Reconstruction for beam blockage of lidar based on generative adversarial networks.

Doppler lidar is an active laser remote sensing instrument. However, beam blockage caused by low-altitude obstacles is a critical factor affecting the quality of lidar data. To reconstruct the line of sight velocities (LOSV) in areas with beam blockages and to evaluate the effectiveness of reconstruction results, the LOSV-filling network (LFnet) approach based on generative adversarial networks (GANs) and an evaluation scheme based on the degree of blockage are proposed in this paper. The LFnet comprises two adversarial models. The first adversarial model captures the structural features of LOSV to output the edge map, and the second adversarial fills in the blockage area using the edge map. We have built a packaged dataset consisting of training, validation and test datasets with mask sets. Then the sensitivity of the reconstruction effectiveness with different shielding conditions is studied, to reveal the mechanism of shielding influencing the reconstruction. A series of indicators were used to evaluate the model's performance, including the traditional indicators and the proposed indicator of root mean square error (RMSE). Finally, LFnet was demonstrated in a practical application in an airport. The complete process of an easterly gust front is reconstructed with RMSE less than 0.85 m/s, which has significance for flight safety.

Three-dimensional detection of CO2 and wind using a 1.57 µm coherent differential absorption lidar.

A 1.57-µm coherent differential absorption lidar is demonstrated for measuring three-dimensional CO2 and wind fields simultaneously. The maximum detection range of CO2 is up to 6 km with a range resolution of 120 m and a time resolution of 1 min. A preliminary assessment of instrument performance is made with a 1-week continuous observation. The CO2 concentration over a column from 1920 to 2040 m is compared with the one measured by an optical cavity ring-down spectrometer placed on a 2 km-away meteorological tower. The concentration is strongly correlated with the in-situ spectrometer with a correlation coefficient and RMSE of 0.91 and 5.24 ppm. The measurement accuracy of CO2 is specified with a mean and standard deviation of 2.05 ppm and 7.18 ppm, respectively. The regional CO2 concentration and the three-dimensional wind fields are obtained through different scanning modes. Further analysis is conducted on vertical mixing and horizontal transport of CO2 by combining with the measured wind fields.

Weak signal detection technique based on Durbin-Watson test and one-bit sampling.

Correlation-based detection techniques are widely used in the weak periodic signal detection field. Traditionally, they are based on extracting the correlation of a weak signal from noise. Considering the impact of a weak signal on the randomness of background noise, this article takes the opposite approach and proposes a weak signal detection technique based on the Durbin-Watson (DW) test and one-bit sampling, detecting the weak signal due to the extent to which the randomness of noise is affected. The randomness of noise is analyzed through the DW test, which is a method for detecting the randomness of data sequences through first-order autocorrelation. One-bit sampling is adopted to reduce the complexity of the sampling circuit and data processing algorithm. The effectiveness of the DW test in the situation of one-bit sampling is demonstrated through simulation and analysis. Simulation results show that the proposed technique is capable of detecting weak sinusoidal and square-wave signals with a signal-to-noise ratio (SNR) above -30 dB, and the frequency or SNR of a weak signal can be further estimated based on mutual constraints. The measured results confirm the capability. In addition, the factors of coherent sampling, noise bandwidth, and comparator threshold that influence the performance of the proposed technique are simulated and discussed in detail.

Estimation of atmospheric refractive index structure constant using an InGaAs/InP single-photon detector.

Remote sensing of atmospheric refractive index structure constant ($\boldsymbol{C}_{\boldsymbol{n}}^2$) using lidar incorporating a single-photon detector (SPD) is proposed. The influence of turbulence on the fiber coupling efficiency with different fiber modes is analyzed. $\boldsymbol{C}_{\boldsymbol{n}}^2$ can be derived from the ratio of the backscattering signals counted on single-mode and multimode fiber-coupling channels of the SPD. In the experiment, by eliminating the shot noise effect on the fluctuation of the ratio, the lowest coupling ratio is used to retrieve $\boldsymbol{C}_{\boldsymbol{n}}^2$ and demonstrated by comparing to the results measured from a large aperture scintillometer (LAS). Good agreement between results from the LAS and the lidar is achieved. The correlation coefficients are 0.90, 0.89, and 0.89, under three different weather conditions.

Stratospheric aerosol lidar with a 300†µm diameter superconducting nanowire single-photon detector at 1064†nm.

