DISTORTION CHARACTERISTICS OF OPTICAL DIRECTIONAL COUPLER MODULATORS

Waveguide electrooptic modulators are of much interest for analog optical transmission. Here, a theoretical analysis 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 is made and the results are compared with those of interferometric modulators. The interferometric, standard 2 x 2 directional coupler and the 1 x 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. © 1990 IEEE