FIBER-OPTIC COMMUNICATION LINKS
Of the two types of photodiodes discussed here, the PIN detector is more economical
and requires less complex circuitry that does its APD counterpart. The PIN diode has
peak sensitivity from about 800 nm to 900 nm for silicon devices.
The overall response time for the PIN diode is good for about 90% of the transient but
sluggish for the remaining 10%, which is a "tail." The power response of the tail portion
of a pulse may limit the net bit rate on digital systems.
The PIN detector does not display gain, whereas the APD does. The response time of
the APD is far better than that of the PIN diode, but the APD displays certain temperature
instabilities where responsivity can change significantly with temperature. Compensation
for temperature is usually required in APD detectors and is often accomplished by a
feedback control of bias voltage. It should be noted that bias voltages for APDs are much
higher than for PIN diodes, and some APDs require bias voltages as high as 200 V. Both
the temperature problem and the high-voltage bias supply complicate repeater design.
Optical Fiber Amplifiers
Optical amplifiers amplify incident light through stimulated emission, the same mechanism
as used with lasers. These amplifiers are the same as lasers without feedback. Optical
gain is achieved when the amplifier is pumped either electrically or optically to realize
There are semiconductor laser amplifiers, Raman amplifiers, Brillouin amplifiers, and
erbium-doped fiber amplifiers (EDFAs). Certainly the EDFAs show the widest acceptance.
One reason is that they operate near the 1.55-
µm wavelength region, where fiber loss is
at a minimum. Reference 11 states that it is possible to achieve high amplifier gains in
the range of 30 dB to 40 dB with only a few milliwatts of pump power when EDFAs are
pumped by using 0.980-
µm or 1.480-µm semiconductor lasers. Figure 9.31 is a block
diagram of a low-noise EDFA.
In Figure 9.31, optical pumping is provided by fiber pigtailed
with typically 100 mW of power. Low-loss wavelength division multiplexers efficiently
combine pump and signal powers and can also be used to provide a pump bypass around
the internal isolator. The EDFA has an input stage that is codirectionally pumped and
an output stage that is counterdirectionally pumped. Such multistage EDFA designs
have simultaneously achieved a low-noise figure of 3.1 dB and a high gain of 54 dB
(Refs. 10, 11).
An EDFA block diagram. (From Ref. 9. Courtesy of Hewlett-Packard.)
A pigtail is a length of fiber factory connected to an active component.