Programmable silicon photonic processors finds widespread applications in classical and quantum information processing because of its CMOS fabrication process compatibility and scalability. The basic building block of a programmable photonic integrated circuit is a tunable 2x2 power splitter. Thermo-optically tunable Mach Zehnder interferometer (MZI) is commonly used because of its robustness and predictable performance over a broad wavelength range. However, its typical footprint is relatively large (∼500 µm) and hence limits large-scale integration. Besides, higher switching power and thermal cross-talk are other hindrances for its large-scale integration. Therefore, a compact device design with equivalent or better performance metrics (in comparison to that of a MZI based TBU) is beneficial for the demonstration of a multipurpose programmable photonic integrated circuits.



In this talk, we will review the state-of-the-art design of tunable basic unit (TBU) based on MZI and discuss about its limitations for large-scale integration in silicon photonics technology platform. We will then discuss about a relatively compact TBU design based on the working principle of multi-mode interferometer (MMI). It will be shown that the MMI based TBU design is relatively compact and exhibits better tolerance to the fabrication process induced phase errors. Preliminary experimental results of a fabricated device will be also presented.