Research On The Installation And Adjustment Technology Of Continuous Wave Ring Triangle Cavity Ring-down Spectrum Device

Absorption spectroscopy is an important branch in the development of spectroscopy.With the emergence of laser technology and the continuous improvement of detection sensitivity requirements,cavity ring-down spectroscopy（CRDS）technology based on resonance enhanced absorption principle can achieve fast response in harsh environments（μs）High precision（ppbv）measurement has been widely used in the field of trace gas detection,such as for measuring the concentration of trace gases in fields such as ground,high altitude,deep sea,aerospace,and for environmental protection and resource detection.The recent use in biomedical and other fields has further expanded its application scope for the prevention and diagnosis of some diseases.The use of CRDS in fields such as isotope ratio measurement places higher demands on the accuracy of instruments.In recent years,under the national"carbon neutrality"strategy,the demand for this instrument has further increased.The triangular ring resonant cavity is widely adopted by CRDS instruments due to its unique optical path advantages.In order to improve the accuracy and stability of the instrument,CRDS has been successfully developed to this day,and its various structures have undergone a series of developments,especially its core structures,such as light sources and ring-down cavities.The light source has undergone a transformation from a pulse light source to a continuous wave laser.The ring-down cavity,as a key structure,has undergone improvements from a straight cavity to a triangular cavity.The triangular cavity improves the optical feedback and etalon effect of the straight cavity,further improving the frequency stability of the light source,but increasing the difficulty of system alignment.The existing triangular cavity assembly and alignment methods use grinding alignment,which has problems of low efficiency and poor accuracy.Therefore,there are problems in the engineering process of this technology,such as the need for further construction of the triangular cavity alignment analysis theory,the difficulty of system alignment,and the low fitting accuracy of the ring-down curve during the ring-down process.This article conducts the following research on the establishment of alignment model,improvement of alignment efficiency and accuracy,and improvement of fitting accuracy in the continuous wave ring triangular cavity ring-down spectroscopy system（CW-RT-CRDS）:Aiming at the construction of alignment theory in the continuous wave ring triangular cavity ring down spectroscopy system,the technical principles and analysis methods in the continuous wave cavity ring down spectroscopy are introduced at first.Based on the eigenmode theory of the resonator,the resonant frequency separation characteristics of Hermite Gaussian（HGS）mode,which is different from the high-f triangular cavity and the straight cavity,are revealed.This theory can guide the rapid and high-finesse alignment of the triangular cavity.Based on these basic theories,the CW-RT-CRDS system structure construction and installation process required cavity transmission and absorption models suitable for ring triangular cavities,as well as sensitivity measurement methods suitable for high-finesse and weak energy systems,were analyzed and constructed.In response to the many problems encountered in the construction and stable operation of a high-finesse and weak energy ring triangular cavity ringdown spectroscopy system,an optimization design model for each cavity mirror index in the high-finesse triangular cavity system was constructed to suppress the transmitted light of the cavity by high reflectivity cavity mirrors.T_I and T₂ were approximately 10²orders of magnitude higher than T₃,achieving maximum power injection.Aiming at the problem of low beam matching accuracy in astigmatic triangular cavities,a matching mirror optimization method suitable for astigmatic triangular cavities is proposed,which reduces the ringdown time from 13μs increased to 20μs.In order to achieve stable operation of the system and accurate measurement of ring down time,an optimization design model for the sweep frequency shutdown scheme was constructed based on the coupling efficiency model.The slow sweep scheme increased the ring down time by 10 times compared to the fast sweep scheme.In response to the difficulty of installing and adjusting the ring triangular cavity system,an analysis method based on Hermite-Gaussian theory was introduced to analyze the transmission characteristics and amplitude excitation trend of HGS modes under various structural parameters of the high-finesse ring triangular cavity.The cavity structure parameters such as cavity length,cavity angle,and cavity mirror curvature were optimized and designed.When the cavity angle is 88.542°and the cavity mirror curvature is 1m,the ring triangular cavity has both resonance stability and high misalignment error tolerance.Based on the resonant frequency separation characteristics of the first order HGS mode in the ring triangular cavity and the monotonicity of the excitation of the fundamental mode,a high-finesse static alignment method for the high-finesse triangular ring down cavity is established,which realizes the directional high-finesse alignment of the misalignment error（axis tilt error（0.1°<α_v<0.5°）and axial displacement error（0.1mm<ε_v<0.7mm））in each plane.Compared with the traditional method,the efficiency and threshold of resonance establishment are improved.In view of the low fitting accuracy of the ring-down curve and the double-e exponential ring-down and other abnormal phenomena in the ring-down process,the same resonance coupling excitation mechanism under different environmental disturbances is clarified,and the relationship between it and the ring-down anomaly is revealed.The conditions and range of resonant coupling in the ring triangular ring-down cavity are systematically analyzed,and the preferred coupling of the middle mode is proposed.Based on the time domain characteristics of CRDS,the HGS modal resonance coupling model in a high finesse triangular cavity system is established.Based on the modal filtering characteristics of the triangular cavity,the order and coupling range of the mid-order modes during resonant coupling were verified.A mid-order mode resonant coupling suppression scheme was proposed,and a mid-order mode suppression aperture was added to improve the fitting accuracy of the ring down curve.In this paper,the theoretical analysis model for the alignment of the triangle ring-down cavity is expounded,and the key index optimization design model of the CW-RT-CRDS system,the high-precision static alignment method and the resonance coupling suppression model of the middle order mode are proposed,which have certain guiding significance and reference value for the engineering of the continuous wave ring triangle cavity ring-down spectrum device.