Heterodyne and optically implemented dual-filter FSK systems: comparison based on a rigorous theory

The performance of heterodyne and optically implemented dual-filter FSK systems are compared based on a new rigorous theory that accounts for noise correlation (due to nonorthogonal signaling), laser phase noise, receiver noise, and amplifier spontaneous emission noise. Ideally, the two implementations have the same sensitivity with and without polarization control. The results emphasize the practical tradeoff between the heterodyne implementation (where receiver noise means that a low modulation index gives the best sensitivity) and the optical one (where the difficulty in making high-quality narrowband optical filters means that a larger modulation index gives the best sensitivity).<>