Lidar is an important device for detecting atmospheric parameters. In this paper, a single channel F-P etalon is used as a narrow-band filter to lock the transmittance of 355 nm wavelength emitted laser by control the transmittance of seed laser with 1064 nm wavelength. Not only the background noise can be suppressed and the signal-to-noise ratio can be improved, but also the transmittance of the locked target in the etalon can be tracked to achieve higher detection accuracy. During the experiment, transmittance is controlled at target transmittance with 0.01 precision.
Lidar and radar are two useful tools for cirus clouds, which can provide the possibility to retrieve the vertical profiles of both the number density of particles and their microphysical characteristics, especially, when they operate together simultaneously. In this study, a multi-wavelength Lidar and a 35 Ghz Radar with polarization channels are employed to measure the properties of cirrus clouds in Hefei. The backscattering coefficient at 0.532 μm from lidar, and the effective reflectivity factor from radar are inversed for use. Furthermore, the quantities responsible for microphysics can be extracted and explained as the dimensionless values, such as the depolarization ratio and color ratio. Then these optical properties for cirrus cloud during campaigns are analyzed and compared with calculating results.
The Fabry-Parot etalon can be widely used in the lidar for Doppler wind measurement and aerosol detection. Due to the very wide application of the FP etalon, the standard transmission curve of the F-P is a very important parameter. The general FP etalon measures the transmittance curve by tuning the wavelength of the laser source or tuning the angle of incident light. The FP etalon generally measures the transmittance curve of the etalon by tuning the wavelength of the laser source or use the frequency comb source. Moreover, the tuning of the wavelength and the angle of incident light are nonlinear, and the measurement accuracy is insufficient, and the frequency comb source is very expensive. This paper proposes a new method for testing the transmittance curve of a FP etalon using a similar frequency comb source. The whispering gallery mode is a typical similar frequency comb laser source which has multiple frequency components, but not equal intervals. The spacing of the frequency of the whispering gallery mode is not equal, but the spacing of the frequency can be determined. So, the transmittance curve of the FP etalon can be measured at one time. And the frequency interval and spectral range are tunable easily. This new method greatly reduces the cost of measuring transmission curve, improve measurement accuracy and effectiveness and has great theoretical and practical value.
As the increasing demands of the environmental protection, meteorological monitoring, ecological detection and other fields, lidars are needed to simultaneously measure a variety of atmospheric parameters (especially simultaneous measurement of various pollutants, such as volatile organic solvent VOCs). Multi-wavelength Raman lidar, differential absorption lidar (DIAL) and high spectrum resolution lidar (HSRL) can be implemented these fields. But either technique needs multiple laser sources to meet their requirements. This paper proposes a new lidar based on Frequency Comb Light Source, which can provide multiple frequency components through a single light source for simultaneously measuring various multiple atmospheric parameters. The frequency comb laser light source is used to emit seed light of frequency ωc from the seed laser, and then after passing through the frequency comb laser, a series of equally spaced spectral frequency components are emitted centered on ωc; then emitted into the atmosphere, and the laser reacts with the atmosphere. The echo signal enters the receiving optical path through the telescope, and is sent through the beam splitter or the discriminator to detect by the detector. Based on different pollutants have different absorption cross sections, similar differential absorption lidar(DIAL), the λon and λoff can be determined. The lidar using the frequency comb light source can be easy designed into a compact structure, which is convenient to carry and maintain. It’s also a very advantageous for the lidar miniaturization and industrialization with a wide application prospect.
In this study, a multi-wavelength Raman-mie Lidar, and a 35 Ghz Radar are employed to measure the properties of aerosol and cloud. The extinction at 0.355μm and 0.532μm and backscattering at 0.355μm, 0.532μm and 1.064μm coefficients from lidar, and the effective reflectivity factor from radar are inversed for use. Furthermore, the quantities responsible for microphysics can be extracted and explained as the dimensionless values, such as the linear depolarization ratio, the color ratio, and the other ratios. Then these microphysical properties for aerosol and cloud during campaigns are analyzed for further use.
As the non-spherical particles could be distinguished from spherical particles easily from polarization lidar measurement, polarization lidar was widely employed in the atmospheric measurements. However, it is very difficult to retrial the particle properties quantitatively from depolarization measurement due to the complex vector scattering model and the calibration of depolarization measurement. In this paper, a method for depolarization calibration of polarization lidar was introduced, and the offset angle of the half-wave plate and gain ratio could be determined exactly by using fitting method. The simulation and lidar measurements indicate that the polarization lidar could be calibrated well by using this method.
The vertical distribution of the aerosol layers is depicted by using the lidar data in Jinhua city from 2013 to 2014. The lidar installed in Jinhua is a dual-wavelength Mie polarization Raman lidar. Aerosol layers are searched through gradient method. At the same time, HYSPLIT model is used to tracing the aerosol trajectories. The results show that different heights of aerosol layers have different transportation route. By a case study, the lidar data on December 30, 2013 and May 1, 2014 reveal several vertical aerosol layers. According to the 24-hour backward trajectory of HYSPLIT model, different aerosol layers comes from different places, and this may relate to the winter monsoon in China.
