Multiple-return laser radar for three-dimensional imaging through obscurations

Multiple-wavelength range-gated active imaging principle integrating spectral information for five-dimensional imaging

The combined multiple-wavelength range-gated active imaging (WRAI) principle is able to determine the position of a moving object in a four-dimensional space and to deduce its trajectory and its speed independently of the video frequency. By combining two wavelength categories, it determines the depth of moving objects in the scene with the warm color category and the precise moment of a moving object's position with the cold color category. Therefore, since each object had the ability to transmit information from different wavelengths, related to the spectral reflectances, it became interesting to identify their spectral signatures from these reflectances. Using a conventional method of spectral classification, it was shown that it is possible to identify objects in a 3D scene from their a priori known spectral signatures and, thanks to this, to reveal especially the fifth dimension in the imaging of the WRAI principle. The experimental tests confirmed that it is possible to record moving objects in a five-dimensional space represented by a single image, thus validating this multi-wavelength imaging method.

Video encryption/compression using compressive coded rotating mirror camera

Compressive coded rotating mirror (CCRM) camera is a novel high-speed imaging system that operates under amplitude optical encoding and frame sweeping modalities in a passive imaging mode that is capable of reconstructing 1400 frames from a single shot image acquisition and achieves the highest compression ratio of 368 compared to the other compressive sensing (CS) based single-shot imaging modalities. The integrated optical encoding and compression adds a strong layer of encryption on the observed data and facilitates the integration of the CCRM camera with the imaging applications that require highly efficient data encryption and compression due to capturing highly sensitive data or limited transmission and storage capacities. CCRM uses amplitude encoding that significantly extends the key space where the probability of having the exact encoder pattern is estimated as [Formula: see text], hence drastically reducing the possibility of data recovery in a brute force manner. Data reconstruction is achieved under CS based algorithms where the obtained amplitude-based pattern from optical encoder operates as the key in the recovery process. Reconstruction on the experimental as well as the synthetic data at various compression ratios demonstrate that the estimated key with less than 95[Formula: see text] matching elements were unable to recover the data where the achieved averaged structural similarity (SSIM) of 0.25 before 95[Formula: see text] encoder similarity and 0.85 SSIM at 100[Formula: see text] encoder similarity demonstrates the high-sensitivity of the proposed optical encryption technique.

Bayesian reconstruction method for underwater 3D range-gated imaging enhancement

We investigate a systematic improvement for 3D range-gated imaging in scattering environments. Drawbacks including absorption, ambient light, and scattering effect are studied. The former two are compensated through parameter estimation and preprocessing. With regard to the scattering effect, we propose a new 3D reconfiguration algorithm using a Bayesian approach that incorporates spatial constraints through a general Gaussian Markov random field. The model takes both scene depth and albedo into account, which provides a more informative and accurate restoration result. Hyper-parameters for the statistical mechanism are evaluated adaptively in the procedure and an iterated conditional mode optimization algorithm is employed to find an optimum solution. The performance of our method was assessed via conducting various experiments and the results also indicate that the proposed method is helpful for restoring the 2D image of a scene with improved visibility.

Pile-up effect in an infrared single-pixel compressive LiDAR system

Compressive sensing (CS) has been used in LiDAR systems utilizing one single-photon-counting avalanche diode. We demonstrate an unexpected grayscale inversed image of an object at an unchosen depth, which appears in the reconstruction of the infrared single-pixel LiDAR system due to the pile-up effect. A correction algorithm and the sparse measurement are proposed and experimentally verified to effectively reduce the photon pile-up influence, so that the negative images can be completely removed. The correction methods in this research can improve the accuracy and the flexibility of the single-pixel LiDAR systems employing detectors with a low maximum light count.

