Laser Detection And Ranging Research Articles

A novel noise-robustness and rotation-invariant LADAR point cloud target classification method

Identifying targets of LAser Detection And Ranging (LADAR) point clouds that are affected by noise and different poses throws a significant challenge. This paper proposes a novel LADAR point cloud target classification method that is both noise-robust and rotation-invariant. Specifically, the proposed method transforms the point cloud target into a slice image that is resilient to noise and holds rotation-invariance. Next, a designed 2D Convolutional Neural Network (CNN) is utilized to classify the corresponding point cloud target based on the slice image. To overcome the challenge of lacking prior knowledge about the discriminative features of the slice image, the 2D CNN is designed with the Local Importance-based Pooling (LIP) layer. This layer extracts the discriminative feature in a data-driven manner, thereby improving the accuracy of the classification process. The proposed method is evaluated through multiple experiments on the public ModelNet40 dataset. The experimental results demonstrate that the designed LIP-CNN can better learn the discriminative features of the slice image, achieving high classification accuracy. Moreover, the proposed slice-image-based method is capable of accurately classifying the target, even in the presence of noise and different poses.

Characterizing the performance impacts of target surface on underwater pulse laser ranging system

Laser-based object detection has been recognized as the most reliable technique for applications such as night vision, 3D (3-Dimensional) imaging, and especially underwater object detection. The key information is extracted from the reflected laser pulse after interacting with the target which surface directly affects the system performance. Due to the variety of the target types, it is necessary to investigate the surface characteristics and their effects on the performance of an underwater pulse laser ranging system. In this paper, the influence of target surface characteristics namely the type of materials, colors, and roughness on the reflectance and system performance are investigated through theoretical analysis using the Bidirectional Reflection Distribution Function (BRDF) and Laser Detection and Ranging (LADAR) model. An underwater peak detection pulse laser ranging system is developed to validate the results of a theoretical study. Both experimental and theoretical results clearly show that the system performance depends on the reflectance caused by the three characteristics of the target surface. This comprehensive research provides a handy reference with regards to the surface material, colors, and roughness for future improvement or correction in this domain.

Effect of optical-scattering characteristics with modified kernel-based model on the triggering probability of Gm-APD

It is not feasible to use the Geiger-mode avalanche photodiode (Gm-APD) detection model to guide detection quantitatively during daytime, because of the incompletely built echo intensity model. In this study, because the background noise and the signal light are both modulated by the target's optical-scattering characteristics (OSCs), a hybrid light-scattering model (the modified kernel-based bidirectional reflection distribution function (BRDF) model) is substituted in the laser detection and ranging (LADAR) ranging equation. In such a BRDF model, the Li-Liang-BRDF model is taken as the kernel, and the background noise equivalent intensity coefficient is employed to link the background noise part to the laser echo part. According to the results of a Gm-APD outdoor experiment, the proposed LADAR ranging equation is applicable to detection during daytime, and the noise scaling factor d(θs) is not a constant with the variation of solar zenith angle; the signal intensities in the theoretical and experimental results only exhibit a difference of 5%. The effects of various factors (e.g., solar zenith angle, LADAR observation angle, and different target materials) on the echo signal-to-noise ratio and triggering probability are analyzed by the proposed LADAR ranging equation. Moreover, a maximum-likelihood estimation-based theoretical method is proposed to calculate the maximum detection range during daytime. Subsequently, the change law of the detection range in daytime is analyzed, and the calculated detection range of the developed Gm-APD LADAR reaches 3.5 km under an actual daytime background. This study can provide theoretical support and formulates several strategies for Gm-APD detection in daytime. Furthermore, novel ideas can be proposed to calculate theoretically the effective detection range of Gm-APD LADAR.

Implementation of a pulse-type laser detection and ranging system based on heterodyne detection for long-range measurement with high repetition rate.

