Tunable multimode interference devices

A tunable multimode interference (MMI) coupler that operates by modifying the phase of the multiple images that are formed around the midpoint of the MMI section is demonstrated. The phase change is achieved by current injection, and therefore minimizing current spreading is crucial for optimal operation. A zinc in-diffusion process has been implemented to selectively define p-i-n regions and effectively regulate the current spreading by. controlling the depth of the zinc doping. Using this process a tunable 3-dB; MMI coupler has been fabricated. Our initial results show that the device can be easily tuned all the way from a 90: 10 to a 30:70 splitting ratio of the optical power transmitted through the two output ports. We believe that further improvement on the device fabrication will lead to a more symmetric tuning response of the device. Nevertheless, the initial results are very encouraging since, to our knowledge, this degree of tuning has never been experimentally demonstrated in similar MMI devices. Furthermore, this device processing technique can easily be applied to a wide variety of semiconductor photonic switches that operate on MMI effects.