Monitoring of CO₂ column concentrations is valuable for atmospheric research. A mobile open-path system was developed based on tunable diode laser absorption spectroscopy (TDLAS) to measure atmospheric CO₂ column concentrations. A laser beam was emitted downward from a distributed feedback diode laser at 2 μm and then reflected by the retroreflector array on the ground. We measured the CO₂ column concentrations over the 20 and 110 m long vertical path. Several single-point sensors were distributed at different heights to provide comparative measurements for the open-path TDLAS system. The results showed that the minimum detection limit of system was 0.52 ppm. Some similarities were observed in trends from the open-path TDLAS system and these sensors, but the average of these sensors was more consistent with the open-path TDLAS system values than the single-point measurement. These field measurements demonstrate the feasibility of open-path TDLAS for measuring the CO₂ column concentration and monitoring carbon emission over large areas.

An open-path atmospheric $CO2$ measurement system was built based on tunable diode laser absorption spectroscopy (TDLAS). The $CO2$ absorption line near $2μm$ was selected, measuring the atmospheric $CO2$ with direct absorption spectroscopy and carrying on the comparative experiment with multipoint measuring instruments of the open-path. The detection limit of the TDLAS system is $1.94×10−6$. The calibration experiment of three AZ-7752 handheld $CO2$ measuring instruments was carried out with the Los Gatos Research gas analyzer. The consistency of the results was good, and the handheld instrument could be used in the TDLAS system after numerical calibration. With the contrast of three AZ-7752 and their averages, the correlation coefficients are 0.8828, 0.9004, 0.9079, and 0.9393 respectively, which shows that the open-path TDLAS has the best correlation with the average of three AZ-7752 and measures the concentration of atmospheric $CO2$ accurately. Multipoint measurement provides a convenient comparative method for open-path TDLAS.

Cantilever enhanced photoacoustic spectroscopy (CEPAS) technology improves the detection ability of weak gas photoacoustic signal effectively utilizing a micro-cantilever acoustic sensor instead of capacitive microphone, which can achieve highly sensitive detection of CO₂ and has great significance for environmental monitoring, industrial production control and regional carbon emission monitoring and other applications. A fiber-optic Fabry-Perot (F-P) sensor with a cantilever structure is designed in this work to overcome the disadvantage of the complication of traditional mechanical cantilever photoacoustic spectroscopy system. The photoacoustic spectroscopy system is established based on the resonance enhancement of the fiber cantilever acoustic sensor with a 2.0 μm distributed feedback (DFB) laser, which combines the technologies of wavelength modulation and second-harmonic detection for measuring CO₂ concentration. The experimental results show that the concentration calibration curve has a good correlation of 0.9765, and the detection limit of CO₂ concentration reaches 0.044×10^-6, which verifies the feasibility of the system for detecting atmospheric CO₂ concentration.

A pulsed-laser frequency-locking technique is proposed for a compact 355-nm direct-detection Doppler wind lidar. The technique employs an integrated locking loop based on a dual-channel Fabry–Perot etalon (DFP) and a seed-tunable 355-nm pulsed laser to support compact airborne deployment and long-term frequency stability. A polarization-controlled beam-splitting structure is introduced before the DFP to suppress splitting-ratio drift and maintain balanced power distribution between the two channels. The reference pulsed laser passes through the same DFP channels as the lidar backscattered signals and serves directly as the frequency-feedback signal, enabling a compact integrated design without a dedicated locking channel. With a cascaded dual-loop proportional–integral–derivative control strategy, the pulsed-laser frequency is stably locked to the DFP reference operating point. During a 6-h temperature-locked closed-loop measurement, the Allan deviations are 1.54 MHz at 1 s and 0.89 MHz at 15 s, corresponding to equivalent wind-speed uncertainties of ∼0.27 and 0.16 m/s at 355 nm, respectively.