This paper presents the implementation of a pulse-type LAser Detection And Ranging (LADAR) system based on heterodyne detection for long-range measurement. A pulse-type LADAR based on an intensity direct-detection is certainly simple and mature, but it requires a high peak-power laser and a low-noise avalanche photodiode for long-range measurement, which restricts the scope of the application due to the weight, power consumption, and cost of the laser and the photodetector. In this work, heterodyne detection using a PIN photodiode is implemented to increase receiver sensitivity instead of using a low-noise avalanche photodiode. An optical phase-locked loop is adopted to generate an optical local oscillator signal for heterodyne detection. The proposed heterodyne detection scheme achieves a minimum detectable signal level of -52.6 dBm at a bandwidth of 1.2 GHz, and it is adopted in a pulse-type LADAR system for long-range measurement. The pulse-type LADAR system can measure a distance of 2.77 km at a repetition rate of 40 kHz, and it demonstrates great advantages for realizing real-time 3D imaging for long-range measurement with a high frame rate.

A Study into the Effects of Factors Influencing an Underwater, Single-Pixel Imaging System’s Performance

Underwater detection has always been a challenge due to the limitations caused by scattering and absorption in the underwater environment. Because of their great penetration abilities, lasers have become the most suitable technology for underwater detection. In all underwater laser applications, the reflected laser pulse which contains the key information for most of the system is highly degraded along the laser’s propagation path and during reflection. This has a direct impact on the system’s performance, especially for single-pixel imaging (SPI) which is very dependent on light-intensity information. Due to the complications in the underwater environment, it is necessary to study the influential factors and their impacts on underwater SPI. In this study, we investigated the influence of the angle of incidence, target distance, and medium attenuation. A systematic investigation of the influential factors on the reflectance and ranging accuracy was performed theoretically and experimentally. The theoretical analysis was demonstrated based on the bidirectional reflection distribution function (BRDF) and laser detection and ranging (LADAR) model. Moreover, 2D single-pixel imaging (SPI) systems were setup for experimental investigation. The experimental results agree well with the theoretical results, which show the system’s dependency on the reflection intensity caused by the angle of incidence, target distance, and medium attenuation. The findings should be a reference for works looking to improve the performance of an underwater SPI system.

Scalable Light-Printing of Substrate-Engraved Free-Form Metasurfaces.

A key challenge for metasurface research is locally controlling at will the nanoscale geometric features on meter-scale apertures. Such a technology is expected to enable large aperture meta-optics and revolutionize fields such as long-range imaging, lasers, laser detection and ranging (LADAR), and optical communications. Furthermore, these applications are often more sensitive to light-induced and environmental degradation, which constrains the possible materials and fabrication process. Here, we present a relatively simple and scalable method to fabricate a substrate-engraved metasurface with locally printed index determined by induced illumination, which, therefore, addresses both the challenges of scalability and durability. In this process, a thin metal film is deposited onto a substrate and transformed into a mask via local laser-induced dewetting into nanoparticles. The substrate is then dry-etched through this mask, and selective mask removal finally reveals the metasurface. We show that masking by the local nanoparticle distribution, and, therefore, the local index, is dependent on the local light-induced dewetting temperature. We demonstrate printing of a free-form pattern engraved into a fused silica glass substrate using a laser raster scan. Large-scale spatially controlled engraving of metasurfaces has implications on other technological fields beyond optics, such as surface fluidics, acoustics, and thermomechanics.

High Spatial Resolution Three-Dimensional Imaging Static Unitary Detector-Based Laser Detection and Ranging Sensor

This paper presents a static unitary detector (STUD)-based laser detection and ranging (LADAR) sensor with a 16-to-1 transimpedance-combining amplifier for high spatial resolution three-dimensional (3-D) applications. In order to readout the large size of a photodetector for better results of 3-D information without any reduction of the bandwidth, the partitioning photosensitive cell method is embedded in a 16-to-1 transimpedance-combining amplifier. The effective number of partitioning photosensitive cells and signal-combining stages are selected based on the analysis of the partitioning photosensitive cell method for the optimum performance of a transimpedance-combining amplifier. A prototype chip is fabricated in a 0.18-μm CMOS technology. The input referred noise is 41.9 pA/√Hz with a bandwidth of 230 MHz and a transimpedance gain of 70.4 dB·Ω. The total power consumption of the prototype chip is approximately 86 mW from a 1.8-V supply, and the TICA consumes approximately 15.4 mW of it.

