Mid-wave (MW) and long-wave infrared (LWIR) spectral bands (3 to 5 μm and 9 to 14 μm) are known for their robust transmission characteristics in free-space optical communications (FSOC) under various weather conditions such as haze, fog, rain, and snow. These bands are also expected to be more tolerant to atmospheric turbulence compared to the shortwave IR region (SWIR) near 1.55 μm. Conversely, low-cost, power efficient laser transmitters (Tx) and receivers (Rx) for the MW-LWIR wavelengths are not as widely available as the 1.55 μm counterpart especially for high bandwidth. Larger aperture sizes are also likely required for MW-LWIR to maintain acceptable beam divergence and adequate receiver signal-to-noise ratios (SNRs). All of these are challenges for the development of the MW-LWIR FSOC technology. In the framework of ARAP DOC-P program (Applied Research for the Advancement of S and T Priorities Defense Optical Channel Program), CCDC-ARL (Combat Capabilities Development Command Army Research Laboratory) has taken on the challenge to investigate and develop ground-to-space FSOC in the MW-LWIR regions with commensurate comparisons of MW-LWIR and SWIR systems. The effort started with a detailed literature survey on the MWIR and LWIR FSOC experiments and the latest progress. CCDC-ARL has conducted investigations of the state-of-the-art MWLWIR laser Tx and MW-LWIR photodetectors including in-house development. An FSOC ground testbed employing MW-LWIR COTS quantum cascade laser (QCL) sources is being developed. The Tx will be directly modulated using electronic circuits built in-house. In a collaborative effort with the Naval Research Laboratory (NRL), CCDC-ARL is testing a free-space link emulator based on 1.55 μm fiber optics components first developed by NRL. CCDC-ARL is also developing atmospheric beam propagation simulation tools based on random phase screens in order to gain insight and compare the performance envelope for MW-LWIR and SWIR.
KEYWORDS: Receivers, LIDAR, Sensors, Signal to noise ratio, Mirrors, Amplifiers, Optical amplifiers, Microelectromechanical systems, Transistors, Control systems
This paper presents research at the Army Research Laboratory (ARL) on a laser radar (LADAR) imager for surveillance from small unmanned air vehicles (UAV). The LADAR design is built around a micro-electro-mechanical system (MEMS) mirror and a low-cost pulsed erbium fiber laser to yield a low-cost, compact, and low-power system. In the simplest sense the LADAR measures the time-of-flight of a short laser pulse to the target and return as a means to determine range to a target. The two-axis MEMS mirror directs the light pulse to a point in the scene and establishes the angular direction to a pixel. The receiver looks over the entire region scanned by the laser and produces a voltage proportional to the amount of laser light reflected from the scene. The output of the receiver is sampled by an analog-to-digital convertor. The net result is a data file containing a range and a horizontal and vertical angle that identifies the position of every image voxel in the scene and its amplitude. This data is displayed on a computer using standard and stereo techniques to render a three-dimensional image of the scene. At this time, the LADAR operating parameters are set to form images of 256 (h) × 128 (v) pixels over a 15° × 7.5° field of view and 50 m range swath at a 5-6 Hz frame-rate to 160 m range. In the prior year, we built an initial flight package that we have flown in an auto-gyro that yielded encouraging imagery of ground targets at an altitude of roughly 100 m. Here we discuss progress to improve the performance of the LADAR to image at an altitude of 160 m and increase its mechanical robustness for extensive data collection activities.
