KEYWORDS: Target detection, Photon counting, Detection and tracking algorithms, Signal to noise ratio, LIDAR, Time correlated single photon counting, Clutter, Tunable filters, Pulse signals, Mirrors
Ultra-long-distance range-finding is often affected by scattering noise, resulting in a low Signal-to-Noise Ratio (SNR) due to the extremely weak retro-reflected light. Due to the low SNR, it is difficult to determine the corresponding distance from the sensor during the final stage of signal processing. It is challenging to adjust the threshold to achieve both a high detection rate and a low false alarm rate. In this study, we propose an effective detector for locating low SNR targets in ultra-long-distance measurements. The detector is based on the Constant False Alarm Rate (CFAR) processing framework widely used in radar target detection. We applied this detector for signal processing in the photon counting system for long-range, small dynamic targets, which we reported in our earlier work. In-site experiments were conducted in Dunhuang, Gansu in September 2021. We successfully performed long-range laser ranging for 60km buildings and ultra-long-range laser ranging for 152km mountain. The results showed that the proposed CFAR detector could quickly and efficiently extract target locations that were difficult to detect using a detector based on constant thresholding.
Lidar is a promising active imaging technology that utilizes laser as the signal source to obtain accurate range, azimuth, speed, and other target characteristics, enabling 3D scene perception. Single photon lidar, using high sensitivity single photon detectors and high-frequency photon counting, has greatly improved the detection range of lidar. However, achieving large field of view and high resolution in single-photon 3D imaging remains a challenge. Therefore, we propose a long-distance single photon 3D imaging lidar based on dual wedge scanning, where the dual wedge scanning method is employed to extend the field of view, and the GM-APD array is utilized to improve imaging speed. In this work, we investigate the distribution characteristics of the point cloud in dual wedge scanning and propose a sub-pixel scanning-based 3D reconstruction and image processing method for point cloud. Through our approach, we successfully achieve large field of view, high resolution, and fast 3D imaging. Our method can be applied in a wide range of applications, including investigation, mapping, unmanned driving, and more.
The time-of-flight method is widely used to measure the range of the targets directly. As the retro-reflected light is so weak in ultra-long-distance measurement, the measurement technique based on the single-photon detector (SPD) and the time-correlated single-photon counting (TCSPC) has been engaged in earth-satellite measurements and altimetry measurements for airborne platforms. But this technique is difficult to applied to dynamic target measuring, especially when the target is moving fast under significant background noise. In this paper, we develop a photon counting system for long-range small dynamic target such as aircraft. This system is based on a dynamic TCSPC method based on point-line duality which has been reported in our early work. Outdoor experiments were conducted to evaluate the ranging ability of our system. The experimental results that the laser ranging system can detect the moving Quadrotor UAV 3.6km away (emission optical attenuation 42.2dB) and the building 21km away (emission optical attenuation 26dB). Through equivalent calculation, it is equivalent to the range measurement of aircraft over 60km. Meanwhile, the photon-counting range finding system could measure the range and velocity of the small target such as moving Quadrotor UAV simultaneously. In addition, all the measurements of our range finding system was in real time.
We proposed a long-distance underwater wireless optical communication with a single photon-counting system in a laboratory environment by using non-return to zero on-off keying modulation. The system successfully achieved a data rate of 1Mbps through a 15m PVC tube by adding in 80dB attenuator and using a pigtailed fiber 450nm blue laser diode. The water channel was measured to have an attenuation coefficient comparable to clean seawater. The experimental results show that under condition of only average 1.2 photons in single optical pulse, photon counting UOWC with a BER of 1. 6×10−4 and a baud rate of 1 Mbps can be achieved. Based on the measured minimum required power in the water channel, the transmission performance was predicted to be 140m and 310m in the clean seawater and pure seawater, respectively.
As a new type of active imaging technology, single photon counting 3D imaging has great application value in the fields of biophotonic imaging, remote sensing mapping, weak target detection, etc. Generally, in order to mitigate the influence of noise photons during the detection, it is necessary to accumulate hundreds of photons per pixel to form a histogram of arrival photons to obtain an accurate 3D image. However, in conditions of low illumination or long distances targets detecting, the available echo energy and the allowable integration time generally do not satisfy the collection of such large number of photons. Therefore, we research on single photon 3D imaging method based on a few echo photons, and several image reconstruction algorithm under strong background noise environment was proposed, including traditional methods based on pixels as well as newly proposed methods based on spatial correlation. We use the 3D point cloud signal to achieve the 3D reconstruction of the target reflectivity signal and distance information.
