Ultrafast optical parametric oscillators for spectroscopy

The combination of high spatial coherence, wide tunability and broad intrinsic bandwidth makes femtosecond optical parametric oscillators (OPOs) uniquely attractive sources for spectroscopy in the visible and infrared. In the mid-infrared the idler pulse bandwidths from such systems can extend over several hundred nanometres, making Fourier-transform spectroscopy possible, and transferring the wavelength calibration and resolution constraints from the OPO to the detection system. Unlike thermal sources of mid-infrared radiation, the spatial coherence of the output from femtosecond OPOs is extremely high, with the potential for spectroscopic measurements to be made over long free-space path lengths, in fiber or at the focus of a microscope objective. Using OPOs based on MgO:PPLN, and pumped by a self-modelocked Ti:sapphire laser, we have shown free-space and photonic-crystal-fiber-based spectroscopy of methane to concentrations as low as 50 ppm. The spectral bandwidth of the idler pulses used for gas sensing exceeds 200 nm, allowing the principal methane absorption lines around 3.3 μm to be acquired without wavelength tuning the OPO. Practical Ti:sapphire and Yb:fiber pumped based OPOs have been demonstrated that offer combinations of flexible tuning, high stability, low-threshold operation and high-energy output pulses.