Coherent detection techniques to eliminate power fading in optical subcarrier systems

摘要

Optical coherent techniques are used to eliminate power fading found in optical subcarrier multiplexed systems. In a double-side band optical subcarrier system with direct detection the signal experiences a periodic power fading that is dependent on the fiber dispersion and subcarrier frequency. This power fading results from interference between the two side-bands following the square-law photodetector. It is shown that the use of an appropriately modulated optical local oscillator to coherently detect the subcarrier channel can eliminate this power fading as well as phase error that gives rise to eye distortion. For homodyne detection an optical local oscillator, centered at the optical carrier, is double-sideband suppressed-carrier (DSB-SC) amplitude modulated by the subcarrier frequency of interest. By independently controlling the phases of the optical local oscillator and the DSB-SC modulation both the phase error and power fading of the detected subcarrier channel can be eliminated. This technique also allows the subcarrier to be selected optically, before the optical-to-electrical conversion.