Design and Performance of Coherent Analog Optical Links

At present, most of the work on analog optical communication links focused on direct detection techniques. Very little attention has been paid to coherent detection where at the receiving end, a local oscillator laser is used and its output is combined with the incoming optical signal before detection. Coherent detection provides several advantages over conventional direct detection techniques including improved receiver sensitivity, inherent frequency translation, and the ability to utilize angle modulation and separate dense WDM signals. Coherent optical links provide all these advantages at low optical powers which makes them less sensitive to various fiber nonlinear effects. However, coherent optical links are potentially susceptible to the effects of phase noise associated with wide laser linewidths. The work presented in this dissertation is the first attempt at a comprehensive study of a coherent analog optical link, from the design stage, its implementation, characterization and investigations into improving its performance. In this work, an experimental coherent analog optical link was constructed, and theoretical and experimental studies were conducted on the performance of coherent analog optical links as compared to conventional direct detection links. The results of these investigations show that the coherent AM-WIRNA heterodyne link can be made insensitive to laser linewidth for linewidths up to 300 MHz. To the author's best knowledge, this is the first ever demonstration of a gigahertz bandwidth linewidth-insensitive coherent analog optical link. This study indicates that the coherent AM link has a higher spurious-free dynamic range (SFDR), by up to 10 dB, than the corresponding direct detection link, when the received optical power is less than 85 muW, and for link losses greater than 7 dB. However, coherent links were found to be more susceptible to the effects of laser RIN, requiring lasers with RIN better than -140 dB/Hz. In addition, coherent links have a lower gain and higher noise figure. The improvements of link performance obtained using optical amplifiers and a linearized modulator were investigated and demonstrated. Results of this work show that it is only for high loss links, link loss greater than 28 dB, that coherent links give the best performance, and this is with an optical amplifier after the optical modulator. For the rest of the cases, the direct detection link, either with an optical amplifier or without it, is the best link. Using a linearized modulator, the highest dynamic range improvement of 11.4 dB was achieved by the work in this thesis. In addition, an SFDR of 115 dB cdotHz^{2/3} was achieved, the best dynamic range ever attained with a coherent analog link.