Denoising coherent Doppler lidar data based on a U-Net convolutional neural network.

The coherent Doppler wind lidar (CDWL) has long been thought to be the most suitable technique for wind remote sensing in the atmospheric boundary layer (ABL) due to its compact size, robust performance, and low-cost properties. However, as the coherent lidar exploits the Mie scattering from aerosol particles, the signal intensity received by the lidar is highly affected by the concentration of aerosols. Unlike air molecules, the concentration of aerosol varies greatly with time and weather, and decreases dramatically with altitude. As a result, the performance of the coherent lidar fluctuates greatly with time, and the detection range is mostly confined within the planetary boundary layer. The original data collected by the lidar are first transformed into a spectrogram and then processed into radial wind velocities utilizing algorithms such as a spectral centroid. When the signal-to-noise ratio (SNR) is low, these classic algorithms fail to retrieve the wind speed stably. In this work, a radial wind velocity retrieving algorithm based on a trained convolutional neural network (CNN) U-Net is proposed for denoising and an accurate estimate of the Doppler shift in a low-SNR regime. The advantage of the CNN is first discussed qualitatively and then proved by means of a numerical simulation. Simulated spectrum data are used for U-Net training and testing, which show that the U-Net is not only more accurate than the spectral centroid but also achieves a further detection range. Finally, joint observation data from the lidar and radiosonde show excellent agreement, demonstrating that the U-Net-based retrieving algorithm has superior performance over the traditional spectral centroid method both in accuracy and detection range.

Lidar system with a fast scanning speed for sea fog detection.

Sea fog changes widely and rapidly, and existing Lidar scanning speeds are insufficient to detect such changes. Therefore, we developed a Lidar system with a fast scanning speed and long detection distance. Experimental results show that at high scanning speeds, the maximum correlation between the Lidar's visibility results and those from two forward scattering visibility meters reaches 0.9537, with a minimum relative error less than 15.31%. The results also show that the visibility of the proposed Lidar system has high accuracy when fast scanning. During the tests, the Lidar system successfully captured sea fog many times and closely tracked the changes of sea visibility, which verifies the feasibility and reliability of the developed Lidar system for obtaining visibility measurements and sea fog detection.

Metastable helium Faraday filter for helium lidar to measure the density of the thermosphere.

We demonstrate a metastable helium Faraday optical filter operating on the 23S1 - 23P1 and 23S1 - 23P2 transition at 1083 nm by using a 3 cm long helium cell. The influence of the magnetic field and gas pressure of the helium cell on the filter characteristics is experimental studied. When the magnetic field is 230 Gs and the gas pressure of helium cell is about 110 Pa, the peak transmission corresponding to the two energy level transitions is about 32% and 57%, respectively. The equivalent noise bandwidth (ENBW) under this working condition is about 1.9 GHz. The metastable helium Faraday filter can be used to improve the optical inefficiency of a helium resonance fluorescence lidar to achieve the metastable helium density detection at 200-1000 km thermosphere.

Parameter optimization of a visibility LiDAR for sea-fog early warnings.

Sea fog represents a significant risk for safe navigation of sea vessels. Visibility LiDAR systems might offer a striking way to reduce the risks associated with sea fog, but they should be appropriately designed to provide a proper level of detection for reliable forewarning of sea fog. Here we analyze the performances of a visibility LiDAR system with the aim of achieving optimal detection operation. A series of echo signals are simulated under different visibility conditions addressing the influence of the various hardware parameters on the final system performances and defining an optimal visibility LiDAR configuration. Using the optimized parameters, a visibility LiDAR system was realized and tested in a field campaign on Hengsha Island (Shanghai). The experimental findings obtained by the visibility LiDAR are compared with results of a forward scattering visibility meter showing good consistency in homogeneous atmosphere, while even superior performances are observed for inhomogeneous atmospheric conditions. Our experimental results indicate that an optimized visibility LiDAR can provide an early warning for light fog located at a distance of 5 km, i.e. about 3.5 hours in advance to the spreading of the fog to the shore. These findings demonstrate the good performances of the visibility LiDAR developed in the present study in performing visibility measurements and its capability of providing sea-fog warning.

Ultraviolet trifrequency Rayleigh DWL for stratosphere atmospheric wind measurements during the daytime based on an ultranarrow-bandwidth optical receiver.

