is the fiber dispersion coefficient in ps/(kmnm), λ
transit times. An optical signal that is sent down such a fiber
is the wavelength spread of the optical signal, and L is the
will therefore break up into some combination of these
length of the fiber. This wavelength spread includes both
modes and become smeared out by the time it reaches the
the intrinsic linewidth of the unmodulated laser and any
end of the fiber. To avoid this dispersion problem of mul-
additional wavelength chirp which may result from
timode fiber, single-mode fiber is always used for Lucent’s
modulating the laser. (This chirp will be discussed more fully
fiberoptic links. Single-mode fiber has a core with a typical
in the section on Distributed-Feedback (DFB) vs. Fabry-Perot
diameter of 9 μm, which allows only a single spatial mode to
(FP) Lasers, page 25.) As an example, a laser with a spectral
propagate straight down the center. This single transverse
width of 1 nm transmitting down a 10 km fiber with a
mode also will experience some chromatic
dispersion coefficient of 5 ps/km-nm would produce a
blurred output for signals faster than 10 GHz. In most cases,
this chromatic dispersion is not a limitation for single-mode
DFB transmitters used with fiber of the correct wavelength.
20
10
DISPERSION
(ps/nm * km)
1300
1500
WAVELENGTH
0
1100
1200
1400
1600
1700
–10
DISPERSION-SHIFTED FIBER
STANDARD TELECOMMUNICATION FIBER
–20
1-1227F
Figure16.WavelengthDependencyofDispersionforStandardandDispersion-ShiftedFIber
20
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Ruggedization
Fusion Splice
Once a particular fiber is chosen, the cable style must be
Several techniques are commonly used to connect fibers and
appropriately specified for the application. One of the more
components. The lowest loss and lowest reflection method
popular styles is the ruggedized simplex shown in Figure
is a fusion splice. In this approach, the ends of two fibers are
17. In this fiber, the 9 μm diameter core is surrounded by a
precisely lined up under a microscope and thermally fused
125 μm cladding of glass. Next, a coating typically 250 μm in
together to produce a nearly seamless connection. Typical
diameter is applied to help keep water and other impurities
optical losses are less than 0.1 dB.
from a layer of nylon up to 900 μm in diameter and a layer
of Kevlar * strands to provide extra strength. Finally, the en-
Mechanical Splices
tire assembly is surrounded by a roughly 3 mm diameter yel-
Mechanical splices also can be used. A number of varieties
low PVC jacket. While simplex cable is convenient for indoor
exist which generally involve cleaving the fiber and guiding
applications, for long haul applications exposed to weather
the fiber ends together in a capillary tube. Once aligned, the
more rugged designs featuring additional water resistant
fibers are glued or mechanically held in place. To improve
layers and more heavily armored jackets are typically used.
the reflection characteristics, an index matching gel fills any
These cables may contain anywhere from 2 to 200 fibers.
gap between the fiber tips. While mechanical splices are
* Kevlar is a registered trademark of E. I. du Pont de Nemours and Co.
fairly inexpensive and are easier to use than fusion splicing
equipment, usually losses of 0.5 dB to 1 dB are observed.
These losses also may vary significantly if the temperature is
changed.
CLADDING
CORE
9 μm
2.9 mm
900 μm
500 μm
125 μm
COATING
NYLON
KEVLAR STRANDS
1-1228F
PVC
Figure17.CrossSectionofTypicalRuggedized,SimplexCable
21
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
slightly, then the reflections increase dramatically, which can
Connectors
Perhaps the easiest method for joining fibers, which also
degrade the performance of a link.
produces good results, is reusable connectors. Although a
wide variety of connectors have been developed for fibers,
In contrast, if a pair of APC connectors is separated slightly,
then the reflected light in the fiber core will still remain low
only a few are appropriate for analog fiber optic
applications. One of the best performers is the FC/APC (angle
physical contact) style of connectors. In this connector, the
fiber is glued inside of a precise ferrule and then polished at
an angle of 8°. When two appropriately keyed connectors
are brought together in a mating sleeve, this angle steers
due to the angle of the polish. Therefore, it is recommended
that angle-polished connectors be used whenever possible
with Ortel links to ensure best performance. The following
table summarizes typical performance of FC connectors,
although performance will vary for different manufacturers.
the majority of any reflected light out of the fiber. As a
Table2.FCConnectorPerformance
result the return loss of APC connectors is typically better
than –60 dB.
Connector
Type
Back
Reflections
Insertion
Loss
FC/PC
< –30 dB
< 0.5 dB, 0.25 dB typ
Another popular set of connectors are the FC/PC (physical
FC/SPC
< –40 dB
< 0.5 dB, 0.25 dB typ
contact) and FC/SPC (super PC). These connectors are
FC/APC
< –60 dB
< 0.5 dB, 0.25 dB typ
identical to the FC/APC except that the polish on the end of
the ferrule is not angled. Instead, the ferrule is polished to
form a convex surface so when two connectors are mated,
the centers of their tips physically contact each other to
provide a smooth optical path. Depending on the quality
of the polish, which is indicated by the PC or SPC name, the
To achieve the performance described above, the connectors
must be kept clean and free from scratches. To clean a
connector, gently wipe it with a cotton swab wetted with
isopropyl alcohol and then dry it with dustfree compressed
air (available in convenient cans).
optical back reflections are better than –30 dB or –40 dB,
respectively.
Although this return loss is acceptable for many applications,
if a speck of dust separates the connectors
22
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
then the entire noise spectrum is enhanced without any
Reflections and Laser Noise
The care that is taken in selecting low-reflection optical
connectors and fiber components minimizes several
phenomena that can degrade the performance of an analog
fiber-optic link. The most noticeable effect of optical back
reflections are those that destabilize the laser itself. If a
spiky features. Although Rayleigh scattering is small (evidenced by the low loss of fiber of 0.4 dB/km),
backscattered light can accumulate over a long length of
fiber. Even reflections as low as –70 dB can cause noticeable
noise enhancement. Since Rayleigh reflections are inevitable,
high-performance transmitters incorporate optical isola-
large amount of light is reflected from a discrete
component, then the laser output will include periodic
spikes, as shown in Figure 18. This noiseenhancement can be
as much as 30 to 40 dB, which is unacceptable for most
tors to block any returning light. The section on Additional
Transmitter Considerations, page 25, describes the
integration of such isolators in transmitters.