A High-Multi Target Resolution Focal Plane Array-Based Laser Detection and Ranging Sensor.

This paper introduces a digital-assisted multiple echo detection scheme, which utilizes the waste time of the full serial data readout period in a focal plane array (FPA)-based laser detection and ranging (LADAR) receiver. With the support of an external digital signal processor (DSP) and additional analog memory inserted into the receiver, the proposed readout scheme can effectively enhance multi-target resolution (MTR) three times higher than the conventional FPA-based LADAR, while maintaining low power consumption and a small area. A prototype chip was fabricated in a 0.18-μm CMOS process with an 8 × 8 FPA configuration, where each single receiver pixel occupied an area of 100 μm × 100 μm. The single receiver achieved an MTR of 20 ns with 7.47 mW power dissipation, an input referred noise current of 4.48 pA/√Hz with a bandwidth 530 MHz, a minimum detectable signal (MDS) of 340 nA, a maximum walk error of 2.2 ns, and a maximum non-linearity of 0.05% among the captured multiple echo images.

Multisensor-Based Target-Tracking Algorithm with Out-of-Sequence-Measurements in Cluttered Environments.

A localization and tracking algorithm for an early-warning tracking system based on the information fusion of Infrared (IR) sensor and Laser Detection and Ranging (LADAR) is proposed. The proposed Kalman filter scheme incorporates Out-of-Sequence Measurements (OOSMs) to address long-range, high-speed incoming targets to be tracked by networked Remote Observation Sites (ROS) in cluttered environments. The Rauch–Tung–Striebel (RTS) fixed lag smoothing algorithm is employed in the proposed technique to further improve tracking accuracy, which, in turn, is used for target profiling and efficient filter initialization at the targeted platform. This efficient initialization increases the probability of target engagement by increasing the distance at which it can be effectively engaged. The increased target engagement range also reduces risk of any damage from debris of the engaged target. Performance of the proposed target localization algorithm with OOSM and RTS smoothing is evaluated in terms of root mean square error (RMSE) for both position and velocity, which accurately depicts the improved performance of the proposed algorithm in comparison with existing retrodiction-based OOSM filtering algorithms. The effects of assisted target state initialization at the targeted platform are also evaluated in terms of Time to Impact (TTI) and true track retention, which also depict the advantage of the proposed strategy.

Research of detecting the laser’s secondary reflected echo from target by using Geiger-mode avalanche photodiode

Abstract When the Geiger-mode avalanche photodiode (Gm-APD) laser detection and ranging (LADAR) carries out the air-to-ground detection, it can detect the echo that is successively reflected by targets and the ground, which causes the false target in the image obtained by the LADAR. In this paper, the echo which is successively reflected by two surfaces is called the secondary reflected echo, and the echo which is just reflected once is called the single reflected echo. Considering the Gm-APD’s superior ability to detect photon, we investigate and analyze the triggering characteristics of the Gm-APD for detecting the secondary reflected laser echo from the targets, also, we carry out the experiments on detecting the secondary reflected echo in near field and far field by using array Gm-APD to verify that the array Gm-APD can effectively detect the secondary reflected echo, and this paper lays the foundation for the research of the secondary reflection’s influence on the images obtained by Gm-APD. In this paper, firstly, we use the echo triggering model, which contains LADAR ranging equation and Poisson probability model, and the Monte Carlo simulating method to completely verify the possibility of detecting the secondary reflected echo by using Gm-APD, in theory. The theoretical results show that Gm-APD can detect the secondary reflected echo, and the wider the field of view(FOV) is, the longer the detecting range is, besides, the higher the LADAR is located at, the lower the target’s secondary reflected echo’s intensity is and the higher the ground’s single reflected echo’s intensity is; then, using the Gm-APD with an array of 64 × 64 and 32 × 32 to detect the indoor scene and the moving van in far field, respectively, the results show that the indoor experiment obtains the complete first and second reflected image and the single pixel can detect the secondary reflected echo from multiple targets, also, for the far-field experiment, the single reflected echo and second reflected echo can be obtained in a single frame image, and under this experimental conditions, two pixels which are studied, realize the range extension of 25.6 m and 41.9 m respectively, which shows that the Gm-APD can likely detect the false target in the far field; finally, the methods that finding the secondary reflected surface by using the multiple-frame statistics for static targets and the image of triggering position in chronological order for moving targets, respectively, are proposed to eliminate the secondary reflected image. This article can provide some theoretical references for eliminating the secondary reflection before target recognition.