Stratospheric aerosols play an important role in the atmospheric chemical and radiative balance. To detect the stratospheric aerosol layer, a 1064 nm lidar with high resolution and large dynamic range is developed using a superconducting nanowire single-photon detector (SNSPD). Measurements are typically performed at 1064 nm for its sensitivity to aerosol, whereas detectors are limited by low efficiency and high dark count rate (DCR). SNSPDs are characterized by high efficiency in the infrared wavelength domain, as well as low noise and dead time, which can significantly enhance the signal quality. However, it is still challenging to build an SNSPD with both large active area and high count rate. To improve the maximal count rate (MCR) so as to avoid saturation in the near range, a 16-pixel interleaved SNSPD array and a multichannel data acquisition system are developed. As a reference, a synchronous system working at 532 nm is applied. In a continuous comparison experiment, backscatter ratio profiles are retrieved with resolutions of 90 m/3 min, and the 1064 nm system shows better performance, which is sensitive to aerosols and immune to the contamination of the ozone absorption and density of molecule change in the lower stratosphere.

Suppression of crosstalk in coding CDWL by active FOV modulation with a deformable mirror.

Coding technology provides new ideas for spatial resolution enhancement of coherent Doppler wind lidar (CDWL). To improve the performance of coding CDWL for ultra-fine-wind field detection, the crosstalk between neighboring laser pulses is analyzed in theory. The strong backscattered signal from aerosols in near field region will interfere with the weak atmospheric signal, making the accuracy of Doppler shift estimation deteriorate seriously. Considering the formation mechanism of crosstalk, a solution based on adaptive field of view (FOV) modulation is proposed to suppress the crosstalk which is validated by numerical simulation and experiment. Dynamic range of the backscatter intensity is controlled from 10 dB to 2 dB within the distance of 50 m to 300 m, thus the crosstalk is accordingly suppressed.

Dark/bright band of a melting layer detected by coherent Doppler lidar and micro rain radar.

Observation of a melting layer using a 1.55 µm coherent Doppler lidar (CDL) is first presented during a stratiform precipitation event. Simultaneous radar measurements are also performed by co-located 1.24 cm micro rain radar (MRR) and 10.6 cm Doppler weather radar (DWR). As a well-known bright band in radar reflectivity appears during precipitation, an interesting dark band about 160 m below that in lidar backscattering is observed. Due to the absorption effect, the backscattering from raindrops at 1.55 µm is found much weaker than that at short wavelengths usually used in direct detection lidars. However, the CDL provides additional Doppler information which is helpful for melting layer identification. For example, a spectrum bright band with broadened width and sign conversion of skewness is detected in this case. After a deep analysis of the power spectra, the aerosol and precipitation components are separated. The fall speed of hydrometeors given by CDL is found smaller than that of MRR, with the differences of approximately 0.5 m/s and 1.5 m/s for the snow and rainfall, respectively. To illustrate the influence of absorption effect, simulations of the backscatter coefficient and extinction coefficient of aerosol and rainfall are also performed at the wavelength range of 0.3 ∼ 2.2 µm using the Mie theory.

Spatial resolution enhancement of coherent Doppler wind lidar using differential correlation pair technique.

A high spatial resolution coherent Doppler wind lidar (CDWL) incorporating the differential correlation pair (DCP) technique is proposed and demonstrated. By employing pulse pair with appropriate window functions, the spatial resolution can be enhanced, as the common parts of the correlation pair can be eliminated in the differential data processing. The performance of the new method is validated in the comparison experiment with the CDWLs adopting conventional schemes. Under a given peak power, the DCP technique provides higher wind velocity accuracy compared with a conventional pulsed CDWL where the laser spectral broadening caused by short pulses can be avoided and the carrier-to-noise ratio is improved. At a laser peak power of 250 W, with a spatial and temporal resolution of 3.3 m and 1 s, continuous radial wind profiling over 700 m is realized with a maximum error of 0.1 m/s.

Photon-counting distributed free-space spectroscopy.

Spectroscopy is a well-established nonintrusive tool that has played an important role in identifying and quantifying substances, from quantum descriptions to chemical and biomedical diagnostics. Challenges exist in accurate spectrum analysis in free space, which hinders us from understanding the composition of multiple gases and the chemical processes in the atmosphere. A photon-counting distributed free-space spectroscopy is proposed and demonstrated using lidar technique, incorporating a comb-referenced frequency-scanning laser and a superconducting nanowire single-photon detector. It is suitable for remote spectrum analysis with a range resolution over a wide band. As an example, a continuous field experiment is carried out over 72 h to obtain the spectra of carbon dioxide (CO2) and semi-heavy water (HDO, isotopic water vapor) in 6 km, with a range resolution of 60 m and a time resolution of 10 min. Compared to the methods that obtain only column-integrated spectra over kilometer-scale, the range resolution is improved by 2-3 orders of magnitude in this work. The CO2 and HDO concentrations are retrieved from the spectra acquired with uncertainties as low as ±1.2% and ±14.3%, respectively. This method holds much promise for increasing knowledge of atmospheric environment and chemistry researches, especially in terms of the evolution of complex molecular spectra in open areas.

