Water vapor (H₂O) is the main greenhouse gas in the atmosphere, which causes a global greenhouse effect and has severely affected human life and health. Thus, accurate monitoring of water vapor concentration is crucial for both environmental monitoring and human health. The heterodyne phase-sensitive dispersion spectroscopy (HPSDS) technique is a dispersion spectroscopy detection technique. It is widely utilized in atmospheric monitoring, medical diagnostics, combustion diagnostics, and other research fields due to its advantages such as high linear dynamic range, zero baseline, effective common-mode noise suppression, and independence from the fluctuation of laser power. In our research, we directly modulate the injection current of a distributed feedback laser with a high modulation frequency to construct a near-infrared HPSDS sensing system for atmospheric H₂O monitoring.

In our study, the gas sensing response of HPSDS is investigated by modulating the injection current of a semiconductor laser. Experiments are conducted using different frequencies to modulate the injection current of distributed feedback lasers. We study the influence on the near-infrared water vapor HPSDS system and select the optimal modulation frequency for the system.

As shown in Fig. 3(a), there is a linear relationship between the peak-to-peak values of the signals and the gas concentration of the NIR HPSDS system at the three modulation frequencies of 1.0, 1.1, 1.2 GHz, and the linear R² values of the three groups reach 0.99, indicating the good linear response of the HPSDS system. Fig. 4 shows the HPSDS system at three different frequencies for a 2 h measurement of H₂O. In terms of long-term stability, the standard deviation is 165.39×10^-6 at a modulation frequency of 1.0 GHz with a relative uncertainty of 0.9%, the standard deviation is 177.71×10^-6 at a modulation frequency of 1.1 GHz with a relative uncertainty of 1.0%, and the standard deviation at a modulation frequency of 1.2 GHz is 126.85×10^-6 with a relative uncertainty of 0.7%. Fig. 5 compares Allen’s variance of the three frequencies and the detection limit when the integration time is all at 156 s. The detection limit is 43.63×10^-6 for 1.0 GHz, 30.87×10^-6 for 1.1 GHz, and 12.77×10^-6 for 1.2 GHz. By comparing the long-term stability and the detection limit, the experiments select 1.2 GHz as the optimal frequency of the system. As shown in Fig. 6, the WMS signal has a good linear response only in the concentration range of 0.74%?1.96% and is clearly nonlinear in 1.96%?2.59%, whereas the HPSDS signal has a good linear response in all concentration ranges of 0.74%?2.59%. As illustrated in Fig. 7, the detection limit can be up to 4.19×10^-6 at an average time of 79 s for the WMS system and 8.82×10^-6 at an average time of 61 s for the HPSDS system. As depicted in Fig. 8, the water vapor concentration obtained from the inversion of the HPSDS sensing system is compared with the water vapor concentration calculated from the temperature and humidity logger, which fits the water vapor profile of the temperature and humidity logger very well.

In our study, the performance of NIR HPSDS sensors at high modulation frequencies is demonstrated by using a low-frequency modulation of the injection current of distributed feedback lasers based on the modulation frequency up to the GHz level, and the performance of NIR HPSDS sensors used for water vapor concentration monitoring is presented, with the optimal modulation frequency of 1.2 GHz and the detection limit of 8.82×10^-6. Experiments comparing the HPSDS system and the WMS system show that the dynamic range of the HPSDS system is better than that of the WMS system, and the precision of the lowest detection limit of the HPSDS system is slightly lower than that of the WMS system. It is verified that injecting a high modulation frequency compared to a low modulation frequency in the direct current modulation distributed feedback laser mode leads to a significant improvement in sensor performance, with an order of magnitude improvement in the minimum detection limit (MDL). Using the NIR HPSDS sensor to monitor water vapor in the atmosphere, by comparing the measurement data from the temperature and humidity loggers, it is obtained that the NIR HPSDS sensor can invert the concentration of water vapor in the atmosphere in real time, which provides a feasible basis for the HPSDS technology in an open-circuit sensor.