Coherent laser ranging for precision imaging through flames

Measuring the deformation of building elements engulfed by flames is essential in fire research to improve safety in the event of a fire. Ideally, the measurement system should be non-contact, able to range at the millimeter to meter scale with sub-mm precision, and have sufficient speed to capture temperature-induced deformations of the target object. To date, no ranging technology has been demonstrated that meets those requirements while imaging through flames. Here, we show that coherent laser detection and ranging (LADAR) can provide three-dimensional images of objects hidden behind methane or acetylene flames with sufficiently high precision to track their deformation. The heterodyne detection of coherent frequency-modulated continuous-wave (FMCW) LADAR allows the ranging signal to be detected in the presence of strong radiation of the flames. We measure three-dimensional point clouds of diffusely scattering complex surfaces with a precision of less than 30 μm at 2-meter stand-off distance, despite soot-induced signal loss and steering (refraction) of the ranging laser by the flames. Movies of the heat-induced surface deformation of objects illustrate the temporal performance. These data show that FMCW LADAR can quantify the deformation and movement of objects in fires, when non-contact shape measurements at stand-off distances of multiple meters are crucial.

4-to-1 Transimpedance combining amplifier-based static unitary detector for high-resolution of LADAR sensor

A fully integrated 4-to-1 transimpedance combining amplifier (TICA)-based static unitary detector (STUD) is developed for high-resolution of laser detection and ranging (LADAR) sensor. With a developed TICA, the STUD is able to have an effective large-area photodetector to enlarge the region of interest (ROI) without the bandwidth deterioration of a receiver for LADAR sensor. The 4-to-1 TICA is fabricated using 0.18-μm standard CMOS technology and it consists of four independent current buffers, a two-stage signal combiner, a balun, an output buffer, and four over-current protectors in one single integrated chip. The core of the TICA dissipates a power of about 7.8 mW. The total power consumption, including that of the balun and the output buffer, is 41 mW from a 1.8-V supply. The average input-referred noise current spectral density is 15.4 pA/?Hz with a bandwidth of 185?MHz and a transimpedance gain of 70?dBΩ. The developed TICA occupies an active area of approximately 107?μm?נ102?μm and the die size, including the I/O pads, is 912?μm?נ1000?μm. From the two-dimensional optical pulse scanning measurements and the three-dimensional (3-D) range measurements, it is verified that the designed TICA is suitable for the receiver front-end of the STUD-based LADAR sensor.

Improved Range Estimation Model for Three-Dimensional (3D) Range Gated Reconstruction.

Accuracy is an important measure of system performance and remains a challenge in 3D range gated reconstruction despite the advancement in laser and sensor technology. The weighted average model that is commonly used for range estimation is heavily influenced by the intensity variation due to various factors. Accuracy improvement in term of range estimation is therefore important to fully optimise the system performance. In this paper, a 3D range gated reconstruction model is derived based on the operating principles of range gated imaging and time slicing reconstruction, fundamental of radiant energy, Laser Detection And Ranging (LADAR), and Bidirectional Reflection Distribution Function (BRDF). Accordingly, a new range estimation model is proposed to alleviate the effects induced by distance, target reflection, and range distortion. From the experimental results, the proposed model outperforms the conventional weighted average model to improve the range estimation for better 3D reconstruction. The outcome demonstrated is of interest to various laser ranging applications and can be a reference for future works.

