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.
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.
In ARL's current chirped amplitude modulation (AM) ladar prototypes using unity gain solid state detectors, amplifier noise limits the receiver sensitivity. This noise is well above the signal shot noise limit. We are developing a method using Geiger-mode avalanche photodiode (Gm-APD) photon counting detectors in the chirped AM ladar receiver to yield sensitivities approaching the signal shot noise limit. This method is based on the fact that the chirped AM waveform that modulates the transmitted laser power also modulates the average photon arrival rate at the receiver with a delay corresponding to the round-trip time between the transceiver and the target. We present the concept, and computer and electronic simulation results for ARL's chirped AM ladar operating with a photon counting receiver. We also present the design and initial results of the proof-of-principle laboratory optical experiment that we recently performed. The simulation and experimental results predict significant improvements in the ladar's receiver sensitivity.
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 (ARL) has developed a three dimensional (3D) imaging ladar based on an amplitude modulated laser for which the frequency of the amplitude modulation (AM) is linearly increased and/or decreased with time (i.e., chirped). The frequency chirped waveform is a standard radar and coherent ladar waveform for high resolution ranging and Doppler frequency shift measurement. ARL first demonstrated the use of this waveform with laser amplitude modulation and optical direct detection for high range resolution 3D imaging ladar. The Doppler frequency shift measurement capability of the AM direct detection ladar had been known previously, but had not been demonstrated until recently. This paper contains the first report of an experimental demonstration of the capability of an AM direct detection ladar to measure the frequency and amplitude of surface vibrations via the phase/frequency modulation induced on the return waveform by the surface motion. In addition, we present data demonstrating the measurement of line-of-sight translational velocities via the Doppler shift of the chirped AM waveform using the same apparatus. We first briefly describe the operating principles of ARL's chirped AM ladar for range and translational Doppler measurement with references to previous papers. We then present the classic theory for vibration induced phase/frequency modulation to explain the operating principles of the AM direct detection ladar vibrometer. We then describe the experimental demonstration of the AM direct detection ladar vibrometer, including descriptions of the experimental setup, data processing and analysis methods, and results.
The U.S. Army Research Laboratory (ARL) has developed a number of near-infrared, prototype laser detection and ranging (LADAR) Systems based on the chirp, amplitude-modulated LADAR (CAML) architecture. The use of self-mixing detectors in the receiver, that have the ability to internally detect and down-convert modulated optical signals, have significantly simplified the LADAR design. Recently, ARL has designed and fabricated single-pixel, self-mixing, InGaAs-based, metal-semiconductor-metal detectors to extend the LADAR operating wavelength to 1.55 mm and is currently in the process of designing linear arrays of such detectors. This paper presents fundamental detector characterization measurements of the new 1.55 mm detectors in the CAML architecture and some insights on the design of 1.55 μm linear arrays.
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.
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.
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.
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.
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.
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.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.