PDH Mode Locking System For Cavity-enhanced Infrared Spectroscopy

Greenhouse gas emissions are the main cause of global warming.The Kyoto Protocol lists the types of gases in detail:carbon dioxide,methane,nitrous oxide,etc.In 2020,President Xi announced that China will adopt strong policies and measures to achieve the peak of carbon emissions by 2030,and finally achieve the ambitious goal of carbon neutrality.Therefore,the development of high-sensitivity greenhouse gas monitoring instruments suitable for my country’s national conditions is of great social value and practical significance for atmospheric environmental quality monitoring and factory pollution gas emission compliance.Based on the principle of infrared gas absorption spectroscopy,this paper combines the cavity-enhanced absorption spectroscopy technology with the Pound-Drever-Hall（PDH）frequency stabilization technology to develop a PDH mode-locking system for cavity-enhanced infrared spectroscopy,which locks the distributed feedback laser with the optical resonator.The designed data acquisition circuit sends the detector data to the host computer for real-time monitoring,and conducts CO₂ gas experiments to verify the stability of the system.First,the infrared spectrum theory and absorption line function are introduced.The basic theory of PDH laser frequency locking technology based on cavity enhancement is studied,and the characteristic parameters and error signal of F-P cavity are theoretically analyzed and simulated.Then,according to the cavity-enhanced absorption spectroscopy technology and the PDH frequency stabilization technology,the overall scheme and device selection of the PDH mode-locking system are designed.A DFB laser with a central wavelength of 1572 nm was used as the light source,and an electro-optical modulator（EOM）was selected for phase modulation of the laser.The high mirror reflectivity of the resonant cavity is 99.3%,and the length of the resonator used is only 0.195 m.Both sides of the gas chamber are sealed by windows,which has passed the underwater air tightness test for up to 10 minutes.In the optical part of the system,three devices including polarizer,polarization beam splitter and quarter wave plate are selected.Through the adjustment of the polarizer,the polarization direction of the incident laser coincides with the principal axis direction of the EOM crystal,effectively reducing the residual amplitude modulation（RAM）.The traditional beam splitter divides the power of two laser beams emitted along the vertical direction into 50:50.In order to reduce the power loss of the laser,the polarization beam splitter（PBS）and 1/4 wave plate are selected to replace the traditional scheme.The parallel linearly polarized light propagates along the main axis and changes into circularly polarized light through a quarter wave plate and enters the resonant cavity for resonance.The horizontally polarized light reflected from the resonant cavity changes into vertically polarized light again through a quarter wave plate time delay?/2.It is received by the detector through the polarization beam splitter,which effectively realizes the collection of the reflected signal of the resonant cavity.In the electrical part,the driving unit Q-Driver is used to generate 24.4 MHz local oscillator signal.The error signal is obtained by mixing and low-pass filtering with the resonator reflection signal collected by the detector.The signal is fed back by PID controller to adjust the cavity length,and the frequency locking between laser and resonator is realized.The transmission signal of the resonator is collected by using photodetector,data acquisition and processing circuit,and the display and data real-time online processing are carried out by Lab VIEW host computer,so as to build a complete sensing system.The noise characteristics of cavity enhanced absorption spectrum gas sensing signal are analyzed,and the denoising capabilities of EMD-SG filter,Wavelet denoising and Kalman filter are compared,which provides a theoretical and experimental basis for the subsequent noise processing of cavity enhanced gas sensing system based on PDH mode locking technology.According to PDH mode locking technology,a cavity-enhanced CO₂ sensing system is established,and the performance of the gas sensor is tested experimentally.Firstly,the optical path collimation is adjusted by visible red light to complete the preliminary preparation of the experiment;1572 nm DFB laser is used as the system light source.CO₂ gas samples with different concentrations were prepared by using the gas distribution system,and the variation relationship between the cavity mode peak voltage and CO₂ gas concentration was calibrated.The upper computer program collected the cavity mode peak voltage in real time and carried out Kalman denoising.Through Allan variance,it was calculated that the lower detection limit of the system after algorithm optimization was 206.5 ppmv,which was 10.4 times lower than that before denoising algorithm optimization.The experimental results show that the system response time optimized by the denoising algorithm is 26 s.The innovation of this paper is:combining cavity-enhanced infrared spectroscopy technology with PDH mode locking technology,a cavity enhanced CO₂ gas sensing system for atmospheric detection is developed,three denoising methods for the application of cavity enhanced spectroscopy technology are designed,a data acquisition circuit is designed to collect the cavity mode signal in the system,and a Lab VIEW host computer software platform is developed to realize the functions of real-time display and online processing of system data.