Development of a Singly-Resonant Optical Parametric Oscillation for a Laser Transmitter of an Eye-Safe Ground-Based Differential Absorption Lidar (DIAL) for Carbon Dioxide (CO2) and Methane (CH4) Studies

Improved understanding of the spatial distribution of the greenhouse gases CO2 and CH4 is necessary to quantify their anthropogenic climate impact. While extensive research has been done on instrumentation for both CO2 and CH4 measurements, the accuracy of below 2% error needed for carbon cycle studies remains a challenge. The goal of our current research is to develop a singly resonant optical parametric oscillator (OPO) for use as a laser transmitter for a ground based differential absorption lidar (DIAL) for the spatial mapping of CO2 and CH4 concentrations. Modeling predicts that the DIAL requires a laser transmitter with over 3 mJ of pulse energy and a repetition rate of 1 kHz at 1.571 μm and 1.654 μm wavelengths for CO2 and CH4 respectively. A periodically poled magnesium-doped lithium niobate (PPLN) crystal is used in the OPO based laser transmitter where quasi-phase matching can be achieved at CO2 and CH4 lines via temperature tuning of the PPLN crystal. The initial laser development for CO2 DIAL applications uses two input laser beams, a 1.064 μm pulsed pump beam and a 1.571 μm continuous wave seed beam. These are guided into the crystal placed within a 99%-50% reflective hemispherical cavity for the seed laser wavelength. Results from the OPO yield an output energy of 2.1 mJ at the 1.571 μm wavelength when operating the pump laser at a 20 Hz pulse repetition rate at 11.5 mJ and the seed laser at 160 mW. Good input/output conversion efficiency of the OPO is observed at greater than 18%. These results predict that an OPO output energy of 3 mJ at 1.571 μm would be possible with an input pump energy of 17 mJ. The CO2/CH4 DIAL architecture, modeling results for the DIAL instrument, and the performance of the OPO based laser transmitter will be presented.