Distortion characteristics of optical directional coupler modulators

A theoretical analysis is made of the nonlinearities of the intensity modulation response of the optical directional coupler, as a function of bias point for the case of phase-mismatch modulation, and the results are compared with those of interferometric modulators. The interferometric, standard 2×2 directional coupler and the 1×2 y-fed directional coupler modulators are shown to exhibit very similar intermodulation distortion effects. At 4% optical modulation depth, the third-order intermodulation products are -74 dB, -72 dB, and -73.6 dB respectively, below the carrier level