Continuous wave cavity ring-down spectroscopy (CW-CRDS) can be utilized in high sensitive trace gas detection. Based on CW-CRDS, using near-infrared distributed feedback diode laser (DFB-DL), semiconductor optical amplifier (SOA), and self-designed high-finesse Fabry-Pérot cavity, laser driver and threshold detection circuit, the compact design of trace gas analyzer have been realized. In the system, SOA not only amplifies the laser power and improves the signal-to-noise ratio of ring-down signal, but also acts as an optical switch to trigger the ring-down time. The whole system is integrated in a 700 mm × 300 mm × 185 mm customized cabinet. Based on this device, CO2 gas is measured at 6359.97 cm-1, and the time series analysis results of the empty cavity ring-down time shows that the detection limit of the system is 3.85×10-8 cm-1.

The emergence of ultrafast laser spectroscopy technology has greatly promoted the deep understanding of the microscopic processes of ultrashort time scale (such as femtosecond) in many research fields. The principle and techniques of femtosecond time-resolved transient absorption spectra are introduced in detail. Combined with our research results, the application of this method in the study of the microscopic mechanism of ultrafast relaxations and interactions in the excited states of condensed molecularsystems is demonstrated, especially for relaxation of excited states, evolution pathway and coherence of wavepacket, energy transfer, proton/charge transfer, molecular structural dynamics and so on. It is indicated that this method will play an importantrole in other related fields such as physics, chemistry, materials, biology, environment and other interdisciplinary fields. Finally, the potential developments and further research trends of the ultrafast spectroscopy are prospected.

Laser absorption spectroscopy (LAS) has the advantages of accurate quantification and straightforward operation. It has become the most widely used laser spectroscopic gas sensing technology. However, in many practical applications, traditional LAS cannot meet the requirements in high-precision optical path length measurement, multi-point gas sensing and large-dynamic-range detection. A spectroscopic gas detection method based on the frequency-modulated continuous-wave (FMCW) technique is introduced. In the method, FMCW is introduced into the LAS, and the absorption spectral information in optical FMCW signal is extracted while making full use of the high-precision positioning capability of FMCW. The basic principle of optical FMCW gas spectrum detection technology is introduced in detail firstly, then the latest research progress in synchronous measurement of gas absorption spectrum and optical pathlength, multi-point gas detection and large-dynamic-range gas detection is reviewed, and the future research work is prospected.

Nitrogen dioxide (NO2) is an important atmospheric pollutant that seriously threats human health and ecosystems. Therefore, the detection of the spatial distribution of NO2 concentration is of great significance for atmospheric environmental monitoring and management. As an active optical remote sensing detection technology, differential absorption lidar (DIAL) technology can realize the detection of horizontal and vertical spatial distribution for atmospheric trace gases and the monitoring of emissions from elevated sources, which has great application value in the field of atmospheric environmental monitoring. DIAL technology uses a tunable laser to alternately emit two laser beams with similar wavelengths into the atmosphere. The wavelength of one laser beam is on the absorption peak of the gas to be measured (λon), and the other deviates from the absorption peak of the gas (λoff). According to the different absorption at the two wavelengths, the concentration of thetarget gas can be evaluated from the ratio of the atmospheric backscatter signals at λon and λoff. The basic principle and measurement error of NO2-DIAL technology are introduced in detail, the recent advancement of NO2-DIAL technology is systematically reviewed. And finally, the research on NO2-DIAL technology is summarized and prospected.

Because of its complex interactions with solar radiation and clouds, aerosols have an important impact on climate. Evaluating the impact of aerosols on climate effects relies on the accurate information of aerosol optical properties. Due to the lack of suitable instruments and methods, the light absorption of aerosols has always been a parameter that is very difficult to measure accurately, resulting in the great uncertainty in current assessment of the impact of aerosols on the climate. Since its signal is only related to the light absorption of the sample and is not affected by the scattering of the sample, photoacoustic spectroscopy is very suitable for measuring the light absorption of aerosols andis considered to be one of the most effective method for measuring the light absorption characteristics of aerosols. The research progress of the photoacoustic spectroscopy in measuring the light absorption characteristics of aerosols is reviewed, and the new technologies developed in recent years are introduced and analyzed, which can provide references for researchers in related fields.

