Title: Development Of A Coherent Differential Absorption Lidar For Range Resolved Atmospheric CO2 Measurements
Primary Author: Yu, Jirong
Organization: NASA LaRC
Co-Author(s): Mulugeta Petros, Songsheng Chen, Yingxin Bai, Paul J Petzar, Bo C Trieu, Grady J Koch, Jeffery J Beyon, Michael J. Kavaya, Upendra N Singh

NASA Langley Research Center (LaRC) has developed a high energy, pulsed 2 µm Differential Absorption Lidar instrument based on coherent heterodyne technique that provides atmospheric CO2 measurements. The CO2 absorption lines in the 2.05 µm band are ideally suited for CO2 concentration measurements. In particular, the weighting function in the 2-µm region is more favorable for making CO2 measurements near the surface where the sources and sinks of CO2 are located We have selected to operate the lidar on the short wavelength wing of R(30) CO2 line at 2050.967 nm (4875.749 cm-1) in the side-line operation mode. The side-line position will be optimized for ideal optical depth of 1.0 over the full range of the measurements. The R(30) line is an excellent absorption line for the measurements of CO2 with regard to the absorption line strength, temperature insensitivity, and lack of interference from other absorption lines.

A high-precision and accurate CO2 measurement impose stringent requirements on the lidar transmitter parameters, such as pulse repetition rate, laser power telescope aperture product, laser frequency control accuracy, and the receiver design. The heart of a CO2 DIAL system is the transceiver which comprises the laser transmitter, the receiver detector and related optics. The exact wavelengths of the Ho laser are controlled by well defined CW seed laser sources to provide the required on-and-off line wavelength pulses sequentially. The compact, rugged, highly reliable transceiver is based on the unique Ho:Tm:YLF high-energy 2-micron pulsed laser technology. The laser architecture is Master Oscillator Power Amplifier (MOPA); both the oscillator and double pass amplifier are side pumped by AlGaAs diode arrays. The temperature controlled rigid optical bench is populated on both sides. The heterodyne receiver and the associated fiber optic couplers, the Faraday isolators to prevent the damage of the seeding lasers, and the laser health diagnostic components are on the back side of the bench while the oscillator and the amplifier are mounted on the front. All the optical mounts are custom designed and have space heritage. They are designed to be adjustable and lockable and hardened to withstand vibrations that can occur in airborne operation. The transmitter is designed to be operated in unique double pulse format to mitigate the measurement error. The transceiver has been integrated into a full functional laser lidar for CO2 concentration measurement by coherent DIAL technique. It is expected to provide unique capbility for supporting NASA Earth Science missions.