Remote sensing of raindrop size distribution using the coherent Doppler lidar.

The coherent Doppler wind lidar (CDL) shows capability in precipitation detection. Retrieval of the raindrop size distribution (DSD) using CDL is still challenging work, as both accurate backscattering cross section at the working wavelength and reflectivity spectrum of raindrop are required. Firstly, the Mie theory and the vectorial complex ray model (VCRM) are applied to calculate backscattering cross section for small spheric raindrops and large oblate raindrops, respectively. Secondly, an iterative deconvolution method is proposed to separate the reflectivity spectrum of raindrop from the lidar power spectrum, which is a superposition of rain and aerosol components. An accompanying aerosol signal model considering the effect of temporal window, from the same height and time, is used to improve the accuracy and robustness of the iteration. In experiment, a co-located micro rain radar (MRR) is used for comparison. Good agreements are obtained despite tremendous differences in wavelength and scattering characteristics. As an example, at 600 m height, the R2 of linear fitting to the mean rain velocity and mean raindrop diameter between CDL and MRR are 0.96 and 0.93, respectively.

Multi-frequency differential absorption lidar incorporating a comb-referenced scanning laser for gas spectrum analysis.

A multi-frequency differential absorption lidar incorporating a tunable laser and an optical frequency comb is demonstrated for precise spectrum analysis of atmospheric gas. The single frequency tunable laser is stabilized by locking to the optical frequency comb, with a standard deviation of 0.5 MHz. To achieve a high signal-to-noise ratio, a multi-mode superconducting nanowire single-photon detector with an active-area diameter of 50 µm, a quantum efficiency of 31.5%, and dark noise of 100 counts per second is implemented, which enables to avoid the need for high energy lasers. In the experiment, the range-resolved spectrum of atmospheric mixture gases (CO2 and HDO) in a region of 1572.2 - 1572.45 nm is obtained. Results show different partially overlapped absorption of two gases in different seasons, with a stronger influence of HDO on CO2 in summer than in winter. The interactions are taken into account by separating the mixture absorption spectrum (one CO2 line and two HDO lines) with triple-peak Voigt fitting. The retrieved concentrations over 6 km with a range resolution of 120 m and a time resolution of 10 min are compared with in-situ sensors. The uncertainties of the retrieved concentrations are as low as 6.5 µmol/mol (ppm) and 1×10-3 g/kg for CO2 and HDO, respectively.

Identifying cloud, precipitation, windshear, and turbulence by deep analysis of the power spectrum of coherent Doppler wind lidar.

Researches on the atmospheric boundary layer (ABL) need accurate measurements with high temporal and spatial resolutions from a series of different instruments. Here, a method for identifying cloud, precipitation, windshear, and turbulence in the ABL using a single coherent Doppler wind lidar (CDWL) is proposed and demonstrated. Based on deep analysis of the power spectrum of the backscattering signal, multiple lidar products, such as carrier-to-noise (CNR), spectrum width, spectrum skewness, turbulent kinetic energy dissipation rate (TKEDR), and shear intensity are derived for weather identification. Firstly, the cloud is extracted by Haar wavelet covariance transform (HWCT) algorithm based on the CNR after range correction. Secondly, since the spectrum broadening may be due to turbulence, windshear or precipitation, the spectrum skewness is introduced to distinguish the precipitation from two other conditions. Whereas wind velocity is obtained by single peak fitting in clear weather condition, the double-peak fitting is used to retrieve wind and rainfall velocities simultaneously in the precipitation condition. Thirdly, judging from shear intensity and TKEDR, turbulence and windshear are classified. As a double check, the temporal continuity is used. Stable wind variances conditions such as low-level jets are identified as windshear, while arbitrary wind variances conditions are categorized as turbulence. In the field experiment, the method is implemented on a micro-pulse CDWL to provide meteorological services for the 70th anniversary of the China's National Day, in Inner Mongolia, China (43°54'N, 115°58'E). All weather conditions are successfully classified. By comparing lidar results to that of microwave radiometer (MWR), the spectrum skewness is found be more accurate to indicate precipitation than spectrum width or vertical speed. Finally, the parameter relationships and distributions are analyzed statistically in different weather conditions.

