Fiber cable basics - Ultra Electronics Nuclear Sensors &

Nuclear Sensors & Process Instrumentation
Fiber Cable Basics
Fiber-optic communication is a method of transmitting information from one place to another by sending light through an
optical fiber. The light forms an electromagnetic carrier wave that is modulated to carry information. First developed in the
1970s, fiber-optic communication systems have revolutionized the telecommunications industry and played a major role in
the advent of the Information Age. Because of its advantages over electrical transmission, the use of optical fiber has largely
replaced copper wire communications in core networks in the developed world.
The process of communicating using fiber-optics involves the following basic steps: Creating the optical signal using a
transmitter, relaying the signal along the fiber, ensuring that the signal does not become too distorted or weak, and
receiving the optical signal and converting it into an electrical signal. The complete fiber optic link is represented in Figure 1
below.
Figure 1: Model of "simple" fiber optic data link
We will look first at the cable, and then the transceivers (which act as both transmitter and receiver on each end of the fiber
cable).
Fiber Optic Cable
A fiber optic cable is a cylindrical pipe. It may be made out of glass or plastic or a combination of glass and plastic. It is
fabricated in such a way that this pipe can guide light from one end of it to the other.
Basically, a fiber optic cable is composed of two concentric layers termed the core and the cladding. These are shown on the
right side of Figure 2. The core and cladding have different indices of refraction with the core having n1 and the cladding
n2. Light is piped through the core. A fiber optic cable has an additional coating around the cladding called the jacket.
Core, cladding and jacket are all shown in the three dimensional view on the left side of Figure 2. The jacket usually consists
of one or more layers of polymer. Its role is to protect the core and cladding from shocks that might affect their optical
or physical properties. It acts as a shock absorber. The jacket also provides protection from abrasions, solvents and other
contaminants. The jacket does not have any optical properties that might affect the propagation of light within the fiber
optic cable.
Figure 2: Fiber Optic Cable, 3 dimensional view and basic cross section
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Tech Notes Bulletin on Fiber Cable Basics
Light is guided down the core of the cable because the core and cladding have different indices of refraction with the index
of the core, n1, always being greater than the index of the cladding, n2. Figure 3 shows how this is employed to effect the
propagation of light down the fiber optic cable and confine it to the core.
Figure 3: Propagation of a light ray down a fiber optic cable
As illustrated a light ray is injected into the fiber optic cable on the right. If the light ray is injected and strikes the core-tocladding interface at an angle greater than an entity called the critical angle then it is reflected back into the core. Since
the angle of incidence is always equal to the angle of reflection the reflected light will again be reflected. The light ray
will then continue this bouncing path down the length of the fiber optic cable. If the light ray strikes the core-to-cladding
interface at an angle less than the critical angle then it passes into the cladding where it is attenuated very rapidly with
propagation distance.
When it comes to mode of propagation, fiber optic cable can be one of two types, multi-mode or single-mode. These
provide different performance with respect to both attenuation and time dispersion.
Multimode
Multimode fiber optic links are the most popular in industrial and customer premise environments because they
generally have the lowest cost cabling and transceivers. However, they have limited lengths of up to 4 or 5 km.
To form a multimode link you must use multimode cable (either 50 or 62.5 µm core diameter) and multimode
transceivers. With multimode fiber the light travels multiple paths down the cable and actually bounces side to
side. Because of the nature of multimode fiber the distance it can go is limited primarily by a phenomenon called
modal dispersion or multimode distortion.
Figure 4 - Multimode Cable - Transmission
Singlemode and Long Haul
Singlemode fiber optic links are less popular because of their higher cost (both cabling and transceivers) but they
can operate over extended distances up to 120 km or more. To form a singlemode link you must use singlemode
cable (8, 9 or 10 µm core diameter; 9 µm is most common) and singlemode transceivers. With singlemode fiber
the light travels in a single path down the cable. This is more efficient and allows for the extended distances.
Singlemode fiber is not affected by modal dispersion so its distance is limited mostly by the power and sensitivity of
the transceivers being used.
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Tech Notes Bulletin on Fiber Cable Basics
Figure 5 – Singlemode Cable - Transmission
Transceivers (Transmitters/Receivers) and Connectors
The Transmitter component of Figure 1 serves two functions. First, it must be a source of the light coupled into the fiber
optic cable. Secondly, it must modulate this light so as to represent the binary data that it is receiving from the Source. With
the first of these functions it is merely a light emitter or a source of light. With the second of these functions it is a valve,
generally operating by varying the intensity of the light that it is emitting and coupling into the fiber.
The Receiver component of Figure 1 again serves two functions. First, it must sense or detect the light coupled out of the
fiber optic cable then convert the light into an electrical signal. Secondly, it must demodulate this light to determine the
identity of the binary data that it represents. In total, it must detect light and then measure the relevant Information
bearing light wave parameters in the premises fiber optic data link context intensity in order to retrieve the Source's binary
data.
