Multi-frequency differential absorption lidar incorporating a comb-referenced scanning laser for gas spectrum analysis

Abstract

A multi-frequency differential absorption lidar incorporating a tunable laser and an optical frequency comb is demonstrated for precise spectrum analysis of atmospheric gas. The single frequency tunable laser is stabilized by locking to the optical frequency comb, with a standard deviation of 0.5 MHz. To achieve a high signal-to-noise ratio, a multi-mode superconducting nanowire single-photon detector with an active-area diameter of 50 µm, a quantum efficiency of 31.5%, and dark noise of 100 counts per second is implemented, which enables to avoid the need for high energy lasers. In the experiment, the range-resolved spectrum of atmospheric mixture gases (CO2 and HDO) in a region of 1572.2 - 1572.45 nm is obtained. Results show different partially overlapped absorption of two gases in different seasons, with a stronger influence of HDO on CO2 in summer than in winter. The interactions are taken into account by separating the mixture absorption spectrum (one CO2 line and two HDO lines) with triple-peak Voigt fitting. The retrieved concentrations over 6 km with a range resolution of 120 m and a time resolution of 10 min are compared with in-situ sensors. The uncertainties of the retrieved concentrations are as low as 6.5 µmol/mol (ppm) and 1×10−3 g/kg for CO2 and HDO, respectively.

Towards a spatially resolved, single-ended TDLAS system for characterizing the distribution of gaseous species

“…(3) www.nature.com/scientificreports/ ToF LiDAR requires that the interrogation beam is modulated into waveforms having rise and fall times on the order of 100 ps to achieve a sub-centimeter spatial resolution, which cannot be done through current or temperature tuning methods since these operate at considerably longer timescales. Instead, the tunable diode laser is operated in cw-mode and paired with a fast-switching optical modulator (e.g., an electro 20,24 or an acousto-optical modulator 17 ) to generates the pulses, as shown in Fig. 1.…”

“…Conceptually, however, two-wavelength DIAL is prone to cross-sensitivities to other process parameters and requires calibration which may limit its applicability to combustors. Multi-frequency DIAL has recently gained increased interest due to its ability to perform measurements at different optical frequencies using tunable external cavity 17 , 18 or current tuned diode laser 10 . These studies showcase the potential of multi-frequency DIAL in atmospheric research.…”

“…These studies showcase the potential of multi-frequency DIAL in atmospheric research. Yu et al 17 demonstrated a system that chops a cavity length-tuned continuous wave (cw)-laser with an acousto-optical modulator to observe atmospheric CO 2 spectra with a range resolution of 120 m. Stroud et al 18 successfully resolved 10 optical frequencies to probe CO 2 and H 2 O. With a range resolution of 250 m, they can distinguish the combustion plume from a power plant from atmospheric background.…”

Results of the development of software and hardware for automation of lidar measurements

“…One of the urgent tasks in the field of greenhouse gas research is the development of lidar systems for remote measurement of their concentrations [1][2][3][4][5]. At the same time, in addition to the creation of new experimental facility, a search is being made for ways to modernize existing approaches and systems for lidar gas analysis of the atmosphere.…”