Optical single sideband for broadband and subcarrier systems

by Davies, Robert James

Radio systems are being deployed for broadband residential telecommunication services such as broadcast, wideband lntemet and video on demand. Justification for radio delivery centers on mitigation of problems inherent in subscriber loop upgrades such as Fiber to the Home (WH) and Hybrid Fiber Coax (HFC). While radio will alleviate sorne of these subscriber loop upgrade problems there are still problerns to be solved, the rnost pressing of which is the propagation difficulties encountered by high frequency wideband radio signals in multipath environments. Deployrnent of the radio structure on optical fiber has been suggested as a mitigation for these path problerns by allowing the reduction the propagation distance between the optical/RF interface point and the consumer prernises. While the radio propagation problem rnay be alleviated by fiber deployrnent, a new problem anses due to the interaction of fiber chromatic dispersion and the spectral characteristics of the optical signal. A dispersion penalty in the forrn of reduced power from the optical Iink results when radio signals are transmitted on fiber in conventional Double Sideband (DSB) Mode. It will be shown that Optical Single Sideband Modulation (OSSB) is an effective tool for combatting dispersion effects on fiber. A new and general theory is presented that addresses OSSB from the optical broadband perspective. The theory is adapted from the studies of Compatible Single Sideband radio modulation for AM radio in the 1960's. The core of the theory involves the behavior of tirne domain signals that have similar amplitude and phase relationships to the 'Minimum Phase' class of signal. It will be shown that immediate dispersion benefits for optical modulation are attained by using these signals and that further benefits are available for post-detection compensation. The theory will then be modified to accommodate optical subcarrier modulation with the end result being a new class of optical link that has general application in fiber optic communications. Predistortion schemes for modulated subcarriers will be developed to efficiently utilize the OSSB theory on optical links that also function as harmonic upconverters. Finally, experimental results will be presented to verify the theoretical daims.