Optically coherent direct modulated FM analog link with phase noise canceling circuit

This paper demonstrates a new phase noise canceling (PNC) circuit for use with an optically coherent analog frequency modulation (FM) link employing directly modulated distributed feedback (DFB) lasers. Direct frequency modulation of a semiconductor laser is a highly efficient optical-to-electrical conversion process, which can lead to very low noise figure (NF) and high dynamic range (DR). However, the large laser phase noise found in semiconductor lasers significantly degrades the FM link performance. The PNC circuit is a simple means for canceling the laser phase noise while extracting the modulated signal and taking advantage of the high FM conversion efficiency of semiconductor lasers. The theoretical performance of the PNC PM link is discussed in terms of the signal-to-noise ratio (SNR), NF, and DR, and is compared to a Mach-Zehnder modulated link which uses high-power, solid-state lasers. Phase noise cancellation is demonstrated in an experimental PNC FM link. Comparison of a PNC FM link to an externally modulated AM (coherent) link shows a 31 dB improvement in the NF of the FM link, and a 10 dB improvement in DR. However at higher received optical powers phase-to-intensity noise limits the performance of both links to well below the theoretical calculations.