As one of the core components of the sensors based on tunable diode laser absorption spectroscopy (TDLAS) technology, multi-pass cell (MPC) is used to increase the path length of a laser beam interacting with gas samples to be measured, so as to improve the detection sensitivity. In recent years, with the increasing demand of small size and long optical path for the laser sensors, the development of MPC with small size and long optical path length has become a research hotspot. This review mainly introduces the current research progress and applications of MPCs, presents the theoretical calculation model of Herriott cell, and then analyzes the theoretical calculation model of MPCs based on the spherical mirror aberration theory. The construction of a sensor system based on a miniature MPC and TDLAS technology is elaborated. At last, the advantages of the miniature MPC and its applications in future are discussed.

Due to the advantages of simple structure, easy alignment, high spectral versatility, good robustness and low cost, optical multi-pass cell is one of the most effective means to increase the optical path length and can significantly improve the detection sensitivity of absorption spectroscopy. The basic principles of different types of optical multi-pass cells are summarized firstly. Then a series of optical multi-pass cells developed by Prof. Weijun Zhang's group of Anhui Institute of Optics and Fine Mechanics, Chinese Academy of Sciences by using ray tracing, including an astigmatic mirror cell with 200 m optical path length, a Herriott cell with 59.7 m optical path length, a dense spot pattern spherical mirror cell with an optical path longer than 200 m and a Chernin cell with portable stabilization adjustment mechanism, are introduced in detail, especially their design methods and applications inlaser absorption spectroscopy.

A new type of ellipsoidal resonant photoacoustic cell is proposed. The finite element analysis software is used to establish the acoustic characteristic model of the cell, and the resonance frequency, the sound pressure distribution in the resonant cavityof the novel photoacoustic cell are calculated. According to the distribution of the absorption line of acetylene gas in the near infrared region, 1532.83 nm is selected as the measurement line of acetylene, and a tunable distributed feedback (DFB) semiconductor laser is selected as the light source. Fabry-Perot (F-P) fiber optic acoustic sensor is used to collect the sound pressure signal, and the performance indicators of the ellipsoidal photoacoustic cell are tested. The experimental results show thatthe signal-to-noise ratio of the ellipsoidal photoacoustic cell is 34, the detection limit sensitivity reaches 1.47×10-9, and the quality factor is 45.5. So it seems that the sensitivity of the system and the performance have been improved significantly by using the proposed structure, which helps to improve the sensitivity of photoacoustic spectroscopy for trace gas detection.

Based on the combination of tunable diode mid-infrared laser absorption spectroscopy technology and long optical path multiple-reflection technology, the fast and real-time detection of trace acetylene in nmol/mol level is studied by using C2H2 gas absorption spectrum near 3025.7 nm in mid-infrared region. The driving current of the interband cascade laser is modulated by the superimposition of the high frequency sine wave generated by DDS AD9958 and triangle wave signal, and then the stable drivingof laser is realized through constant current circuit. HgCdTe photodetector is used to receive mid-infrared laser, and target signal is extracted by integrated lock-in amplifier. A long optical path absorption cell is designed based on the combination ofWhite multiple-reflection and plane reflection, and its measured optical path reaches 19.2 m, which further reduces the detection limit of C2H2. In laboratory, trace C2H2 gas sample is measured with the concentration ranging from 0 to 1000 nmol/mol. It shows that the linear error is less than ±1% F.S., and the detection limit is 0.29 nmol/mol, which indicates that the method has the advantages of high accuracy, no cross interference from background gas and convenient use.

