To address the issue of regional open-path benzene concentration detection, we utilize a 10 kHz integrated path differential absorption (IPDA) lidar to experimentally probe the benzene detection performance through openpaths. This system is divided into a direct-current IPDA (DC-IPDA) and a power-modulated IPDA (PM-IPDA). For the first time, several experiments through multiple open-path lengths are conducted. We compare and analyze the effects of lateral wind and open-path lengths on the benzene diffusion process and the fluctuation level of the echo signal. Their signal-to-noise ratio (SNR), minimum detectable concentrations (sigma(CL)) and the detection limit all show distinct segmented characteristics with respect to the open-path length. The power modulation can optimize the sigma(CL) and detection limit of IPDA for benzene detection by up to approximately 30.3 %. The achievable concentration detection range represented by CL is 18.80-9470 mg/m2. An open-path length of 37 m is the critical point for the detection performance of IPDAs, at which the lowest detection limit achieved is 0.15 ppm and the performance reached its optimum, providing an important reference for the location layout of the IPDA in the practical applications.

Objective Lidar is an active remote sensing instrument that detects information such as the distance and physical properties of targets by emitting laser beams and receiving their reflected or backscattered echo signals.It has achieved rapid development and wide application in the field of atmospheric environment detection.Factors such as the structural relationship between the transmitting and receiving modules,the laser speckle effect,and the detector's receiver efficiency all affect the target signal's receiving efficiency.The signal loss in the near field,caused by factors like laser beam divergence,telescope field of view,and the axial spacing between modules,is quantified by the overlap factor.This factor represents the signal receiving efficiency at a given detection range.For the near-field detection problem of the lidar system,a normalized optical imaging model of the overlap factor is proposed.The model's greatest advantage lies in its comprehensive consideration of influencing factors,wide applicability,and high-precision overlap factor correction. Methods This research investigates the normalized optical imaging model for the lidar overlap factor.First,from the perspective of energy loss,the overlap factor can be defined as the ratio of the energy received by the detector to the energy of the echo signal at the transmission distance.Second,based on optical imaging principles,an overlap factor model for lidar is established by considering the laser beam energy distribution,field overlap,central obstruction,and detector reception.Based on the idea of parameter normalization,the 6-dimensional component parameters of the optical model are reduced to a 5-dimensional structural parameter model.Furthermore,by incorporating the laser beam energy factor,a comprehensive 6-dimensional structural parameter model is obtained.Finally,this model is used in the self-developed forest fire early warning lidar,and its validity is verified by the horizontal experimental method. Results and Discussions To address the near-field detection problem in lidar,we propose a normalized optical imaging model for the overlap factor that incorporates six parameters.The influence of these parameters on the overlap factor is analyzed(Fig.4).The beam energy factor,defined by the difference between uniform and Gaussian distributions,leads to a key characteristic:overlap factor curves corresponding to different normalized parameter sets intersect at a common point[Fig.4(e)].The position of this intersection point varies with the deviation angle of the laser emission axis(Fig.6).Based on this common intersection point,a method for correcting system parallelism is proposed.Furthermore,the model is applied to a self-developed forest-fire early-warning lidar,and its correction validity is verified through horizontal experiments(Fig.7).The model is used to calculate theoretical overlap factor curves,which are then compared with curves derived from three sets of horizontal experimental data.The results show that the model calculations agree well with the experimental results.Multi-angle,multi-temporal atmospheric aerosol extinction profiles are inverted using the Klett method to verify the effectiveness of the normalized optical imaging model for near-field signal correction(Fig.8). Conclusions Based on the normalized optical imaging model,the effects of key factors—including the beam collimation coefficient,transceiver module radius ratio,field ratio,axial distance ratio,beam energy factor,and central obstruction ratio—on the overlap factor are analyzed. The analysis shows that,using normalized parameter metrics,the overlap factor curves for different beam energy factors intersect at a value of 0.454 for a detection distance of 340 m.The position of this intersection point varies with the deviation angle of the laser emission axis.This point can be used as an indicator to judge the parallelism of the receiving and transmitting optical axes.This model is used in the self-developed forest-fire early-warning lidar,and the validity of the model correction is verified by the horizontal experimental method when the beam energy factor is 0.95.The average error of the overlap factor in the transition zone is less than 0.025.Multi-angle,multi-temporal atmospheric aerosol extinction profiles are inverted using the Klett method.The results show that after correction with the overlap factor,the effective near-field detection range is reduced from 560 m to 223 m at the end of the blind zone.Therefore,this model demonstrates substantial theoretical and practical value,effectively advancing the engineering maturation of lidar technology.

