Impact of Lock-In Time Constant on Remote Monitoring of Trace Gas in the Atmospheric Column Using Laser Heterodyne Radiometer (LHR)

Abstract

The time constant selected for lock-in amplification (LIA) has a crucial impact on observed line shapes in laser heterodyne spectroscopy, in particular in the case of ground-based remote monitoring of trace gas in the atmospheric column using laser heterodyne radiometer (LHR). Conventional simulation could not allow validation of LHR spectra measured in a real and complex atmospheric environment exhibiting large temporal and spatial variability (humidity, temperature, pressure, etc) that impact significantly the measured LHR spectra profiles. High-precision spectral measurement is thus crucial to avoid any spectral distortion resulting from the measurement. In this paper, the impact of LIA time constant on spectral line shape is investigated for LHR operating in continuous laser tuning mode, based on analysis of laboratory heterodyne spectra, in terms of signal-to-noise ratio (SNR), line width broadening, absorption depth and line shift. With respect to the given frequency scanning speed in continuous mode and to the halfwidth of the absorption feature to scan, a reasonable scanning time ΔTscan, the time needed for scanning laser frequency through the halfwidth ΔνHWHM of the absorption line, equal to or longer than 14 times of the LIA time constant τ is concluded in order to efficiently reduce the noise while without significant shift and distortion of the line shape. Experimental validation was carried out using a laser heterodyne absorption spectroscopy approach in the laboratory. Four different combinations of time constants τ and scanning time ΔTscan were used to record heterodyne spectra of a CH4 absorption line near 1242.00 cm−1 in continuous laser tuning mode. An optimal combination of a scanning time of 137 ms with a time constant of 1 ms was obtained. This optimal combination was used for ground-based measurements of CH4 and N2O in the atmospheric column by LHR. The extracted LHR spectrum is in good agreement with a referenced TCCON (Total Carbon Column Observing Network) FT-IR (Fourier-transform infrared) spectrum.

Performance Characterization of a Fully Transportable Mid-Infrared Laser Heterodyne Radiometer (LHR)

“…In order to obtain a high-precision heterodyne spectrum (with a minimized line shape shift and distortion), the scanning time Δ T scan across the halfwidth of the absorption line Δ ν HWHM (in (cm −1 )), which is also dependent on the scanning speed of the LO frequency υ sc (in (cm −1 /s)), should be appropriately adapted to an LIA time constant τ, such that [ 33 ] …”

Design of an all hollow fiber‐coupled middle infrared laser heterodyne radiometer (LHR)

“…To estimate the transmission loss in hollow fiber theoretically, it is necessary to consider the attenuation and coupling efficiency of each mode at the same time. Hence, the transmission of the length z, P z can be represented by 3,26…”

“…When calculating the theoretical loss of hollow fibers operating under poor launch conditions, it is especially important to add up all possible modes, because lots of higher-order modes could be excited. The total loss L z is calculated by 3,31…”

“…16 With the development of telecommunication technology in the near-IR, 17 high-performance DFB lasers, photodetectors, and fiber optics have a great attraction in developing portable LHRs for on-site measurements. 3,18 The atmospheric windows of 3-5 μm and 8-12 μm correspond to absorption of a large number of key atmospheric molecules, which are very attractive for sensitive remote measurement with LHR. 2,3 Recently, a mid-IR LHR using a 3.53 µm interband cascade laser (ICL) as LO has been developed for remote sensing atmospheric water vapor and CH 4 .…”

“…2,3 Recently, a mid-IR LHR using a 3.53 µm interband cascade laser (ICL) as LO has been developed for remote sensing atmospheric water vapor and CH 4 . 19 17,20 However, in the mid-IR, unlike the near-IR, commercially fiber optics are not available, 3 coherent photomixing of the incoming incoherent solar radiation and the LO beam, which is a key aspect of mid-IR LHR performance, is achieved with a spatially separated optical assembly. 17,[19][20][21] Therefore a mid-IR LHR involving an all-hollow fiber-coupled photomixing assembly in principle has great potential to highly improve photomixing efficiency.…”