In order to realize high sensitivity detection of low concentration NO, a tunable diode laser absorption spectroscopy (TDLAS) detection system is designed by using a quantum cascade laser (QCL) with a central wavelength of 5.18 μm and a single opticalpath gas with an optical path of 25 cm. In order to reduce the influence of system noise so as to improve the sensitivity of NO detection and reduce the detection limit, the empirical mode decomposition (EMD) algorithm is used to filter the second harmonic (2f) signal of wavelength modulation spectroscopy (WMS) to identify and remove the high-frequency noise and optical fringes hidden in the detected signal, and then the noise reduction effect of (2f) signal of EMD algorithm is compared with that of the other common filtering methods. Based on this, NO continuous monitoring experiment is carried out, and Allan variance analysis is used to compare the stability and detection limit of the system before and after EMD filtering. The results show that the response linearity of the detection system reaches 0.999 afterEMD noise reduction. The fluctuation range of NO detection concentration at 30×10-6 is reduced from 29.424?×10-6～33.184×10-6 to 29.585×10-6～31.273×10-6, and the relative error range is reduced from 0.17%～10.61% to 0～4.24%. In the average time of 1.5 s, the detection limit of the system is reduced from 653×10-9 to 442×10-9. Under the optimal average time, the detection limit of the system is reduced from 272.7×10-9 to 185.5×10-9, which proves that this method can effectively improve the detection accuracy and flexibility of the detection system and further reduce the detection limit.

As a potential energy carrier and industrial material, the increasing importance of hydrogen calls for trace gas detection technique with higher sensitivity, faster response and wider dynamic measurement range. Based on the Pound-Drever-Hall (PDH) laser frequency stabilization technique, laser is coupled into a high-finesse optical resonant cavity in cavity-enhanced Raman spectroscopy (CERS) system, resulting in a power gain factor of 1900 for trace hydrogen gas sensing. With 7 mW input laser power and 100 s exposure time, a detection limit of 2 Pa for hydrogen gas is achieved for the home-made CERS system. Measurements also show that the signal of Raman scattering has an excellent linear relationship with the gas pressure and laser power, which indicatesCERS has the potential for quantitative analysis of gases with high precision.

Mehane (CH4) and nitrogen (N2) are two main gas components of many terrestrial stars, and HCN, CN and C2H2 et?al are the main products from the mixture of these two gases. The formation process of the main products is simulated by CH4/N2 discharge plasma, and the relative concentrations of HCN and C2H2 are obtained by measuring the absorption spectra based on mid-infrared wavelength modulation absorption spectroscopy. The results show that the relative concentrations of HCN and C2H2 depend on the ratio of C/N in the mixed gases, and excess N2 is beneficial to the formation of HCN, while excess CH4 is beneficial to the formation of C2H2. Further study shows that the main formation pathway of HCN is CH3+Nhtarrow HCN+H2. In addition, C2H2 generated by CH4/N2 glow discharge may further combine with N2 to form HCN.

The highly sensitive laser absorption spectroscopy technology is applied to the field of microbial growth measurement to realize the real-time monitoring of the microbial growth process and the drawing of growth curves taking Escherichia coli as an example. A set of experimental measurement equipment is designed and built to monitor and fit the second harmonic signal of carbon dioxide (CO2) generated by metabolism of Escherichia coli in real time. The growth curves of E. coli are measured at 25, 26, 29, 32, 34, 37 and 43 °C. The experimental results show that the measured growth curve can accurately reflect the adjustment period, logarithmic phase, stable period and decay period of E. coli growth at different temperatures, which proves that the microbial growth metabolite (CO2) method based on laser absorption spectroscopy technology can be well applied to the field of microbial growth measurement.

Density functional theory has been applied to in-depth study on photoelectron spectrum of BCl+3 cation in the C2?A''2 state. With the aug-cc-pvtz basis set, the geometries of neutral BCl3, BCl+3 cation in ground state, X2?A2', and an electronically excited state, C2?A''2, are optimized using three theory levels of PBE0, ωB97XD and M06-2X. Then the corresponding vibrational frequencies and the Franck-Condon factors of the BCl3(X1A1)→ BCl+3(C2?A''2) transition are calculated, and the photoelectron spectrum is simulated. By comparing with the high-resolution photoelectron spectrum obtained previously in experiment, the photoelectron spectrum of the C2?A''2 band is successfully simulated, then reliable assignments for the observed vibrational progression are obtained. Based on these vibrational assignments, the adiabatic ionization energy (AIE) and vertical ionization energy (VIE) for BCl+3 cation in the C2?A''2 state have been determined directly as AIE(C2?A''2)=(14.298 ±0.028) eV and VIE(C2?A''2)=(14.405 ±0.028) eV, respectively, and their wrong values reported in previous experiments have been corrected.