All Photons Imaging Through Volumetric Scattering

Imaging through thick highly scattering media (sample thickness ≫ mean free path) can realize broad applications in biomedical and industrial imaging as well as remote sensing. Here we propose a computational "All Photons Imaging" (API) framework that utilizes time-resolved measurement for imaging through thick volumetric scattering by using both early arrived (non-scattered) and diffused photons. As opposed to other methods which aim to lock on specific photons (coherent, ballistic, acoustically modulated, etc.), this framework aims to use all of the optical signal. Compared to conventional early photon measurements for imaging through a 15 mm tissue phantom, our method shows a two fold improvement in spatial resolution (4db increase in Peak SNR). This all optical, calibration-free framework enables widefield imaging through thick turbid media, and opens new avenues in non-invasive testing, analysis, and diagnosis.

Fast differential discrimination approach to improve time resolution with multiple returns for time-of-flight system

A differential discrimination approach has been proposed in this paper to make fast discrimination from multi-returns in time-of-flight (TOF) systems. The time resolution of classical direct discrimination in a laser TOF system has been analyzed. A theoretical model of the differential discriminator has been established. As theoretical calculations and numerical simulations have shown, time resolution can be improved by using a differential discriminator instead of a classical direct discriminator, especially with large-amplitude signals. However, the relationship between the time resolution and the amplitude is not monotonic. The time resolution decreases as the amplitude decreases when the signals are small. But, through further analysis and simulations, a method to find an appropriate parameter, which can optimize the relationship curve, has been proposed. The theory and the simulations were verified by electrical circuit experiments, and the optical circuit experiments taken with two close targets of about 8.3 m. The FWHM of the laser is about 1.9 ns. The measured time resolution was better than 2.2 ns when the amplitude is 30 mV.

Recovering three-dimensional shape through a small hole using three laser scatterings

An approach was proposed and developed to recover the 3D shape concealed in a shelter with a small hole only using three laser scatterings. This approach extends reconstruction of concealed 3D shape from "around a corner" to "through a small hole". Based on principle of rectilinear propagation of light, a simple geometric mapping tentative theoretical frame independent of scene was proposed to extract 3D information of concealed objects. Experimental setup mainly consists of a nanosecond laser and a single-photon APD, both of which are commercially available. The 3D reconstructions of three hidden objects were acquired with a resolution of centimeters.

Approach for LIDAR signals with multiple returns

Time-correlated single photon counting (TCSPC) and burst illumination laser (BIL) data can be used for depth reconstruction of a target surface; the problem is how to analyze the response for the reconstruction. We propose a fast-approach STMCMC (Simulated Tempering Markov Chain Monte Carlo) for LIDAR signals with multiple return, in order to obtain a complete characterization of a 3D surface by the laser range system. STMCMC is used to explore the spaces by the preset distributions instead of the prior distributions. Added active intervention tempering makes the Markov chain mix better through the temporary expansion of the solutions. The added step keeps the operation under control and yet retains the Markov characteristic of the operation. The theoretical analysis and the demonstrations on the practical data show flexible operation, and the parameters can be estimated to a high degree of accuracy.

Spectral LADAR: active range-resolved three-dimensional imaging spectroscopy

We present the concept and experimental results for Spectral LADAR, an augmented LADAR imager combining three-dimensional (3D) time-of-flight ranging with active multispectral sensing in the shortwave infrared (1080-1620 nm). The demonstrated technique is based on a nanosecond regime pulsed supercontinuum transmitter and spectrally multiplexed receiver that computes a high-resolution range value for each of 25 spectral bands. A low frame-rate prototype unit is described. Results demonstrating 3D imaging and material type classification of objects, especially those obscured by camouflage, are shown at effective stand-off ranges exceeding 40 m. These capabilities and the highly eye safe wavelengths at which the system operates make it suitable for applications in military imaging and robotic perception.

Photon-counting compressive sensing laser radar for 3D imaging

We experimentally demonstrate a photon-counting, single-pixel, laser radar camera for 3D imaging where transverse spatial resolution is obtained through compressive sensing without scanning. We use this technique to image through partially obscuring objects, such as camouflage netting. Our implementation improves upon pixel-array based designs with a compact, resource-efficient design and highly scalable resolution.