Inversion probability enhancement of all-fiber CDWL by noise modeling and robust fitting.

Accurate power spectrum analysis of weak backscattered signals are the primary constraint in long-distance coherent Doppler wind lidar (CDWL) applications. To study the atmospheric boundary layer, an all-fiber CDWL with 300µJ pulse energy is developed. In principle, the coherent detection method can approach the quantum limit sensitivity if the noise in the photodetector output is dominated by the shot noise of the local oscillator. In practice, however, abnormal power spectra occur randomly, resulting in error estimation and low inversion probability. This phenomenon is theoretically analyzed and shown to be due to the leakage of a time-varying DC noise of the balanced detector. Thus, a correction algorithm with accurate noise modeling is proposed and demonstrated. The accuracy of radial velocity, carrier-to-noise ratio (CNR), and spectral width are improved. In wind profiling process, a robust sine-wave fitting algorithm with data quality control is adopted in the velocity-azimuth display (VAD) scanning detection. Finally, in 5-day continuous wind detection, the inversion probability is tremendously enhanced. As an example, it is increased from 8.6% to 52.1% at the height of 4 km.

Simultaneous wind and rainfall detection by power spectrum analysis using a VAD scanning coherent Doppler lidar.

Doppler wind lidar is an effective tool for wind detection with high temporal and spatial resolution. However, precise wind profile measurement under rainy conditions is a challenge, due to the interfering signals from raindrop reflections. In this work, a compact all-fiber coherent Doppler lidar (CDL) at working wavelength of 1.5 µm is applied for simultaneous wind and precipitation detection. The performance of the lidar is validated by comparison with the weather balloons. Thanks to the ability of precise spectrum measurement, both aerosol and rainfall signals can be detected by the CDL under rainy conditions. The spectrum width is used to identify the precipitation events, during which the two-peak Doppler spectrum is observed. The spectrum is fitted by a two-component Gaussian model and two velocities are obtained. By using the velocity-azimuth display (VAD) scanning technique, wind speed and rainfall speed are simultaneously retrieved. The false detection probability of wind speed in the rainy conditions is thus reduced.

Meter-scale spatial-resolution-coherent Doppler wind lidar based on Golay coding.

Generally, the pulse duration of a coherent Doppler wind lidar (CDWL) is shortened to minimize the spatial resolution at the sacrifice of carrier-to-noise ratio, since the peak power of a laser source is limited by the stimulated Brillouin scattering or other nonlinear optical phenomena. To solve this problem, an all-fiber CDWL incorporating Golay coding is proposed and demonstrated. Given the peak power of the laser pulse, the Golay coding method can improve the measuring precision by improving the pulse repetition frequency of the outgoing laser. In the experiment, the Golay coding implementation is optimized by normalizing the intensity of every single pulse of the outgoing laser with a closed-loop feedback, achieving a spatial resolution of 6 m and a temporal resolution of 2 s with a maximum detection range of 552 m. The wind profile in line of sight and the result derived from another noncoding CDWL show good agreement.

Compact and lightweight 1.5 µm lidar with a multi-mode fiber coupling free-running InGaAs/InP single-photon detector.

We present a compact and lightweight 1.5 µm lidar using a free-running single-photon detector (SPD) based on a multi-mode fiber (MMF) coupling InGaAs/InP negative feedback avalanche diode. The ultimate light detection sensitivity of SPD highly reduces the power requirement of the laser, whilst the enhanced collection efficiency due to MMF coupling significantly reduces the volume and weight of telescopes. We develop a specific algorithm for the corrections of errors caused by the SPD and erbium-doped fiber amplifier to extract accurate backscattering signals. We also perform a comparison between single-mode fiber (SMF) coupling and MMF coupling in the lidar receiver, and the results show that the collection efficiency with MMF coupling is five times higher than that with SMF coupling. In order to validate the functionality, we use the lidar system for the application of cloud detection. The lidar system exhibits the ability to detect both the cloud base height and the thickness of multi-layer clouds to an altitude of 12 km with a temporal resolution of 1 s and a spatial resolution of 15 m. Due to the advantages of compactness and lightweight, our lidar system can be installed on unmanned aerial vehicles for wide applications in practice.

Micro-pulse polarization lidar at 1.5 µm using a single superconducting nanowire single-photon detector.