applications. If the source of reflections is due to
unavoidable Rayleigh scattering in long optical fibers rather
than distinct reflectors (such as poor connectors),
–140 dBc
WITHOUT
REFLECTION
FREQUENCY
AS MUCH
AS 30 dB
–140 dBc
WITH
REFLECTION
FREQUENCY
1-1229F
Figure18.PeriodicSpikesCanDegradePerformanceinAnalogFiber-OpticLinks
23
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
As an example of how interferometric noise is avoided, the
Reflections and Interferometric Noise
spare fibers from an optical splitter in Figure 19 are polished
Even with an optical isolator, care must be taken to
at an angle to provide a clean optical termination. If this
minimize reflections from connectors or optical splitters
were not done, some of the laser light would be reflected
because of the effect of interferometric noise. In fiberoptic
from the termination, pass back through the optical splitter,
links, the signal that arrives at the photodetector is a
reflect off the termination near the laser, pass again through
combination of the primary signal plus a low-level signal
the splitter, finally striking the receivers to create noise.
that is the result of reflections and re-reflections
When such discrete reflections are eliminated by careful de-
throughout the length of the fiber. These various doubly
sign practices, then only the much smaller effects of Rayleigh
reflected versions of the primary signal mix electrically with
scattering remains.
the signal and cause excess noise. The total amount of this
noise depends on the percentage of doubly reflected light,
Polarization Mode Dispersion
which can be significant for discrete reflections from poor
The fiber itself can also distort a signal by the effect of
connectors, splices, or other optical components.
polarization-mode dispersion. This problem tends to occur
only in some older fibers that were not manufactured to the
higher-quality standards used today. For newer fibers this
should not be an issue.
FIBER-OPTIC
RECEIVER
LASER
OPTICAL
SPLITTER
FIBER-OPTIC
RECEIVER
REFLECTION
<–45 dB
OPTICAL
TERMINATION
FIBER-OPTIC
RECEIVER
OPTICAL
TERMINATION
Figure19.Low-ReflectionComponents(TerminatorsandOpticalNoiseSplitters)
Help To Avoid Interferometric Noise
24
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Selection of Optical Fiber Components
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
fiber-coupled with lower efficiency to help attenuate any
Optical Attenuators
Once a fiber and style of connector have been chosen, a link
also might need optical attenuators, as mentioned in section
on Laser Noise, page 9. For a short link, the power from the
laser can be high enough at the receiver to saturate or
damage the photodetector. In these cases where an
attenuator is needed, it should be chosen to provide
sufficient attenuation for the maximum laser power, which
may be significantly more than the minimum specified on
the data sheet. The attenuator also should have a low back
reflection to avoid creating noise.
reflected light. As a result, the gain of transmitters using
isolators is higher than those without isolators.
Distributed-Feedback (DFB) vs. Fabry-Perot (FP)
Lasers
Of the two types of laser diodes used in analog fiberoptic
links, DFBs provide superior performance for lownoise, high
dynamic range systems. Fabry-Perot type lasers provide a
more economical alternative for less demanding
applications. A DFB laser is a special type of laser diode
designed to operate in a single longitudinal mode (optical
frequency), as compared to a conventional F-P laser, which
The section on Link Design Calculations, page 5, discusses
the basic equations necessary to predict the performance
of a link based on the values available in the data sheets.
Although for many applications this information should be
sufficient to allow for a good choice of components, this
section discusses a few of the more subtle considerations
that can affect the performance of a given link.
typically lases in many longitudinal modes (i.e., wavelengths)
at once. DFB lasers acquire this singlemode behavior because
of a grating structure in the laser chip design. The resulting
single-mode output is therefore immune to chromatic dispersion and allows for wider bandwidths over longer fibers
than can be achieved with F-P lasers. DFBs are clearly superior for longer distances. For comparably shorter distances,
however, it might appear that F-P lasers would perform
Additional Transmitter
Considerations
as well despite their multimode pattern, since bandwidth
spreading due to chromatic dispersion would be small. In
Optical Isolators
fact, this is not the case due to the effect of mode partition
One of the simplest methods to improve the performance of
noise present with F-P lasers (see the section on Unconverted
a laser is the use of an optical isolator. These isolators take
Noise and SNR, page 12). In an F-P laser, power constantly
advantage of a phenomenon known as the Faraday effect
shifts between the modes in such a way that the total power
to pass light in one direction, but block light in the other
remains relatively constant. Therefore, when the total
direction. Moreover, they can be made small enough to be
optical power is detected directly from an F-P transmitter,
incorporated into the laser module itself.
little noise is observed. However, when propagated in a
The more common isolator used in Ortel transmitters is a
single-stage isolator, which provides approximately 35 dB
of isolation. For systems requiring long lengths of fiber or
exceptionally good noise performance a 1.5 stage isolator
can be used instead, which provides approximately 50 dB of
fiber, dispersion causes the modes to walk off in time relative to one another. The modes’ fluctuations now no longer
compensate for one another and the total optical power
shows fluctuations originating from this power exchange
process. The resulting noise occurs at a low frequency, but
is upconverted to higher frequencies when a modulating
isolation.
signal is present. Besides this noise, low-frequency modeAlternatively, many applications that have lower noise
hopping noise can manifest itself directly from the FP, even
requirements and less reflection do not require the use of a
without fiber dispersion.
discrete isolator. Some isolation is still needed even in these
applications, so lasers without isolators are purposely
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Additional Transmitter Considerations
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Distributed-Feedback (DFB) vs. Fabry-Perot (FP)
Lasers (continued)
The original RF signals plus the distortion signals are then
For single-frequency lasers such as DFBs, both modeparti-
the primary device will then cancel the injected distortion,
tion and mode-hopping noises are absent. Figure 20 shows
resulting in a highly linear output signal.
applied to the primary device. The distortion generated by
how this noise affects the comparative performance of F-P
The challenge for linearizing DFB lasers is that the distortion
and DFB lasers. Of particular note for the F-P is the dramatic
is often strongly frequency-dependent and is often not
increase in the noise level near the signal for longer lengths
strictly in phase or 180° out of phase with the input signals.
of fiber. When the signal modulation is turned off, this noise
Ortel has pioneered the development of predistortion
disappears as shown in the clean spots in the Figure 20. In
circuits to linearize devices with complex distortion
contrast, sideband noise is negligible for DFB lasers with and
characteristics, such as DFB lasers. The complex nature of the
without modulation and for distances as long as 20 km. The
nonlinearities of DFB lasers does not limit the effectiveness
only additional noise for the DFB is the comparably small
of predistortion, but it is necessary to generate a similarly
interferometric noise from Rayleigh scattering.
complex distortion signal to linearize the DFB lasers.