Transceiver is the industry term for the transmitter/receiver unit as a pair. When manufacturers refer to a fiber optic “port,”
they are typically referring to both transmit and receive connection required for a duplex fiber optic link. Thus a switch with
2 fiber ports will have 2 complimentary pairs of transmit and receive connections, or 2 “transceivers.”
Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard
type such as SC, ST, or LC (the 3 types featured in Ultra Electronics, NSPI’s switches).
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. The end face is where
light (and subsequently the fiber optic signal) enters and leaves the optic core. A fiber-optic connector is basically a rigid
cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. The mating mechanism can be "push and
click", "turn and latch" ("bayonet"), or screw-in (threaded).
Regardless of what type of connection, it is essential that the connector used is matched to the type of fiber (MM or SM)
it will terminate, that the connector is installed per manufacturer specifications, and that the end face of the connection
remains clean at all times.
Ultra Electronics offers two distinct types of transceivers; 1x9 (SC or ST connectors) and SFP (LC connectors).
1x9 Transceivers with SC or ST Connectors
These are offered on the fiber optic fast Ethernet (100 Mbps) ports. The “1x9” refers to the industry-standard
pin-out of 1 row by 9 pins. Ultra Electronics offers these transceivers with dual ST or SC style connectors. They are
available as Multimode, Singlemode, or Singlemode Long Haul. Other variations are available as special order.
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Tech Notes Bulletin on Fiber Cable Basics
1x9 Transceiver with SC Connectors
1x9 Transceivers with ST Connectors
SFP (Small Form Pluggable) Transceivers (aka Mini-Gbic) with LC Connectors
These are offered on the fiber optic gigabit Ethernet (1000 Mbps) ports. These transceivers plug into a cage
assembly that is already in place in the Ultra switch. They are more compact than the more traditional 1x9 style
transceivers. Ultra offers these transceivers with dual LC connectors. They are available as Multimode, Singlemode,
or Singlemode Long Haul. Other variations such as CDWM (Coarse Wavelength Division Multiplexing) are available
as special order.
SFP Transceiver out of cage
SFP Transceiver inserted in cage
Selecting the Proper Components for a Fiber Optic Network Link
There are 4 primary considerations when selecting the proper components for a fiber optic link:
1.
The length of the fiber link from end to end
2.
The bandwidth (i.e. 100mbps or 1Gbps) required based on the applications and information that will be sent over the
link
3.
The estimated amount of total attenuation over the link (a factor of fiber optic cable properties and the amount and
quality of the splice/connector points along the fiber from end to end).
4.
The power budget of the transceiver pairs being used to generate and accept the fiber optic signal.
Using the Reference Tables at the end of this note as guidelines (and using attenuation characteristics for the specific
fiber optic cable installed or proposed for use), a user can develop an acceptable link-loss budget and select the proper
transceivers for their system. Please note that type of connector has no influence on the loss along a system.
Example
An industrial user has a series of Ethernet devices operational at Location A. They are building a new facility 8km
away. At this distance they are certainly going to have to plan on using single mode fiber. They want to transmit
(or plan to transmit) at Gigabit speeds. The SM fiber that they have decided to use attenuates at a rate of .3dB/km.
There will be 3 splices along the route and then connectors at each end.
Cable attenuation = 8km x .3dB/km = 2.4dB
Splice loss = 3 splices x .2dB/splice = .6dB
Connector loss = 2 connectors x 1dB/connector = 2dB
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Tech Notes Bulletin on Fiber Cable Basics
Total loss along physical link = 5dB
Safety factor of 25% = 1.25dB
Total loss budget = 6.25dB
Looking at the Gigabit fiber transceiver performance specifications, we can see that the single mode Gigabit fiber
transceiver has a worse-case power budget of 11dB so this should work satisfactorily in this proposed fiber link.
Troubleshooting Fiber Optic Links
If installed correctly, fiber optic links can provide years of worry free connectivity for users, but like all communications
systems, there are times when some troubleshooting may be required. There is expensive cable test equipment such as
power source/meters and OTDRs available on the market from a number of manufacturers which are optimal for testing
and verifying physical links. For most users, the economy of owning equipment like this does not make sense, so the
following are some quick and easy steps to try before calling in a vendor to test your system.
1.
Make sure that all of your connectors are clean. Even a little bit of dust, dirt or grease on a connector face can
significantly degrade a fiber signal. This includes the main fiber optic link as well as any patch cables that you may be
using. When cleaning, it is important to use lint free swabs or wipes, preferably of a clean room quality. These can be
used dry or wet (with 99% isopropyl alcohol solutions).
