Ultra-compact multimode interference InGaAsP multiple quantum well modulator

We propose a new structure for an ultra-compact multimode interference (MMI) InGaAsP multiple quantum well modulator. The operating principle is based on restricting the coupling of the self-image produced by the MMI region into a single mode output waveguide. The key is to excite only the even modes within the MMI region, and this is achieved by operating the MMI waveguide under the condition of restricted symmetric interference. By asymmetrically inducing a phase change of along a selected area within the MMI region, mode conversion of all the even modes to odd modes is achieved. Since only the fundamental mode can be coupled to the output waveguide, neither an individual mode, nor any combination of the modes will be coupled, and therefore the injected light is fully attenuated. The modulation characteristics are analyzed using the finite-difference beam propagation method. Extinction ratios as low as –37 dB are demonstrated without electro-absorption effects. For the case of low electro-absorption, which corresponds to a more realistic situation, this value is only increased to –35 dB.