Mid-infrared absorption spectroscopy of methane across a 14.4THz spectral range using a broadband femtosecond optical parametric oscillator based on aperiodically poled lithium niobate

Conventionally optical parametric oscillators (OPOs) have been used in high-resolution absorption-spectroscopy as narrow-band tuneable sources where the measurement resolution is determined by the OPO output linewidth, rather than the wavelength resolution of the detector. In contrast, the absorption spectroscopy of gases and other media has for many years been carried out using instruments such as Fourier-transform infrared (FTIR) spectrometers or high-resolution diffraction-grating-based tuneable monochromators. These techniques commonly utilise broadband thermal sources with highly-divergent illumination beams limiting their use in remote sensing or fibre delivery applications. The work presented here reports a new approach to FTIR spectroscopy based around a novel Ti:sapphire pumped, signal-resonant OPO that uses a 10mm crystal of aperiodically-poled lithium niobate (APPLN) as the gain medium producing an idler output covering a 3.2-3.85µm tuning range with a typical full-width-half-maximum bandwidth of 85nm. Methane was used to demonstrate the technique since the OPO tuning range almost completely covers the strongest mid-infrared absorption lines of methane from 3.0 - 3.7µm (limited only by the available resonator optics). A double-beam Michelson interferometer was built around the OPO idler beam using a helium-neon laser as the second beam to self-calibrate each trace. Course tuning of the OPO resulted in the measurement of absorption data across the 3.2-3.85µm tuning range using methane held at pressures ranging from 2000mbar down to 25mbar. A maximum resolution of around 1cm was achieved using a simple rapidly scanning mirror assembly indicating that with further development this approach could yield very high-resolution measurements.