High-Resolution and Broadband Two-Stage Speckle Spectrometer

Motivated by handheld optical sensing applications, spectrometers with small footprint, broadband operating bandwidth, and low cost are highly desired. In this paper, we propose and experimentally test a fully integrated high-resolution and broadband two-stage spectrometer based on a multimode waveguide (MMW). MMW was used to enhance the resolution of the system while the integrated photodiodes provide a fully integrated solution. Six waveguide modes were launched into the multimode waveguide via asymmetric directional couplers and each mode could be individually enabled via its respective optical switches. The output mode field of the multimode waveguide was spatially sampled by six integrated photodiodes. Switch arrays increase the physical channels from 6 to 384 possible states (26 switch states × 6 detectors), therefore significantly enhance the bandwidth. Using compressive sensing algorithms, 5000 spectral channels of the optical signal can be reconstructed which is an order of magnitude higher than the cutting edge method. We measured the spectrometer to have a spectral resolution of 0.02-nm and optical bandwidth of 100 nm. Both the sparse and continuous spectra were reconstructed successfully.