Application of Lock-in Amplifier in Absorption Spectroscopy

Keywords: Laser heterodyne radiometer, stable isotope, reverse trajectory analysis, lock-in amplifier

Note: This article uses Saluki SE1201 lock-in amplifier to measure

【Overview】

In January 2024, the team of Huang Yinbo and Cao Zhensong from the Anhui Institute of Optics and Precision Mechanics of the Chinese Academy of Sciences published an article titled “Atmospheric HDO Abundance Measurements in the Tibetan Plateau Based on Laser Heterodyne Radiometer” in the journal of Remote Sensing published by MDPI. The article studied the relationship between water vapor density and atmospheric motion by analyzing the reverse trajectory of atmospheric transport, and determined that changes in H2O column density and HDO/H2O ratio are related to atmospheric motion.

The Qinghai-Tibet Plateau is known as the “third pole” of the world, and its environmental changes have a profound impact on the climate of East Asia and even the world. HDO is a stable isotope of water vapor, an ideal tracer for studying the water cycle, and is often used in atmospheric circulation and climate research. In August 2019, the team led by Huang Yinbo and Wu Pengfei used a portable laser heterodyne radiometer (LHR) in Golmud to obtain the absorption spectra of HDO and H2O in the atmosphere, and used the optimal estimation method to detect the density of HDO and H2O. The average columnar density of H2O was 1.22. During the observation period, the HDO/H2O ratio in Golmud was 178±15×10-6.

【Test methods & some test results】

Laser heterodyne spectroscopy uses a narrow linewidth laser to mix with the input optical signal. The total photocurrent value that a heterodyne detector can theoretically generate is:?

Where ALO and AS are the amplitudes of the laser signal and the input optical signal, νLO and νS are the frequencies of the laser signal and the input optical signal, and η is the efficiency of the detector. The second term of the formula is the mixing signal, whose power is:

When the input optical signal power is weak, the light intensity information it carries can be amplified by the optical signal generated by the local oscillator (LO). The generated mixing signal is filtered, detected and demodulated to obtain the absorption spectrum signal of atmospheric gas.

The laser heterodyne radiometer consists of three modules: a solar tracking module, a mixing module, and a signal processing module. The sunlight is captured by the solar tracker and mixed with the optical signal generated by the local oscillator. In order to enhance the mixing signal, the mixing ratio of the oscillating light signal and the sunlight signal is about 10%:90%. The mixed light is focused by an off-axis parabolic (OAP) and input into the detector. The dielectric signal is then processed by a radio frequency device and demodulated using a lock-in amplifier SE1201.

The measured mixed spectrum is inverted by the Optimal Estimation Method (OEM), which was developed by C. Rodgers and is widely used in atmospheric inversion. Reliable results can be obtained by setting parameters (such as atmospheric layer, number of iterations, and iteration type). The process of HDO and H2O inversion is shown in Figure 3.

After adopting the OEM method, a more accurate fit to the measured spectrum was obtained. The absorption spectra of HDO and H2O were obtained during the inversion process. One set of measured and inverted fitting spectra is shown in Figure 4: the blue line is the fitting spectrum of HDO, the green line is the fitting spectrum of H2O, and the red line is the synthetic spectrum of the inverted fitting. The residual spectrum is less than ±0.1 V.

The measurement results in Figure 5 provide information on the distribution of HDO and H2O in the atmosphere below 10 km. In the upper atmosphere, the density of HDO and H2O is significantly lower, and the sensitivity of the laser heterodyne radiometer is not enough to detect them. The values inverted in this range are almost the same as the prior profiles, indicating that the profiles of H2O and HDO can be inverted using laser heterodyne radiometers in the lower troposphere based on higher sensitivity and more comprehensive inversion algorithms.

【Summarize】

In August 2019, the team of Huang Yinbo and Cao Zhensong successfully used a self-built 3.66μm laser heterodyne radiometer in the Golmud area to obtain the absorption spectra of HDO and H2O in the atmosphere. The average columnar density during the experiment was 1.07 to 1.4g/cm2, and the HDO/H2O ratio in Golmud during the observation period was 163×10-6 to 193×10-6. During the spectral research process, the team of Huang Yinbo and Wu Pengfei optimized the altitude interval in the retrieval process based on the analysis of the altitude grid, and reduced redundant information through the refined altitude layer. At the same time, the study used reverse trajectory analysis to study the relationship between H2O (HDO) density and atmospheric motion, and based on the observation results, it was shown that the origin and path of the airflow can affect the H2O density and isotopic abundance.

【References】

?Remote Sensing | Free Full-Text | Atmospheric HDO Abundance Measurements in the Tibetan Plateau Based on Laser Heterodyne Radiometer