Objective Lidar,a high-performance active sensing technology,has gained significant traction in recent years across fields such as meteorology,climate science,and environmental monitoring.It serves as a principal methodology for detecting atmospheric physical properties by analyzing echo signals generated from interactions between emitted narrow-pulse lasers and atmospheric constituents,offering high resolution and extended detection range.The quadratic attenuation of lidar echo signals with distance causes single-pulse returns to be overwhelmed by ambient noise,necessitating multiple accumulations(time integration)to enhance the signal-to-noise ratio(SNR).However,conventional lidar data acquisition and integration methods involve onboard storage of data chains collected from a single trigger event.After reaching the predetermined number of accumulations,the stored data are sequentially read out and averaged.This approach introduces a time overhead during data readout,as data acquisition cannot occur simultaneously with readout,creating an acquisition dead time.When the repetition frequency of the lidar system increases to the kHz level,the interplay between acquisition and readout becomes a limitation,resulting in extended dead time and potential pulse omissions. Methods In this paper,a"read-accumulate-store"intellectual property(IP)was proposed,which was developed within a field-programmable gate array(FPGA)with the dual-port RAM and an adder,enabling temporal integration of corresponding points in a data linked list.Its core innovation lies in concurrent acquisition and integration:during each acquisition cycle,prior results are retrieved,accumulated with current data,and stored iteratively until the preset accumulation count is achieved.The temporal integration IP architecture,implemented in FPGA,comprises the components of photoelectric conversion module(transforms optical signals into electrical signals),analog-to-digital(A/D)module,accumulation(ADD)module,dual-port RAM storage and control unit.To achieve the designed function,the control unit executes the following steps.First,prior to the first trigger,all storage units in the linked list are reset to zero.Second,the first data point is acquired when the first trigger arrives,and the value stored in the first position of the linked list is read.These two values are summed and stored back in the first position.This process repeats N times to complete the sampling for the first trigger.Since the storage units were cleared before the first trigger,each unit contains the result of a single acquisition after this step.For the second trigger,the same procedure as in the previous step is repeated.As the values in each storage unit of the linked list are read during this acquisition,each unit now contains the cumulative result of the corresponding points from the first and second acquisitions after completion.For the M-th trigger,this process continues,with each storage unit holding the cumulative result of the corresponding points from the previous M acquisitions.Finally,once the predetermined number of accumulations,P,is reached,the accumulated data are read out and transmitted to the host computer,yielding the final integrated results.This method ensures efficient data processing by integrating acquisition,accumulation,and storage,thereby facilitating high-fidelity temporal integration for lidar systems. Results and Discussions To verify the effectiveness of the proposed design,a square wave signal,superimposed with a 0.2 V Gaussian white noise,with a period of 100 Hz and an amplitude of 1 V,was used as the input signal for testing.The results demonstrate that the signal becomes progressively smoother with the increase in accumulations,indicating significant noise signal attenuation(Fig.5).The SNR increased by 42 dB after 1000 accumulations,confirming that the data acquisition and integration module can achieve multiple acquisition accumulation to reduce background noise and improve SNR.For further verification,this module was compared with the DPO5104 digital oscilloscope of Tektronix and the PXI-9826 acquisition card of ADLINK Technology in actual measurements of laser radar echo signals.After correcting the signals obtained by the three devices by the square of the distance,The RSCS,which eliminates the factor of attenuation factor of light transmission,revealed that despite some variations,the waveform trends were fundamentally consistent,and the position data of the thin cloud layer measurements were largely concordant.Finally,this method was applied to a high-repetition-frequency polarized Mie lidar system,with a 5 kHz repetition frequency,achieving data acquisition at a 50 MHz sampling rate and performing over 80000 cumulative averages,successfully determining the extinction and depolarization ratio coefficients(Fig.8). Conclusions A distinctive requirement in the digitization of lidar echo signals,setting it apart from other methods,is the need for temporal integration.While current acquisition techniques effectively handle data collection and temporal integration at low repetition frequencies,limited research addresses these processes at lidar repetition frequencies in the kHz range.This paper presents a novel"read-accumulate-store"method that enables temporal integration of corresponding data points within a linked list structure.This approach simultaneously reads previous acquisition results,accumulates them with current acquisition data,and stores the resulting sum,achieving seamless temporal integration.To implement this method,an intellectual property architecture for temporal integration was developed within a FPGA,utilizing dual-port RAM and an adder.SNR analyses and practical testing demonstrate that this method enables high-speed data acquisition and temporal integration in high-repetition-frequency lidar systems.Additionally,the method offers flexible configuration of parameters,as channels,sampling frequency,sampling length,and integration settings can be adjusted by the hardware description language.Its single-chip integration capability enhances both cost-effectiveness and compactness.Given its versatility and performance,this method shows potential for standardization as a modular component in lidar systems,promoting widespread adoption in advanced sensing applications.