Compared with traditional Lidar, photon counting laser radar uses a high repetition rate and low pulse energy detection mechanism. When the echo photon is very weak, the echo signal can be extracted by increasing the counting time of TCSPC. However, it becomes difficult to detect fast-moving targets within a long counting time, especially under the interference of high background noise. In this article, we propose a dynamic ranging algorithm based on target motion parameter matching for the application scenario of long-distance moving target ranging, which solves the problem of fast real-time ranging of remote dynamic small targets in noisy environment, and realizes the distance Measure synchronously with speed. Further, in order to improve the application effect of the dynamic ranging algorithm, we transplanted the algorithm to the FPGA platform.
Underwater imaging always suffers strong absorption and scattering of water, leading to unsatisfactory imaging quality. Among these factors, scattering is considered as the main factor that makes underwater imaging fuzzy. Various methods have been introduced to eliminate the influence of scattering, and play a positive role in image enhancement, such as rangegated imaging, active polarization imaging and synchronous scanning technology. However, these methods appear limited effects in a higher level of turbidity conditions as it is generally believed that the multiple scattering photons are ascendant in a higher level of turbidity. In this paper, a new underwater image enhancing method was proposed. This method combined the tail-gating technology and the active polarization imaging technology. A range-gated imaging system was upgraded for verifying the novel method by adding polarizer in the light source unit and polarization analyzer in the light collecting unit. Experiments were conducted in a water tank size of 3m×0.6m×0.6m (L × W × H) that filled with running water is to simulate sea water condition in laboratory condition. The turbidly of sea water condition was simulated by adding Magnesium Hydroxide as scattered particles. Several groups of experiments under different range-gate setting and polarization strategy were conducted for comparison. The enhancement of the underwater imaging resolution was evaluated by computing the MTF (Modulation Transfer Function) values. The results said that the method combined the tail-gating and active polarization technology was capable of improving the imaging resolution in turbid underwater environment.
Single-photon LiDAR offers single-photon sensitivity and picoseconds timing resolution, leading to high-resolution images. In this paper, we investigate the reconstruction of depth and intensity profiles from data acquired using a custom designed time-of-flight scanning transceiver based on the time-correlated single-photon counting LiDAR. The system comprises a pulsed laser source at 1064nm wavelength, a monostatic scanning transceiver and a Si single-photon detector. The experimental result is present for reconstruction of high resolution depth and intensity profiles of targets obscured by camouflage at a distance of 425m in daylight. A total variation restoration optimization algorithm is performed to restore profiles of targets, and the results show that it maintains image quality for multi-depth targets while reducing acquisition times.
Compared with traditional Lidar, photon counting laser radar uses a high repetition rate and low pulse energy detection mechanism. The efficiency of laser echo can be improved by more than two orders of magnitude compared with traditional Lidar systems benefitting from the use of extremely sensitive Gm-APD (Geiger mode avalanche photodiode), which greatly improves the detection probability of the system. However, it becomes difficult to detect fast-moving targets within a long counting time, especially under the interference of high background noise. In order to detect high background, long distance and fast moving targets, we propose a dynamic TCSPC method based on point-line duality. The proposed method clusters the point clouds with a certain relationship in the original point cloud space, and finds the corresponding points in the parameter space that can connect some point clouds in a certain analytical form. Simulation results show that highspeed moving target can be detected.
In this paper, we designed a compact Mie scattering lidar system for ocean-atmospheric horizontal visibility measuring and an algorithm used for obtaining the visibility of the aerosol. The effective range of our lidar system was from 300 m to 3 km with a 7.5-m horizontal range resolution and a 1-min time resolution. To reduce the uncertainty caused by using slope method which based on the hypothesis that either molecule or aerosol components exist in the atmosphere, we used the two-component fitting method to retrieve the aerosol extinction coefficient and the calculated the visibility based on Koschmieder’s theory. The whole system was powered by electricity supply which made it easy mounting on a ship or observation station by the sea. Lots of experiments were conducted laboratory to ensure the veracity and stability. In 2016 summer, we joined the cruise survey in China Bo-Hai and Huang-Hai Sea. Site experiments were carried out on the research vessel ‘Dongfanghong 2’. The results showed that the visibility values obtained by our system were in good agreement with the value set by the visibility meter and our lidar system was able to achieve visibility measuring under different weather conditions.
