Page 124 - Optical Communications Essentials

Photodiodes and Receivers

114 Chapter Seven

TABLE 7.3. Generic Operating Parameters of Si, Ge, and GaAs Avalanche Photodiodes
Parameter Symbol Unit Si Ge InGaAs
Wavelength range λ nm 400–1100 800–1650 1100–1700
Avalanche gain M — 20–400 10–200 10–40
Dark current I D nA 0.1–1 50–500 10–50 @ M=10
Rise time τ r ns 0.1–2 0.5–0.8 0.1–0.5
Gain bandwidth M B GHz 100–400 2–10 20–250
Bias voltage V B V 150–400 20–40 20–30

For short-distance applications, Si devices operating around 850nm provide
relatively inexpensive solutions for most links. Longer links usually require
operation in the higher spectral bands (O-band through L-band). In these bands
one normally uses InGaAs-based photodiodes.

7.4. Optical Receiver

An optical receiver consists of a photodetector and electronics for amplifying
and processing the signal. In the process of converting the optical signal power
emerging from the end of an optical fiber to an electric signal, various noises
and distortions will unavoidably be introduced due to imperfect component
responses. This can lead to errors in the interpretation of the received signal.
The most meaningful criterion for measuring the performance of a digital
communication system is the average error probability. In an analog system the
fidelity criterion usually is specified in terms of a peak signal-to-noise ratio. The
calculation of the error probability for a digital optical communication receiver
differs from that of its electronic counterpart. This is a result of the discrete
quantum nature of the optical signal and also because of the probabilistic char-
acter of the gain process when an avalanche photodiode is used.
This section first gives a definition of the signal-to-noise ratio and then looks
at the origins of noises. Given this knowledge, one then can characterize the
performance of an optical fiber communication system. Chapter 14 looks at this
topic in greater detail.

7.4.1. Photodetector noise
In fiber optic communication systems, the photodiode must detect very weak
optical signals. Detection of the weakest possible optical signals requires that
the photodetector and its associated electronic amplification circuitry be opti-
mized so that a specific signal-to-noise ratio is maintained. The term noise
describes unwanted components of a signal that tend to disturb the transmis-
sion and processing of the signal in a physical system. Noise is present in every
communication system and represents a basic limitation on the transmission

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