Abstract

Frequency noise degrades the performance of optical heterodyne communications systems: the error rate performance of an FSK system is limited to a floor caused by cross talk due to nonzero laser linewidth. Recently it was pointed out that the use of nonlinear heterodyne detection coupled with differential FSK modulation can alleviate this limitation.¹ We present preliminary experimental results confirming this prediction. Two semiconductor lasers were used in a heterodyne communications experiment. Commercial acoustooptic modulators were used to modulate one laser in a binary differential FSK format at 1 Mbps. At the receiver, the intermediate frequency signal was squared and the difference-frequency component extracted. This component is free from the effects of frequency noise. Error rate measurements showed that the performance of this system was not limited by a floor as in a conventional case. The effective signaling linewidth was <100 kHz, even for laser linewidths of 100 MHz.

© 1985 Optical Society of America