Some Considerations of Broadband Noise Performance of Optical Heterodyne Receivers

Abstract

We derive an explicit expression in this paper for the spot noise factor of a perfectly aligned optical heterodyne receiver consisting of a semiconductor photodiode followed by an IF amplifier. We show that this noise factor F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> , which is a function of the admittance of the diode, varies as a function of the modulation frequency. We obtain constraints imposed by the photodiode on the broadband noise performance of the optical receiver for any arbitrary lossless interstage network. The integral form of the constraint shows that the noise factor F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R</inf> cannot be made equal to its optimum value F <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">RO</inf> over any nonzero band of frequencies. We give explicit expressions for the amount of tolerance of broadband noise performance obtained with lossless interstage networks. We show that for certain types of approximations, and for a certain transistor IF amplifier usually used in practice, the interstage network which achieves broadband signal performance for the receiver also obtains broadband noise performance. The theory of broadband noise performance we present for optical heterodyne receivers can also be applied to the study of broadband noise performance of other linear systems normally encountered in practice.