Scanning Spectrometer-on-a-Chip Using Thermo-Optical Spike-Filters or Vernier-Comb Filters

This theoretical modeling and simulation paper presents designs and projected performance of an on-chip scanning spectrometer using thermo-optical (TO) spike-or Vernier-comb filters operating around 3600 nm for the germanium-on-nitride technological platform. Raman spectra and absorption spectra are targeted. The spike filter consists of a thermo-optically tunable waveguide Bragg grating resonator (WBGR). The length of the WBGR is chosen to realize a Lorentzian resonant spectral profile with a bandwidth changing between 0.1 and 1 nm. The Verniercomb filter is obtained using the cascade of two multi-Sagnac loop Mach-Zehnder interferometers, in which through the spectrum of the second MZI is shifted along the wavelength axis by means of a low-power TO heater stripe atop each loop-connector. The sagnac loop reflectors length is designed according to the wavelength-channel spacing requirements in order to induce the Type-1 Vernier effect. Time-scanned discrete sampling of the unknown input spectrum is obtained by driving the TO heater stripe with a periodic ramp current. We examined the device performance in terms of signal reconstruction in the mid-infrared region. The investigation demonstrated that high quality spectrum reconstruction is obtained for arbitrary input signals with an observation wavelength range from 23.4 to 77.4 nm. We also show that spectrum reconstruction of arbitrary signals is feasible in the germanium on nitride chips even in presence of additive noise.