•
Make certain that you are not cleaning an active fiber as the laser can cause permanent damage to your eyes
should you look into the end face.
•
Additionally, it is not necessary to scrub the end face, rather to just gently wipe it clean and then recheck the link. If
additional cleaning is required simply repeat this process.
2.
Ensure that the cable type you are using matches the transceiver type. That is, MM cable requires MM transceivers.
3.
Ensure that the patch cords that you are using match the link fiber cable. Again MM needs to be used with MM.
Additionally, it is important that 62.5um is used with 62.5um and 50um with 50um. This is a very common and easy to
make mistake. If the fiber cores are not aligned correctly significant attenuation will occur.
4.
Make sure that all connectors are plugged completely into their proper ports. Again if end faces are lined up correctly
with transceivers and/or mated fiber ends, the system may fail due to excess attenuation.
5.
Make sure that the transmit cable at the near end is the receive cable at the far end. Their needs to be a crossover for a
fiber link to work correctly. Be sure to factor in all patch cords that may be used.
Fiber Transceiver Performance Specifications – Reference Tables
Ultra Electronics Fiber Transceivers Performance Specifications:
Ethernet Type
Mode
Data Rate
(Mbps)
Signal Rate
(MHz)
Wave-length
(nm)
IEEE
Standard
Fast Ethernet
Multi
100
125
1310
100BaseFX
Fast Ethernet
Single
100
125
1310
100BaseFX
Fast Ethernet
Single – long haul
100
125
1310
100BaseFX
Gigabit Ethernet
Multi
1000
1250
850
1000BaseSX
Gigabit Ethernet
Single
1000
1250
1310
1000BaseLX
Gigabit Ethernet
Single – long haul
1000
1250
1310
1000BaseLX
Gigabit Ethernet
Single – long haul
1000
1250
1550
1000BaseLH
Gigabit Ethernet
Single – long haul
1000
1250
1550
1000BaseLH
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Tech Notes Bulletin on Fiber Cable Basics
Ultra Electronics Fiber Transceivers Performance Specifications (continued):
Ethernet Type
Power Budget (Power - Sensitivity)
Transmitter Power*
Receiver Sensitivity*
Typical
Worst
Min.
dB
Typ.
dB
Max.
dB
Min.
dB
Typ.
dB
Max.
dB
Mode
Fast Ethernet
Multi
14 (-17 minus -31)
10 (-21 minus -31)
-21
-17
-14
--
-34
-31
Fast Ethernet
Single
20 (-11 minus -31)
16 (-15 minus -31)
-15
-11
-8
--
-36
-31
Fast Ethernet
Singlelong haul
31 (-3 minus -34)
29 (-5 minus -34)
-5
-3
0
--
-36
-34
Gigabit Ethernet
Multi
12 (-6 minus -18)
9 (-9 minus -18)
-9
-6
-3
--
--
-18
Gigabit Ethernet
Single
14 (-6 minus -20)
11 (-9 minus -20)
-9
-6
-3
--
--
-20
Gigabit Ethernet
Singlelong haul
22 (-1 minus -23)
19 (-4 minus -23)
-4
-1
2
--
--
-23
Gigabit Ethernet
Singlelong haul
22 (-1 minus -23)
19 (-4 minus -23)
-4
-1
2
--
--
-23
Gigabit Ethernet
Singlelong haul
25 (2 minus -23)
23 (0 minus -23)
0
2
5
--
--
-23
*Note: For transmitter power, the higher the number the better. The opposite is true for receiver sensitivity, the lower the
number the better.
Fiber Cable Parameters (typical)
Mode
Connector Losses Splice
Wavelength
Loses (db
(dB per
(nm)
per splice)
connection)
Distance
Losses
(dB per km)
Multimode
Modal Dispersion*
(MHz x km)
Singlemode
Dispersion
(ps/nm x km)
62.5/125 µm
Multi
850 nm
1 dB
0.2 dB
3.3 dB
300
--
50/125 µm
Multi
850 nm
1 dB
0.2 dB
2.7 dB
700
--
62.5/125 µm
Multi
1310 nm
1 dB
0.2 dB
1 dB
500
--
50/125 µm
Multi
1310 nm
1 dB
0.2 dB
0.8 dB
800
--
9/125 µm
Single
1310 nm
1 dB
0.2 dB
0.5 dB
--
3.5
9/125 µm
Single
1550 nm
1 dB
0.2 dB
0.25 dB
--
19
8/125 µm
Single
1550 nm
1 dB
0.2 dB
0.2 dB
--
19
Cable Size
(core/cladding)
(µm)
•Note: These are just guideline numbers. Refer to your cable specifications for more accurate values.
