On-chip Spectrometer with Cascaded Micro-ring Resonator and Mach-Zehnder Interferometers

We demonstrate a compact on-chip spectrometer by cascading a tunable micro-ring resonator (MRR) and a Mach-Zehnder interferometer (MZI)-based 4-channel demultiplexer. The MRR acts as a high-fineness optical filter with tunable periodic narrow passbands. The function of the MZI-based demultiplexer is to separate passbands of the MRR to be measured by different photodetectors. The 3 dB bandwidth of the MRR's passband is 0.15 nm, which determines the resolution of the spectrometer. We fabricate the device on SOI substrate and verify its feasibility by measuring two laser lines with different wavelength spacing. We also discuss possible ways to further improve the resolution of the device.