Principles and design of a planar waveguide Fourier transform spectrometer for remote-sensing applications

摘要

This paper presents the design and operating principles of an advanced Fourier transform (FT) microspectrometer. The microspectrometer is a static Fourier transform instrument based on the principle of spatial heterodyne spectroscopy (SHS), affording high optical throughput (étendue) as compared with an arrayed waveguide (AWG) or planar waveguide echelle grating spectrometer. The instrument is realized as a densely-packed array of Mach-Zehnder interferometers (MZIs) with linearly increasing optical path delays (OPDs). Each MZI in the array constitutes a sampling point in the discrete Fourier-transform of the optical spectrum. The use of discrete MZIs in this device makes the selection of Fourier samples straightforward, and the throughput advantage permits the development of very high-resolution devices. Using this approach we have developed a 100-MZI FT chip with high resolution (0.05 nm) over a free spectral range (FSR) centered on the Q-branch absorption features of atmospheric methane (1667.75 nm-1665.25 nm). This FT chip is the central component of an integrated microspectrometer payload for measuring greenhouse gas emissions in the Canadian oil sands. The MZI array is realized in silicon nitride and has an overall footprint of 12mm × 22 mm with OPDs ranging from 0.32 mm to 32 mm. The two outputs of each MZI are re-balanced in multi-mode interference (MMI) devices but not combined, resulting in a system with 100 inputs and 200 outputs. Separating the outputs of the MZIs in this manner allows for normalization between MZI arms in order to correct for asymmetric loss in the MZI. High-resolution spectrometers of this type require highly unbalanced MZIs, which may be unstable with respect to temperature. Temperature induced variations may be corrected in postprocessing provided that the variations are made small, this can be accomplished either by active cooling via Peltier system, or passively by the use of athermal waveguides. We have designed athermal waveguides through the use of a cladding material with a negative thermo-optic coefficient (TOC). We balance the modal confinement between the core (positive TOC), lower cladding (positive TOC), and upper cladding (negative TOC) to lower the effective TOC of the device. By design of these athermal waveguides we eliminate the need for a precise active cooling system, reducing the mass, volume, and power requirements of the integrated payload.