KEYWORDS: 3D image processing, 3D acquisition, LIDAR, Single photon, Single photon detectors, Sensors, Superconductors, Nanowires, Imaging systems, Mirrors
Single-photon LiDAR offers single-photon sensitivity and picoseconds timing resolution, leading to high-resolution three-dimensional (3D) images. In this paper, we investigate the reconstruction of 3D images from data acquired using a custom designed time-of-flight scanning transceiver based on the time-correlated single-photon counting LiDAR. The system comprises a pulsed laser source at 1064nm wavelength, a monostatic scanning transceiver and a superconducting nanowire single-photon detector. The maximum distance of ranging finding under the experimental environments is up to 95.1km, and high-resolution 3D image of a pavilion is reconstructed at the range of 2.9km. A total variation restoration optimization algorithm is performed to reduce the acquisition time of the entire 3D images. Experimental results show that this system is feasible for imaging at longer range by refining the setup, and have potential for target recognition.
Based on optical total reflection critical Angle method, we have designed a refractive index measurement system. It adopted a divergent light source and a CCD camera as the occurrence and receiver of the signal. The divergent light source sent out a bunch of tapered beam, exposure to the interface of optical medium and sulfuric acid solution. Light intensity reflected from the interface could be detected by the CCD camera and then sent to the embedded system. In the DSP embedded system, we could obtain the critical edge position through the light intensity distribution curve and converted it to critical angle. Through experiment, we concluded the relation between liquid refractive index and the critical angle edge position. In this system, the detecting precision of the refractive index of sulfuric acid solution reached 10-4. Finally, through the conversion of the refractive index and density, we achieved high accuracy online measurement of electrolyte density in lead-acid battery.
In this paper, we designed a pint-sized underwater pulsed lidar system for underwater obstacles detection based on a 532nm Nd-YAG pulsed laser as a source and a Hamamatsu photomultiplier tube (PMT) as a detector. In order to acquire the location of the obstacles, an algorithm was devised to handle the echo signal. Through this algorithm, the background noise was suppressed and the accurate range information of the target was obtained. A high-capacity lithium battery was employed to support this lidar system operating as long as eight hours continuously. To ensure our lidar system working steady in the natural underwater environment, a stable waterproof housing was designed for the system which has good water-tightness at 40m depth underwater. This system is small, compact and hand-held. An experiment was conducted in laboratory which proof that the system can achieve target detection within 25m. At last, this lidar system was tested in natural underwater environment of Fuxian Lake in Yunnan Province. There are lots of organic particles and other impurity particles in Fuxian Lake and the attenuation coefficient of the lake is about 0.67m-1. The results showed that this small-size lidar system was able to catch sight of the target within 20 meters and perform smoothly in the natural underwater environment.
Multiangle dynamic light scattering (MDLS) compensates for the low information in a single-angle dynamic light scattering (DLS) measurement by combining the light intensity autocorrelation functions from a number of measurement angles. Reliable estimation of PSD from MDLS measurements requires accurate determination of the weighting coefficients and an appropriate inversion method. We propose the Recursion Nonnegative Phillips-Twomey (RNNPT) algorithm, which is insensitive to the noise of correlation function data, for PSD reconstruction from MDLS measurements. The procedure includes two main steps: 1) the calculation of the weighting coefficients by the recursion method, and 2) the PSD estimation through the RNNPT algorithm. And we obtained suitable regularization parameters for the algorithm by using MR-L-curve since the overall computational cost of this method is sensibly less than that of the L-curve for large problems. Furthermore, convergence behavior of the MR-L-curve method is in general superior to that of the L-curve method and the error of MR-L-curve method is monotone decreasing. First, the method was evaluated on simulated unimodal lognormal PSDs and multimodal lognormal PSDs. For comparison, reconstruction results got by a classical regularization method were included. Then, to further study the stability and sensitivity of the proposed method, all examples were analyzed using correlation function data with different levels of noise. The simulated results proved that RNNPT method yields more accurate results in the determination of PSDs from MDLS than those obtained with the classical regulation method for both unimodal and multimodal PSDs.
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