Theoretical analysis of ASK heterodyne optical receivers with tuned front ends

Heterodyne optical receivers designed with high intermediate frequency or with large IF bandwidth suffer from frequency-squared noise spectral density components if a base-band receiver design is used. This noise component can be reduced usefully if the front-end is tuned to the IF. We present detailed calculations of noise spectral densities for an ASK receiver using square-law IF detection. An example of a 140 Mbit/s receiver with an IF filter suitable for lasers with significant phase noise is considered. The tuned receiver may be up to 6 dB more sensitive than the base band design for the range of receiver parameters considered.