Oxygen (O₂) is one of the most important components in the atmosphere and plays a key role in the survival of all living organisms on Earth. However, the atmospheric O₂ concentration is decreasing due to the rapid expansion of human activities. The Tibet Plateau, known as the third pole, is a plateau with the highest altitude in the world. For a long time, as the O₂ content of the local atmosphere is lower than that of other regions, the ecosystem is extremely fragile and sensitive. Long-term quantitative monitoring of atmospheric O₂ concentration is crucial to understand the evolution pattern of decreasing O₂ and its effects on the ecosystem of the Tibet Plateau. However, it requires a very challenging detection precision of 10^-6 level. Various O₂ measurement methods have been proposed. The commonly employed electrochemical techniques and magnetic dynamics methods are compact and easily commercial available but suffer from long-term stability and vibration. In contrast, laser absorption spectroscopic techniques based on the Beer-Lambert law can provide high reliability, completely non-contact measurement, and long-term performance. In particular, Faraday rotation spectroscopy further offers high selectivity for paramagnetic molecules and is a powerful tool for O₂ measurement with high precision. Our paper develops a simply easy-to-deploy and maintenance-free O₂ sensor based on FRS and provides a feasible sensor scheme for O₂ detection in Tibetan Plateau.

A 762.3 nm continuous wave distributed-feedback laser working at room temperature is employed as the probe laser. The laser current and temperature are controlled by a commercial laser controller. O₂ measurement selects the ^PP(1)(J=1) line with a strong near-infrared magnetic effect and a strength of 3.063×10^-24 cm/molecule at 13118.04 cm^-1. The static magnetic field is generated by a solenoid coil under constant current excitation. A Herriott optical multi-pass cell with two 8 cm diameter spherical mirrors separated at 35 cm is adopted to provide an effective absorption path length of 7 m. The cell is made of aluminum alloy which is oxidized and blackened to reduce stray light. A polarizer is utilized before the laser beam incidence in the cell to clean the polarization state, and a second polarizer, placed after the light beam exits the cell, acts as a polarization analyzer. Various parameters of the sensor are optimized to ensure that the sensor operates in optimal conditions, including magnetic field strength, offset angle, and modulation amplitude. Finally, the performance of the sensor performance is assessed by continuous O₂ measurement with a fixed concentration. The system stability and detection precision are analyzed by Allan deviation and a histogram of frequency counts.

The parameters of the sensor are optimized. The noise measurement shows that the optimal offset angle is 10° and the corresponding total noise of the system is 0.33 μV/Hz^1/2 (Fig. 7). The optimal modulation amplitude for O₂ detection at atmospheric pressure is 18 mV (Fig. 8). We find that the measured Faraday rotation spectral signals are proportional to magnetic field strength in the range of 0 to 540 Gs (Fig. 9). The 180 Gs field strength is chosen due to the safety and heat. The stability of the magnetic field strength is tested continuously for 12 hours by a Gaussmeter with a resolution of 0.1 Gs and an accuracy of ±0.3% of the reading (Fig. 5). The results indicate high stability. System calibration is performed with a strong linear relationship between Faraday rotation spectral signals and O₂ concentrations. A fixed volume ratio of about 5.36% is continuously measured for 3600 s and the time resolution is 1 s (Fig. 12). A Gaussian profile is fitted to the frequency distribution histogram. The standard deviation value which corresponds to the actual instrument precision is 149×10^-6. Allan deviation evaluation demonstrates that the optimal average integration time of the system is 60 s, at which the detection precision can be improved to 32×10^-6.

A high-precision atmospheric O₂ sensor based on Faraday rotation spectroscopy is developed. The measurement selects the ^PP(1)(J=1) line with a strong near-infrared magnetic effect at 13118.04 cm^-1. A Herriott optical multi-pass cell with coils wounded is designed specifically for Faraday rotation spectroscopy to offer an absorption path length of 7 m and a magnetic field strength of 180 Gs, which effectively enhances the detection signal and improves the system performance. The operating parameters of the sensor system are optimized, and the performance is evaluated. As a result, a detection precision of 32×10^-6 with the acquisition time of 60 s is achieved, thereby confirming the precision and reliability of the sensor, and providing a feasible scheme for long-term O₂ detection in Tibet Plateau. In future work, we will develop permanent rare-earth magnets instead of solenoid coils, try to provide higher-strength constant magnetic fields, and pursue lower power consumption and higher performance. We hope that our future research can achieve the networking O₂ measurement in the Tibet Plateau.