Predistortion
1310 nm vs. 1550 nm Wavelengths
While DFB lasers provide intrinsically high dynamic ranges
Recently, DFB lasers also have been developed for the 1550
because of their low noise and distortion, their performance
nm wavelength to complement the capabilities of
is improved still further for some transmitters by adding
established 1310 nm systems. The strongest advantage of
predistortion electronics. Predistortion is a linearization
1550 nm is its lower attenuation in single-mode fiber, as
technique that has been known for many years in the fields
shown in Figure 15 on page 19 and in Table 3. For a given
of RF and microwave electronics. With predistortion, the RF
input power, a 1550 nm signal can travel approximately 60%
signals are first passed through a device carefully designed
farther in a fiber than a similar 1310 nm signal can. This is
to generate distortion, which is equal in amplitude and op-
especially advantageous in long links or delay lines. How-
posite in sign to that produced by the primary device (the
ever, since 1310 nm lasers can produce more power than can
laser in this case).
1550 nm lasers, shorter links still favor 1310 nm transmitters.
6 km
6 km
km
11km
0.0
0.0
–10.0
-10.0
–20.0
-20.0
–30.0
-30.0
–40.0
-40.0
–50.0
-50.0
–60.0
-60.0
–70.0
-70.0
–80.0
-80.0
–90.0
-90.0
100.0
100.0
RBW=1
RBW = 1 MHz
MHZ
F-P
FP
Noise
w/o
NOISE W/O
Signal
SIGNAL
5.5
5.5
6.5
6.5
7.5
7.5
0.0
10.0
–10.0
0.0
–20.0
-10.0
–30.0
-20.0
–40.0
-30.0
–50.0
-40.0
–60.0
-50.0
–70.0
-60.0
–80.0
-70.0
–90.0
-80.0
100.0
-90.0
F-P
FP
NOISE W/O
Noise
w/o
SIGNAL
Signal
5.5
5.5
FREQUENCY (GHz)
FREQUENCY
(GHz)
0.0
0.0
–10.0
-10.0
–20.0
-20.0
–30.0
-30.0
–40.0
-40.0
–50.0
-50.0
–60.0
-60.0
–70.0
-70.0
–80.0
-80.0
–90.0
-90.0
100.0
100.0
DFB
DFP
5.5
5.5
6.5
6.5
0.0
10.0
RBW=1
–10.0
RBW = 1 MHz
MHZ
0.0
F-P
FP
–20.0
-10.0
–30.0
-20.0
–40.0
-30.0
–50.0
-40.0
–60.0
-50.0
–70.0
-60.0
–80.0
-70.0
Noise
NOISEw/o
W/O
–90.0
-80.0
Signal
SIGNAL
100.0
-90.0
7.5
5.5
6.5
7.5
5.5
6.5
7.5
7.5
FREQUENCY (GHz)
FREQUENCY
(GHz)
0.00.0
–10.0
-10.0
–20.0
-20.0
–30.0
-30.0
–40.0
-40.0
–50.0
-50.0
–60.0
-60.0
–70.0
-70.0
–80.0
-80.0
–90.0
-90.0
100.0
100.0
5.5
7.5
7.5
5.5
RBW=100
RBW = 100 kHz
kHZ
6.5
6.5
km
2020km
RBW=1
RBW = 1MHz
MHZ
FREQUENCY (GHz)
FREQUENCY
(GHz)
DFB
DFP
FREQUENCY (GHz)
FREQUENCY
(GHz)
RBW=100
RBW = 100 kHz
kHZ
6.5
6.5
7.5
7.5
0.0
0.0
–10.0
-10.0
–20.0
-20.0
–30.0
-30.0
–40.0
-40.0
–50.0
-50.0
–60.0
-60.0
–70.0
-70.0
–80.0
-80.0
–90.0
-90.0
100.0
100.0
RBW = 100 kHz
kHZ
RBW=100
DFB
DFP
5.5
5.5
FREQUENCY (GHz)
FREQUENCY
(GHz)
6.5
6.5
7.5
7.5
FREQUENCY (GHz)
FREQUENCY
(GHz)
26
Figure20.ComparativeNoisePerformanceforDFBandF-PLasers
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Additional Transmitter Considerations
(continued)
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
1310 nm vs. 1550 nm Wavelengths (continued)
laser transmitting down a standardfiber due to the fact that
presently 1310 nm lasers have inherently lower chirp and
Table 3. Comparison of 1310 nm and 1550 nm Fiber-Optic Links
noise than comparable 1550 nm lasers.
Parameter
1310 nm
1550 nm
Fiber Attenuation
0.4 dB/km
0.25 dB/km
Fiber Dispersion
Nearly Zero
Zero Only for Special Fiber
Laser Chirp
Lower
Higher
different advantage in their capability to be used with
Power
Higher
Lower
optical amplifiers. Such amplifiers increase the power of an
Noise
Lower
Higher
Optical Amps
Laboratory Based
Commercially Mature
While 1310 nm lasers have better noise and distortion
performance than 1550 nm lasers, 1550 nm lasers have a
optical signal without the need for traditional repeaters,
which convert the light to an RF signal, amplify it electrically,
and then reconvert it back into light. Due to the physics of
Another advantage of 1310 nm lasers is the zero dispersion
the materials used in these fiber-optic amplifiers, 1550 nm
point in single-mode fibers, which naturally occurs near this
amplifies much better than does 1310 nm, therefore nearly
wavelength. For lasers with wide optical bandwidths,
all systems incorporating optical amplifiers are designed for
chromatic dispersion (see the section on Dispersion, page 20)
1550 nm.
will limit the transmission distance for high frequency 1550
nm systems using standard single-mode fiber. By using a
Another application where 1550 nm lasers complement
1310 nm lasers is in wavelength-division multiplexing
single wavelength 1550 nm laser and/or special
dispersion-shifted fiber, this particular limit usually can be
overcome. Even with single-frequency DFBs at 1550 nm, the
high dispersion of standard fiber can still pose a problem
at this wavelength due to laser chirp. When the intensity of
the light from a DFB is modulated, the optical frequency
(wavelength) is inadvertently modulated as well. As light
from such a modulated laser travels down the fiber, the
(WDM). In WDM, a single fiber can be used to send two sets
of signals, one at 1310 nm and the other at 1550 nm. By
using wavelength-selective components, these signals can be
separated from each other and returned to their original RF
format. This type of application is especially popular where
the cost of laying additional fibers is prohibitive. In a typical
implementation, a 1550 nm transmitter sends a signal on
a fiber to a remote antenna where another transmitter at
portions of the signal corresponding to different
wavelengths will travel at different speeds due to the
chromatic dispersion of the fiber. By the time the signal
1310 nm sends a return signal down the same fiber back to
the base station.
reaches the receiver, these various wavelengths will be
Summary
spread out in proportion to both the intensity of the
In this guide, the primary technical issues involved in using
modulating signal and the value of the fiber dispersion.
