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.
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.
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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