Direct-detection Laser Radar Research Articles

Laser radar: historical prospective—from the East to the West

This article discusses the history of laser radar development in America, Europe, and Asia. Direct detection laser radar is discussed for range finding, designation, and topographic mapping of Earth and of extraterrestrial objects. Coherent laser radar is discussed for environmental applications, such as wind sensing and for synthetic aperture laser radar development. Gated imaging is discussed through scattering layers for military, medical, and security applications. Laser microradars have found applications in intravascular studies and in ophthalmology for vision correction. Ghost laser radar has emerged as a new technology in theoretical and simulation applications. Laser radar is now emerging as an important technology for applications such as self-driving cars and unmanned aerial vehicles. It is also used by police to measure speed, and in gaming, such as the Microsoft Kinect.

Range uncertainty distribution of direct-detection laser radar with a peak-detecting routine

A stochastic model was firstly developed to derive the range uncertainty distribution of a peak detecting laser radar. An analytical solution was obtained, which can describe the probability density distribution of range data obtained by a peak detecting laser radar under different signal amplitudes, waveforms as well as noise distributions. The results show that the probability density distribution is proportional to the second derivative of the laser signal waveform and the probability distribution of differential noise.

Theoretical distribution of range data obtained by laser radar and its applications

This paper addresses the distribution of range data obtained by laser radar. An analytical solution of the range distribution was obtained for direct detection laser radar using constant threshold discriminator based on the time-of-flight principle. The analytical solution was verified by experiments and simulations. The results show that the derived analytical function can describe the probability density distribution of the range data obtained by laser radar with a constant threshold discriminator. The probability density distribution of the range data is proportional to the probability density function of the noise and to the slope of the rising edge of the laser echo pulse. The probability density distributions of the range data obtained by laser radar with different pulse shapes, amplitudes, widths and thresholds are also presented. These factors are important for improvements in the design of laser radar systems.

Autofocus technique for three-dimensional imaging, direct-detection laser radar using Geiger-mode avalanche photodiode focal-plane array

An autofocus technique is proposed for a three-dimensional imaging, direct-detection laser radar system that uses a Geiger-mode avalanche photodiode focal plane array (GmAPD-FPA). This technique is implemented by pointing laser pulses on a target of interest and observing its scattered photon distribution on a GmAPD-FPA. Measuring the standard deviation of the photon distribution on a GmAPD-FPA enables the best focus condition to be found. The feasibility of this technique is demonstrated experimentally by employing a 1 × 8 pixel GmAPD-FPA. It is shown that the spatial resolution improves when the GmAPD-FPA is located in the best focus position found by the autofocus technique.

Multihit mode direct-detection laser radar system using a Geiger-mode avalanche photodiode

In this paper, a direct-detection laser radar system that uses a Geiger-mode avalanche photodiode (GAPD) of relatively short dead time (45 ns) is described. A passively Q-switched microchip laser is used as a laser source and a compact peripheral component interconnect system, which includes a time-to-digital converter (TDC), is set up for fast signal processing. With both the GAPD and the TDC functioning multistop acquisition, the system operates in a multihit mode. The software for the three-dimensional visualization and an algorithm for the removal of noise are developed. It is shown that the single-shot precision of the system is approximately 10 cm (sigma) and the precision is improved by increasing the number of laser pulses to be averaged so that the precision of approximately 1 cm (sigma) was acquired with more than 150 laser pulses scattered from the target. The accuracy of the system is measured to be 12 cm when the energy of the emitted laser pulse varies with a factor of 7.

Detection and false-alarm probabilities for laser radars that use Geiger-mode detectors.

For a direct-detection laser radar that uses a Geiger-mode detector, theory shows that the single-pulse detection probability is reduced by a factor exp(-K), where K is the mean number of primary electrons created by noise in the interval t between detector turn-on and arrival of laser photons reflected from the target. The corresponding false-alarm probability is at least 1 - exp(-K). For fixed-rate noise, one can improve the detection and false-alarm probabilities by reducing t. Moreover, when background-light noise is significant and dominates dark-current noise and when the laser signal is of the order of ten photoelectrons or more, the probabilities can be improved by reducing the amount of light falling on the detector, even if the laser signal is reduced by the same factor as the background light is. Additional analytical calculations show that identifying coincidences in data from as few as three pulses canreduce the false-alarm probability by orders of magnitude and, for some conditions, can also improve the detection probability.

Three-dimensional imaging laser radar with a photon-counting avalanche photodiode array and microchip laser.

We have developed a threedimensional imaging laser radar featuring 3-cm range resolution and single-photon sensitivity. This prototype direct-detection laser radar employs compact, all-solid-state technology for the laser and detector array. The source is a Nd:YAG microchip laser that is diode pumped, passively Q-switched, and frequency doubled. The detector is a gated, passively quenched, two-dimensional array of silicon avalanche photodiodes operating in Geigermode. After describing the system in detail, we present a three-dimensional image, derive performance characteristics, and discuss our plans for future imaging three-dimensional laser radars.