Ortel’s linear fiber-optic links are discussed. By taking
This leads primarily to second-order distortion, which in-
advantage of the high performance and reliability of solid-
creases as the square of the frequency of the
state transmitters and receivers, and the low loss and high
distortion products.
bandwidth of optical fibers, a diverse set of applications has
A 1550 nm laser transmitting down a sufficiently long length
of standard single-mode fiber intended for 1310 nm will
therefore have higher distortion products than a similar
1310 nm laser. In fact, even with dispersion-shifted fiber
intended for 1550 nm, both the distortion and the noise of a
1550 nm laser will be worse than that for a 1310 nm
been enabled, including RF and microwave antenna
remoting, broadband CATV distribution, and compact
optical delay lines. In addition, due to the modularity of
these fundamental optical components, other forthcoming RF and microwave applications can be expected to take
advantage of these capabilities as well.
27
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Appendix
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Performance Characteristics
Gain
The following curves are calculated from the gain equation below. In all cases, the links are assumed to have ROUT = RIN.
GLINK, dB = 20 log(
Tx RF)( Rx, RF)
Electrical/Optical
Efficiency
– 2 LO, dB + 10 log(ROUT/RIN)
Impedance
Difference
Optical
Loss
Where
= transmitter RF efficiency (mW/mA),
receiver RF efficiency (mW/mA),
RPD = photodiode responsivity (mW/mA),
2 LO, dB = optical losses in dB
ROUT = output impedance
RIN = input impedance
Tx, RF
Rx, RF =
For some receivers, in cases where a link is too short, it might be necessary to add optical attenuators to prevent damage to the
photodiode, which thereby determines a minimum optical loss for a given transmitter/receiver pair. Reactively matched or amplified transmitters and receivers provide higher gain due to these extra components. To calculate the total gain for such links,
use the unmatched or unamplified efficiencies then add the improvement specified in the data sheet.
–20
LINK GAIN (dB)
–30
Tx, RF
= 0.1
Tx, RF
= 0.075
–40
–50
Tx, RF
= 0.06
–60
–70
Tx, RF
= 0.02
–80
0
5
10
15
20
OPTICAL LOSSES (dB)
Note: RPD = 0.75;
= 0.375.
Figure21.GainCurve1,ResistivelyMatchedPhotodiodefor50Ω
RX, RF
1-1119F
–20
Tx, RF
= 0.1
LINK GAIN (dB)
–30
Tx, RF
= 0.075
–40
Tx, RF
–50
= 0.06
–60
Tx, RF
= 0.02
–70
–80
0
5
10
15
20
OPTICAL LOSSES (dB)
Note: RPD = 0.75;
RX, RF
= 0.75.
1-1120F
Figure22.GainCurve2,UnmatchedPhotodiode
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Appendix
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Gain (continued)
–20
Tx, RF
= 0.1
LINK GAIN (dB)
–30
Tx, RF
= 0.075
–40
Tx, RF
= 0.06
–50
–60
Tx, RF
= 0.02
–70
–80
0
5
10
15
20
OPTICAL LOSSES (dB)
Note: RPD = 0.85;
RX, RF
= 0.85.
Figure21.GainCurve1,ResistivelyMatchedPhotodiodefor50Ω
Total Link Noise
The following equivalent input noise (EIN) curves represent the combined effect of shot noise, receiver thermal noise, and
laser noise when referenced to the input of the transmitter. Neither fiber-induced effects nor up-converted noise, most notably
mode-partition noise for Fabry-Perot lasers, are accounted for in these calculations. To find the EIN or noise figure for a given
link, first determine the appropriate set of curves based on the transmitter and receiver efficiencies and the laser power. Next,
determine the EIN of the transmitter for the frequency of interest and locate the appropriate curve. Each curve below corresponds to a specific laser EIN, separated by increments of 5 dB, and corresponds to the value at the intersection of the left
axis. (For 0 dB, optical losses, the total EIN is approximately equal to the EIN of only the laser.) For example, if a transmitter and
receiver corresponds to Noise Curve 1, and if the transmitter EIN is –110 dBm/Hz and the optical link losses are 10 dB, then the
–95
79
–100
74
–105
69
–110
64
–115
59
–120
54
–125
0
10
5
15
20
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
graph predicts an EIN for the complete optical link of –107.5 dBm/Hz and a noise figure of (NF) of 66.5 dB.
49
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.02 mW/mA; PLASER = 0.4 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1122F
29
Figure24.NoiseCurve1,EquivalentInputNoisevs.OpticalLosses
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
–95
79
–100
74
–105
69
–110
64
–115
59
–120
54
–125
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Total Link Noise (continued)
49
0
5
15
10
20
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.02 mW/mA; PLASER = 0.8 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1123F
–100
74
–105
69
–110
64
–115
59
–120
54
–125
49
–130
44
0
Note:
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Figure25.NoiseCurve2,EquivalentInputNoisevs.OpticalLosses
Tx, RF
5
10
15
20
OPTICAL LOSSES (dB)
= 0.05 mW/mA; PLASER = 3.0 mW; Rx, RF = 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1124F
Figure 26. Noise Curve 3, Equivalent Input Noise vs. Optical Losses
30
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MICROWAVE
–100
74
–105
69
–110
64
–115
59
–120
54
–125
49
–130
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Total Link Noise (continued)
44
0
5
15
10
20
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.06 mW/mA; PLASER =2.4 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1125F
–100
74
–105
69
–110
64
–115
59
–120
54
–125
49
–130
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Figure27.NoiseCurve4,EquivalentInputNoisevs.OpticalLosses
44
0
5
10
15
20
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.06 mW/mA; PLASER = 3.0 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1126F
Figure 28. Noise Curve 5, Equivalent Input Noise vs. Optical Losses
31
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MICROWAVE
–100
74
–105
69
–110
64
–115
59
–120
54
–125
49
–130
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Total Link Noise (continued)
44
0
5
10
15
20
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.75mW/mA; PLASER = 3.0 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1127F
–100
74
–105
69
–110
64
–115
59
–120
54
–125
49
–130
NOISE FIGURE (dB)
EIN TOTAL (dBm/Hz)
Figure29.NoiseCurve6,EquivalentInputNoisevs.OpticalLosses
44
0
5
10
15
20
OPTICAL LOSSES (dB)
Note:
Tx, RF
= 0.1 mW/mA; PLASER = 4.0 mW;
Rx, RF
= 0.375 mA/mW; RPD = 0.75 mA/mW.