Simultaneous ranging and velocimetry of fast moving targets using oppositely chirped pulses from a mode-locked laser

A lidar system based on the coherent detection of oppositely chirped pulses generated using a 20 MHz mode locked laser and chirped fiber Bragg gratings is presented. Sub millimeter resolution ranging is performed with > 25 dB signal to noise ratio. Simultaneous, range and Doppler velocity measurements are experimentally demonstrated using a target moving at > 330 km/h inside the laboratory.

Range resolved lidar for long distance ranging with sub-millimeter resolution

A lidar technique employing temporally stretched, frequency chirped pulses from a 20 MHz mode locked laser is presented. Sub-millimeter resolution at a target range of 10.1 km (in fiber) is observed. A pulse tagging scheme based on phase modulation is demonstrated for range resolved measurements. A carrier to noise ratio of 30 dB is observed at an unambiguous target distance of 30 meters in fiber.

Improvement of range accuracy of range-gating laser radar using the centroid method

An improved processing approach based on the relation between range accuracy and slicing number is proposed to improve the range accuracy of range-gating laser radar. The sequence of time-slice images is segmented according to their optimal slicing number and processed in segments to achieve the range information of objects. Experimental results indicate that the slicing number has a significant impact on range accuracy, and the highest range accuracy can be achieved when the systems work with an optimal slicing number.

Monte Carlo simulation of multiple photon scattering in sugar maple tree canopies

Detecting objects hidden beneath forest canopies is a difficult task for optical remote sensing systems. Rather than relying upon the existence of gaps between leaves, as other researchers have done, our ultimate goal is to use light scattered by leaves to image through dense foliage. Herein we describe the development of a Monte Carlo model for simulating the scattering of light as it propagates through the leaves of an extended tree canopy. We measured several parameters, including the gap fraction and maximum leaf-area density, of a nearby sugar maple tree grove and applied them to our model. We report the results of our simulation in both the ground and the receiver planes for an assumed illumination angle of 80 degrees. To validate our model, we then illuminated the sugar maple tree grove at 80 degrees and collected data both on the canopy floor and at our monostatic receiver aperture. Experimental results were found to correlate well with our simulated expectations.

Long-range time-of-flight scanning sensor based on high-speed time-correlated single-photon counting

We describe a scanning time-of-flight system which uses the time-correlated single-photon counting technique to produce three-dimensional depth images of distant, noncooperative surfaces when these targets are illuminated by a kHz to MHz repetition rate pulsed laser source. The data for the scene are acquired using a scanning optical system and an individual single-photon detector. Depth images have been successfully acquired with centimeter xyz resolution, in daylight conditions, for low-signature targets in field trials at distances of up to 325 m using an output illumination with an average optical power of less than 50 microW.

Bayesian analysis of Lidar signals with multiple returns

Time-Correlated Single Photon Counting and Burst Illumination Laser data can be used for range profiling and target classification. In general, the problem is to analyse the response from a histogram of either photon counts or integrated intensities to assess the number, positions and amplitudes of the reflected returns from object surfaces. The goal of our work is a complete characterisation of the 3D surfaces viewed by the laser imaging system. The authors present a unified theory of pixel processing that is applicable to both approaches based on a Bayesian framework which allows for careful and thorough treatment of all types of uncertainties associated with the data. We use reversible jump Markov chain Monte Carlo (RJMCMC) techniques to evaluate the posterior distribution of the parameters and to explore spaces with different dimensionality. Further, we use a delayed rejection step to allow the generated Markov chain to mix better through the use of different proposal distributions. The approach is demonstrated on simulated and real data, showing that the return parameters can be estimated to a high degree of accuracy. We also show some practical examples from both near and far range depth imaging.

