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 mm 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.