1-1128F
Figure 30. Noise Curve 7, Equivalent Input Noise vs. Optical Losses
32
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
ac
Alternating current.
ALC
Automatic leveling control. A feedback circuit that
maintains constant laser optical power.
AM
Amplitude modulation.
Amplification
Strengthening of a signal by an electrical device.
Analog
A type of signal that represents information as a continuously varying voltage,
current, or intensity level. Analog transmission is typically used for video and
POTS (plain old telephony service).
Available output power
The maximum RF power that can be delivered by a device to a load; occurs when
the load impedance is the complex conjugate of the device output impedance.
Backbone
The part of the communications network intended to carry the bulk of the
traffic.
Bandwidth
The information-carrying capacity of a communications channel measured in bitsper-second for digital systems or in megaHertz for analog systems.
Fiber bandwidth-length product
Fiber acts as a low-pass filter on the optical modulation because of dispersion.
The bandwidth-length product gives the frequency at which the modulation is
effectively attenuated by 3 dB after a given length of fiber (usually 1 km). Note
that dispersion does not attenuate the light, only the modulation. (See Dispersion, page 20.)
Laser modulation bandwidth
Modulation of the laser input injection current causes the laser optical output
intensity to be modulated at the same rate. The frequency at which the amplitude of the optical modulation has dropped to 0.707 (1/√2) of its mean value is
the laser modulation bandwidth. When detected by a photodiode with a flat
frequency response, the laser modulation bandwidth corresponds to the 3 dB
electrical bandwidth.
Photodiode modulation bandwidth
Intensity modulation of the light incident on a photodiode causes the photodiode output current to be modulated at the same frequency. The photodiode
modulation bandwidth is that frequency at which the square of the amplitude
of the photodiode output current is one-half its mean value (given a light source
with a flat frequency response).
Noise-equivalent bandwidth
Given the amplitude frequency response of an RF device, the noise-equivalent
bandwidth is defined as the width of a rectangle whose area is equal to the
total area under the response curve and whose height is that of the maximum
amplitude of the response. This is the bandwidth used to compute the total noise
power passed by a device and is generally not the same as the 3 dB bandwidth.
33
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Baseband
A transmission scheme in which the signal is sent in its native format, without
modulation to a higher frequency carrier.
Bellcore
Bell Communications Research, Inc. Focuses on standards, procedures and
research and development of interest to the RBOCs. Now called Telcordia
Technologies*.
Bend radius
The radius to which either a coaxial or fiber cable can be bent before breaking.
More typically, it is the radius to which a cable can be bent before the signal
being carried is affected.
Buffered fiber
Also called tight buffered fiber. Coated fiber surrounded by a 900 μm layer of
plastic or nylon.
Broadband
A communication network or channel capable of carrying large amounts of
information.
Broadcast
To send information over a data communications network to two or more
devices simultaneously.
Cable Wires
Wires or groups of wires carrying voice or data transmissions.
CATV
Community antenna television; cable television.
C band
In electrical networks, a portion of the radio frequency spectrum.
Communication satellites operate on C-band frequencies from 5.925 to 6.425
GHz for uplinks and 3.700 to 4.200 GHz for downlinks. In optical networks, a
range of wavelengths between 1535 nm and 1565 nm.
CE certified
Signifies that a company has met the applicable health, safety, and conformity
requirements to market its products in the European Union.
CENELEC
Comite European de Normalisation Electrotechnique or the European Committee
for standardization. Responsible for European standards in the electrotechnical
field.
Central office
The building that houses the switching equipment to which are connected
circuits of business and residence phones; also called exchange.
Cladding
The glass that surrounds the central fiber core and has a lower index of
refraction than the core.
C/N
Carrier-to-noise ratio. The ratio of carrier power to noise power in a
communication channel.
Coaxial cable
A transmission medium consisting of insulated core surrounded by a braided
shield.
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Coaxial network
A network that uses coaxial cable.
Coaxial run
Any length of coaxial cable that connects devices.
Composite fiber cable
A fiber-optic cable with copper conducts included for dc transmission.
Compression
Refers to the nonlinear behavior of the RF device. As the input RF power is
increased, the output power increases by less and less. When the output power is
1 dB lower than it would be if the device were perfectly linear, the device is said
to be operating at its 1 dB compression point.
Conductor
Any material, such as aluminum or copper, capable of transmitting electric
current.
Convergence
The gradual blurring of telecommunications, computers, cable, and the Internet
into a single system.
CSO
Composite second-order distortion. In CATV systems or devices, a measure of the
undesired second-order distortion.
CTB
Composite triple-beat distortion. In CATV systems or devices, a measure of the
undesired third-order distortion.In electrical networks, a portion of the radio
frequency spectrum.
Core
Central glass (in an optical fiber) having an index of refraction larger than that of
the surrounding cladding glass. The region of the core serves as a waveguide for
the light propagating through the fiber.
Data communications
The transfer of data between points.
Dark current
The current through a photodiode when no light is present.
dB
Decibel, defined as 10 multiplied by the logarithm of the ratio of two power
levels.
DBS
Direct broadcast satellite.
dc
Direct current.
Delay
Wait time between two events. For example, the time between when a signal is
sent and when it is received.
Direct broadcast satellite
System of delivering satellite television signals direct to the home.
Diplexer
A device that allows the feeding of signals from two transmitters to a single
antenna at the same time without interference.
Direct modulation
The process in which an RF or digital signal is applied directly to a laser for
transmission.
35
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Dispersion
The delay distortion in a fiber that results in the spreading of a narrow pulse as it
propagates through the fiber. Specified in ps/(km-nm); the amount of spreading
of a 1 nm-wide pulse through 1 km of fiber.
Chromatic (spectral) dispersion
This dispersion is the sum of the material and waveguide dispersions and occurs
in both multimode and single-mode fiber. Since fiber has optical
wavelength-dependent propagation characteristics, this phenomenon occurs
when using a multiwavelength laser.