Objective Meteorological factors have a significant effect on the vertical distribution of aerosols,and the formation,accumulation,and dissipation of heavy pollution processes in winter are usually controlled by meteorological conditions.There is a high correlation between meteorological factors and the vertical structure of aerosols.Obtaining detailed evolution characteristics of meteorological factors is of great research value for studying the process of haze generation and dissipation.Among various meteorological factors,the vertical distribution of temperature plays a crucial role in the aggregation of aerosols in the boundary layer.Currently,ground meteorological stations,meteorological satellites,radiosondes,and microwave radiometers are the main means of detecting vertical temperature profiles,but none of them can achieve high spatiotemporal resolution detection of temperature profiles within the boundary layer.Rotational Raman lidar,as an effective technique for atmospheric temperature measurement,offers high temporal and spatial resolution,which is advantageous for studying atmospheric physical processes within the boundary layer during haze conditions.However,the system's detection performance within the boundary layer is significantly affected by the inconsistency of the bottom detection blind zone and signal attenuation caused by aerosols within the boundary layer,as well as interference from elastic scattering.To enable atmospheric temperature measurements in the bottom layer of haze conditions,we propose a temperature correction technique based on the backscatter ratio.We hope to effectively obtain the vertical structure of atmospheric temperature within the boundary layer of haze weather through this technique,thereby providing data support for the refined study of atmospheric physical processes under haze conditions. Methods The core of this technique involves constructing a linear functional relationship between the backscatter ratio and the elastic scattering crosstalk ratio and using the backscatter ratio to correct the rotational Raman ratio.The consistency of the geometric overlap factor of the rotational Raman channels significantly affects the bottom detection performance of the lidar.Therefore,accurately obtaining the ratio of high and low quantum number rotational Raman channels is a prerequisite for implementing rotational Raman temperature measurements.First,we use experimental data under clear sky conditions without haze to calibrate the geometric overlap factor ratio of the rotational Raman channel signals.Then,we calibrate the inversion function using a radiosonde under simultaneous spatial conditions and obtain the theoretical Raman ratio within the haze layer based on the temperature data from the radiosonde.Based on this theoretical ratio and the rotational Raman ratio that includes elastic scattering crosstalk,we calculate the elastic scattering crosstalk ratio.We perform a linear regression analysis on both the backscatter ratio and the elastic scattering crosstalk ratio to derive the corresponding system calibration constant.Finally,using this calibration constant and the measured backscatter ratio,we complete the correction of the rotational Raman ratio,allowing for the retrieval of the true atmospheric temperature data in the elastic scattering region. Results and Discussions Numerical simulation results indicate that inconsistencies in the geometric overlap factor and the elastic scattering crosstalk can lead to retrieval biases in atmospheric temperature measurements within the boundary layer during haze events.After applying dual corrections for the geometric overlap factor and the rotational Raman ratio,the temperature retrieval bias is reduced to less than 0.2 K(Fig.4).Experimental results show that the corrected temperature profile from the morning of of December 26,2023 exhibits a high degree of consistency with the radiosonde data obtained under simultaneous spatial conditions,with a maximum temperature bias of less than 1.4 K,while the uncorrected temperature bias is up to 7 K(Fig.8).On the evening of of December 24,2023,the maximum temperature bias is less than 0.6 K,while the uncorrected temperature bias is approximately 4 K(Fig.9).Additionally,continuous observation results clearly illustrate the vertical distribution of the temperature field.A comparison of the backscatter ratios of aerosols reveals a close correlation between the inversion temperature features and the vertical distribution of aerosols(Fig.10). Conclusions In our study,we propose a temperature correction technique based on the backscatter ratio to achieve atmospheric temperature measurements within the bottom layer of haze conditions.Simulation and experimental results indicate that this technique can effectively achieve precise measurements of atmospheric temperature within the boundary layer during haze events,clearly illustrating the vertical structure of atmospheric temperature and the characteristics of inversion.Since the fundamental basis for temperature correction relies on the correlation between the elastic scattering crosstalk ratio and the backscatter ratio,it is crucial to accurately obtain the backscatter ratio and the rotational Raman ratio within the haze layer.Therefore,highly consistent preprocessing of the lidar echo signals is necessary.Additionally,the stability of parameters such as laser energy,the geometric overlap relationship of the light transmission and reception system,the elastic scattering suppression ratio of the rotational Raman channel,and the photoelectric conversion efficiency also influence the implementation of this technique.Variations in these parameters can directly affect system stability,thereby affecting the temperature correction results.Therefore,a lidar system with high stability is an essential prerequisite for conducting atmospheric temperature corrections.Any minor adjustments to system parameters necessitate a reevaluation of the system calibration.In summary,the introduction of this correction technique provides scientific data and technical means for studying atmospheric physical processes within the boundary layer during haze events,facilitating a detailed investigation and analysis of the formation and evolution characteristics of haze.

