Abstract
We demonstrate a passively thermally-stabilized planar waveguide Fourier-transform spectrometer for remote detection of atmospheric methane. The device is implemented as a spatial heterodyne spectrometer using an array of 100 Mach-Zehnder interferometers (MZIs) on an integrated photonic chip. The spectrometer is buffered against temperature fluctuations by using waveguides with a carefully engineered, athermal geometry. The achieved waveguide thermooptic optic coefficient is 3.5×10−6K−1. Effective entrance aperture is increased over dispersive element spectrometers, without sacrificing spectral resolution, by coupling light independently to each of the 100 MZIs. The output of each MZI is sampled in quadrature, to compensate for non-uniform illumination across the MZI input apertures. The spectrometer is validated using a methane reference cell in a benchtop setup: an interferogram is inverted via least-squares spectral analysis (LSSA) to retrieve multiple absorption lines at a spectral resolution of 50 pm over a 1 nm free spectral range (FSR) centered at λ0 = 1666.5 nm. The retrieved spectrum is compared against the Beer-Lambert absorption law and is found to provide a correct measurement of the volume mixing ratio (VMR) in the optical path.
Highlights
Methane is a greenhouse gas that contributes significantly to global climate change through its absorption of outgoing infrared (IR) radiation and interaction with atmospheric aerosols [1]
The device is implemented as a spatial heterodyne spectrometer using an array of 100 Mach-Zehnder interferometers (MZIs) on an integrated photonic chip
Effective entrance aperture is increased over dispersive element spectrometers, without sacrificing spectral resolution, by coupling light independently to each of the 100 MZIs
Summary
Methane is a greenhouse gas that contributes significantly to global climate change through its absorption of outgoing infrared (IR) radiation and interaction with atmospheric aerosols [1]. One promising technique for inexpensively monitoring sparsely distributed sources of methane is remote detection of atmospheric methane via observation of molecular absorption lines using high-resolution IR spectrometers [4]. Such spectrometers may be housed on highly mobile platforms such as unmanned aerial vehicles (UAVs), and micro and nanosatellites in order to provide global coverage of methane emission sources. The first on-chip SHFTS was proposed in Michelson configuration [10] and was subsequently implemented as an array of MZIs with linearly increasing optical path delays (OPDs) [11,12,13,14,15] These MZIs constitute a discrete Fourier-transform in which the output of each MZI corresponds to a point in the spatial interferogram. We use the waveguide SHFTS and a reference gas cell to demonstrate on-chip SHFTS detection of atmospheric absorption features for the first time
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