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Tech Notes Bulletin on Fiber Cable Basics
Calculating Fiber Optic Distances
There are two primary ways to calculate how far you can go with your fiber optic links. To be safe you should go by the
shortest result from the two methods, if you use both such as you can with Multimode fiber. Also, you should design for up
to a 25% safety margin to be conservative and allow for degradation of the signal and cable over time.
Method 1: Modal Dispersion for Multimode links only
Maximum Distance = modal Dispersion/Signal rate
Speed
Mode
Wavelength
(nm)
Cable
Diameter
(µm)
Modal
Dispersion*
Signal
Rate
(MHz)
Max. Distance
Based on
Modal Dispersion
Fast Ethernet
Multi
850 nm
62.5/125
300
125
2.4 km
Fast Ethernet
Multi
850 nm
50/125
700
125
5.6 km
Fast Ethernet
Multi
1310 nm
62.5/125
500
125
4 km
Fast Ethernet
Multi
1310 nm
50/125
800
125
6.4 km
Gigabit Ethernet
Multi
850 nm
62.5/125
300
1250
240 m
Gigabit Ethernet
Multi
850 nm
50/125
700
1250
560 m
Gigabit Ethernet
Multi
1310 nm
62.5/125
500
1250
400 m
Gigabit Ethernet
Multi
1310 nm
50/125
800
1250
640 m
Method 2: Based on Optical Budget
Power Budget = Transmitter Power – Receiver Sensitivity
Spare Optical Budget = Power Budget – Power Losses (splices and connectors)
Maximum Distance = Spare Optical Budget/Distance Losses
Speed
Mode
Cable Size
Wavelength
Power Budget
(Worst case)
Typical
Losses*
Spare
Power
Distance
Losses
Max.
Distance
Fast Ethernet
Multi
62.5/125 µm
1310 nm
10 dB
6 dB
4 dB
1 dB
4 km
Fast Ethernet
Multi
50/125 µm
1310 nm
10 dB
6 dB
4 dB
0.8 dB
5 km
Fast Ethernet
Single
9/125 µm
1310 nm
16 dB
6 dB
10 dB
0.5 dB
20 km
Fast Ethernet
Long haul
9/125 µm
1310 nm
29 dB
6 dB
23 dB
0.5 dB
46 km
Gigabit Ethernet
Multi
62.5/125 µm
850 nm
9 dB
6 dB
3 dB
3.3 dB
0.9 km
Gigabit Ethernet
Multi
50/125 µm
850 nm
9 dB
6 dB
3 dB
2.7 dB
1.1 km
Gigabit Ethernet
Single
9/125 µm
1310 nm
11 dB
6 dB
5 dB
0.5 dB
10 km
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
19 dB
6 dB
13 dB
0.5 dB
26 km
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
19 dB
6 dB
13 dB
0.25 dB
52 km
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
23 dB
6 dB
17 dB
0.2 dB
85 km
Note: Typical losses include 2 dB (two connectors), 3 dB (safety margin) & 0.4 (two splices) = 6 dB (rounded up)
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Nuclear Sensors & Process Instrumentation
Fiber Optic Maximum Distance Summary
Speed
Mode
Cable Size
Wavelength
IEEE
Recommended
Distance
Max. Distance
Based on Power
Budget*
Max. Distance
Based on Modal
Dispersion*
Fast Ethernet
Multi
62.5/125 µm
1310 nm
2 km
4 km
4 km
Fast Ethernet
Multi
50/125 µm
1310 nm
2 km
5 km
6.4 km
Fast Ethernet
Single
9/125 µm
1310 nm
15 km
20 km
--
Fast Ethernet
Long haul
9/125 µm
1310 nm
--
46 km
--
Gigabit Ethernet
Multi
62.5/125 µm
850 nm
220 m
0.9 km
240 m
Gigabit Ethernet
Multi
50/125 µm
850 nm
550 m
1.1 km
560 m
Gigabit Ethernet
Single
9/125 µm
1310 nm
5 km
10 km
--
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
--
26 km
--
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
--
52 km
--
Gigabit Ethernet
Long haul
9/125 µm
1310 nm
70 km
85 km
--
*Note: These numbers are just guidelines and are highly dependent on your cable and transceiver specifications.
Ultra Electronics
NUCLEAR SENSORS & PROCESS INSTRUMENTATION
707 Jeffrey Way, PO Box 300
Round Rock, TX 78680-0300 USA
Tel: +1 512 434 2850
Fax: +1 512 434 2901
e-mail: fiberop@ultra-nspi.com
www.ultra-nspi.com
Ultra Electronics, Nuclear Sensors & Process Instrumentation
is a business name of Weed Instrument Co., Inc.
Ultra Electronics reserves the right
to vary these specifications
without notice.
© Ultra Electronics 2010.
Printed in the USA.
03/10
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