Singly-resonant optical parametric oscillators with pump-modulation transfer for frequency-modulated spectroscopy in the mid-infrared

Frequency modulation (FM) techniques are well known methods for improving signal-to-noise ratios in laser spectroscopy. Such techniques have proven particularly effective with diode lasers due to the ease with which they can be frequency modulated via their injection current. Although singly-resonant optical parametric oscillators (OPOs) are flexible, powerful and widely-tunable sources for mid-infrared laser spectroscopy, the utilization of FM techniques with OPOs suffers from the inconvenience of requiring an external mid-IR modulator. As a consequence, FM techniques have not been implemented with such devices. In this paper we describe the implementation of wavelength-modulation spectroscopy (WMS) and frequency-modulation spectroscopy (FMS) using a singly-resonant OPO pumped by a fiber-amplified diode laser. The OPO was capable of producing output powers of up to 1W in the 3.15-3.8 μm range with continuous tuning over >100GHz on millisecond timescales. Frequency modulation, via injection current, of the diode laser transferred directly to the OPO's idler output, allowing mid-IR FM to be achieved without external modulation devices. WMS and FMS spectra of methane were then recorded, clearly demonstrating that this approach provides a means of implementing these important techniques with powerful, widely tunable, mid-IR sources while retaining the simple, flexible modulation properties of diode lasers.