The Army Research Laboratory (ARL) has continued to research a short-range ladar imager for use on small unmanned ground vehicles (UGV) and recently small unmanned air vehicles (UAV). The current ladar brassboard is based on a micro-electro-mechanical system (MEMS) mirror coupled to a low-cost pulsed erbium fiber laser. It has a 5-6 Hz frame rate, an image size of 256 (h) x 128 (v) pixels, a 42º x 21º field of regard, 35 m range, eyesafe operation, and 40 cm range resolution with provisions for super-resolution. Experience with driving experiments on small ground robots and efforts to extend the use of the ladar to UAV applications has encouraged work to improve the ladar’s performance. The data acquisition system can now capture range data from the three return pulses in a pixel (that is first, last, and largest return), and information such as elapsed time, operating parameters, and data from an inertial navigation system. We will mention the addition and performance of subsystems to obtain eye-safety certification. To meet the enhanced range requirement for the UAV application, we describe a new receiver circuit that improves the signal-to-noise (SNR) several-fold over the existing design. Complementing this work, we discuss research to build a low-capacitance large area detector that may enable even further improvement in receiver SNR. Finally, we outline progress to build a breadboard ladar to demonstrate increased range to 160 m. If successful, this ladar will be integrated with a color camera and inertial navigation system to build a data collection package to determine imaging performance for a small UAV.
This paper introduces a new concept that applies computational imaging techniques to laser radar for robotic perception. We observe that nearly all contemporary laser radars for robotic (i.e., autonomous) applications use pixel basis scanning where there is a one-to-one correspondence between world coordinates and the measurements directly produced by the instrument. In such systems this is accomplished through beam scanning and/or the imaging properties of focal-plane optics. While these pixel-basis measurements yield point clouds suitable for straightforward human interpretation, the purpose of robotic perception is the extraction of meaningful features from a scene, making human interpretability and its attendant constraints mostly unnecessary. The imposing size, weight, power and cost of contemporary systems is problematic, and relief from factors that increase these metrics is important to the practicality of robotic systems. We present a system concept free from pixel basis sampling constraints that promotes efficient and adaptable sensing modes. The cornerstone of our approach is agile and arbitrary beam formation that, when combined with a generalized mathematical framework for imaging, is suited to the particular challenges and opportunities of robotic perception systems. Our hardware concept looks toward future systems with optical device technology closely resembling modern electronically-scanned-array radar that may be years away from practicality. We present the design concept and results from a prototype system constructed and tested in a laboratory environment using a combination of developed hardware and surrogate devices for beam formation. The technological status and prognosis for key components in the system is discussed.
LADAR is among the pre-eminent sensor modalities for autonomous vehicle navigation. Size, weight, power and cost constraints impose significant practical limitations on perception systems intended for small ground robots. In recent years, the Army Research Laboratory (ARL) developed a LADAR architecture based on a MEMS mirror scanner that fundamentally improves the trade-offs between these limitations and sensor capability. We describe how the characteristics of a highly developed prototype correspond to and satisfy the requirements of autonomous navigation and the experimental scenarios of the ARL Robotics Collaborative Technology Alliance (RCTA) program. In particular, the long maximum and short minimum range capability of the ARL MEMS LADAR makes it remarkably suitable for a wide variety of scenarios from building mapping to the manipulation of objects at close range, including dexterous manipulation with robotic arms. A prototype system was applied to a small (approximately 50 kg) unmanned robotic vehicle as the primary mobility perception sensor. We present the results of a field test where the perception information supplied by the LADAR system successfully accomplished the experimental objectives of an Integrated Research Assessment (IRA).
Future robots and autonomous vehicles require compact low-cost Laser Detection and Ranging (LADAR) systems for
autonomous navigation. Army Research Laboratory (ARL) had recently demonstrated a brass-board short-range eye-safe
MEMS scanning LADAR system for robotic applications. Boeing Spectrolab is doing a tech-transfer (CRADA) of this
system and has built a compact MEMS scanning LADAR system with additional improvements in receiver sensitivity,
laser system, and data processing system. Improved system sensitivity, low-cost, miniaturization, and low power
consumption are the main goals for the commercialization of this LADAR system. The receiver sensitivity has been
improved by 2x using large-area InGaAs PIN detectors with low-noise amplifiers. The FPGA code has been updated to
extend the range to 50 meters and detect up to 3 targets per pixel. Range accuracy has been improved through the
implementation of an optical T-Zero input line. A compact commercially available erbium fiber laser operating at 1550
nm wavelength is used as a transmitter, thus reducing the size of the LADAR system considerably from the ARL brassboard
system. The computer interface has been consolidated to allow image data and configuration data (configuration
settings and system status) to pass through a single Ethernet port. In this presentation we will discuss the system
architecture and future improvements to receiver sensitivity using avalanche photodiodes.
