Development of a singly-resonant optical parametric oscillator for carbon cycle science

Abstract

The human impact on the global carbon cycle is affecting the health of the environment by changing the balance between incoming and outgoing radiation as well as altering other geochemical cycles such as the nitrogen and water cycles. Although carbon dioxide makes up most of the greenhouse gas emissions, methane has a much greater impact on climate change due to its warming potential on a per molecule basis. Improved understanding of the spatial distribution of methane is necessary to quantify the anthropogenic impacts and mitigate future damage. A differential absorption lidar (DIAL) is proposed for spatially mapping methane concentrations. The system requires a laser transmitter that can produce over 3 mJ of pulse energy with a repetition rate of 1 kHz and output wavelength of 1.654 micron as well as a narrow linewidth on the order of 3 MHz. Modeling predicts that a system with these specifications can achieve measurement error of less than 2% relative to ambient levels of methane. Laser sources with these specifications are not commercially available, and the goal of this work is to evaluate the potential for a singly-resonant optical parametric oscillator (OPO) for the DIAL laser transmitter. The OPO is based on large-aperture periodically-poled magnesium-oxide-doped lithium niobate as the nonlinear optical material. Results from the OPO indicate that energies on the order of 1 mJ are possible with the experimental setup presented when operating at 20 Hz repetition rate. The OPO produced a linewidth of 10.5 GHz, measured on a system with resolution of 6.6 GHz. Future work includes optimization of the OPO to increase the output energy from the system to 3 mJ by improving mode-matching into the cavity and increasing the energy into the system. Additionally a method to precisely measure the linewidth of the OPO output is necessary as well as a pump laser that operates at 1 kHz to test the performance of the OPO at 1 kHz. The initial results show promise for the use of the OPO as the DIAL laser transmitter and with improvements, the OPO should meet the requirements for the DIAL system.