In order to monitor forest fires, a high-repetition-rate polarization Lidar system was developed based on the light scattering and polarization characteristics of smoke particles generated during fires. The system consists of subsystems for laser emission, optical reception, echo signal acquisition and processing, and scanning control. To meet the demands for large-scale, high-resolution, and rapid forest fire detection, a high-power, high-repetition-rate laser was selected as the probing source, coupled with a high-resolution gimbal for precise scanning. With a Lidar repetition rate of 5 kHz, the system can perform patrol scanning a forest area with a 10-km radius in 48 minutes at an angular resolution of 1 degrees. To address the challenges of echo signal acquisition and cumulative averaging during high-repetition-rate detection, a novel "readout-accumulation-storage" IP (Intellectual Property) architecture was designed, enabling efficient echo signal processing and improving the signal-to-noise ratio. The completed high-repetition-rate polarization Lidar underwent near-field and far-field simulation experiments, with detected signal peaks corresponding to fire locations. When deployed in Yan'an City, the Lidar successfully detected simulated fires at distances of 5.4 km and 8. 1 km, validating the system's effective detection capability.

We report on the application of two tunable interband cascade lasers (ICLs) with about 3.2-μm wavelength in integrated-path differential absorption (IPDA) lidar and power-modulation integrated-path differential absorption (PM-IPDA) lidar experiments for probing open-path benzene concentration. To demonstrate the performance of the PM-IPDA lidar, we investigate the performance in an open-path atmosphere according to the comparative experiment of the IPDA lidar and the PM-IPDA lidar. To the best of our knowledge, it is the first comparative experiment of the IPDA lidar and the PM-IPDA lidar in the mid-infrared band for probing open-path benzene concentration. Data from these laboratory experiments indicate the effectiveness of the IPDA lidar and the PM-IPDA lidar, and also that the PM-IPDA lidar can reduce the impact of 1/ f noise on detection performance. Data from these open-path atmosphere experiments with a time resolution of 0.1 s indicate that the detection limit of the IPDA lidar is about 0.89 mg·m -3 and that the detection limit of the PM-IPDA lidar is approximately 0.66 mg·m -3 . Hence the detection limit of the PM-IPDA lidar is approximately 26% lower than that of the IPDA lidar.

Aimed at the stability of calibration coefficients in a general non-orthogonal retrieval algorithm (NRA) of pure rotational Raman lidars (PRRLs), an orthogonal retrieval algorithm (ORA) of atmospheric temperature profiles based on the orthogonal basis function is proposed. This algorithm eliminates the correlation between the calibration coefficients in the NRA to reduce the influence of the number of calibration points and the selection scheme on the calibration coefficients. In this paper, the stabilities of calibration coefficients in the NRA and ORA are compared and analyzed, and the data analysis for atmospheric temperature profiles with a time resolution of minute-level are given, based on the developed Cloud Precipitation Potential Evaluation (CPPV) lidar data and the parallel radiosonde temperature data. The analysis results show that coefficients of variation (CVs) of ORA calibration coefficients are one order of magnitude smaller than those of NRA coefficients. The mean deviation of the ORA retrieval results is roughly reduced by 16.1% compared with the NRA, and the root-mean-square deviation is roughly reduced by 15.0% compared with the NRA. Therefore, the temperature retrieval performance of the ORA is better than that of the NRA.

This study proposed an inversion method for atmospheric-aerosol or cloud microphysical parameters based on dual-wavelength lidar data. The matching characteristics between aerosol and cloud particle size distributions and gamma distributions were studied using aircraft observation data. The feasibility of the retrieval of the particle effective radius from lidar ratios and backscatter ratios was simulated and studied. A method for inverting the effective radius and number concentration of atmospheric aerosols or small cloud droplets using the backscatter ratio was proposed, and the error sources and applicability of the algorithm were analyzed. This algorithm was suitable for the inversion of uniformly mixed and single-property aerosol layers or small cloud droplets. Compared with the previous study, this algorithm could quickly obtain the microphysical parameters of atmospheric particles and has good robustness. For aerosol particles, the inversion range that this algorithm can achieve is 0.3–1.7 µm. For cloud droplets, it is 1.0–10 µm. An atmospheric-observation experiment was conducted using the multi-wavelength lidar developed by Xi'an University of Technology, and a thin cloud layer was captured. The microphysical parameters of aerosol and clouds during this process were retrieved. The results clearly demonstrate the growth of the effective radius and number concentration.

