Abstract

Heterodyne transmissions at moderate bit rates (≤ 1 Gb/s) offer a high power budget and are promising for long-haul repeaterless transmissions. DPSK modulation allows very high sensitivity but, up to now, has required an external phase modulator, which leads to additional implementation difficulties and 2–3-dB insertion loss. CPFSK modulation could reach comparable sensitivity without external modulation but needs a laser transmitter with a flat frequency response down to dc with a negligible thermal condition.

© 1991 Optical Society of America