Equivalence between preamplified direct detection and heterodyne receivers

The interrelationship of heterodyne receivers and preamplified direct-detection receivers is explained. The authors compare the performances of the two lightwave receiver schemes, taking into account all of the essential functional blocks needed in a real receiver. This includes analysis of the laser phase noise, the limiting noise processes, the optical and electrical carrier wave filters, the nonlinear demodulation functions, and the baseband filters. Even with these realistic effects included, the two receivers have mathematically equivalent performance under certain conditions, which are rigorously identified. It is shown that this is a general result which is independent of the modulation format used. The established equivalence suggests that the analytical and practical design criteria used for one scheme can also be used for the other.<>