Fiber Optic Transmitters and Receivers (Transceivers)
Fiber Optic Datalink
Fiber optic transmission systems (datalinks) all work similar to the
diagram shown
above. They consist of a transmitter on one end of a fiber and a
receiver on the other end. Most systems operate by transmitting in one
direction on one fiber and in the reverse direction on another fiber
for full duplex operation.
Fiber Optic Transceiver
Most
systems use a "transceiver" which includes both transmission and
receiver in a single module. The transmitter takes an electrical
input and converts it to an optical output from a laser diode
or LED. The light from the transmitter is coupled into the fiber
with a connector and is transmitted through the fiber optic cable
plant. The light from the end of the fiber is coupled to a receiver
where a detector
converts the light into an electrical signal which is then conditioned
properly for use by the receiving equipment.
Sources for Fiber Optic Transmitters
The
sources used for fiber optic transmitters need to meet several
criteria: it has to be at the correct wavelength, be able to be
modulated fast enough to transmit data and be efficiently coupled into
fiber.
Four types of sources are commonly
used, LEDs, fabry-perot (FP) lasers, distributed feedback (DFB) lasers and vertical cavity surface-emitting
lasers (VCSELs). All convert electrical signals into optical
signals, but are otherwise quite different devices. All three are tiny
semiconductor devices (chips). LEDs and VCSELs are fabricated on
semiconductor wafers such that they emit light from the surface of the
chip, while f-p lasers emit from the side of the chip from a laser
cavity created in the middle of the chip.
LEDs
have much lower power outputs than lasers and their
larger, diverging light output pattern makes them harder to couple
into fibers, limiting them to use with multimode fibers. Laser have
smaller tighter light outputs and are easily coupled to singlemode
fibers, making them ideal for long distance high speed links. LEDs have
much less bandwidth than lasers and are limited to systems operating up
to about 250 MHz or around 200 Mb/s. Lasers have very high bandwidth
capability, most being useful to well over 10 GHz or 10 Gb/s.
Because of their fabrication methods, LEDs
and VCSELs are cheap to make. Lasers are more expensive because
creating the laser cavity inside the device is more difficult, the chip
must be separated from the semiconductor wafer and each end coated
before the laser can even be tested to see if its good.
Typical Fiber Optic Source Specifications
| Device Type |
Wavelength (nm) |
Power into
Fiber (dBm) |
Bandwidth |
Fiber Types |
| LED |
850, 1300 |
-30 to -10 |
<250 MHz |
MM |
| Fabry-Perot Laser |
850,
1310 (1280-1330) 1550 (1480-1650) |
0 to +10 |
>10 GHz |
MM, SM |
| DFB Laser |
1550 (1480-1650) |
0 to +25 |
>10 GHz |
SM |
| VCSEL |
850 |
-10 to 0 |
>10 GHz |
MM |
LEDs
have a limited bandwidth while all types of lasers are very fast.
Another big difference between LEDs and both types of lasers is the
spectral output. LEDs have a very broad spectral output which causes
them to suffer chromatic dispersion in fiber, while lasers have a
narrow spectral output that suffers very little chromatic dispersion. DFB lasers, which are used in long distance and DWDM systems, have the narrowest spectral width which minimizes chromatic dispersion on the longest links. DFB lasers are also
highly linear (that is the light output directly follows the electrical
input) so they can be used as sources in AM CATV systems.
The
choice of these devices is determined mainly by speed and fiber
compatibility issues. As many premises systems using multimode
fiber have exceeded bit rates of 1 Gb/s, lasers (mostly VCSELs) have
replaced LEDs. The output of the LED is very broad but lasers are
very focused, and the sources will have very different modal fill in
the fibers. The restricted launch of the VCSEL (or any laser) makes the effective
bandwidth of the fiber higher, but laser-optimized fiber, usually OM3,
is the choice for lasers.
The
electronics for a transmitter are simple. They convert an incoming
pulse (voltage) into a precise current pulse to drive the source.
Lasers generally are biased with a low DC current and modulated above
that bias current to maximize speed.
Detectors for Fiber Optic Receivers
Receivers
use semiconductor detectors (photodiodes or photodetectors) to convert
optical signals to electrical signals. Silicon photodiodes
are used for short wavelength links (650 for POF and 850 for glass MM
fiber). Long wavelength systems usually use InGaAs (indium gallium
arsenide) detectors as they have lower noise than germanium which
allows for more sensitive receivers.
Very high speed systems sometimes use avalanche photodiodes (APDs) that are biased at high voltage to create gain in the photodiode. These devices are more expensive and more complicated to use but offer significant gains in performance.
Packaging
Transcivers
are usually packaged in industry standard packages like these XFP
modules for gigabit datalinks(L) and Xenpak (R). The XFP modules
connect to a duplex LC connector on the optical end and a standard
electrical interface on the other end. The Xenpak are for 10 gigabit
networks but use SC duplex connection. Both are similar to media converters but are powered from the equipment they are built into.
Performance
Just
as with copper
wire or radio transmission, the performance of the fiber optic
data link can be determined by how well the reconverted electrical
signal out of the receiver matches the input to the transmitter.
The discussion of performance on datalinks applies directly to
transceivers which supply the optical to electrical conversion.
Every
manufacturer of transcivers specifies
their product for receiver sensitivity (perhaps a minimum power
required)
and minimum power coupled into the fiber from the source. Those
specifications will end up being the datalink specifications on the
final product used in the field.
Test Your Comprehension
After you study this page and "More on fiber optic datalinks", you should test your comprehension here.
Table of Contents: The FOA Reference Guide To Fiber Optics