KEYWORDS: LIDAR, Mirrors, Field programmable gate arrays, Sensors, Receivers, Transmitters, Data acquisition, Signal to noise ratio, Microelectromechanical systems, Robots
The Army Research Laboratory (ARL) is researching a short-range ladar imager for navigation, obstacle/collision
avoidance, and target detection/identification on small unmanned ground vehicles (UGV).To date, commercial UGV
ladars have been flawed by one or more factors including low pixelization, insufficient range or range resolution, image
artifacts, no daylight operation, large size, high power consumption, and high cost. ARL built a breadboard ladar based
on a newly developed but commercially available micro-electro-mechanical system (MEMS) mirror coupled to a lowcost
pulsed Erbium fiber laser transmitter that largely addresses these problems. Last year we integrated the ladar and
associated control software on an iRobot PackBot and distributed the ladar imagery data via the PackBot's computer
network. The un-tethered PackBot was driven through an indoor obstacle course while displaying the ladar data realtime
on a remote laptop computer over a wireless link. We later conducted additional driving experiments in cluttered
outdoor environments. This year ARL partnered with General Dynamics Robotics Systems to start construction of a
brass board ladar design. This paper will discuss refinements and rebuild of the various subsystems including the
transmitter and receiver module, the data acquisition and data processing board, and software that will lead to a more
compact, lower cost, and better performing ladar. The current ladar breadboard has a 5-6 Hz frame rate, an image size of
256 (h) × 128 (v) pixels, a 60° × 30° field of regard, 20 m range, eyesafe operation, and 40 cm range resolution (with
provisions for super-resolution or accuracy).
The Army Research Laboratory (ARL) is researching a short-range ladar imager for small unmanned ground vehicles for
navigation, obstacle/collision avoidance, and target detection and identification. To date, commercial ladars for this
application have been flawed by one or more factors including, low pixelization, insufficient range or range resolution,
image artifacts, no daylight operation, large size, high power consumption, and high cost. In the prior year we conceived
a scanned ladar design based on a newly developed but commercial MEMS mirror and a pulsed Erbium fiber laser. We
initiated construction, and performed in-lab tests that validated the basic ladar architecture. This year we improved the
transmitter and receiver modules and successfully tested a new
low-cost and compact Erbium laser candidate. We further
developed the existing software to allow adjustment of operating parameters on-the-fly and display of the imaged data in
real-time. For our most significant achievement we mounted the ladar on an iRobot PackBot and wrote software to
integrate PackBot and ladar control signals and ladar imagery on the PackBot's computer network. We recently remotely
drove the PackBot over an inlab obstacle course while displaying the ladar data real-time over a wireless link. The ladar
has a 5-6 Hz frame rate, an image size of 256 (h) × 128 (v) pixels, a 60° x 30° field of regard, 20 m range, eyesafe
operation, and 40 cm range resolution (with provisions for super-resolution or accuracy). This paper will describe the
ladar design and update progress in its development and performance.
The Army Research Laboratory (ARL) is researching a short-range ladar imager for small unmanned ground vehicles for
navigation, obstacle/collision avoidance, and target detection and identification. To date, commercial ladars for this
application have been flawed by one or more factors including, low pixelization, insufficient range or range resolution,
image artifacts, no daylight operation, large size, high power consumption, and high cost. The ARL conceived a
scanned ladar design based on a newly developed but commercial MEMS mirror and a pulsed Erbium fiber laser. The
desired performance includes a 6 Hz frame rate, an image size of 256 (h) × 128 (v) pixels, a 60° × 30° field of regard, 20
m range, eyesafe operation, and 40 cm range resolution (with provisions for super-resolution or accuracy). The ladar will
be integrated on an iRobot PackBot. To date, we have built and tested the transceiver when mounted in the PackBot armmounted
sensor head. All other electronics including the data acquisition and signal processing board, the power
distribution board, and other smaller ancillary boards are built and operating. We are now operating the ladar and
working on software development. This paper will describe the ladar design and progress in its development and
performance.
