LINEWIDTH-INSENSITIVE COHERENT AM OPTICAL LINKS - DESIGN, PERFORMANCE, AND POTENTIAL APPLICATIONS

The use of coherent detection in analog optical links offers several a dvantages over direct detection: improved receiver sensitivity, inhere nt frequency translation, and the ability to utilize angle modulation and separate wavelength division multiplexed (WDM) signals. In this pa per, we investigate an externally modulated coherent AM optical link. We study the dynamic range of the coherent AM link, considering receiv er noise, laser phase noise, laser relative intensity noise (RIN), and system nonlinearities. With proper selection of the receiver's IF ban dwidth, the coherent AM link can be made insensitive to the laser line width. For optical powers less than 5 mW, RIN of less than -160 dB/Hz reduces the spurious-free dynamic range (SFDR) by less than 3 dB with the use of a balanced receiver. The external modulator nonlinearity is the dominant nonideal effect; it reduces the SFDR by 5-19 dB from the theoretical limit for 100% modulation index. We compare the performan ce of the coherent AM link with that of a conventional direct detectio n link for two applications: point-to-point links and distribution net works. When the received optical power is less than 1 mW, the coherent link can provide higher SFDR than the direct detection link. Thus, co herent links are well-suited for long distance point-to-point links an d FM video distribution systems.