Multichannel Coherent Lightwave Systems: Practical Problems and Possible Solutions

An extensive field deployment of optical fiber has already undergone, and it is expected to replace the copper within the next twenty years. The ultimate goal in communications and computing industry is to make all optical networks possible in the near future. In this context, certain important practical problems that exist in multichannel coherent lightwave systems are studied, and possible solutions are provided in this dissertation. It is shown that the capacity of dual-filter FSK heterodyne lightwave systems can be enhanced by exploiting the interrelationship between the frequency separation and the amount of laser phase noise. Optimum choice of intermediate frequency filter bandwidth also improves the system capacity. The effect of finite intermediate frequency on the performance of ASK heterodyne lightwave systems is also studied. The results obtained show that certain finite choices of intermediate frequency allows ideal envelope detection. Thus, one can design a multichannel ASK heterodyne lightwave system with relatively small optical domain channel spacings as long as optimum values of intermediate frequency is used. Otherwise, either the channel spacings should be increased, which, in turn, reduces the system capacity, or an extra sensitivity penalty should be tolerated which translates into an increase in the system cost. It is also shown that the effect of finite intermediate frequency is more significant in negligible linewidth systems. On the other hand, non-negligible linewidth systems are more immune to the effects of finite intermediate frequency. However, the amount of channel spacing in a multichannel system significantly increases in the case of non-negligible linewidth systems due to spectral broadening of information bearing signal. The effect of crosstalk in multi-channel ASK heterodyne lightwave systems with polarization control is also studied, and the results obtained show that choice of intermediate frequency filter bandwidth, in addition to channel spacing, has a significant impact on system performance. It is shown that most of the crosstalk is observed when system evolves from single-channel to two-channel system. Further additions of new channels have incremental effects on system performance, assuming that nonlinear effects of optical fiber are negligible. The interrelationship between optical domain channel spacing, electrical domain channel spacing, and location of local oscillator laser frequency is also quantified. Finally, it is shown that using optical polarization diversity technique, instead of polarization control, introduces an additional sensitivity penalty of less than 0.5 dB only.