Shipboard infrared search and track (IRST) systems can detect sea-skimming anti-ship missiles at long ranges, but cannot distinguish missiles from slowly moving false targets and clutter. In a joint Army-Navy program, the Army Research Laboratory (ARL) is developing a ladar to provide unambiguous range and velocity measurements of targets detected by the distributed aperture system (DAS) IRST system being developed by the Naval Research Laboratory (NRL) sponsored by the Office of Naval Research (ONR). By using the ladar's range and velocity data, false alarms and clutter objects will be distinguished from incoming missiles. Because the ladar uses an array receiver, it can also provide three-dimensional (3-D) imagery of potential threats at closer ranges in support of the force protection/situational awareness mission. The ladar development is being accomplished in two phases. In Phase I, ARL designed, built, and reported on an initial breadboard ladar for proof-of-principle static platform field tests. In Phase II, ARL was tasked to design, and test an advanced breadboard ladar that corrected various shortcomings in the transmitter optics and receiver electronics and improved the signal processing and display code. The advanced breadboard will include a high power laser source utilizing a long pulse erbium amplifier built under contract. Because award of the contract for the erbium amplifier was delayed, final assembly of the advanced ladar is delayed. In the course of this year's work we built a "research receiver" to facilitate design revisions, and when combined with a low-power laser, enabled us to demonstrate the viability of the components and subsystems comprising the advanced ladar.
Shipboard infrared search and track (IRST) systems can detect sea-skimming anti-ship missiles at long ranges. Since IRST systems cannot measure range and line-of-sight velocity, they have difficulty distinguishing missiles from slowly moving false targets and clutter. In a joint Army-Navy program, the Army Research Laboratory (ARL) is developing a chirped amplitude modulation ladar to provide range and velocity measurements for tracking of targets handed over to it by the distributed aperture system IRST (DAS-IRST) under development at the Naval Research Laboratory (NRL) under Office of Naval Research (ONR) sponsorship. By using an array receiver based on Intevac Inc.'s Electron Bombarded Active Pixel Sensor (EBAPS) operating near 1.5 μm wavelength, ARL's ladar also provides 3D imagery of potential threats in support of the force protection mission. In Phase I, ARL designed and built a breadboard ladar system for proof-of-principle static platform field tests. In Phase II, ARL is improving the ladar system to process and display 3D imagery and range-Doppler plots in near real-time, to re-register frames in near real-time to compensate for platform and target lateral motions during data acquisition, and to operate with better quality EBAPS tubes with higher quantum efficiency and better response spatial uniformity. The chirped AM ladar theory, breadboard design, performance model results, and initial breadboard preliminary test results were presented last year at this conference. This paper presents the results of tests at the Navy's Chesapeake Bay Detachment facility. The improvements to the ladar breadboard since last year are also presented.
Shipboard infrared search and track (IRST) systems can detect sea-skimming, anti-ship missiles at long ranges. Since IRST systems cannot measure range and line-of-sight (LOS) velocity, they have difficulty distinguishing missiles from false targets and clutter. In a joint Army-Navy program, the Army Research Laboratory (ARL) is developing a ladar based on the chirped amplitude modulation (AM) technique to provide range and velocity measurements of potential targets handed-over by the distributed aperture system - IRST (DAS-IRST) being developed by the Naval Research Laboratory (NRL) and sponsored by the Office of Naval Research (ONR). Using the ladar's range and velocity data, false alarms and clutter will be eliminated, and valid missile targets' tracks will be updated. By using an array receiver, ARL's ladar will also provide 3D imagery of potential threats for force protection/situational awareness. The concept of operation, the Phase I breadboard ladar design and performance model results, and the Phase I breadboard ladar development program were presented in paper 5413-16 at last year's symposium. This paper will present updated design and performance model results, as well as recent laboratory and field test results for the Phase I breadboard ladar. Implications of the Phase I program results on the design, development, and testing of the Phase II brassboard ladar will also be discussed.
