Deficiency monitoring of DPSK signal using coherent hybrid

A method called coherent hybrid is proposed to monitor the deficiency of differential phase shifted keying (DPSK) either generated by the bias offset drift of Mach-Zehnder modulator or an unmatched drive voltage applied to a phase modulator (PM) is proposed and demonstrated. The hybrid power ratio of the maximum to the minimum after mixing the unmodulated local oscillation lightwave with the modulated signal from the same light source is employed to measure the degree of deficiency, simultaneously eliminate the dependence on light source power. Our experimental results reveal that a±5% bias offset of MZI can produce 0.3dB power variation, larger than that of 0.02dB in the traditional method by measuring the signal average power, and a drive voltage with a more than 10 percent deviation from the halfwave voltage can be detected when it is applied to a PM. A feedback and control circuit based on this scheme is developed and applied in our system. The impairments given by the unparallel orientation between the polarizations of signal and local oscillation lightwave and the transient time of modulation signal pulse is discussed. By properly setting the control parameters in the feedback control process, the impairments can be minimized.