The Near-Earth Asteroid Rendezvous Laser Altimeter

In 1999 after a 3-year transit, the Near-Earth Asteroid Rendezvous (NEAR) spacecraft will enter a low-altitude orbit around the asteroid, 433 Eros. Onboard the spacecraft, five facility instruments will operate continuously during the planned one-year orbit at Eros. One of these instruments, the NEAR Laser Rangefinder (NLR), will provide sufficiently high resolution and accurate topographical profiles that when combined with gravity estimates will result with quantitative insight into the internal structure, rotational dynamics, and evolution of Eros. Developed at the Applied Physics Laboratory (APL), the NLR instrument is a direct-detection laser radar using a bistatic arrangement. The transmitter is a gallium arsenide (GaAs) diode-pumped Cr:Nd:YAG (1.064-µm) laser and the separate receiver uses an extended infrared performance avalanche-photodiode (APD) detector with 7.62-cm clear aperture Dall–Kirkham telescope. The lithium-niobate (LiNbO3) Q-switched transmitter emits 15-ns pulses at 15.3 mJ pulse-1, permitting reliable NLR operation beyond the required 50-km altitude. With orbital velocity of 5 m s-1 and a sampling rate of 1 Hz, the NLR spot size provides high spatial sampling of Eros along the orbital direction. Cross-track sampling, determined by the specific orbital geometry with Eros, defines the resolution of the global topographic model; this spacing is expected to be <500 m on the asteroid's surface. Combining the various sources of range errors results with an overall range accuracy of 6 m with respect to Eros' center-of-mass. The NLR instrument design, perfomance, and validation testing is decribed. In addition, data derived from the NLR are discussed. Using altimetry data from the NLR, we expect to estimate the volume of 433 Eros to 0.01% and its mass to 0.0001% accuracies; significantly greater accuracies than ever possible before NEAR.

Imaging laser radar in the near and far infrared

Recent advances in laser transmitter technology have made available to the laser radar system designer a multitude of wavebands from which to choose. There are four infrared wavebands receiving emphasis for modest to long range (1-10 km) imaging applications each of which contains efficient, mature source technologies suitable for commercial or tactical military applications. These wavebands include far-IR band centered at 10.6 /spl mu/m and the near-IR band with three subbands located near 2 /spl mu/m, 1.5 /spl mu/m, and 1 /spl mu/m. Visible wavelengths are usually avoided for reasons of eye-safety or detectability. The near-IR subbands coincide with the peak spectral responses of the three most suitable photodetector materials in the near-IR band and include a variety of lasing molecules, ions, transitions, and host materials. The most efficient and mature of these include the CO/sub 2/ gas laser at 10.59 /spl mu/m, the holmium, thulium solid-state laser(s) near 2 /spl mu/m, the optical parametric down converted neodymium solid-state laser(s) operating near 1.5 /spl mu/m, and the neodymium laser(s) operating near 1 /spl mu/m. Diode lasers are not considered due to their low peak power capabilities. A brief comparative performance analysis is presented for the ground-to-ground scenario which discusses the key tradeoff issues between the various wavebands with emphasis on atmospheric effects, including atmospheric turbulence. A nominal set of system requirements and design parameters are chosen which, when used in the analysis, lend insight into the performance trends expected for the various wavebands. Atmospheric propagation is modeled using MODTRAN and FASCOD3 for medium and high resolution spectral absorption profiles, respectively. Example field data is presented from the Raytheon Electronic Systems 10.59 /spl mu/m coherent Tri-Service Laser Radar (TSLR), a Hercules Defense Electronic Systems, Inc. 1.064 /spl mu/m Neodymium direct detection laser radar and a Fibertek, Inc. 1.54 /spl mu/m OPO:Nd direct detection system.

An Airborne Passive/Active Electro-Optic Sensor System for Theater Ballistic Missile Defense

An airborne passive/active surveillance aircraft-based, electro-optic sensor system, named Gatekeeper, is being developed for the US Navy Theater Ballistic Missile Defense Program. The sensor is designed to detect theater ballistic missiles (TBMs), either in boost or post-boost phase, and make precision three-dimensional measurements of the TBM's post-boost, ballistic trajectory with sufficient state-vector accuracy for handover to naval, air and land based missile defense systems. The sensor includes a dual-band IRST for acquisition, a precision angle tracker (MWIR focal-plane array and high-bandwidth mirror), and a short-pulse, direct detection laser radar. The sensor subsystems are coupled and controlled by a sophisticated multiprocessor computer control system. The system has a highly compressed engagement timeline resulting in a substantial target handling capacity. This paper describes the sensor, its specifications, and performance in terms of the accuracy of the state-vector, and its target handling capability in a realistic engagement scenario.