Modal dispersion
This dispersion occurs in cases where the multimode fiber core diameter is large
compared to the wavelength of light. The light inside the fiber breaks into
numerous spatial modes, each arriving at the fiber output a different time.
Distortion
The variance between two measures of a signal; for example, a signal as it is
transmitted versus the signal as it is received.
Distributed-feedback (DFB) laser
Instead of using end reflectors (see Fabry-Perot lasers), the DFB laser structure
uses a periodic variation of the refractive index within the optical waveguide.
The result is a laser with single optical wavelength, which produces a lower-noise
output that is usually observed with multimode lasers.
DTH
Direct to home. Predecessor to DBS (direct broadcast satellite).
DWDM
Dense wavelength-division multiplexing. Using optical multiplexers and
optical amplifiers, DWDM increases capacity by combining multiple optical
signals so they can be amplified as a group and transported over a single
fiber.
Dynamic range
The range of a signal power (input or output) that can be handled by a
system. It is limited by the sensitivity requirement at the low end and the
linearity requirement at the high end. The linearity limit is usually specified
by the 1 dB compression or the third-order intercept. (See also spur-free
dynamic range.)
E2000 *
A type of optical connector.
Earth station
A ground-based antenna system used to send or receive signals to or from
a satellite.
EDFA
Erbium-doped fiber amplifier. A means of fiber optic amplification. The
transmitted light signal is passed through a section of fiber doped with
erbium, a rare earth element, and is amplified by a pump diode.
EMI
Electromagnetic interference.
External modulation
Modulation of a light source by an external device such as an
interferometricor electoabsorbtive modulator.
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Fabry-Perot (FP) laser
A laser that uses the reflection from the laser facets to provide the feedback
necessary for the lasing process. Many varieties of these lasers simultaneously
emit multiple optical modes. (See laser noise.)
FC/PC, /APC, or /APC
A popular style of fiber-optic connectors, commonly used with linear fiberoptic
links. The terms following the slash indicate the type and quality of polish, for
example, FC/APC is face-contacted/angled polished connector.
Ferrule
A component of a fiber optic connection that holds the fiber in place and aids in
its alignment.
Fiber
In fiber optics terms, refers to fiber made of very pure glass.
Fiber length
The physical length of the fiber. This length can then be translated to optical loss.
Fiber optics
A technology that uses light to transmit information from one point to
another.
Flatness
The peak-to-peak deviation in amplitude of a signal over a given frequency
or wavelength range.
Frequency stacking
The process in which allows two identical frequency bands can be sent over a
single cable. This occurs by upconverting one of the frequencies and stacking
it with the other.
FTTC
Fiber to the curb.
FTTH
Fiber to the home.
Fusion splice
A splice created by applying heat to fuse or melt the ends of two optical
fiber cables, forming a continuous single fiber.
Gigabit
One thousand million bits.
GHz
Gigahertz (Hz x 10 ). 9
GPS
Global positioning satellite.
Headend
The originating point of a signal in a cable TV system.
Hot swappable
The ability of a component to be added or removed from a device without
requiring that the device be powered down.
Intermediate frequency. Typically refers to 70 MHz/140 MHz for satcom
IF
applications. In headend applications, it refers to the 950 MHz to 2050 MHz
(L band).
IGC
Input gain control.
Index of refraction
The ratio of the phase velocity of light in a vacuum to that in the medium.
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Input power
See optical input.
IP telephony
The transmission of voice over an internet protocol (IP) network. Also called voice
over IP (VOIP), IP telephony allows users to make phone calls over the Internet,
intranets, or private LANs and WANs that use the TCP/IP protocol.
ITU
A committee of the International Telecommunications Union that
recommends international standards for radio, television, and
telecommunication signals.
ITU grid
In DWDM systems, an assignment of standard wavelengths.
Ku band
A portion of the radio-frequency spectrum. Communication satellites operate
on Ku-band frequencies from 14.0 MHz to 15.5 MHz for uplinks and 12.2 MHz
to 12.7 for downlinks. DBS satellites operate on Ku-band frequencies from
17.3 GHz to 17.8 GHz for uplinks and 12.2 GHz to 12.7 GHz for downlinks.
LAN
Local area network. A network that interconnects devices over a
geographically small area, typically in one building or a part of a building.
Laser
A device that produces a single frequency of light.
L band
In electrical systems, a portion of the radio-frequency spectrum, 950 MHz to
2050 MHz, used in satellite, microwave, and GPS applications. In optical
systems, the range of wavelengths between 1570 nm and 1620 nm.
LEC
Local exchange carrier. An organization that provides local telephone service
and includes the RBOCs, large companies such as GTE, and hundreds of small,
rural telephone companies. A LEC controls the service from its central office
(CO) to subscribers within a local geographic area.
Link budget
A means of calculating the overall system’s performance based upon a set
criteria.
Link gain
The amount a signal’s power has gained, expressed in dB. This occurs when
the loss in the components in the system is less than the gain of the system’s
components.
Link loss
The amount of a signal’s power lost, expressed in dB. This occurs when the
loss in the components in the system exceeds the gain of the system’s
components.
Link margin
The available power remaining in the system once its operating point has
been obtained.
LNB
Low-noise block-converter.
LNBF
Low-noise block-converted frequencies.
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Mbits/s
A measurement of the speed of digital communications channel; one million bits
per second.
MDU
Multiple dwelling units. Apartment and condominium complexes, hospitals,
dormitories.
Mechanical splicing
An optical fiber splice accomplished by fixtures or material rather than
thermal fusion.
Metro
Metropolitan fiber ring. High-speed local network that connects local
telephone users to long distance networks.
MHz
Megahertz. Refers to a frequency equal to one million hertz.
Micron
A unit of measurement, equaling one-millionth of a meter.
Microwave
Line of sight, point-to-point transmission of signals at high frequency.
Modem
Modulator/demodulator. A DCE (data circuit-terminating equipment)
installed between a DTE (data terminal equipment) and an analog
transmission channel, such as a telephone line. A DTE refers to a device that
an operator uses, such as a computer or a terminal.
Modulation depth
The ratio of the peak amplitude of the laser intensity modulation to the average laser intensity.
Modulation gain
The efficiency with which the laser converts input injection current to light
output power coupled into the fiber.
Monitor photodiode
A photodiode included in a laser module to convert the detected laser power
to a current that is used for automatic leveling control.
MSO
Multiple system operator. MSO modulation gain is a function of the
frequency of the modulation. At dc, it is equal to the light vs. current slope;
specified in mW/mA.