Design of a CMOS ROIC for InGaAs Self-Mixing Detectors Used in FM/cw LADAR

A scannerless laser detection and ranging (LADAR) system based on frequency modulation/continuous wave (FM/cw) technique is attractive due to a reduced bandwidth requirement of the receiver circuit. This paper presents the $64\times 64$ InGaAs-based focal plane array (FPA) self-mixing detectors and the $1\times 32$ element readout integrated circuit (ROIC) array based on transimpedance amplifier for the FM/cw LADAR. Different from conventional detector, the InGaAs self-mixing detector presents a low impedance and a complex electrical model. The linear chirp local-oscillator (LO) electrical signal through the detector causes a large dc current and a unique dc offset voltage (Vos) error in the ROIC. To overcome these problems, the blocking capacitors and the novel multiple low-pass filters are applied in the ROIC. The ROIC array was fabricated in 0.18- $\mu \text{m}$ CMOS process. The measured results show that the Vos error is eliminated within the limited LO amplitude range and the ROIC achieves a 140-dB $\Omega$ transimpedance gain with a bandwidth of 1 MHz. Compared with the state-of-the-art FM/cw LADAR, the ROIC accurately detects the signal of interest of the InGaAs FPA self-mixing detectors and miniaturizes the LADAR receiver.

The Development of a 3D LADAR Simulator Based on a Fast Target Impulse Response Generation Approach

A new laser detection and ranging (LADAR) simulator has been developed, using MATLAB and its graphical user interface, to simulate direct detection time of flight LADAR systems, and to produce 3D simulated scanning images under a wide variety of conditions. This simulator models each stage from the laser source to data generation and can be considered as an efficient simulation tool to use when developing LADAR systems and their data processing algorithms. The novel approach proposed for this simulator is to generate the actual target impulse response. This approach is fast and able to deal with high scanning requirements without losing the fidelity that accompanies increments in speed. This leads to a more efficient LADAR simulator and opens up the possibility for simulating LADAR beam propagation more accurately by using a large number of laser footprint samples. The approach is to select only the parts of the target that lie in the laser beam angular field by mathematically deriving the required equations and calculating the target angular ranges. The performance of the new simulator has been evaluated under different scanning conditions, the results showing significant increments in processing speeds in comparison to conventional approaches, which are also used in this study as a point of comparison for the results. The results also show the simulator’s ability to simulate phenomena related to the scanning process, for example, type of noise, scanning resolution and laser beam width.

Enhancement of quantum-enhanced LADAR receiver in nonideal phase-sensitive amplification

The phase-sensitive amplification (PSA) with an injected squeezed vacuum field is theoretically investigated in quantum-enhanced laser detection and ranging (LADAR) receiver. The theoretical model of the amplified process is derived to investigate the quantum fluctuations in detail. A new method of mitigating the unflat gain of nonideal PSA is proposed by adjusting the squeezed angle of the squeezed vacuum field. The simulation results indicate that signal-noise ratio (SNR) of system can be efficiently improved and close to the ideal case by this method. This research will provide an important potential in the applications of quantum-enhanced LADAR receiver.

