On-chip optical spectrometer with a tunable micro-ring resonator and a Mach-Zehnder interferometer lattice filter

We propose and demonstrate a compact on-chip optical spectrometer by integrating a tunable micro-ring resonator (MRR) with a 4-channel wavelength demultiplexer (DEMUX) based on a Mach-Zehnder interferometer (MZI) lattice filter. The MRR with a 3-dB bandwidth of 0.15 nm ensures the high resolution of the spectrometer. The 4-channel DEMUX is designed with channel spacing equal to the free spectral range (FSR) of the MRR, providing effective wavelength separation with crosstalk less than -18 dB. The thermally tuned MRR across its FSR, together with the synchronized tuning MZIs, enables wavelength scanning within the bandwidth of 40 nm. With such a design, a compact on-chip spectrometer with a footprint of about 0.2 mm² is implemented to demonstrate the ability to retrieve the spectra of two laser lines separated by 0.2 nm. Our studies shed light on the configuration design of chip-scale spectrometers.