Objective   The high spectral resolution lidar (HSRL) based on Rayleigh scattering spectroscopy is currently one of the most effective equipment for remote sensing of atmospheric temperature below 20 km. Traditional HSRL for temperature measurement requires a single longitudinal mode laser source, which leads to the defects of high system cost, poor environmental adaptability and low laser energy utilization. Therefore, it is of great scientific significance and practical application value to study atmospheric temperature detection technology with high detection accuracy, high spatial and temporal resolution, strong environmental adaptability and low cost. For this purpose, the HSRL with multi-longitudinal mode (MLM) laser, i.e. MLM-HSRL technology based on two-stage Fabry-Perot interferometer (FPI) for temperature measurement is proposed and studied.

  Methods   The temperature detection principle of MLM-HSRL based on two-stage FPI is analyzed (Fig.1). The theoretical model of temperature detection is constructed accordingly, and the measurement error formulas of temperature and backscatter ratio are derived. The frequency matching and locking conditions are studied, and the temperature measurement deviation caused by frequency matching error and locking error is analyzed. The frequency matching calibration method and steps based on the combination of FPI cavity length coarse scanning and fine scanning are presented (Fig.5-6). The MLM-HSRL system parameters (Tab.1) are designed, and its detection performance is simulated using the 1976 USA atmospheric model and simulated cumulus and cirrus clouds.

  Results and Discussions   The frequency matching condition is that the longitudinal mode interval of the MLM laser is an integer multiple of the free spectral spacing of the two-stage FPI. When this condition is satisfied, the MLM temperature measurement is equivalent to the superposition of each single longitudinal mode (SLM) temperature measurement. The analysis results show that the larger the backscatter ratio is, the greater the temperature measurement deviation caused by the same frequency matching error and locking error is; the frequency matching error has a great impact on temperature measurement; the frequency matching error and locking error should be less than 5 MHz and 10 MHz, respectively (Fig.4). The simulation results of MLM-HSRL detection performance show that in the altitude of 0-20 km, the temperature measurement deviation caused by the frequency matching error and locking error is usually very small, and it can be neglected above 2 km; If there are clouds, dust, etc., this deviation will be larger at the corresponding altitude (Fig.8); When the vertical resolution is 30 m at 0-12 km and 60 m at 12-20 km, and the time resolution is 1 min, the temperature measurement errors caused by noise during the day and night are below 3.7 K and 3.5 K, respectively, and the backscatter ratio relative measurement errors are below 0.40% and 0.38%, respectively (Fig.9).

  Conclusions   A MLM-HSRL technology for temperature measurement based on two-stage Fabry-Perot interferometer (FPI) is proposed and studied. This technology requires that the longitudinal mode spacing of the laser source is matched with the free spectral spacing of the two-stage FPI, and the center frequency of each longitudinal mode is locked at the peak of the periodic spectrum of the first stage FPI. When the frequency matching condition is satisfied, the MLM temperature measurement is equivalent to the superposition of each SLM temperature measurement. The frequency matching error and locking error will cause additional temperature measurement error, and they should be less than 5 MHz and 10 MHz, respectively, in order to ensure the accuracy of the low-altitude atmospheric temperature measurement, which can be achieved through frequency matching calibration. The simulation results show that the MLM-HSRL system based on this technology is capable of measuring temperature and backscatter ratio at the altitudes up to 20 km with high accuracy in all weather. These conclusions fully demonstrate the feasibility of this technology.