Objective Lidar remote sensing technology possesses significant advantages in detecting atmospheric parameters(such as clouds and aerosols,temperature,and wind speed)with high precision and high timeliness.However,atmospheric turbulence can affect the transmission characteristics of laser in the atmosphere,causing a series of turbulence effects such as light intensity fluctuations,phase fluctuations,beam wander,and beam spread.Based on the backscattering enhancement effect of laser transmission in turbulent atmosphere,a double-telescope hard-target reflection lidar system for detecting atmospheric turbulence intensity is proposed.The system realizes the measurement of backscattering enhancement coefficient by receiving the diffuse reflection echo signals of hard-target with the double telescope.The biggest advantage of this system is the utilization of small-aperture double-telescope receiving channels,greatly simplifying the complexity of the system and reducing equipment costs. Methods The application of double-telescope lidar technology based on backscattering enhancement effect in atmospheric turbulence detection is studied.First,a double-telescope hard-target reflection lidar for detecting atmospheric turbulence intensity is proposed and designed based on the backscattering enhancement effect.The system consists of one transmission channel and two receiving channels,where one receiving channel aligns with the transmitting channel and the other is offset by 15 cm.Then,the backscattering enhancement coefficient of hard-target reflected signal at the receiving telescope is established based on the generalized Huygens-Fresnel principle.Finally,the experimental system is constructed,and the preliminary experiments are conducted in calm weather with uniform turbulence intensity.The effects of turbulence intensity,laser transmission distance,temperature,and wind speed on the backscattering enhancement coefficient are studied. Results and Discussions The detection principle of the proposed backscattering enhancement effect lidar is presented.It breaks through the limitation of traditional lidar using a large-aperture receiving telescope,which leads to complex system structures,particularly in terms of the optical path.Meanwhile,this system features simple structure,mobility,and low cost(Fig.1).Table 1 presents the main parameters of the lidar system.By simulating various turbulence intensity changes through the distance between the beam and the heater,the relationship between backscattering enhancement coefficient and turbulence intensity is analyzed(Fig.3).Under the same observation conditions,the relationship between different laser integration paths and the backscattering enhancement coefficient is studied(Fig.4).The correlation between nighttime wind speed and temperature changes with the backscattering enhancement coefficient is observed and analyzed.The results show that,due to the gradual decline trend of night ground temperature is consistent with the conventional turbulence intensity,they show a strong correlation[Fig.5(b)].However,the random variation of wind speed is different from the decline trend of conventional turbulence intensity,and the correlation is poor[Fig.6(b)]. Conclusions We propose a double-telescope hard-target reflection lidar based on the backscattering enhancement effect to study the transmission characteristics of laser beam in turbulent atmosphere.The biggest advantage of this system is that a small-aperture double-telescope receiving channel can be employed to simplify the structure and reduce costs.The theoretical analysis of the backscattering enhancement coefficient of the echo signal is performed by receiving reflected signal of the double telescope.Additionally,the preliminary experiments are conducted in calm weather with uniform turbulence intensity.The results show that the backscattering enhancement coefficient increases monotonically with the increase of simulated turbulence intensity,exhibiting a saturation trend.Under the same observation conditions,the backscattering enhancement coefficient also shows a saturation trend as the integral path increases.The nighttime temperature shows a good correlation with backscattering enhancement coefficient,and its Pearson correlation coefficient R is 0.95.However,the correlation between wind speed and backscattering enhancement coefficient is relatively poor,and its Pearson correlation coefficient R is 0.67.The double-telescope hard-target reflection lidar proposed in this paper possesses significant research and practical value for the detection of atmospheric turbulence.

Objective More than 50% of atmospheric water vapor exists mainly in the lower atmosphere within 2 km. Vibrational Raman scattering lidar is an important remote sensing tool for atmospheric water vapor measurement. However, the traditional vibrational Raman scattering lidar mainly adopts a coaxial and non-coaxial parallel transceiver system structure, and the system detection blind zone and transition zone limit their effectiveness in ground atmospheric water vapor detection. We propose a novel detection technique of lateral vibrational Raman scattering lidars based on the structure of a bistatic system, where the lateral vibrational Raman scattering signals of N-2 and H2O at different heights are detected by the elevation angle scanning of the lateral receiver system. Finally, it realizes fine detection of near-surface atmospheric water vapor without a blind zone from the ground to the height of interest. Methods We study the lateral vibrational Raman scattering lidar technique in the application of accurate measurements of atmospheric water vapor from the ground to the height of interest. First, a novel lateral scanning vibrational Raman scattering lidar technique is proposed and designed. Two telescopes combined with specified narrow-band interference filters are utilized to detect the lateral scattering signals of the vibrational Raman scattering spectra of N-2 and H2O respectively. Then, the inversion algorithm of atmospheric water vapor using the lateral vibrational Raman scattering lidar is established. Vibrational Raman scattering spectra of N-2 and H2O have large wavelength differences, which lead to large differences between atmospheric transmissivity of the slant path in these two detection channels, and the aerosol extinction coefficients inverted by Raman method are adopted to correct atmospheric transmissivity of the slant path and improve the detection accuracy of the atmospheric water vapor mixing ratio. Finally, the construction of the experimental system is completed, and the preliminary experiments are conducted via the lateral scanning vibrational Raman scattering lidar. Two different rotation schemes including the continuous equidistant resolution and segmented equidistant resolution are employed during the experimental observations. Results and Discussions The detection principle of the lateral vibrational Raman scattering lidar is innovatively proposed. It breaks through the traditional backward vibrational Raman scattering lidar by a monostatic transceiver system structure, which produces the blind zone and transition zone without effective detection of near- surface atmospheric water vapor. Meanwhile, this technology can utilize a continuous-wave laser featuring light weight, portability, mobility, and low cost (Fig. 1). Data correction of atmospheric water vapor is realized by analyzing the atmospheric molecular scattering phase function and the difference in slant path atmospheric transmissivity caused by the wavelength difference between the vibrational Raman scattering spectra of N-2 and H2O. The aerosol extinction coefficient obtained from the inversion of the lateral N-2 vibrational Raman scattering signal is employed for real-time correction of the slant path atmospheric transmissivity, which improves the accuracy of atmospheric water vapor mixing ratio detection (Figs. 2-4). Preliminary experimental observational studies of a lateral scanning pure rotational Raman scattering lidar are performed by two different rotation schemes including the continuous equidistant resolution and segmented equidistant resolution, which are employed during the experimental observations. The experimental results show that both rotation schemes can realize atmospheric water vapor detection from the ground to the height of interest. In particular, the segmented equidistant resolution scheme can realize more fine detection of atmospheric water vapor distribution in the ground zone (Figs. 5- 8). Conclusions We focus on the detection demand for atmospheric water vapor from the ground to the height of interest using the lidar technique. Based on the theoretical basis of vibrational Raman scattering, the innovative technology of lateral scanning Raman scattering lidar for detecting atmospheric water vapor at the ground surface is proposed. This technology combines the elevation angle scanning function of the lateral receiver system to achieve non-blind scanning detection of water vapor in the lower atmosphere. Due to large differences between the wavelengths of the vibrational Raman scattering spectra of N-2 and H2O, the aerosol extinction coefficients obtained by inverting the lateral N-2 vibrational Raman scattering signals are adopted to make real-time corrections to the slant path atmospheric transmissivity, which improve the accuracy of atmospheric water vapor mixing ratio. If a high-power pulsed laser is applied, it can be simultaneously observed with a backward vibrational Raman scattering lidar to construct a joint detection system to realize the measurement of atmospheric water vapor from the ground to the height of interest. The experimental results show that the lateral vibrational Raman scattering lidar can detect atmospheric water vapor mixing ratios up to 1400 m with a horizontal distance of 60 m between the laser transmitter system and the lateral telescope receiver system. Additionally, the segmented equidistant resolution scheme has variable resolutions at different heights to show more details of water vapor distribution in the ground zone.

