The Long Wave Infrared (LWIR) Profile Feature Extractor (PFx) sensor has evolved from the
initial profiling sensor that was developed by the University of Memphis (Near IR) and the Army
Research Laboratory (visible). This paper presents the initial signatures of the LWIR PFx for
human with and without backpacks, human with animal (dog), and a number of other animals.
The current version of the LWIR PFx sensor is a diverging optical tripwire sensor. The LWIR
PFx signatures are compared to the signatures of the Profile Sensor in the visible and Near IR
spectral regions. The LWIR PFx signatures were collected with two different un-cooled micro
bolometer focal plane array cameras, where the individual pixels were used as stand alone
detectors (a non imaging sensor). This approach results in a completely passive, much lower
bandwidth, much longer battery life, low weight, small volume sensor that provides sufficient
information to classify objects into human Vs non human categories with a 98.5% accuracy.
The optical performance of an infrared sparse sensor detector system is modeled. Such a system, due to its low cost, uses single element, spherical, off-the-shelf optical components that may produce poor quality off-axis images. Since sensors will not populate the entire focal plane, it is necessary to evaluate how the optics will affect sensor placement. This analysis will take into account target location, optical system aberrations, and wavelength, in an effort to determine the proper placement of the sparsely populated sensors.
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 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.
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.
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 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.
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.
We report on temporal response measurements of InGaAs metal-semiconductor-metal photodetectors (MSM-PDs) under high-illumination conditions. The peak current efficiency of the MSM-PDs decreases as the optical pulse energy increases due to space-charge-screening effects. The screening effects begin to occur at an optical pulse energy as low as 1.0 pJ/pulse, as predicted by a recent two-dimensional model. The fall time and full width at half maximum of the impulse response increase as the optical pulse energy increases and decrease as the bias voltage increases. For optical pulse energies between 1.0 pJ and 100 pJ, the rise time displays a U-shaped behavior as the bias voltage increases. This may be associated with the shape of the electron velocity-field characteristic in conjunction with screening of the dark field by optically generated carriers.
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 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 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.
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.