Aerosol and water vapor are two important atmospheric parameters. The accurate quantification of diurnal variation of these parameters are very useful for environment assessment and climate change studies. A moveable, compact and unattended lidar system based on modular design is developed for aerosol extinction coefficients and water vapor mixing ratios measurements. In the southern suburbs of Beijing, the continuous observation was carried out by this lidar since the middle of the year of 2017. The lidar equipment is presented and the case study is also described in this paper. The observational results show that the lidar kept a very good status from the long-time continuous measurements which is suitable for networking especially in meteorological research field.
Simulation of retrieving bimodal log-normal aerosol size distribution is accomplished using the algorithm of nonlinear optimal estimation with the aerosol optical parameters combined with three aerosol backscattering coefficients in 355nm, 532nm, 1064nm wavelength (3β) and two extinction coefficients in 355nm, 532nm wavelength (2α). The results show that the parameters characterized the bimodal log-normal function can be successfully retrieved using this algorithm of high accuracy and stability. Given the ±10% uncertainty of the prior value the retrieving results error of the total number of particles and the effective radius are 9.4%, 19% and 9.5%, 22.1% respectively in Bimodal 1 and Bimodal 2. And the geometrical standard deviation brings greater error.
Lidar is a kind of active optical remote sensing instruments , can be applied to sound atmosphere with a high spatial and temporal resolution. Many parameter of atmosphere can be get by using different inverse algorithm with lidar backscatter signal. The basic setup of a lidar consist of a transmitter and a receiver. To make sure the quality of lidar signal data, the lidar must be calibrated before being used to measure the atmospheric variables. It is really significant to character and analyze lidar optical subsystem because a well equiped lidar optical subsystem contributes to high quality lidar signal data. we pay close attention to telecover test to character and analyze lidar optical subsystem.The telecover test is called four quadrants method consisting in dividing the telescope aperture in four quarants. when a lidar is well configured with lidar optical subsystem, the normalized signal from four qudrants will agree with each other on some level. Testing our WARL-II lidar by four quadrants method ,we find the signals of the four basically consistent with each other both in near range and in far range. But in detail, the signals in near range have some slight distinctions resulting from overlap function, some signals distinctions are induced by atmospheric instability.
A high-spectral-resolution lidar (HSRL) can measure vertical profiles of the aerosol backscatter ratio and the aerosol extinction coefficient simultaneously. Fabry-Perot (F-P) interferometer used as a spectrum filter can provide fine frequency resolution with an important impact on HSRL retrieval accuracy. However, in practical applications, the spectral characteristics of the interferometer are affected by many factors. In this article, the impact of several factors on F-P interferometer spectral properties, such as the temperature change, the beam divergence angle, the absorption loss, surface defects and non-parallelism of the interferometer, were analyzed in depth. The performances of F-P interference spectrum filters are modeled and simulated by analyzing the characteristics of lidar echo, respectively using Gaussian model and Bruneau model. Finally, designed the sandwich structure of F-P etalon encapsulation, and the finite element method is used for thermal analysis.
Three microphysical backscatter ratios (color ratio, depolarization ratio, and lidar ratio) widely used for interpretation of lidar signals returned from cirrus clouds have been calculated for the first time. The physical-optics code developed earlier by the authors is applied. Though the data are obtained for the hexagonal ice plates and columns, that are the simplest crystal shapes, their arbitrary spatial orientation has been taken into account. The lidar experimental data measuring simultaneously the depolarization ratio and color ratio in cirrus clouds are also presented.
A Dual-wavelength Mie Polarization Raman Lidar has been developed for cloud and aerosol optical properties measurement. This idar system has built in Hefei and passed the performance assessment in 2012, and then moved to Jinhua city to carry out the long-term continuous measurements of vertical distribution of regional cloud and aerosol. A double wavelengths (532 and 1064 nm) Nd-YAG laser is employed as emitting source and four channels are used for detecting back-scattering signals from atmosphere aerosol and cloud including 1064 nm Mie, 607 nm N2 Raman, two 532 nm Orthogonal Polarization channels. The temporal and spatial resolutions for this system, which is operating with a continuing mode (24/7) automatically, are 30s and 7.5m, respectively. The measured data are used for investigating the aerosol and cloud vertical structure and cloud phase from combining of cloud signal intensity, polarization ratio and color ratio.
Extensive studies have illustrated the importance of obtaining exact vertical structures of clouds
and aerosols for satellite and relevant climate simulations. However, challenging exists, for example, in
distinguishing clouds from aerosols at times. Accurate cloud vertical profiles are mainly determined by cloud
bases and heights. Based on the ground-based lidar observations in Hefei Radiation Observatory (HeRO), the
vertical structures of clouds and aerosols in Hefei area(31.89°N,117.17°E) during May 2012-May 2014
have been investigated. The results show that the cloud fraction in the autumn and winter is less than that in
the summer and spring, and is largest in the spring followed by the summer. The cloud fractions in the autumn
and winter are comparable. The low cloud accounts for the most portion of the total. Compared with the cloud
of the other heights, the high cloud is the least in the winter. Nearly 50% of the total vertical profiles can be
detected by lidar as clouds and the proportion of the cloud of different heights seems to be stable annually.
The fraction of low cloud is nearly 45%, medium cloud is nearly 35%, and high cloud is nearly 20%. In
comparison with the results derived from CALIPSO, it is found that high cloud is usually missed for the
ground-based lidar, while low cloud is usually omitted for the satellite observations. A combination of
ground-based and space-borne lidar could lead to more reliable results. Further analysis will be performed in
future studies.
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