21 - Detection of optical radiation

Summary

Introduction

In this chapter we will review some of the fundamentals of the optical detection process. This will include a discussion of the randomly fluctuating signals, known as noise, that appear at the output of any detector. We will then examine some of the practical characteristics of various types of optical detector. The chapter will conclude with a discussion of the limiting detection sensitivities of important detectors used in various ways.

Photon detectors operate by absorbing the photons coming from a source and using the absorbed energy to produce a change in the electrical characteristics of their active element(s). This can occur in many ways. In a photomultiplier or vacuum photodiode the incoming photons are absorbed in a photoemissive surface, and free electrons are produced by the photoelectric effect. These electrons can be accelerated and detected as an electric current. In a semiconductor photodiode or photovoltaic detector, absorption of a photon at a p–n or p–i–n junction creates an electron–hole pair. The electron and hole separate because of the energy barrier at the junction – each carrier moves to the region where it can reduce its potential energy, as shown in Fig. 21.1.

Thermal radiation detectors use the heating effect produced by absorbed photons to change some characteristic of the detector element. In a bolometer, the heating of carriers changes their mobility and consequently the resistivity of the detector element. In a thermopile, the heating effect is used to generate a voltage through the thermoelectric (Seebeck) effect.