Some Considerations of Broadband Noise Performance of Optical Heterodyne Receivers

Semiconductor photocliodes like Schottky barrier diodes or conventional p-n or p-i-n diodes are increasingly being used for detection in optical heterodyne (double detection) receivers.1-12 They normally are fast and efficient, converting up to 70 per cent of the photons of the light beam into photoelectric current.11 Because of the intensity of the light beam, almost all photodiodes used in optical detection give an output proportional to the intensity of light.1 However, this output is normally so small that further amplification is required, and 429 430 THE BELL SYSTEM TECHNICAL JOURNAL, MARCH 1!)68 so any practical receiver consists of a photodiode followed by a highgain low-noise IF amplifier. We have already considered in detail the signal performance of such receivers.13 In this paper we shall deal only with the noise performance of the receiver. The output of the diode is usually corrupted by noise generated within the diode and elsewhere in the system. We discuss briefly the characteristics of the photodiode in Section II and give its equivalent circuit. In Section I I I we discuss the noise performance of the IF amplifier and show that its noise factor is a function of its source admittance.14 We show that the noise factor Fn of the optical receiver is a function of frequency in spite of the fact that the IF amplifier has a broadband noise performance characteristic. In Section IV we discuss the role of the lossless interstage network in achieving broadband noise performance of the optical receiver and show that it is impossible to make the noise factor FR equal to its optimum value Fro over any nonzero band of frequencies.