298
7 Optical Receiver Operation
P r = (SNR) 4qB e /(m
2
R) =
1 × 10
5
4
1.6 × 10
−19
5 × 10
6
(0.5)
2
(0.9)
= 1420 nW = 1.42 × 10
−3 mW or in dBm
P r (dBm) = 10 logP r = 10 log 1.42 × 10
−3
= −28.5 dBm
7.6 Summary
The function of an optical receiver is first to convert the optical energy emerging
from the end of a fiber into an electrical signal. The receiver then must amplify this
signal to a large enough level with an appropriate fidelity so that the information
content can be processed by the electronics following the receiver amplifier. In these
processes various noise and distortion effects will unavoidably be introduced, which
can lead to errors in the interpretation of the received signal. The three basic stages
of an optical receiver are a photodetector, an amplifier, and an equalizer. The design
of the amplifier that follows the photodiode is of critical importance, because it is in
this amplifier where the major noise sources are expected to arise. The equalizer that
follows the amplifier is normally a linear frequency-shaping filter, which is used to
mitigate the effects of signal distortion and intersymbol interference.
In a digital receiver the amplified and filtered signal emerging from the equalizer
is compared with a threshold level once per time slot to determine whether or not
a pulse is present at the photodetector in that time slot. Various noises, interference
from adjacent pulses, and conditions wherein the light source is not completely
extinguished during a 0 pulse can cause errors in the decision-making process. To
calculate the error probability, it is necessary to know the mean square noise voltage
that is superimposed on the signal voltage during the decision time.
Because the statistics of the output voltage at the sampling time are rather complex,
approximations are used to calculate the performance of a binary optical fiber
receiver. In applying these approximations, a tradeoff is needed between computational simplicity and accuracy of the results. The simplest method is based on
a Gaussian approximation. In this method it is assumed that when the sequence of
optical input pulses is known, the equalizer output voltage is a Gaussian random variable. Thus, to calculate the error probability, only the mean and standard deviation
of the output voltage need to be known.
The three basic approaches to the design of preamplifiers for optical fiber receivers
are the low-impedance, the high-impedance, and the transimpedance configurations.
These categories are not actually distinct because a continuum of intermediate configurations is possible, but they illustrate design approaches. The low-impedance preamplifier is the most straightforward, but not necessarily the optimum approach. This
design is limited to special short-distance applications in which high receiver sensitivity is not a major concern. The high-impedance design produces the lowest noise,
7 Optical Receiver Operation
P r = (SNR) 4qB e /(m
2
R) =
1 × 10
5
4
1.6 × 10
−19
5 × 10
6
(0.5)
2
(0.9)
= 1420 nW = 1.42 × 10
−3 mW or in dBm
P r (dBm) = 10 logP r = 10 log 1.42 × 10
−3
= −28.5 dBm
7.6 Summary
The function of an optical receiver is first to convert the optical energy emerging
from the end of a fiber into an electrical signal. The receiver then must amplify this
signal to a large enough level with an appropriate fidelity so that the information
content can be processed by the electronics following the receiver amplifier. In these
processes various noise and distortion effects will unavoidably be introduced, which
can lead to errors in the interpretation of the received signal. The three basic stages
of an optical receiver are a photodetector, an amplifier, and an equalizer. The design
of the amplifier that follows the photodiode is of critical importance, because it is in
this amplifier where the major noise sources are expected to arise. The equalizer that
follows the amplifier is normally a linear frequency-shaping filter, which is used to
mitigate the effects of signal distortion and intersymbol interference.
In a digital receiver the amplified and filtered signal emerging from the equalizer
is compared with a threshold level once per time slot to determine whether or not
a pulse is present at the photodetector in that time slot. Various noises, interference
from adjacent pulses, and conditions wherein the light source is not completely
extinguished during a 0 pulse can cause errors in the decision-making process. To
calculate the error probability, it is necessary to know the mean square noise voltage
that is superimposed on the signal voltage during the decision time.
Because the statistics of the output voltage at the sampling time are rather complex,
approximations are used to calculate the performance of a binary optical fiber
receiver. In applying these approximations, a tradeoff is needed between computational simplicity and accuracy of the results. The simplest method is based on
a Gaussian approximation. In this method it is assumed that when the sequence of
optical input pulses is known, the equalizer output voltage is a Gaussian random variable. Thus, to calculate the error probability, only the mean and standard deviation
of the output voltage need to be known.
The three basic approaches to the design of preamplifiers for optical fiber receivers
are the low-impedance, the high-impedance, and the transimpedance configurations.
These categories are not actually distinct because a continuum of intermediate configurations is possible, but they illustrate design approaches. The low-impedance preamplifier is the most straightforward, but not necessarily the optimum approach. This
design is limited to special short-distance applications in which high receiver sensitivity is not a major concern. The high-impedance design produces the lowest noise,