wireless at the speed of light ……………. Technical note

wireless at the speed of light …………….
Technical note:
Fiber connectivity to SONAbeam terminals
Date of Author:
9-June-2003
(95-0245 Rev A)
This technical note provides connectivity details between SONAbeam fiber transceivers and
customer premise equipment (CPE). All SONAbeam fiber transceivers operate at a center
wavelength of 1310 nm.
Overview of Fiber Types, Lasers, and Fiber Budgets
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The most common fiber types available today are Multimode (MM) and Single mode (SM) fibers .
MM fibers have a wide core (e.g. 62.5 um or 50 um) allowing light to take many paths (i.e.
modes) as it travels through the media. Because a light path straight through the fiber is shorter
than paths reflecting within the fiber, some paths reach the end of the fiber faster. This results in
a spreading effect (i.e. modal dispersion) limiting the maximum speed of light changes that the
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fiber will allow (i.e. the bandwidth) . SM fibers have a much smaller core (e.g. 9 um) allowing
only one path to propagate through the fiber media. Reducing the number of paths through the
fiber decreases modal dispersion and increases bandwidth.
Lasers launch light in high-powered, concentrated beams for further distance, and they modulate
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(i.e. turn on and off) optical pulses at fast rates to increase bandwidth . Laser standards over
fiber optic media include short-wavelength SX and long-wavelength LX operating at 850 nm and
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1310 nm , respectively. LX lasers (e.g. FP or DFB) are typically used with SM fiber cable, but
can also be used with MM fiber. SX lasers (e.g. VCSEL) are typically only used with MM fiber.
Generally, SX lasers traverse shorter distances than LX lasers, but cost less.
Distances
achieved through a fiber type depend on the attenuation at the wavelength selected. The table
below summarizes typical fiber losses at different wavelengths.
Attenuation
(db/km)
MM Fiber
SM Fiber
850 nm
1.5-2.5
N/A
1300 nm
1
0.35-0.5
1550 nm
N/A
0.2-0.3
1
Single mode fiber types also include: 1) Dispersion shifted fibers (DSF) making operation at 1550nm
attractive; 2) Non-zero DSF (NZ-DSF) making operation within the DWDM range of 1510-1600nm
attractive.
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SM fiber has a bandwidth greater than 1 THz per km at 1310 nm. Standard 62.5 MM fiber usually has a
bandwidth limitation of 160 MHz per km and 500 MHz per km at 850 nm and 1310 nm, respectively.
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In comparison, light emitting diodes (LEDs) modulate at slower speeds and diffuse beams at lower power.
In addition, narrower spectral widths for lasers result in less chromatic dispersion; approximate spectral
widths of LEDs, Fabry-Perot lasers, and Distributed Feedback lasers are 40nm, 4nm, and <0.1nm,
respectively.
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SONET/SDH standard defines operation at 1310nm, and 100BaseFX (Fast Ethernet over fiber) defines
operation at 1310nm. Note that dispersion and attenuation are higher at 1310 nm (in comparison to
operating at 1550nm on SM fiber).
wireless at the speed of light …………….
Fiber budgets are used to estimate minimum and maximum distances supported over a specific
fiber type. The minimum fiber length is typically calculated as follows:
([max Power] – [Rx saturation point]) / (Attenuation)
The maximum fiber length is typically calculated as follows:
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([min. Power] – [Receiver Sensitivity] – [Overhead ]) / (Attenuation )
SONAbeam Connector Types
fSONA terminals provide SC connector types for connectivity between the SONAbeam’s fiber
transceiver and customer premise equipment (CPE). All SONAbeam fiber transceivers operate
at a center wavelength of 1310 nm, and can connect to customer’s fiber optic network using
either multimode (MM) or single mode (SM) fiber. The terminals contain a SC connector block
with two duplex SC connectors: one for the customer side (CPE) and one for the SONAbeam
transceiver side.
The customer’s side of the SC connector block has two SC connector labeled “TX” and “RX”.
The connector labeled “TX” connects to a fiber optic cable carrying traffic destined towards the
customer network; this traffic received from the SONAbeam terminal at the other end of the free
space optics (FSO) link. The connector labeled “RX” connects to a fiber optic cable carrying
traffic received from the customer network; this traffic is destined for the SONAbeam terminal at
the other end of the FSO link.
On the SONAbeam transceiver’s side of the SC connector block, two short fiber optic cables
connect to the transceiver (located in the optical head): a MM fiber cable connects to the
terminal’s receiver and a yellow SM fiber cable connects to the terminal’s transmitter. This fiber
optic pigtail is approximately 2 meters and 0.5 meters in length for the –M product and –S
product, respectively.
At the SONAbeam transmitter, the short SM fiber cable allows connectivity to CPE receivers
using either MM or SM fiber. When MM fiber is used between the CPE device and the
SONAbeam terminal (i.e. at the customer side of the SC connector block), attenuation may be
required for shorter distances (e.g. less than 100 meters) and a mode-conditioning patch cord
should be used for longer distances (e.g. greater than 300 meters). The following table
summarizes the output launch power of the fiber transceiver for various SONAbeam models.
Fiber Transceiver
Launch Power (dBm)
Max Output Power
Min Output Power
Model
52/155
-8
-15
Model
622/1250
-3
-11
At the SONAbeam receiver, the short MM fiber cable also allows connectivity to CPE transmitters
using either MM or SM fiber. Received optical power must not exceed the receiver saturation
point of the SONAbeam transceiver to avoid damage. The following tables summarizes receiver
saturation points for various SONAbeam transceivers.
Receiver Saturation
(dBm)
Maximum Input Power
5
Model
52/155
-8
Model
622/1250
-3
5dBm overhead for connector losses is typical
For the maximum fiber length calculation, the worst-case attenuation for a specific fiber type (e.g. 0.5
dB/km for 1300nm SM fiber) should be incorporated to provide a conservative estimate.
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