Abstract

The best optical detection sensitivities and longest fiber transmission distances^1-3 are now obtained by techniques which employ local oscillator lasers and optical mixers to convert an optical signal to radio frequencies for detection. Such systems (generally called coherent even when phase is not detected) have proved to be the most successful route to near-quantum limited sensitivities since they provide a means for overcoming the thermal noise which limits direct photon detection schemes. For example, in a transmission experiment at 400 Mbit/s a sensitivity of 45 photons per bit was obtained⁴ compared with the quantum limited value of 20. This corresponds to an improvement of a factor of 10 over direct detection at this rate.⁵

© 1986 Optical Society of America