Switched 4-to-1 Transimpedance Combining Amplifier for Receiver Front-End Circuit of Static Unitary Detector-Based LADAR System

Laser detection and ranging (LADAR) systems are commonly used to acquire real-time three-dimensional (3D) images using the time-of-flight of a short laser pulse. A static unitary detector (STUD)-based LADAR system is a simple method for obtaining real-time high-resolution 3D images. In this study, a switched 4-to-1 transimpedance combining amplifier (TCA) is implemented as a receiver front-end readout integrated circuit for the STUD-based LADAR system. The 4-to-1 TCA is fabricated using a standard 0.18 μm complementary metal-oxide-semiconductor (CMOS) technology, and it consists of four independent current buffers, a two-stage signal combiner, a balun, and an output buffer in one single integrated chip. In addition, there is a switch on each input current path to expand the region of interest with multiple photodetectors. The core of the TCA occupies an area of 92 μm × 68 μm, and the die size including I/O pads is 1000 μm × 840 μm. The power consumption of the fabricated chip is 17.8 mW for a supplied voltage of 1.8 V and a transimpedance gain of 67.5 dBΩ. The simulated bandwidth is 353 MHz in the presence of a 1 pF photodiode parasitic capacitance for each photosensitive cell.

Influence of target reflection on three-dimensional range gated reconstruction.

The range gated technique is a promising laser ranging method that is widely used in different fields such as surveillance, industry, and military. In a range gated system, a reflected laser pulse returned from the target scene contains key information for range reconstruction, which directly affects the system performance. Therefore, it is necessary to study the characteristics and effects of the target reflection factor. In this paper, theoretical and experimental analyses are performed to investigate the influence of target reflection on three-dimensional (3D) range gated reconstruction. Based on laser detection and ranging (LADAR) and bidirectional reflection distribution function (BRDF) theory, a 3D range gated reconstruction model is derived and the effect on range accuracy is analyzed from the perspectives of target surface reflectivity and angle of laser incidence. Our theoretical and experimental study shows that the range accuracy is proportional to the target surface reflectivity, but it decreases when the angle of incidence increases to adhere to the BRDF model. The presented findings establish a comprehensive understanding of target reflection in 3D range gated reconstruction, which is of interest to various applications such as target recognition and object modeling. This paper provides a reference for future improvement to perform accurate range compensation or correction.

Pixel multiplexing technique for real-time three-dimensional-imaging laser detection and ranging system using four linear-mode avalanche photodiodes.

The avalanche-photodiode-array (APD-array) laser detection and ranging (LADAR) system has been continually developed owing to its superiority of nonscanning, large field of view, high sensitivity, and high precision. However, how to achieve higher-efficient detection and better integration of the LADAR system for real-time three-dimensional (3D) imaging continues to be a problem. In this study, a novel LADAR system using four linear mode APDs (LmAPDs) is developed for high-efficient detection by adopting a modulation and multiplexing technique. Furthermore, an automatic control system for the array LADAR system is proposed and designed by applying the virtual instrumentation technique. The control system aims to achieve four functions: synchronization of laser emission and rotating platform, multi-channel synchronous data acquisition, real-time Ethernet upper monitoring, and real-time signal processing and 3D visualization. The structure and principle of the complete system are described in the paper. The experimental results demonstrate that the LADAR system is capable of achieving real-time 3D imaging on an omnidirectional rotating platform under the control of the virtual instrumentation system. The automatic imaging LADAR system utilized only 4 LmAPDs to achieve 256-pixel-per-frame detection with by employing 64-bit demodulator. Moreover, the lateral resolution is ∼15 cm and range accuracy is ∼4 cm root-mean-square error at a distance of ∼40 m.

Chromatic methodology for laser detection and ranging (LADAR) image description

Chromatic methodology for information extraction has evolved from a number of origins, some based upon human perceptions and the others on scientific and analytical approaches. From the applications of this methodology, a new robust LADAR image descriptor is proposed. This descriptor is developed to has a high discrimination capability, robust to the effects that disturb LADAR images, and requires less storage space and computational time for recognition. The performance of the proposed LADAR descriptor is evaluated using simulated LADAR images. Experimental tests are also being undertaken on the new descriptor to validate its ability with processing real LADAR images. The results show high discrimination capability for the new descriptor over the traditional techniques such as Moments descriptor, which is used to benchmark the results. The results also show the robustness of the new descriptor in the presence of noise, view change, low resolution, translation, scaling, and rotation effects.