The Army Research Laboratory is researching system architectures and components required to build a 32x32 pixel scannerless ladar breadboard. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (RF) subcarrier that is linearly frequency modulated (i.e. chirped amplitude modulation). The backscattered light is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. Pixel read-out is achieved using code division multiple access techniques as opposed to the usual time-multiplexed techniques to attain high effective frame rates. The raw data is captured with analog-to-digital converters and fed into a PC to demux the pixel data, compute the target ranges, and display the imagery. Last year we demonstrated system proof-of-principle for the first time and displayed an image of a scene collected in the lab that was somewhat corrupted by pixel-to-pixel cross-talk. This year we report on system modifications that reduced pixel-to-pixel cross-talk and new hardware and display codes that enable near real-time stereo display of imagery on the ladar's control computer. The results of imaging tests in the laboratory will also be presented.
Shipboard infrared search and track (IRST) systems can detect sea-skimming anti-ship missiles at long ranges. Since IRST systems cannot measure range and velocity, they have difficulty distinguishing missiles from slowly moving false targets and clutter. ARL is developing a ladar based on its patented chirped amplitude modulation (AM) technique to provide unambiguous range and velocity measurements of targets handed over to it by the IRST. Using the ladar's range and velocity data, false alarms and clutter objects will be distinguished from valid targets. If the target is valid, it's angular location, range, and velocity, will be used to update the target track until remediation has been effected. By using an array receiver, ARL's ladar can also provide 3D imagery of potential threats in support of force protection. The ladar development program will be accomplished in two phases. In Phase I, currently in progress, ARL is designing and building a breadboard ladar test system for proof-of-principle static platform field tests. In Phase II, ARL will build a brassboard ladar test system that will meet operational goals in shipboard testing against realistic targets. The principles of operation for the chirped AM ladar for range and velocity measurements, the ladar performance model, and the top-level design for the Phase I breadboard are presented in this paper.
We analyze the optoelectronic mixing characteristics of InAlAs, Schottky-enhanced, InGaAs-based, metal-semiconductor-metal photodetectors. For devices with Schottky-enhancement layers (SELs) of about 500 Å, the measured frequency bandwidth is less than that of a corresponding photodetector. The mixing efficiency decreases with decrease in optical power, decreases with increase in local oscillator frequency and decreases with decrease in mixed signal frequency. We attribute this behavior to the band-gap discontinuity associated with the SEL. For devices with thinner SELs (≈100 Å), the mixing characteristics are greatly improved: the bandwidth of the optoelectronic mixer (OEM) is similar to that of a corresponding photodetector and the mixing efficiency decreases only slightly with decrease in optical power. We attribute these results to the enhancement of the thermionic/tunneling current through the thinner SEL. We also present a circuit model of the Schottky-enhanced, InGaAs-based OEM to explain the experimental results.
The Army Research Laboratory is researching a focal plane array (FPA) ladar architecture that is applicable for smart munitions, reconnaissance, face recognition, robotic navigation, etc.. Here we report on progress and test results attained over the past year related to the construction of a 32x32 pixel FPA ladar laboratory breadboard. The near-term objective of this effort is to evaluate and demonstrate an FPA ladar using chirped amplitude modulation; knowledge gained will then be used to build a field testable version with a larger array format. The ladar architecture achieves ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated (chirped amplitude modulation). The diode's output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of optoelectronic mixing, metal-semiconductor-metal detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal resulting from the mixing process whose frequency is proportional to the target range. This IF signal is continuously sampled over a period of the rf modulation. Following this, a signal processor calculates the discrete fast Fourier transform over the IF waveform in each pixel to establish the ranges and amplitudes of all scatterers.
