Optical Communications, 2nd Edition

3.2 PHOTODETECTORS

Practical photodetectors are of two basic types: phototube devices using vacuum tube construction and solid-state devices using junction effects [2–5]. The phototube, shown in Figure 3.3a uses a photosensitive material as its receiving surface. Excited electrons are released into a vacuum as a space charge and are collected at an anode plate to produce the current flow. A photomultiplier tube (PMT) is a phototube with multiple plates (dynodes) that each produce secondary emissions (Figure 3.3b), thereby multiplying up the current flow. Because secondary emissions can be made extremely high, a PMT can have mean gains in the 10³ to 10⁵ range. The gain variance is often described by a spreading factor ξ such that . This means the excess noise factor in Eq. (3.1.3) has the form F = 1 + ξ² for the PMT. Spreading factors of approximately 1 to 30 percent are typical.

The photosensitive materials used in phototubes are generally composed of silicon or germanium components. Figure 3.4 shows the quantum efficiency factor of some of these materials as a function of wavelength. These efficiency factors are peaked in the visible range, and fall off rapidly for wavelengths exceeding 2 to 3 μm. Hence, efficient optical photodetectors do not exist for the longer wavelengths.

A photodiode (Fig. 3.5) is a solid-state PN junction device (commonly called a PIN diode), in which the ...