Multimode fiber
Optical fiber with a core diameter that is large compared to the wavelength
of light. This results in many (hundreds) spatial modes inside the fiber. For
this reason, multimode fiber is not used with linear fiber-optic links.
Multiple system operator
Cable television company that operates more than one cable system.
Multiple-user
The ability to have simultaneous users.
mW
Milliwatt. A unit of measure for power equal to one one-thousandth of a
watt.
Noise
On a communications channel, extraneous signals that degrade the quality or
performance of the link.
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Equivalent input noise (EIN)
The noise power at the input of an RF device that would produce the
observed available output noise power if the device itself were noiseless.
Specified as noise density in mW/Hz or dBm/Hz.
Laser noise
Fluctuations in the optical output of a laser generally due to any of three
major effects:
First, the intrinsic laser noise is due to a number of fundamental processes
within the laser diode. Second, optical feedback noise is due to light reflected
back into the laser diode, disturbing the laser oscillation. Third,
partition-mode noise occurs with Fabry-Perot lasers with multiple
longitudinal modes coupled into a long length of fiber. Optical power is
continuously and randomly being transferred from one mode to another
in the new laser spectrum. This phenomenon, together with the chromatic
dispersion of the fiber, produces noise at the fiber output. Using a fiber with
a zero dispersion point at the laser center wavelength minimizes this noise.
Noise-equivalent power (NEP)
The amount of optical power incident on a photodiode that would generate
a photocurrent equal to the total photodiode noise current.
Noise figure
The ratio of the available output noise power of an RF device to that of a
noiseless but identical device when the input is passively terminated with a
conjugate match at a standard temperature (290 K).
Relative intensity noise (RIN)
When a laser diode is biased above the threshold, the emitted light exhibits
small intensity fluctuations around the average value. The RIN is defined as
the ratio of the mean square intensity fluctuations to the square of the
average intensity.
Shot noise
The noise generated in a photodiode (when detecting optical signals) due
to the discrete photon nature of light.
Thermal noise (Johnson or Nyquist noise)
Voltage fluctuations at the terminals of a device due to the random thermal
motion of charge carriers.
NTSC
National Television Systems Committee. Sets television standards in the
United States. Also the video format used in the United States.
OC-192
Optical carrier level 192. SONET channel capable of carrying 10 gigabits per
second.
OC-48
Optical carrier level 48. SONET channel capable of carrying 2.488 gigabits
per second.
Optical fiber
Long strands of glass, thinner than a human hair, that propagate a lightwave
signal for use in broadband communications.
40
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Optical input
The optical input to a photodiode or optical receiver.
Optical isolator
A device used to suppress or redirect backscattered or backreflected light.
Optical loss
The amount of a signal’s power lost, expressed in dB. This is due to the length
of a fiber, amount of splices, bend radius, or any mechanical external factors
placed on the fiber.
Optical output
The optical output power of a laser or optical transmitter.
Optical reflections
The optical power (expressed in dB) reflected by a component or an assembly.
Optical return loss
The ratio (expressed in dB) of optical power reflected by a component or an
assembly to the optical power incident on a component port when that
component or assembly is introduced into a link or system.
Optoelectronics
Materials and devices associated with fiber-optic and infrared transmission
systems. Optoelectronic light sources convert electrical signals to an optical
signal that is transmitted to a light receiver and converted back to an
electrical signal.
Output power
See optical output.
PAL
Phase-alternate line, a video format used in most parts of the world.
Photodiode
Light detector in a fiber-optic signal transport system that generates an
electric current in proportion to the intensity of the light falling on it.
APD
Avalanche photodiode. A semiconductor device that combines the detection
of optical signals with internal amplification of the photocurrent.
PIN Photodiode
A particular type of photodiode structure (positive-intrinsic-negative)
characterized by favorable noise, bandwidth, and linearity properties.
Photodiode current monitor
A device that converts electrical current to a dc voltage used to monitor
optical power. This conversion is typically a 1:1.
Photonics
In communication technology, the use of fundamental particles of light,
called photons, to form coded light pulses that convey information in digital
form.
Pigtail
A short piece of unconnectorized optical fiber connected to either the transmitter or receiver. The pigtail, in turn, would be spliced to the transmission
fiber.
Polarization
A technique used in satellite communications to increase the capacity of the
satellite transmission channels by reusing the satellite transponder
frequencies. Also, the direction of the electric field in the lightwave.
41
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Radio frequency
The RF carrier frequency of a given device.
RBOC
Regional Bell operating company.
Receiver
A device that receives a transmitted signal.
Receiver gain
The difference between the RF signal presented to the input of the receiver
versus the RF signal at the receiver output. This gain will not always represent
itself as an increase in signal, since the losses in the receiver may be greater
than its internal amplifier.
Redundancy
Having one or more backup systems available in case the primary system fails.
Redundant link
A second connection between transmission and receive devices that operates
only when integrity is lost on the active link.
Responsivity
The efficiency with which a photodiode converts incident optical power into
output electrical current. Specified in units of mA/mW.
Return loss
A comparison of impedance at the point of transmission and at termination.
RF
Radio frequency.
RF efficiency, receiver
The efficiency with which a complete optical receiver, including the
photodiode and any other electronics, converts light into usable RF signals;
specified in mA/mW.
RF efficiency, transmitter
The efficiency with which a complete optical transmitter, including the laser
diode and any other electronics, converts RF signals into modulated light;
specified in mA/mW.
RF Input
The signal in dBm as presented to the input of a device.
RF Loss
A measure of RF signal loss through a device or link.
RF Output
The signal, measured in dBm, at the output of a device.
RG
Receiver gain.
SBS
Stimulated Brillouin scattering.
SC/APC
Subscription channel/angled polished connector.
Semiconductor
A crystalline solid that offers the behavior properties of a conductor (e.g.,
iron) and an insulator (e.g., glass). Semiconductors are the raw materials used
in active electronic and optical devices.
Sensitivity
The input level required for a device to provide a predetermined output.
Short haul
According to some definitions, a distance between several hundred yards and
42
20 miles.
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RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Single-mode (monomode) fiber
Optical fiber having a core diameter small enough to allow only one spatial
mode (of each polarization) to propagate at the wavelengths of interest.
Single thread
A connection between transmission and receive device that operates without
the benefit of redundancy.
SONET
Synchronous optical network. SONET is a Telcordia Technologies (formerly
Bellcore) specification currently used in worldwide public data networks
(PDNs). It defines a synchronous optical network-based user-network
interface (UNI), either public or private, operating at speeds from 51 Mbits/s.