Interdigitated-finger metal-semiconductor-metal photodetectors (MSM-PDs) are widely used for high-speed optoelectronic applications. Recently, GaAs MSM-PDs have been utilized as optoelectronic mixers (OEMs) in an incoherent laser radar (LADAR) system. InGaAs MSM-PDs would allow LADAR operation at eye-safe wavelengths, mainly 1.55 μm. Unfortunately, the Schottky barrier height on InGaAs is quite low (~0.1-0.2eV) leading to high dark current and, hence, low signal-to-noise ratio. To reduce dark current, the Schottky barrier is typically “enhanced” by employing a high-band-gap lattice-matched Schottky enhancement layer (SEL). Detectors using SELs yield low dark current, high responsivity, and high bandwidths. In this paper we analyze the mixing effect in InAlAs Schottky-enhanced InGaAs-based MSM-PDs. We find that the measured frequency bandwidth of such a mixer is smaller than when used as a photodetector. Moreover, the mixing efficiency depends on the light modulation and mixed signal frequencies and decreases non-linearly with decrease in optical power. This is not observed in GaAs-based and non-Schottky-enhanced InGaAs MSM-PDs. We present a circuit model of the MSM-PD OEM to explain the experimental results.
KEYWORDS: LIDAR, Sensors, Receivers, Modulation, Interference (communication), Signal to noise ratio, Signal detection, Signal processing, Prototyping, Optical amplifiers
The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with an optically detected, time-delayed UHF chirp scattered from a target. ARL is also investigating the use of metal-semiconductor-metal (MSM) detectors as unique self-mixing detectors, which have the ability to internally detect and down-convert the modulated optical signals. ARL has recently incorporated a 1x32 element linear MSM self-mixing detector array into a prototype FM/cw ladar system and performed a series of characterization and outdoor image collection experiments using this prototype. This paper discusses the basic performance of the prototype system and presents some fundamental measurements as well as ladar imagery taken on the ARL Adelphi campus.
Variation in rectification current with ac-bias frequency has recently been observed in metal-semiconductor-metal (MSM) detectors when utilized as optoelectronic mixers in a frequency-modulated continuous-wave (FM/cw) LADAR System. This current degrades the performance of the LADAR System by inducing false targets. In this paper, we present a detailed experimental and theoretical investigation on the origin of this current. We find that MSM detectors exhibit asymmetric current-voltage characteristics that are related to imperfections in device design and processing. We also find that, although the asymmetry is usually small, a rectification current may exist even under zero mean ac bias. Both the dark current and the photocurrent exhibit asymmetric behavior, but have opposite asymmetry with respect to one another. Under transient bias voltage the device shows two transient current responses: (1) a fast one related to the displacement current and (2) a slow one related to the removal of carriers from the device. The asymmetry in current related to the slow process is opposite to the dc asymmetry, while the asymmetry in current related to the fast process is more symmetric. The rectification current varies not only with ac voltage and optical power, but also with ac bias frequency.
The U.S. Army Research Laboratory (ARL) is investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference ultra-high frequency (UHF) chirp with a detected, time-delayed UHF chirp. ARL is also investigating the use of unique self-mixing detectors that have the ability to internally detect and down-convert light signals that are amplitude modulated at UHF. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver thereby reducing both the cost and complexity of the system. ARL has fabricated a 32 element linear array of self-mixing detectors and incorporated it into a breadboard ladar using the ARL FM/cw architecture. This paper discusses the basic theory of detector operation, a description of the breadboard ladar and its components, and presents some fundamental measurements and imagery taken from the ladar using these unique detectors.
KEYWORDS: LIDAR, Modulation, Amplifiers, Signal processing, Staring arrays, Digital signal processing, Sensors, Imaging systems, Semiconductor lasers, Data storage
The Army Research Laboratory is developing scannerless ladar systems for smart munition and reconnaissance applications. Here we report on progress attained over the past year related to the construction of a 32x32 pixel ladar. The 32x32 pixel architecture achieves ranging based on a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated. The diode's output is collected and projected to form an illumination field in the downrange image area. The returned signal is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. This IF signal is continuously sampled over each period of the rf modulation. Following this, a N channel signal processor based-on field-programmable gate arrays calculates the discrete Fourier transform over the IF waveform in each pixel to establish the ranges to all the scatterers and their respective amplitudes. Over the past year, we have built one and two-dimensional self-mixing MSM detector arrays at .8 and 1.55 micrometers , designed and built circuit boards for reading data out of a 32x32 pixel array, and designed an N channel FPGA signal processor for high-speed formation of range gates. In this paper we report on the development and performance of these components and the results of system tests conducted in the laboratory.