Aimed at the regional open -path detection of benzene (C 6 H 6 ) in the atmosphere, a power -modulated integrated path differential absorption (PM-IPDA) lidar is introduced and demonstrated. Two tunable interband cascade lasers (ICLs) with about 3.2 mu m wavelength are utilized to generate the required PM optical signal. These two operation central wavelengths (CWs) of the PM-IPDA lidar are, respectively, 3236.6 and 3187.1 nm, which can mitigate the influence of significant gases such as H 2 O, CH 4 , and HCl on the detection performance. In this work, the fast Fourier transform algorithm is used to retrieve the measured values with the time resolution of 0.1 s corresponding to 10 4 sampling bins at the sampling rate of 100 kSps/s. The modulated frequency of the PM-IPDA lidar is selected as 10 kHz by laboratory experiments. The slow fluctuation characteristic of the benzene absorption spectrum within the vicinity region of 3.2 mu m reduces the impact of small wavelength fluctuations on the performance of PM-IPDA lidar, although a scheme modulated only the driving current causes wavelength fluctuations of similar to +/- 0.2 nm. These laboratory experiments also indicate the PM-IPDA lidar can reduce the error resulting from 1 / f noise. Open -path observation experiments show that the detection limit is about 0.60 mg m - 3 and that the PM-IPDA lidar can be used for the regional open -path real-time detection of benzene. (c) 2024 Optica Publishing Group

To address the challenges of overlap factor analysis and correction in noncoaxial lidar,we introduce a normalized overlap factor model analysis method.This approach simplifies the traditional five-dimensional geometric area overlap factor model into a four-dimensional structural parameter description.This simplification helps reduce the number of influencing factors and facilitates the identification of key features.The analysis examines the impact of four structural parameters(beam collimation coefficient,transceiver module radius ratio,transceiver module field-of-view ratio,and transceiver module spacing radius ratio)on the overlap factor.Notably,within a specific range of beam collimation coefficient variations,a distinct feature point emerges in the overlap factor curve at a certain detection distance.This feature point serves as a crucial alignment indicator for the biaxial adjustment of the lidar.The validity of our normalized model and its correction method is demonstrated through horizontal experimentation.Experimental results reveal that,compared to traditional methods,the normalized model achieves an average error of 0.016 for the overlap factor within the 0-1 km range.Furthermore,following overlap factor correction,the effective detection distance in the near-field is reduced from 510 m to 107 m.This normalized model offers valuable insights for assessing the parallelism accuracy of noncoaxial lidar adjustments,thereby advancing the practical implementation of automatic lidar product adjustment.

To address the issue of limited detection of water vapor in the near-surface area by existing Lidar technology, a side-scanning continuous light method is proposed. This method uses a high-power continuous light laser as the detection source and obtains the water vapor concentration contour by side-scanning the light scattering and absorption signals on the detection path. The water vapor concentration contour is then obtained through inversion based on the differential absorption method. The model establishes the detection of water vapor concentration using side-scanning continuous light. The validity of the method is verified through simulation, which shows that semiconductor continuous-wave lasers with detection wavelength pairs of 880 nm and 940 nm and energy of 5 W can effectively detect water vapor within 500 m near the ground. This method does not require expensive narrow-pulse lidar as the detection light source. It has the advantages of being easy to implement and cost-effective. It can be a useful supplement to existing water vapor detection technology for near-surface detection.

