Self-calibration and laser energy monitor validations for a double-pulsed 2-μm CO₂ integrated path differential...

Double-pulsed 2-μm integrated path differential absorption (IPDA) lidar is well suited for atmospheric CO₂ remote sensing. The IPDA lidar technique relies on wavelength differentiation between strong and weak absorbing features of the gas normalized to the transmitted energy. In the double-pulse case, each shot of the transmitter produces two successive laser pulses separated by a short interval. Calibration of the transmitted pulse energies is required for accurate CO₂ measurement. Design and calibration of a 2-μm double-pulse laser energy monitor is presented. The design is based on an InGaAs pin quantum detector. A high-speed photoelectromagnetic quantum detector was used for laser-pulse profile verification. Both quantum detectors were calibrated using a reference pyroelectric thermal detector. Calibration included comparing the three detection technologies in the single-pulsed mode, then comparing the quantum detectors in the double-pulsed mode. In addition, a self-calibration feature of the 2-μm IPDA lidar is presented. This feature allows one to monitor the transmitted laser energy, through residual scattering, with a single detection channel. This reduces the CO₂ measurement uncertainty. IPDA lidar ground validation for CO₂ measurement is presented for both calibrated energy monitor and self-calibration options. The calibrated energy monitor resulted in a lower CO₂ measurement bias, while self-calibration resulted in a better CO₂ temporal profiling when compared to the in situ sensor.