Source: http://aoot.osa.org/oe/abstract.cfm?uri=oe-25-26-33018
Timestamp: 2019-04-21 16:15:47+00:00

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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.
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Fig. 1 Diagram showing the operating principle for an on-chip SHFTS. MZIs are implemented as squared spirals, enabling high-density packing. Each MZI has an independent aperture and splits its outputs into two ports which are 180° out-of-phase as shown in the lower inset. The continuous interferogram, I(x), is sampled at locations defined by the OPDs of each MZI, as shown in the upper inset.
Fig. 2 Waveguide cross-section showing modal power distribution and waveguide dimensions. The waveguides are formed on an 8 μm thick layer of SiO2, and the waveguide core consists of two 300 nm thick layers of PECVD Si3N4 separated by a 90 nm thick SiO2 layer. The upper cladding is formed by a 7 μm thick layer of SU-8 polymer.
Fig. 3 (a) Light from a tunable laser is coupled into a single-mode fiber (yellow), and passes through a 7.5 cm methane cell. The optical power is tapped at 10% and monitored using an InGaAs photodiode (PD1). The single mode fiber is split by a polarizing beam splitter (PBS) into two polarization-maintaining fibers (blue). One arm leaving the PBS is directed to a wavemeter and photodiode (PD2). The second arm is directed to the chip via a fiber collimator and focused in the horizontal and vertical axes by two cylindrical lenses (L1, L2). The output signal is passed through a linear polarizer (LP) and captured by an InGaAs camera. (b) Output signal captured by camera. (c) Photograph of FTS chip next to a 25-cent Canadian coin.
Fig. 4 Measured calibration matrix of the SHS chip showing the wavelength-dependent response of each of the 100 MZIs obtained during a high-resolution scan over a 2.5 nm bandwidth between 1666 nm and 1668.5 nm. MZIs are ordered sequentially according to their OPDs, with MZI 1 having the shortest OPD and MZI 100 having the longest.
Fig. 5 Experimental observation of methane absorption features using on-chip SHFTS over 1 nm bandwidth with 50 pm resolution. The spectrum is obtained from the interferogram via pseudoinverse of the calibration matrix shown in Fig. 4. The signal collected by the reference photodiode (PD1 in Fig. 3(a)) is also displayed. The methane absorption features in the reference cell are retrieved with SNR > 3 : 1.
Fig. 6 Modulation function of a high-sensitivity MZI as a function of wavenumber at T1 = 25 °C and T2 = 15 °C. The modulation function is obtained as the difference in intensity between the two out-of-phase MZI outputs normalized by their sum. The OPD of the device (1.22 cm) is first constrained by a cosine fit to the modulation function at 25 °C. The TOC is then determined by fitting the modulation function at 15 °C (i.e. ΔT = −10 °C) to Eq. (4), yielding the TOC αeff = −(3.52 ± 0.05) × 10−6K−1.

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