This publisher's note contains corrections to Opt. Lett.48, 2595 (2023).10.1364/OL.488924.

A novel lateral scanning Raman scattering lidar (LSRSL) system is proposed, aiming to realize the accurate measurement of atmospheric temperature and water vapor from the ground to a height of interest and to overcome the effect of a geometrical overlap function of backward Raman scattering lidar. A configuration of the bistatic lidar is employed in the design of the LSRSL system, in which four horizontally aligned telescopes mounted on a steerable frame to construct the lateral receiving system are spatially separated to look at a vertical laser beam at a certain distance. Each telescope, combined with a narrowband interference filter, is utilized to detect the lateral scattering signals of the low- and high-quantum-number transitions of the pure rotational Raman scattering spectra and vibrational Raman scattering spectra of N2 and H2O. The profiling of lidar returns in the LSRSL system is performed by the elevation angle scanning of the lateral receiving system, in which the intensities of the lateral Raman scattering signals at each setting of elevation angles are sampled and analyzed. Preliminary experiments are carried out after the construction of a LSRSL system in Xi'an city, whose retrieval results and statistical error analyses present a good performance in the detection of atmospheric temperature and water vapor from the ground to a height of 1.11 km and show the feasibility for combination with backward Raman scattering lidar in atmospheric measurement.

Objective Multi-longitudinal-mode (MLM) high-spectral-resolution lidar (HSRL) is a novel laser remote sensing technique for realizing fine detection of aerosol optical properties. The Mach-Zehnder interferometer (MZI) with a periodic transmittance is selected as the spectral discriminator for directly separating aerosol Mie scattering and molecular Rayleigh scattering spectra excited by the MLM laser. In principle, the design of the MZI for the application of the MLM-HSRL should meet two conditions. One is that the optical path difference of the MZI is twice the laser cavity length, and the other is that the optical path difference of the MZI is an integer multiple of the laser wavelength. The laser elastic echo scattering signal received by the MLM-HSRL has Gaussian transmission characteristics consistent with the laser beam, which makes the incident light beam on the MZI have a divergence angle. The divergence angle of the incident light beam leads to a deviation in the optical path difference of the MZI, which makes the optical path difference of the MZI fail to equal an integer multiple of the laser wavelength and then makes the discrimination capability of the MZI worse. As the divergence angle of the incident light beam cannot be eliminated, a field-widening technique for the MZI with a large optical path difference based on compensated glasses is proposed to reduce the influence of the divergence angle of the incident beam on the discrimination capability of the MZI. Methods In this study, a field- widening technique for the MZI with a large optical path difference is studied for the application of the MLM-HSRL for realizing fine detection of aerosol optical characteristics. First, the required design parameters of the MZI with a large optical path difference and an inversion method of aerosol optical properties in the MLM-HSRL system are analyzed. Second, the mathematical relationship between the divergence angle of the incident light beam and the effective transmittance of the MZI with a large optical path difference is established, and the maximum allowed divergence angle for the MZI (OPD= 1000 mm) is calculated. Third, a field-widening technique for the MZI with a large optical path difference based on compensated glasses is proposed. The principle of the field-widening technique is explained, and the mathematical model between the optical path difference of the MZI with the field-widening technique and the divergence angle of the incident light beam is established. According to such analysis, compensated glasses are selected, and their length is calculated. Fourth, the theoretical modeling and simulation verification of the proposed fieldwidening technique are carried out. Results and Discussions The discrimination capability of the MLM- HSRL system is affected by the divergence angle generated by the Gaussian transmission distribution of the laser elastic echo scattering signal. Theoretical analysis shows that the maximum allowed divergence angle of the MZI (OPD= 1000 mm) is no more than 0. 4 mrad, so as to ensure an excellent discrimination capability of the MZI with a large optical path difference (Fig. 4). A field-widening optical path of the MZI with a large optical path difference based on compensated glasses is shown in Fig. 5. Compensated glasses are chosen to be the HK9LGT glass, whose refractive index ( standard state @532. 0 nm) is 1. 517, and the coefficient of thermal expansion is 7. 6x10-6. The total length of the required compensated glasses is 1165. 767 mm. In addition, the transmittance of the compensated glasses decreases as the length increases, and the transmittance decreases by 0. 2% when the length increases by 10 mm (Table 1). The theoretical analysis shows that the allowed divergence angle of the MZI with a large optical path difference (OPD= 1000 mm) should be less than 25. 6 mrad after the field widening (Fig. 6). Zemax simulation results show that the effective transmittance of T aa is lower than 0. 7, and positive discrimination effect cannot be achieved when the divergence angle is greater than 0. 4 mrad before the field widening. The effective transmittance of T aa is 0. 825-0. 793, and MZI has excellent discrimination capacity (Fig. 9) when the divergence angle is 0-5 mrad after the field widening. Conclusions MZI, as a spectral discriminator, is the core device in the MLM-HSRL system. The Gaussian transmission distribution of the laser elastic echo scattering signal makes the incident light beam on the MZI with a large optical path difference inevitably have a divergence angle. In this paper, the influence of the divergence angle on the discrimination capability of the MZI with a large optical path difference is analyzed in detail, and the maximum allowed divergence angle of the MZI ( O PD=1000 mm) is 0. 4 mrad. In order to reduce the influence of the divergence angle on the transmittance of the MZI, a field- widening technique for the MZI with a large optical path difference based on compensated glasses is proposed. The theoretical analysis results show that the maximum allowed divergence angle of the MZI (O PD= 1000 mm) is 25. 6 mrad after the field widening. The proposed field-widening technique enlarges the allowed divergence angle range of the system by nearly 50 times. The simulation results show that the effective transmittance of T aa decreases rapidly with the increase in the divergence angle, and the discrimination capability of the MZI becomes worse at a divergence angle greater than 0. 4 mrad before the field widening, while T aa decreases slightly with the increase in the divergence angle, and the discrimination capability of the MZI is positive at a divergence angle ranging from 0 to 5 mrad after the field widening. The proposed field-widening technique can extend the received field angle of the MZI with a large optical path difference and improve the discrimination capability of the MZI with a large optical path difference.