An ultranarrow-bandwidth-optical-receiver-based ultraviolet trifrequency Rayleigh Doppler wind lidar (DWL) technology is proposed that is able to simultaneously detect stratospheric wind with high precision during the daytime. The lidar system is designed, and the principle of wind measurement is analyzed. An ultranarrow-bandwidth element used for suppressing strong background light is designed as an important part of the ultranarrow-bandwidth optical receiver. A three-channel Fabry-Perot interferometer (FPI) is capable of measuring wind speed. A non-polarized beam splitter cube optically contacted on the three-channel FPI can offer a stable splitting ratio. The parameters of the three-channel FPI are optimized. The structure and parameters of the ultranarrow-bandwidth element are designed, and the transmission curve is measured. The transmission curve and stability of the three-channel FPI are validated. The background photon number is collected with the ultranarrow-bandwidth element and with an interference filter (IF) alternately from 08:00 to 18:00. Based on the selected system parameters and measured background photon number, the detection performance of the proposed lidar is simulated. Simulation results show that with 200 m range resolution from 15 to 25 km, 500 m range resolution from 25 to 40 km, and 30 min total accumulation time for paired line-of-sight (LOS) measurement, within $\pm {100}\;{\rm m/s}$±100m/s LOS wind speed range, the daytime LOS wind speed error is below 4.77 m/s from 15 to 40 km altitude. Compared with the traditional IF-based dual-FPI Rayleigh Doppler lidar, the wind speed accuracies are improved by 1.29-16.29 times and the detection altitudes are improved from 23.55 to 40 km with the same wind-detecting precision.

Fine gust front structure observed by coherent Doppler lidar at Lanzhou Airport (103°49$

Coherent Doppler lidar (CDL) has long been used to automatically identify gust front-induced wind shear signatures from the velocity data, but rare attention has been given to the fine structure of wind gust fronts. In this work, a compact and robust CDL with high efficiency and accuracy is equipped at Lanzhou Airport (103°49$^{\prime}$'E, 36°03$^{\prime}$'N) to conduct interpretation of wind gust front structures by using high-resolution CDL data. Outflows of gust fronts could be detected reliably from radial velocities, spectral widths, as well as radial shears. For the case study presented here, photographs of the velocity and spectrum width capacitates gust front characteristics such as height, advance speed, and radial shear, as well as vertical structure to be displayed in minute detail. Besides, the quasi-continuous vertical wind reveals the potential turbulent mixing and vertical transport process during the gust front event, which makes CDL a very attractive and essential technique for future development of gust front automatic detection systems.

Urban air pollution monitoring using scanning Lidar.

We have developed a scanning Lidar system in this work to detect urban air pollution changes in real time and locate the sources of urban air pollution. We first proposed an algorithm to retrieve atmospheric extinction coefficients, which we used to create Lidar maps. Using Lidar map of the average extinction coefficients, we identified the locations of the local maximum values, and hence, the positions of the urban air pollution sources. Experimental results indicate that this method is effective for urban air pollution monitoring.

Demonstration of daytime wind measurement by using mobile Rayleigh Doppler Lidar incorporating cascaded Fabry-Perot etalons.

A well-designed filter assembly is incorporated to an earlier mobile Rayleigh Doppler Lidar developed at University of Science and Technology of China (USTC) for wind measurement round the clock. The filter assembly consists of two cascaded Fabry-Perot Etalons (FPEs) and a narrow-band interference filter (IF), which are optimized to filter out strong solar background radiation during daytime. The high resolution FPE is mainly used to compress the whole bandwidth of the filter assembly, whereas the low resolution FPE with relatively large free spectral range (FSR) is primarily used to block the unwanted periodic transmission peaks of high resolution FPE arising within the narrow-band IF passband. Some test experiments are carried out and demonstrate that the filter assembly have an overall peak transmission of 33.32% with a bandwidth of 2.41 pm at 355 nm. When applying it to the USTC mobile Rayleigh Doppler Lidar, the daytime background is only 3% or less than before. Consequently, the detectable altitude during daytime increases to ∼51 km with wind velocity accuracy of ±7.6 m/s.

Novel Lidar algorithm for horizontal visibility measurement and sea fog monitoring.

Traditionally, Klett and Fernald inversion estimates an initial value using the slope method for horizontal visibility, which causes inversion uncertainty. We proposed an algorithm to retrieve the extinction coefficient and visibility distribution information from scanning Lidar to overcome instability due to initial atmospheric extinction coefficient choice and assuming the Lidar ratio. Numerical simulations showed that extinction coefficient maximum relative was much larger for inhomogeneous atmosphere using the Klett method, reaching 0.31. In contrast, it is only 0.049 using the proposed algorithm. Experimental showed that the proposed algorithm and scanning Lidar system provide very high stability and accuracy, can work in different weather conditions and monitor sea fog evolution over real time, and is suitable for various situations with different visibility.

Development and accuracy of a multipoint method for measuring visibility.