Monolithic optical parametric oscillator using chirped quasi-phase matching

We describe a highly efficient monolithic, Q-switched, nanosecond optical parametric oscillator based on a magnesium-oxide-doped periodically poled lithium niobate crystal and containing multiple quasi-phase-matched gratings. The crystal consisted of a single unchirped grating and five gratings containing progressively increasing amounts of longitudinal chirp. The monolithic design makes the device highly compact, stable, and robust, and it demonstrated a pump-to-signal conversion efficiency of around 50%, generating 50 microJ pulses at 1.55 microm with a spectral bandwidth of 20 nm. Sonogram traces are presented showing the effect of crystal chirp on the temporal and spectral performance.

Pulsed Raman fiber laser and multispectral imaging in three dimensions

Raman scattering in single-mode optical fibers is exploited to generate multispectral light from a green nanolaser with high pulse repetition rate. Each pulse triggers a picosecond camera and measures the distance by time-of-flight in each of the 0.5 Mpixels. Three-dimensional images are then constructed with submillimeter accuracy for all visible colors. The generation of a series of Stokes peaks by Raman scattering in a Si fiber is discussed in detail and the laser radar technique is demonstrated. The data recording takes only a few seconds, and the high accuracy 3D color imaging works at ranges up to approximately 200 m. Applications for optical tomography in highly scattering media such as water and human tissue are mentioned.

Intensity-invariant nonlinear filtering for detection in camouflage

We introduce a method based on an orthonormal vector space basis representation to detect camouflaged targets in natural environments. The method is intensity invariant so that camouflaged targets are detected independently of the illumination conditions. The detection technique does not require one to know the exact camouflage pattern, but only the class of patterns (e.g., foliage, netting, woods). We use nonlinear filtering and the calculation of several correlations. The nonlinearity of the filtering process also allows high discrimination against false targets. Several experiments confirm the target detectability where strong camouflage might delude even human viewers.

Detection probabilities for photon-counting avalanche photodiodes applied to a laser radar system

Arrays of photon-counting avalanche photodiodes with time-resolved readout can improve the performance of three-dimensional laser radars. A comparison of the detection and false-alarm probabilities for detectors in linear mode and in Geiger mode is shown. With low background radiation their performance is comparable. It is shown that in both cases it will be necessary to process several laser shots of the same scene to improve detection and reduce the false-alarm rate. Additional calculations show that the linear mode detector is much better at detecting targets behind semitransparent obscurations such as vegetation and camouflage nets.

Gated viewing and high-accuracy three-dimensional laser radar

We have developed a fast and high-accuracy three-dimensional (3-D) imaging laser radar that can achieve better than 1-mm range accuracy for half a million pixels in less than 1 s. Our technique is based on range-gating segmentation. We combine the advantages of gated viewing with our new fast technique of 3-D imaging. The system uses a picosecond Q-switched Nd:Yag laser at 532 nm with a 32-kHz pulse repetition frequency (PRF), which triggers an ultrafast camera with a highly sensitive CCD with 582 x 752 pixels. The high range accuracy is achieved with narrow laser pulse widths of approximately 200 ps, a high PRF of 32 kHz, and a high-speed camera with gate times down to 200 ps and delay steps down to 100 ps. The electronics and the software also allow for gated viewing with automatic gain control versus range, whereby foreground backscatter can be suppressed. We describe our technique for the rapid production of high-accuracy 3-D images, derive performance characteristics, and outline future improvements.

Efficient storage and transmission of ladar imagery

We develop novel methods for compressing volumetric imagery that has been generated by single-platform (mobile) range sensors. We exploit the correlation structure inherent in multiple views in order to improve compression efficiency. We show that, for lossless compression, three-dimensional volumes compress more efficiently than two-dimensional (2D) images by a factor of 60%. Furthermore, our error metric for lossy compression suggests that accumulating more than nine range images in one volume before compression yields as much as a 99% improvement in compression performance over 2D compression.