emi- biased. The operation is very simple. The radiation penetrates the conductor lasers and the interested reader is advised to consult thin p+layer with minimum absorption and it is totally, or nearly manufacturer's catalogues for further information. totally, absorbed in the i layer. The threshold wavelength for absorp- tion is given by the Equation 53.42, which is the same equation met in the interaction of radiation with matter, in the section on optical sources. Optical detectors (53.42) 53.3.1 The basics of optical detection The function of an optical detector is to recover the information Photodetectors are transducers in the sense that they convert which was imposed on the optical carrier. There are many effects which can be used for optical detection but in terms of performance optical power (photons) into electrical power (electrons). A critical detectors based on the photoelectric effects and using semiconduc- parameter of a detector is the efficiency with which it performs this tor material outperform all others, at least in the field of fibre conversion. This parameter is known as the quantum efficiency and is defined as in Equation 53.43. communication, and we will concentrate on them. The principle of photodetection is simple. Photons are absorbed by the material and electron from the valence band are raised in the (53.43) conduction band leaving behind holes. If an electric field is applied q electrons available collected = across the device both electrons and holes are swept out of the device creating the photocurrent. The value of n with wavelength depends on the detector material Since the generated photocurrent is proportional to the intensity and is a function of wavelength. Figure 53.19 shows the absorption of the irradiance, that is the square of the optical field, the detection of some typical detector materials and for fibre communications Si, process is also known as direct detection and is the most widely used Ge and the quarternary/ternary InGaAsP cover the wavelength form of optical 