Accurate measurements of visibility are of great importance in many fields. This paper reports a multipoint visibility measurement (MVM) method to measure and calculate the atmospheric transmittance, extinction coefficient, and meteorological optical range (MOR). The relative errors of atmospheric transmittance and MOR measured by the MVM method and traditional transmissometer method are analyzed and compared. Experiments were conducted indoors, and the data were simultaneously processed. The results revealed that the MVM can effectively improve the accuracy under different visibility conditions. The greatest improvement of accuracy was 27%. The MVM can be used to calibrate and evaluate visibility meters.

Gravity waves observation of wind field in stratosphere based on a Rayleigh Doppler lidar.

Simultaneous wind and temperature measurements in stratosphere with high time-spatial resolution for gravity waves study are scarce. In this paper we perform wind field gravity waves cases in the stratosphere observed by a mobile Rayleigh Doppler lidar. This lidar system with both wind and temperature measurements were implemented for atmosphere gravity waves research in the altitude region 15-60 km. Observations were carried out for two periods of time: 3 months started from November 4, 2014 in Xinzhou, China (38.425°N,112.729°E) and 2 months started from October 7, 2015 in Jiuquan, China (39.741°N, 98.495°E) . The mesoscale fluctuations of the horizontal wind velocity and the two dimensional spectra analysis of these fluctuations show the presence of dominant oscillatory modes with wavelength of 4-14 km and period of around 10 hours in several cases. The simultaneous temperature observations make it possible to identify gravity wave cases from the relationships between different variables: temperature and horizontal wind. The observed cases demonstrate the Rayleigh Doppler Lidar's capacity to study gravity waves.

Stratospheric temperature measurement with scanning Fabry-Perot interferometer for wind retrieval from mobile Rayleigh Doppler lidar.

Temperature detection remains challenging in the low stratosphere, where the Rayleigh integration lidar is perturbed by aerosol contamination and ozone absorption while the rotational Raman lidar is suffered from its low scattering cross section. To correct the impacts of temperature on the Rayleigh Doppler lidar, a high spectral resolution lidar (HSRL) based on cavity scanning Fabry-Perot Interferometer (FPI) is developed. By considering the effect of the laser spectral width, Doppler broadening of the molecular backscatter, divergence of the light beam and mirror defects of the FPI, a well-behaved transmission function is proved to show the principle of HSRL in detail. Analysis of the statistical error of the HSRL is carried out in the data processing. A temperature lidar using both HSRL and Rayleigh integration techniques is incorporated into the Rayleigh Doppler wind lidar. Simultaneous wind and temperature detection is carried out based on the combined system at Delhi (37.371°N, 97.374°E; 2850 m above the sea level) in Qinghai province, China. Lower Stratosphere temperature has been measured using HSRL between 18 and 50 km with temporal resolution of 2000 seconds. The statistical error of the derived temperatures is between 0.2 and 9.2 K. The temperature profile retrieved from the HSRL and wind profile from the Rayleigh Doppler lidar show good agreement with the radiosonde data. Specifically, the max temperature deviation between the HSRL and radiosonde is 4.7 K from 18 km to 36 km, and it is 2.7 K between the HSRL and Rayleigh integration lidar from 27 km to 34 km.

Mobile Rayleigh Doppler lidar for wind and temperature measurements in the stratosphere and lower mesosphere.

A mobile Rayleigh Doppler lidar based on the molecular double-edge technique is developed for measuring wind velocity in the middle atmosphere up to 60 km. The lidar uses three lasers with a mean power of 17.5 W at 355 nm each and three 1 m diameter telescopes to receive the backscattered echo: one points to zenith for vertical wind component and temperature measurement; the two others pointing toward east and north are titled at 30° from the zenith for zonal and meridional wind component, respectively. The Doppler shift of the backscattered echo is measured by inter-comparing the signal detected through each of the double-edge channels of a triple Fabry-Perot interferometer (FPI) tuned to either side of the emitted laser line. The third channel of FPI is used for frequency locking and a locking accuracy of 1.8 MHz RMS (root-mean-square) at 355 nm over 2 hours is realized, corresponding to a systematic error of 0.32 m/s. In this paper, we present detailed technical evolutions on system calibration. To validate the performance of the lidar, comparison experiments was carried out in December 2013, which showed good agreement with radiosondes but notable biases with ECMWF (European Centre for Medium range Weather Forecasts) in the height range of overlapping data. Wind observation over one month performed in Delhi (37.371° N, 97.374° E), northwest of China, demonstrated the stability and robustness of the system.

Mid-altitude wind measurements with mobile Rayleigh Doppler lidar incorporating system-level optical frequency control method.