Source laser
A laser used as a light source in an externally modulated system.
Stimulated Brillouin scattering
A nonlinear phenomenon that can cause distortion of an optical signal in a
fiber.
Spectral width
The full-width half maximum (FWHM) of the output optical spectrum of a
laser.
Splice
Any number of permanent and semipermanent fiber connections.
Spur-free dynamic range (SFDR)
The range in power of a pair of equilevel input signals in which the signals
are above the noise floor and the third-order products are above the noise
floor.
TEC
Thermoelectric cooler. A device that uses the Peltier effect to heat or cool
as necessary to keep the laser temperature constant.
Telecommunications
The transmission, reception, and switching of electrical or optical signals
over wire, cable, or air.
TG
Transmitter gain.
Third-order intercept (TOI)
A specification used to characterize the small signal nonlinearity of an RF
device. An extrapolation of the results of a small signal two-tone test to the
point where the level of the third-order intermodulation would be equal to
that of the two tones.
Third-order intermodulation product
A spurious output signal produced by the nonlinear mixing of multiple input
signals.
Threshold current (ITH)
The input electrical current into the laser diode, above which the diode
emits light as a laser.
TIA
Transimpedance amplifier. An electrical circuit or device that accepts a current
at its input and generates a voltage at its output.
43
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Glossary
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
Transmitter
In fiber-optic communications, a light source that emits a beam that can be
modulated and sent along an optical fiber, and the electronics that support it.
TVRO
Television, receive only. An earth station designed to handle downlink signals
only.
Twisted pair cable
A cable consisting of two or more copper wires twisted together in pairs.
Telephone wiring is an example of twisted-pair cable.
Vdc
Volts dc.
Video on demand
The ability for a subscriber to an interactive TV service to select and view a
specific program provided by a device such as an interactive video server.
Voice over IP
The delivery of voice information in digital form over the Internet instead of
in analog form over the public switched telephone system.
Volt
The force required to produce a current of 1 A through a resistance or
impedance of 1 Ω.
VSAT
Very small aperture terminal. Relatively small satellite antenna used for
satellite-based, point-to-multipoint data communications applications.
WAN
Wide area network. A data network typically extending a LAN outside a
building or beyond a campus, over IXC or LEC lines to link to other LANs at
remote sites. Typically created by using bridges or routers to connect geo
graphically separated LANs.
Wavelength
The physical distance between two adjacent peaks or valleys in a wave; the
property of light that determines its color.
Wideband
Digital communication between 1.5 Mbits/s and 45 Mbits/s.
X band
A portion of the radio frequency spectrum, 7 GHz and 8 GHz, used by military
satellites.
Zero dispersion point
For single-mode fiber, the wavelength at which the chromatic dispersion is
zero.
44
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References
RF and Microwave Fiber-Optics Design Guide
MICROWAVE
E. Ackerman, et.al., “A 3 to 6 GHz Microwave/Photonic Transceiver for Phased-Array Interconnects,” Lightwave Journal, April
1992, pp. 60–71.
H. Blauvelt, N.J. Kwong, P.C. Chen, and I. Ury, “Optimum Range for DFB Laser Chirp for Fiberoptic AM Video Transmission,” IEEE
J. Lightwave Electronics, vol. 11, pp 55—59, 1993.
J.A. Chiddix, and D.M. Pangrac, “Fiber Backbone: A Proposal for and Evolutionary CATV Architecture,” Communications Tech.,
Oct. 1988, pp. 36—47.
C.M. Gee, T.R.Chen, N. Bar-Chaim, I. Ury, K.Y. Lau, “Designing High Dynamic Range Fiber-optic Links: A Comparison Between
Directly-Modulated Fabry-Perot and Distributed-Feedback Laser Diodes,” Microwave Journal, May 1993.
G.J. Hansen, Evaluation of Midband Analog Fiber Optic Telemetry Links, Sandia National Lab., SAND93-0910-UC-706, Unlimited
Release, May 1993.
D. Huff, et.al., “High Performance Analog Fiber Optic Links for Radio Applications,” Internations Conference on
Communications, Chicago, IL, Num. 304.5, 1992.
B. Kanack, and C.L. Goldsmith, “Improved Semiconductor Based Analog Fiber Optic Link Performance Through Reactive
Matching,” Proc. DOD Fiber Optics, AFCEA, pp. 391—396, 1994.
H. Lewis, et.al., “Fiber Optics Technology Improves Shipboard Antenna Links,” Deckplate (USN), vol. 13, no. 13, May—June 1993.
G.F. Lutes and R.T. Logan, “Status of Frequency and Timing Reference Signal Transmission by Fiber Optics,” 45th IEEE Annual
Symp. of Frequency Control, Los Angeles, CA, May 1991.
S.E. Miller, I.P. Kaminow, eds., Optical Fiber Telecommunications II, Academic Press Inc., Boston, 1988.
J. Paslaski, et.al., “High Power Microwave Photodiode for High Dynamic Range Analog Transmission,” 1994 Optical Fiber
Conference, num. ThG5.
Poole and Darcie, “Distortion Related to Polarization Mode Dispersion in Analog Lightwave Systems,” IEEE Journal of Lightwave
Technology, Nov. 1993, pp 1749–1759.
M. Shibutani, et.al., “Reflection Induced Degradation in Optical Fiber Feeder for Microcellular Mobile Radio Systems,” IEICE
Trans. Electron., vol. E76-C, no. 2, Feb. 1993, pp. 287—292.
W.E. Stephens, T.R. Joseph, “System Characterization of Direct Modulated and Externally Modulated RF Fiber-Optic Links,”
Journal of Lightwave Technology, vol. LT-5, no. 3, March 1987, pp. 380—387.
H. Suzuki, et.al., “Characteristics of RF Reference and Timing Signal Distribution for Spring-8,” 9th Symp. on Accel. Science and
Tech., July 1993.
W.C. Turner and G.R. Balke, “Implementationic of Remotely Operated Unmanned Telemetry Tracking Systems with Fiber Optic
Cable,” European Telemetry Conference, Garmisch-Partenkirchen, Germany, May 1994.
S.E. Wilson, “Evaluate the Distortion of Modular Cascades,” Microwaves, March 1981, pp. 67—70.
H. Zarem, “Fiberoptic Antennas—A New Tool for Providing In-Building Cellular Coverage,” Cellular Business, Sept.1994.
Rev - November 19, 2010
45
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