【摘要】 针对腔衰荡光谱（CRDS）中激光频率漂移、耦合效率低及现有稳频技术成本高、操作复杂等问题，本文提出了一种基于频率调制的激光稳频技术，并将其应用于气体检测。基于频率调制光谱原理的理论分析与数值模拟结果显示，在9 MHz调制频率下，吸收相位信号优于色散信号，可提供更高幅值的误差信号。搭建了基于频率调制的激光稳频实验系统，采用电光调制器（EOM）实现了9 MHz激光调制，并结合多通池、解调系统与现场可编程门阵列（FPGA）中的比例-积分-微分（PID）模块，将激光频率锁定至乙炔（C₂H₂）吸收线的中心频率，频率偏差稳定在3.6 MHz内。利用稳频后的激光，通过控制采集板参数分析了谱线测量和单点测量的速率极限，测量了空腔衰荡时间并进行了Allan方差分析。结果表明，系统可实现10 Hz的快速单点检测，在146 s积分时间下的C₂H₂检测极限为5.15×10^-10 cm^-1，对应最低检测摩尔分数为1.04×10^-11，较未稳频的系统灵敏度减小了近一个量级。此外，6种不同摩尔分数C₂H₂气体的测量结果表明，摩尔分数与吸收系数之间呈现良好的线性关系（拟合度R²=0.9998），证明该系统对摩尔分数变化具有优异的线性响应特性。设计并搭建了一套基于吸收相位频率调制的小型化CRDS系统，提升了系统的稳定性和灵敏度，为气体传感等相关领域提供了重要技术支撑。更多还原

【Abstract】 Objective Cavity ring-down spectroscopy（CRDS） is widely used in trace gas detection research because of its insensitivity to light source intensity fluctuations, low detection limits, and suitability for various gases and complex background environments. However, it typically measures by scanning the entire absorption spectral line, which results in long measurement time. Therefore, this paper selects a more rapid single-point measurement method. Existing CRDS approaches also face problems such as low coupling efficiency, laser frequency drift, high cost of frequency stabilization technology, and complex operation. To address these limitations, this study proposes a laser frequency stabilization method based on frequency modulation of absorption phase: by applying a 9 MHz lowfrequency modulation and using the absorption phase signal as the error signal, the laser frequency is locked at the center of the acetylene absorption line, thereby achieving stable frequency control and narrowing of the laser linewidth. The stabilized laser is applied to CRDS measurement, effectively improving the response speed and measurement accuracy of the single-point measurement system.Methods The absorption and dispersion phase spectroscopic signals were simulated at a modulation frequency of 9 MHz, and the absorption phase demonstrated clear advantages. A distributed-feedback（DFB） semiconductor laser with a central wavelength of 1531.58 nm was used in the experiment. After being emitted from the laser, the output beam was split into two paths by a fiber-optic beam splitter at a 9∶1 splitting ratio. Ninety percent of the optical power was directed into the laser frequency stabilization module, where the frequency was locked to the center of the target absorption line. An electro-optic modulator（EOM） was used for modulation. The modulated light entered a Herriott cell with an effective optical path length of 28 m. The transmitted signal was detected, amplified, and demodulated to produce an error signal. After low-pass filtering, this signal was fed to a proportionalintegral-derivative（PID） controller implemented on a field-programmable gate array（FPGA）, and the resulting feedback signal was applied to the laser driver to maintain locking at the C₂H₂ line center. The remaining ten percent of the laser output power was delivered to the CRDS module for acetylene detection. The stabilized laser passed through a fiber isolator, an amplifier, an acoustooptic modulator, and a collimator before entering the resonant cavity. The transmitted signal was detected by a photodetector, collected by a data acquisition card, and processed on a computer.Results and Discussions Firstly, based on the laser with a stable frequency and analysis of the parameters of the control acquisition board, the rate limits for spectral line and single-point measurements were determined, achieving a rapid single-point detection of 10 Hz. The laser frequency was locked to the center frequency of the C₂H₂ absorption line. From the frequency distribution histogram, the frequency deviation after locking was found to be 3.59 MHz. The laser frequency fluctuation decreased from 180 MHz before locking to 8.6 MHz. Based on the frequency-stabilized laser, the cavity ring-down time was measured. The detection limit of the system was 5.15×10^-10 cm^-1 at integration time of 146 s, which was nearly an order of magnitude lower in accuracy compared to 1.55×10^-9 cm^-1 of the unstable system at integration time of 155 s, corresponding to the minimum detectable molar fraction of 1.04×10^-11 of C₂H₂ gas. To further evaluate the influence of the frequency-stabilized laser on the system stability and detection sensitivity, the ring-down time in the presence of gas inside the cavity was measured, and Allan variance analysis was performed. After frequency locking, the system achieved a detection limit of 7.27×10^-10 cm^-1 at integration time of 95 s. In addition, by measuring C₂H₂ gas samples with six different mole fractions, the good linear relationship was observed between the absorption coefficient and mole fraction（the goodness of fit is R²=0.9998）, verifying the excellent quantitative detection capability and accuracy of the system.Conclusions The laser frequency stabilization based on frequency modulation proposed in this paper can effectively suppress frequency drift when applied to the CRDS detection system. This system has the advantages of high sensitivity, strong linearity, and good stability, making it a reliable tool for trace gas detection and analysis. This provides a method for rapid and high-precision measurement of gas concentration and can be applied in environmental monitoring and industrial gas sensing.更多还原