A mobile Rayleigh Doppler lidar based on double-edge technique is developed for mid-altitude wind observation. To reduce the systematic error, a system-level optical frequency control method is proposed and demonstrated. The emission of the seed laser at 1064 nm is used to synchronize the FPI in the optical frequency domain. A servo loop stabilizing the frequency of the seed laser is formed by measuring the absolute frequency of the second harmonic against an iodine absorption line. And, the third harmonic is used for Rayleigh lidar detection. The frequency stability is 1.6 MHz at 1064 nm over 2 minutes. A locking accuracy of 0.3 MHz at 1064 nm is realized. In comparison experiments, wind profiles from the lidar, radiosonde and European Center for Medium range Weather Forecast (ECMWF) analysis show good agreement from 8 km to 25 km. Wind observation over two months is carried out in Urumqi (42.1°N, 87.1°E), northwest of China, demonstrating the stability and robustness of the system. For the first time, quasi-zero wind layer and dynamic evolution of high-altitude tropospheric jet are observed based on Rayleigh Doppler lidar in Asia.

[Structures and bioactivity of polysaccharides from isatidis radix].

OBJECTIVE: To investigated the chemical structures and bioactivity of polysaccharides from Isatidis Radix. METHOD: Polysaccharides were extracted and purified by column chromatograph and their chemical structures were identified by UV, IR, NMR, periodic acid oxadation and Smith degradation method and their stimulation effects to macrophage were evaluated by using MTT method. RESULT: Five polysaccharides, polysaccharide A , B, C, D and E were gotten and their molecular weights were 2 000, 1 757.1, 1 34 2.7, 955.6, 11.7 kDa, respectively. Polysaccharide A was composed of arabinose, polysaccharide E was composed of arabinose and galactose, polysaccharides B, C, D were composed of glucose and 1 --> 2, 1 --> 3, 1 --> 4, 1 --> 6 linkages existed in polysaccharides A-E, of A, B, C, D, E were alpha-configurations. Polysaccharides B, C and D showed better bioactivity than polysaccharides A and E with stimulation index (SI) of 5.31, 4.76, 5.17. CONCLUSION: Five polysaccharides are seperated firstly from Isatidis Radix.

Edge technique for direct detection of strain and temperature based on optical time domain reflectometry.

A hybrid technique for real-time direct detection of strain and temperature along a single-mode fiber is proposed. The temperature is directly detected from the Raman backscattering in the time domain. To retrieve the strain profile from the Brillouin backscattering, an edge technique is introduced and a response function of the Fabry-Perot interferometer for the Brillouin backscattering is defined for the first time to our knowledge. The outgoing laser and the Brillouin backscattering are measured on different interference orders through different channels of the Fabry-Perot interferometer. A low-resolution reference channel and a high-resolution Brillouin channel are designed to keep both a high measurement sensitivity and a wide dynamic range. The measurement is based on detecting the bandwidth changes and the frequency shifts of the Brillouin backscattering; thus the resulting measurement is insensitive to the power fluctuation of the backscattering and the laser frequency jitter or drift. Neither time-consuming frequency scanning nor heavy data processing is needed, which makes real-time detection possible. The dynamic range of the edge technique can be increased substantially by using a piezoelectric tunable and capacitive-servo-stabilized Fabry-Perot interferometer. We highlight the potential of this technique by numerical simulations. Given that the uncertainty of the temperature measurement is 0.5 degrees C and that the spatial and temporal resolutions are 10 cm and 1 s, the strain uncertainty is less than 20 microepsilon within a 2 km distance when the strain is below 0.4%, and it is not more than 110 microepsilon within a 4 km distance when the strain is below 0.6%.

Fabry-Perot interferometer based Mie Doppler lidar for low tropospheric wind observation.

Similar in principle to recent implementations of a lidar system at 355 nm [Opt. Lett. 25, 1231 (2000), Appl. Opt. 44, 6023 (2005)], an incoherent-detection Mie Doppler wind lidar at 1064 nm was developed and deployed in 2005 [Opt. Rev. 12, 409 (2005)] for wind measurements in the low troposphere, taking advantage of aerosol scattering for signal enhancement. We present a number of improvements made to the original 1064 nm system to increase its robustness for long-period operation. These include a multimode fiber for receiving the reference signal, a mode scrambler to allow uniform illumination over the Fabry-Perot interferometer, and a fast scannable Fabry-Perot interferometer for calibration and for the determination of outgoing laser frequency during the wind observation. With these improvements in stability, the standard deviation of peak transmission and FWHM of the Fabry-Perot interferometer was determined to be 0.49% and 0.36%, respectively. The lidar wind measurements were validated within a dynamic range of +/-40 m/s. Comparison experiments with both wind profiler radar and Vaisala wiresonde show good agreement with expected observation error. An example of 24 h continuous observations of wind field and aerosol backscatter coefficients in the boundary layer with 1 min and 30 m temporal and spatial resolution and 3 m/s tolerated wind velocity error is presented and fully demonstrates the stability and robustness of this lidar.