Aiming at the real-time detection of toluene concentration in the atmosphere, an integrated-path differential absorption (IPDA) lidar is proposed based on inter-band cascade lasers. Since the C-H bond of toluene has a slowly-changing absorption spectrum in the mid-infrared band, this IPDA lidar is designed using 2935.5cm-1 and 3192.0cm-1 as the operating wavelength by considering the influence of the main interfering gases such as H2O, CH4, and HCl. A spectroscopic system with a mid-infrared diffraction grating is configured to realize synchronous detection of dual-wavelength received signals. A retrieval algorithm and its error analysis model for atmospheric toluene concentration are presented. And then the performance of lidar is analyzed and discussed under the conditions of different visibilities, path lengths, and water vapor concentrations by combining with the mid-latitude standard atmospheric model. These results show that the relative error of toluene concentration is less than 10% within the concentration range of 20ppb to 10ppm under the condition of atmospheric visibility of 5km, path length of 1.6km, and the water vapor concentration of less than 0.4%. This IPDA lidar can provide an effective scheme for real-time detection of atmospheric toluene concentration.

Benzene is an important component of volatile organic compounds (VOCs), and its pollution of the atmosphere has attracted increasing attention. The mid-infrared band is usually the fundamental frequency fingerprint absorption region of molecules, so it has become an important band for detecting trace gas molecules. Moreover, the differential absorption lidar is an important means of detecting atmospheric trace gases. Therefore, aiming at the problem of real-time remote sensing of regional benzene concentration, an integral path differential absorption (IPDA) lidar for detecting atmospheric benzene concentration based on inter-band cascade lasers (ICLs) is proposed. Firstly, we construct the retrieval algorithm of IPDA lidar and its error analysis model based on analyzing the detection principle of IPDA lidar. Secondly, the absorption spectra of benzene and major interfering gases (such as HCl, CH4 and H2O) near the mid-infrared vicinity region of 3 100 cm i from the HITRAN database are analyzed in detail. By considering comprehensively the influence of HCl, CH4 and H-2 on the detection results, the measurement wavelength and reference wavelength of the IPDA lidar are selected to be 3 090. 89 and 3 137. 74 cm(-1) respectively. Thirdly, we designed an IPDA lidar for detecting atmospheric benzene concentration based on two continuous-wave ICLs. The output wavelengths of these ICLs can be tuned by controlling the temperature and driving curren, so that their wavelengths can be stabilized in the strong absorption spectrum region and the weak absorption spectrum region respectively. And then, a spectroscopic system with a mid-infrared diffraction grating as the core is designed to realize synchronous detection of dual-wavelength receiving signals. Finally, combined with the mid-latitude standard atmospheric model, the performance of lidar under the conditions of different visibilities, path lengths, and water vapor concentrations is analyzed and discussed. And then, we carry out test experiments by building a mid-infrared band detection gas cell to verify the feasibility of the IPDA lidar. These results from simulations and experiments show that the relative error of benzene concentration is less than 10% within the concentration-path length product (CL) range of 0.1 similar to 24 mg center dot m(3) center dot km, and the relative error of detection is better than 1%0, while the CL of benzene is 5 mg center dot m(3) center dot km, under the condition of atmospheric visibility of 5 km, and the water vapor concentration of less than 0. 4%; and that the linear correlation coefficient R-2 of differential absorption lidar detection in the midinfrared band is about 98. 7% by preliminary experiments.