 amplifier Filter Signal processing (Equaliser) Figure 53.22 Analogue optical reveiver 53.4 Optical receivers and transmitters impedance. The resistive load input optical receiver is the simplest possible, as shown in Figure 53.24. The detector is terminated on a The function of the optical receiver is to recover the information imparted on one of the parameters of the optical carrier and in our case this means the intensity of the carrier. An optical receiver consists of an optical detector (the transducer) and a low noise electronic amplifier which raises the signal level to a value where further signal processing is possible without deterioration of the S/N ratio. After the low noise amplifier the details of the receiver depend 50 ohm on the application. An analogue receiver will include stages of amplification, filtering, electronic signal detection (whose function depends on the modulation format) and some additional signal processing such as filtering, and only after this is the information available to the user, as in Figure 53.22. A digital receiver will include some more amplification, filtering, detection, clock recovery circuits, and finally the regenerator, after which the information is available to the user as in Figure 53.23. A combination is possible by using digital modulation on a subcarrier. In such a situation the subcarrier is first processed by an analogue receiver which recovers the digital information, but in analogue Figure 53.24 Resistive input optical receiver form, and this is then processed by a digital receiver now minus the optical front end. resistor and the voltage developed across it is applied to the ampli- The measure of performance used in the design of optical re- fier. The bandwidth is limited by the detector bandwidth, the band- ceivers depends on the application. For analogue receivers the width of the input circuit and the bandwidth of the amplifier itself. signal to noise ratio (SNR) is in widespread use. For digital re- With a 50 ohm load the bandwidth can be very large