Heterodyne phase dedimming photopyrogenic elastic spectrum

Trace gases usually refer to gases with volume concentrations on the order of ppt (10-12) to ppm (10-6). Trace gas detection has important uses in environmental monitoring, life medicine, fire warning, and combustion diagnosis. In 2018, the photo-induced thermoelastic spectroscopy (LITES) proposed by Professor Ma Yufei’s research group has the advantages of high sensitivity, fast response speed and non-contact measurement, and has become one of the research hotspots in the field of trace gas sensing.

In LITES, it is common to boost the system signal by increasing the laser power, but when the laser power increases, the thermal noise generated by the laser directly hitting the tuning fork surface also increases. The presence of thermal noise limits the purpose of LITS technology to obtain a higher signal-to-noise ratio and improve system detection performance by increasing the laser power.

In view of the thermal noise problem in the traditional LITS system, the Fabry-Perot Interferometer (FPI) composed of tuning forks and optical fiber ends can be used to demodulate the vibration information of quartz tuning forks, which is conducive to avoiding thermal noise in traditional LITS and achieving more sensitive gas detection at high power.

Figure 1 (cover image) Schematic diagram of photopyrogenic elastic spectroscopy. Source: Harbin Institute of Technology

In acoustic sensors based on laser absorption spectroscopy, the FPI is usually subjected to intensity demodulation method, which can achieve fast response to signals and has a simple structure and easy to operate. However, this method has some limitations, such as the cavity length must operate in the linear region of the interference spectrum, and the detection laser wavelength must be at the orthogonal operating point (Q point) of the interference spectrum to ensure the highest sensitivity of the system. Due to environmental interference and detection laser light source disturbance, the Q point of the system will continue to drift, which will seriously affect the long-term stability of the system. In order to solve the problem that FPI is susceptible to disturbance, it is usually necessary to cooperate with Q-point stabilization algorithm to improve the long-term stability of the sensor, but this will increase the complexity of the system and have more requirements and limitations on the performance of the device.

Recently, the team of Professor Ma Yufei of the School of Astronautics, Harbin Institute of Technology, theoretically and experimentally verified that the phase of FPI is only related to micro-vibration, independent of detection laser power, and insensitive to small changes in detection laser wavelength under certain conditions, thereby greatly improving the long-term stability of the system. Compared with traditional intensity demodulation, this heterodyne phase demodulation system is simple and has good immunity to light sources and environmental disturbances.

The work was published in Light: Advanced Manufacturing under the title “Fabry-Perot based phase demodulation of heterodyne light-induced thermoelastic spectroscopy.” Corresponding author: Professor Ma Yufei, Harbin Institute of Technology; First author: Dr. Lang Ziting, Harbin Institute of Technology.

A schematic diagram of the heterodyne photopyrotropic spectroscopy (H-LITES) phase demodulation system based on Fabry-Perot (F-P) is shown in Figure 2. The laser is absorbed by gas molecules and focused on the surface of the quartz tuning fork, due to the photothermal elastic energy conversion, the tip of the quartz tuning fork produces mechanical vibration, the length of the F-P cavity composed of the end of the single-mode fiber and the side wall of the quartz tuning fork changes, and the reflected light intensity obtained by the photodetector is sent to the lock-in amplifier for heterodyne phase demodulation to obtain the vibration information of the tuning fork, thereby inverting to obtain the concentration of the corresponding gas.

Figure 2: Schematic diagram of the structure of the H-LITES phase demodulation system based on F-P. Light: Advanced Manufacturing 4, 23(2023).

The F-P-based H-LITES signal was obtained by electrolytic modem and F-P demodulation method, respectively, and the results verified that the FPI-based heterodyne phase demodulation method had a higher signal-to-noise ratio and obtained better detection performance.

Subsequently, the power response and wavelength response of the FPI-based H-LITS sensor were studied by F-P intensity and phase demodulation methods. When the intensity demodulation method is used, the peak-to-peak is linearly related to the detection laser power and is easily affected by laser power fluctuations. The peak-to-peak signal varies as a cosine function with wavelength, and the sensor has the best sensitivity only when the wavelength is at the Q point. When the phase demodulation method is used, the peak-to-peak is approximately the same at different powers and wavelengths, which means that the phase demodulation method is not only less susceptible to light source disturbances, but also maintains approximately consistent sensitivity as the wavelength changes, without the need to fix the wavelength at the Q point. The phase demodulation method in FPI can fundamentally solve the problem of system instability caused by environmental interference and laser disturbance. The long-term stability of the F-P-based H-LITS sensor was tested by intensity and phase demodulation methods, respectively. The peak-to-peak of the intensity demodulated signal gradually decreases with time, and the stability is poor. In contrast, the peak-to-peak value of the phase demodulation signal remains approximately unchanged and has good system stability.

Based on the high-sensitivity LITS technology, this study aims to solve the problem that thermal noise at high power significantly affects the detection limit of the sensor, and effectively combines the F-P cavity to improve the signal-to-noise ratio of the system. Aiming at the instability in the traditional F-P cavity intensity demodulation method, which is easily affected by environmental and light source disturbances, a heterodyne phase demodulation method is proposed, and it is verified in experiments that the proposed method is not only immune to light source disturbance, but also can fundamentally solve the problem of Q point drift caused by environmental interference. This achievement is expected to play an important role in related fields such as high-sensitivity LITS gas sensing and F-P cavity sensors. (Source: Advanced Manufacturing WeChat public account)

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