Optical Communications, 2nd Edition

3.1 THE PHOTODETECTION PROCESS

Photodetection is achieved by having a photosensitive material respond to incident light by producing free electrons. These electrons are then susceptible to an externally applied electric potential that forces the free electrons to drift in a given direction. This electron flow is then exhibited at the detector output as a current flow. Hence photodetection converts impinging light fields to output current. In a vacuum tube, the free electrons are produced from a photosensitive surface material and released into a vacuum cavity, where they are collected by a charged anode plate. In a solid-state detector, the light excites electrons at a positive-negative (PN) junction, and the current corresponds to an electron flow across the junction gap. The general photodetection operation can therefore be represented by the simplified diagram in Figure 3.1 showing the incident light and the external bias circuitry producing the output current flow. This current can then be converted to a voltage by passing through a load resistor.

Photomultiplication is achieved during photodetection by having the excited electrons regenerate additional free electrons so that the resulting accumulated current flow is many times higher than for the primary electrons alone. This electron enhancement can be obtained by using multiple anode plates to 75 produce secondary electron emissions in a vacuum tube or by using avalanching effects in doped semiconductor compounds placed ...