Mid-infrared dual comb spectroscopy with asynchronous optical parametric oscillators
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Dual-comb spectroscopy (DCS) is a novel approach that uses asynchronous broadband coherent sources to achieve Fourier-transform-like spectroscopy but with no moving parts and at kHz acquisition rates. To date, fully resolved and accurate dual-comb spectrometers have been demonstrated in the near-infrared and applied to broadband spectroscopy for precise measurement of molecular centerlines, spectral lidar, and greenhouse gases from the near- to mid-IR. This thesis describes DCS with asynchronous optical parametric oscillators and explores their applications in rapid, high-resolution broadband spectroscopy in the mid-infared. Initially a dual-comb spectrometer was designed with two identical optical parametric oscillators (OPOs) pumped by two identical Yb:fibre lasers and its stability performance was characterized measuring relative intensity noise. First experiments were accomplished by using free-running independent MgO:PPLN based OPOs with a repetition-rate difference of 500 Hz, achieving resolutions of 0.2 cm-1 across a wavelength range 3.1 to 3.5 μm; an absolute wavelength calibration technique was employed to allow registration and averaging of consecutively acquired dual-comb spectra. Then experiments were repeated with a dual-comb source for the spectral fingerprint region based on a pair of entirely free-running OPOs, each pumped by a 1-µm femtosecond laser and utilizing the new gain medium orientation-patterned gallium phosphide (OPGaP) to produce broadband idler pulses tunable from 6–8 µm. Methane absorption spectroscopy in the deep infrared region was demonstrated with the same wavelength calibration approach for both dual-comb spectrometers, leading to a high quality and low-noise absorbance measurement with spectral coverage simultaneously spanning the methane P, Q and R branches in good agreement with the Hitran database.