Tunable multimode interference coupler

Introduction: Multimode interference (MMI) couplers have been widely used in integrated photonic devices because of their compact size, wide optical bandwidth, polarisation independence and relaxed fabrication tolerances [1]. In most applications they operate as passive components, and only recently has their use as active components for the development of photonic switches been investigated [2, 3]. Switching is achieved by modifying the phase of the multiple self-images that occur at different lengths along the MMI waveguide. Modifying the phase relation between the self-images leads to a modified output image, and light can then be directed to a specific output waveguide. The approach works properly as long as the refractive index change is entirely confined within the areas containing the principal self-images. However, owing to the amount of refractive index change required, the devices are intended to operate by electric current injection. In this case, current spreading becomes a serious issue since the phase change is applied to a bigger area than the one delineated by the electrical contact, and thus deteriorates the optimum device performance. This has been a key factor in experimentally demonstrating such MMI devices. In this Letter, we report the application of an area selective zinc in-diffusion process to selectively define p–n junctions across a semiconductor wafer. The isolated junction acts as a channel for the injected electrical current, and thus current spreading can be regulated by simply changing the zinc depth. Using this process we demonstrate a tunable 3 dB MMI coupler that can be easily tuned from a 90:10 to a 30:70 splitting ratio. To our knowledge, this amount of tuning has never been demonstrated before in MMI structures, and demonstrates the potential of the technique for more sophisticated MMI-based devices.