The optoelectronic mixing effect in metal-semiconductor-metal photodetectors (MSM-PDs) is studied. Numerical results, using the Scharfetter-Gummel scheme, are presented for gallium-arsenide (GaAs) MSM-PDs with different donor concentrations and analytical results are presented for devices with high background donor concentration operating below the flat-band condition and for low background donor concentration operating above the flat-band condition. MSM-PDs with unequal Schottky barrier heights at the electrodes (asymmetric MSM-PDs) are also studied. We find that asymmetric detectors exhibit asymmetric dc characteristics with the photocurrent asymmetry opposite to the dark-current asymmetry. We also find that the mixing efficiency of the MSM-PD increases with increase in applied ac voltage and decreases with increase in ac frequency. For asymmetric detectors, a rectification current exists even under zero mean ac bias that varies not only with ac voltage and optical power but also with ac-bias frequency. The theoretical results agree with observed experimental results.
The Army Research Laboratory is researching scannerless ladar systems for smart munition and reconnaissance applications. Here we report on progress attained over the past year related to the systems architectures, component development, and test results of the scannerless ladars. The imaging system architectures achieve ranging based on a frequency modulation/continuous wave technique implemented by directly amplitude modulation a near-IR diode laser transmitter with a radio frequency subcarrier that is linearly frequency modulated. The diode's output is collected and projected to from an illumination field in the downrange image area. The returned signal is focused onto an array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency signal whose frequency is proportional to the target range. This IF signal is continuously sampled over each period of the rf modulation. Following this, an N channel signal processor based-on field-programmable gate arrays calculates the discrete Fourier transform over the IF waveform in each pixel to establish the ranges to all the scatterers and their respective amplitudes. Over the past year, we have continued development of laser illuminators at .8 and 1.55 micrometers , built 1D self-mixing MSM detector arrays at .8 and 1.55 micrometers and built an N channel FPGA signal processor for high-speed formation of range gates. In this paper we report on the development and performance of these components and the results of system test conducted in the laboratory.
Physical Optics Corporation (POC) is developing an innovative light illumination system for a continuous wave imaging laser radar that is being investigated at the Army Research Laboratory. The illumination system will combine the output power from a number of laser diodes into one highly collimated beam with a divergence of three angular minutes. This will provide a 10-m diameter illumination spot at a distance of 5 km, and therefore, a high-power illumination field at the object of interest. The illumination system consists of several fiber-coupled laser diodes, mechanical and optical assemblies for focusing light from every fiber to a collimator focus point, and a non-imaging beam combiner-collimator with 180 degree acceptance aperture. The outgoing clear aperture of the combiner-collimator element is about 80 mm; overall the entire illuminator is compact, light-weight, and cost- effective in mass production.
Barry Stann, Ahmed Abou-Auf, William Ruff, Dale Robinson, Brian Liss, William Potter, Scott Sarama, Mark Giza, Deborah Simon, Scott Frankel, Zoltan Sztankay
We describe the technical approach, component development, and test results of a line imager laser radar (ladar) being developed at the Army Research Laboratory (ARL) for smart munition applications. We obtain range information using a frequency modulation/continuous wave (FM/cw) technique implemented by directly amplitude modulating a near-IR diode laser transmitter with a radio frequency (rf) subcarrier that is linearly frequency modulated. The diode's output is collimated and projected to form a line illumination in the downrange image area. The returned signal is focused onto a line array of metal-semiconductor-metal (MSM) detectors where it is detected and mixed with a delayed replica of the laser modulation signal that modulates the responsivity of each detector. The output of each detector is an intermediate frequency (IF) signal (a product of the mixing process) whose frequency is proportional to the target range. This IF signal is continuously sampled over each period of the rf modulation. Following this, a N-channel signal processor based on field- programmable gate arrays (FPGA) calculates the discrete Fourier transform over the IF waveform in each pixel to establish the ranges to all the scatterers and their respective amplitudes. Over the past year, we constructed the fundamental building blocks of this ladar, which include a 3.5-W line illuminator, a wideband linear FM chirp modulator, a N-pixel MSM detector line array, and a N-channel FPGA signal processor. In this paper we report on the development and performance of each building block and the results of system tests conducted in the laboratory.