An all-fiber, eye-safe and micro-pulse polarization lidar is demonstrated with a polarization-maintaining structure, incorporating a single superconducting nanowire single-photon detector (SNSPD) at 1.5 µm. The time-division multiplexing technique is used to achieve a calibration-free optical layout. A single piece of detector is used to detect the backscatter signals at two orthogonal states in an alternative sequence. Thus, regular calibration of the two detectors in traditional polarization lidars is avoided. The signal-to-noise ratio of the lidar is guaranteed by using an SNSPD, providing high detection efficiency and low dark count noise. The linear depolarization ratio (LDR) of the urban aerosol is observed horizontally over 48 h in Hefei [N31°50'37'', E117°15'54''], when a heavy air pollution is spreading from the north to the central east of China. Phenomena of LDR bursts are detected at a location where a building is under construction. The lidar results show good agreement with the data detected from a sun photometer, a 532 nm visibility lidar, and the weather forecast information.

1.5 µm polarization coherent lidar incorporating time-division multiplexing.

Atmospheric depolarization ratio and wind velocity are measured simultaneously by a single versatile coherent Doppler lidar (CDL). Backscattering components at parallel and perpendicular polarization states are obtained by using a single balanced detector, adopting time-division multiplexing technique. Thus systematic error induced by the non-uniform response of different detectors in traditional lidars is avoided. The operation mode of the instrument can be switched from polarization CDL to traditional CDL by the user depending on atmospheric conditions and desired performance. As demonstrated, the perpendicular component of the backscattering, usually wasted, not only can be used to retrieve the ADR, but also can be used to improve the carrier to noise ratio in wind detection. In the traditional mode, given a tolerance of 0.5 m/s precision, a detection range of 6 km is achieved by using a 300 ns laser pulse with energy of 100 µJ, where the temporal and spatial resolution of 2 s and 60 m, respectively. Continuous wind detection of the atmospheric boundary layer over 26 hours is presented to demonstrate the robustness and stability of the system. Dynamic evolution and wind structure are recorded.

Dual-frequency Doppler lidar for wind detection with a superconducting nanowire single-photon detector.

A dual-frequency direct detection Doppler lidar is demonstrated using a superconducting nanowire single-photon detector (SNSPD) at 1.5 µm. The so-called double-edge technique is implemented by using a dual-frequency laser pulse, rather than using a double-channel Fabry-Perot interferometer. Such a modification to the reported lidars enhances the frequency stability in the system level. Using the time-division multiplexing method, only one piece of SNSPD is used in the optical receiver, making the system simplified and robust. The SNSPD is adopted to enhance the temporal resolution since it offers merits of high quantum efficiency, low dark count noise, no after-pulsing probability, and a high maximum count rate. Two telescopes that point westward and northward at a zenith angle of 30° are used to detect the line-of-sight wind components, which are used to synthesize the horizontal wind profile. Horizontal wind profiles up to an altitude of about 2.7 km are calculated with vertical spatial/temporal resolution of 10 m/10 s. Wind dynamic evolution and vertical wind shears are observed clearly.

Fully integrated free-running InGaAs/InP single-photon detector for accurate lidar applications.

We present a fully integrated InGaAs/InP negative feedback avalanche diode (NFAD) based free-running single-photon detector (SPD) designed for accurate lidar applications. A free-piston Stirling cooler is used to cool down the NFAD with a large temperature range, and an active hold-off circuit implemented in a field programmable gate array is applied to further suppress the afterpulsing contribution. The key parameters of the free-running SPD including photon detection efficiency (PDE), dark count rate (DCR), afterpulse probability, and maximum count rate (MCR) are dedicatedly optimized for lidar application in practice. We then perform a field experiment using a Mie lidar system with 20 kHz pulse repetition frequency to compare the performance between the free-running InGaAs/InP SPD and a commercial superconducting nanowire single-photon detector (SNSPD). Our detector exhibits good performance with 1.6 Mcps MCR (0.6 µs hold-off time), 10% PDE, 950 cps DCR, and 18% afterpulse probability over 50 µs period. Such performance is worse than the SNSPD with 60% PDE and 300 cps DCR. However, after performing a specific algorithm that we have developed for afterpulse and count rate corrections, the lidar system performance in terms of range-corrected signal (Pr2) distribution using our SPD agrees very well with the result using the SNSPD, with only a relative error of ∼2%. Due to the advantages of low-cost and small size of InGaAs/InP NFADs, such detector provides a practical solution for accurate lidar applications.