Page 127 - Optical Communications Essentials

Photodiodes and Receivers

Photodiodes and Receivers 117

for relatively strong, clean signals to complex circuitry if the receiver needs to
interpret weak, distorted signals at high data rates.
Semiconductor-based photodiodes are the main devices that satisfy optical
receiver requirements. The two basic types of devices used are called a pin photo-
diode and an avalanche photodiode (APD).
The choice of a photodetector material determines the wavelength range over
which the device will operate. Material choices include Si, Ge, or GaAs, InGaAs,
and InGaAsP. Of these the most widely used are Si for operation in the 850-nm
region and InGaAs for both long-wavelength pin and avalanche photodiodes.
Table 7.1 summarizes the wavelength ranges over which various materials are
sensitive.
Other important characteristics of a photodetector include

■ Its quantum efficiency, which is the number of electron-hole pairs that are
generated per incident photon of energy hν
■ Its responsivity, which specifies the photocurrent generated per unit optical
power
■ Its response speed, which is particularly important to properly interpret high
data rate signals
■ The gain and its associated noise figure for avalanche photodiodes

Tables 7.2 and 7.3 list representative performance values for pin and ava-
lanche photodiodes, respectively. The values were derived from various vendor
data sheets and from performance numbers reported in the literature. They are
given as guidelines for comparison purposes.

Further Reading
1. D. Neamen, Semiconductor Physics and Devices, 3d ed., McGraw-Hill, New York, 2003.
2. S. O. Kasap, Optoelectronics and Photonics, Prentice Hall, Upper Saddle River, N.J., 2001.
3. G. Keiser, Optical Fiber Communications, 3d ed., McGraw-Hill, Burr Ridge, Ill., 2000, Chap. 6.
4. S. R. Forrest, “Optical devices for lightwave communications,” in S. E. Miller and I. P. Kaminow,
eds., Optical Fiber Telecommunications—II, Academic, New York, 1988.
5. M. C. Brain and T. P. Lee, “Optical receivers for lightwave communication systems,” J. Lightwave
Technology, vol. 3, pp. 1281–1300, December 1985.

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