The U.S. Army Research Laboratory (ARL) is currently investigating unique self-mixing detectors for ladar systems. These detectors have the ability to internally detect and down-convert light signals that are amplitude modulated at ultra-high frequencies (UHF). ARL is also investigating a ladar architecture based on FM/cw radar principles, whereby the range information is contained in the low-frequency mixing product derived by mixing a reference UHF chirp with a detected, time-delayed UHF chirp. When inserted into the ARL FM/cw ladar architecture, the self-mixing detector eliminates the need for wide band transimpedance amplifiers in the ladar receiver because the UHF mixing is done internal to the detector, thereby reducing both the cost and complexity of the system and enhancing its range capability. This fits well with ARL's goal of developing low-cost, high-speed line array ladars for submunition applications and extremely low-cost, single pixel ladars for ranging applications. Several candidate detectors have been investigated for this application, with metal-semiconductor-metal (MSM) detectors showing the most promise. This paper discusses the requirements for a self-mixing detector, characterization measurements from several candidate detectors and experimental results from their insertion in a laboratory FM/cw ladar.
We report on the fabrication and characterization of interdigitated finger, optical detectors/mixers. These devices are used in an FM/cw ladar system to detect and demodulate low intensity amplitude-modulated optical signals. Three different types of interdigitated finger structure were tested and compared in this study. We also present a theory to explain the asymmetry observed in the devices and discuss its implication in an FM/cw ladar application.
We describe the research and development of a scannerless three- dimensional (3-D) imaging laser radar (ladar) performed at the Army Research Laboratory for reconnaissance applications. Range information is obtained by a frequency modulation/continuous wave (FM/cw) radar technique implemented by amplitude modulation of a near-IR diode laser with an rf subcarrier that is linearly frequency modulated. The diode's output is projected to floodlight the downrange image area. The returned signal is focused onto the cathode of an image intensifier tube (IIT) where it is mixed with a delayed replica of the laser modulation applied to the cathode bias to modulate the tube gain. The output image of the IIT is modulated at an intermediate frequency (IF) that is sampled in time by a conventional charge-coupled device (CCD) camera. Image frames over one period of the frequency modulation are collected and stored. A discrete Fourier transform is calculated over the IF waveform to establish the ranges to all scatterers in a pixel. This processing scheme yields a scannerless ladar possessing high range resolution with no range ambiguities. We constructed a breadboard version of this ladar and used it to collect 256 X 256 pixel images of targets at 1-km ranges with 0.375-m range resolution. We present imagery collected during field experiments and discuss the direction of future research to enhance the ladar's performance.
This paper treats a practical adaptation of frequency modulation (FM) radar ranging principles to an incoherent laser radar (ladar). In the simplest sense, the ladar's laser transmitter output is amplitude modulated with a radio-frequency subcarrier which itself is linearly frequency modulated. The subcarrier signal may have a start frequency in the tens to low hundreds of megahertz and stop frequency in the hundreds of megahertz to low gigahertz. The difference between the start and stop frequency, (Delta) F, is chosen to establish the desired range resolution,(Delta) R, according to usual equation from FM radar theory, (Delta) R equals c/(2(Delta) F), where c is the velocity of light. The target-reflected light is incoherently detected with a photodiode and converted into a voltage waveform. This waveform is then mixed with an undelayed sample of the original modulation waveform. The output of the mixer is processed to remove `self-clutter' that is commonly generated in FM ranging systems and obscures the true target signals. The clutter-free waveform is then processed coherently using the discrete Fourier transform to recover target amplitude and range. A breadboard of the ladar architecture was developed around a 30-mW GaAlAs diode laser operating at 830 nm. Imagery and range responses obtained show that the theoretical range resolution of 0.25 m was attained for a (Delta) F of 600 MHz. Embodiments of this ladar are likely to be practical and economical for both military and commercial applications because low-cost continuous wave laser diodes are used, coherent optical mixing is not required, and the post- mixing processor bandwidth is low.
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