Sidebands imposed on the output of a stable CO2 laser have been heterodyne mixed (on a 77K HgCdTe detector) with blackbody emission and alternately with radiation from a stable laser oscillator. The sidebands are generated upon passage of the beam from the laser through an electro-optic modulator consisting of: A GaAs waveguide structure (prism coupled) which has been etched with a 1 mm wide microwave microstrip transmission circuit - designed for use in the 12-18 GHz range - through which propagates the output from a traveling wave tube amplifier (TWTA). We have found that the heterodyne signal-to-noise ratio (SNR) depends in large part on the quality of the TWTA, and only under a unique set of circumstances can the theoretical limit for the ideal SNR (ηPr/hvB) be approached.
As CO2 laser-based systems have increased in sophistication, performance, and producibility, more attention has been focused on the detector unit. These units have typically been photoconductive (PC) HgCdTe detectors for low frequency systems, such as rangefinders, and photovoltaic (PV) HgCdTe detectors for heterodyne detection systems. Recent systems applications have, however, demanded more from the detector unit. Requirements have included both direct and heterodyne detection capability, small arrays of detector elements, and rugged, long-life cryogenic packages. This paper discusses the performance, packaging, and producibility of both types of detectors in current systems, and analyzes PC/PV technology trends for laser detector applications.
The frequency chirp in the output of a hybrid TE CO2 laser has been measured using a heterodyne technique. Due to the low operating pressure of about 35 Torr, laser kinetic and hydrodynamic processes differ considerably in time scale from those in regular TEA CO2 lasers. We have observed that the chirps in laser pulses up to 150 μs long could not be described adequately with existing theories. Almost complete cancellation of the laser chirp was achieved by appropriate use of acousto-optic modulators external to the hybrid TE CO2 laser. The operating and output characteristics of this laser and other parameters related to chirp will be discussed.
An agile, lightweight, and diffraction-limited beam director has been developed for a proposed space based 10.6μ laser radar. The system is diffraction limited through its 30° field of regard. Preliminary tests on the 0.5 meter development device suggest that diffraction-limited optical performance and pointing accuracy can be maintained at repointing rates of 50 per second.
The features of the Digicon detector concept are described and related to the requirements for a direct-imaging laser radar receiver. Recent experimental results on sensitivity and time resolution of a prototype detector are reported. The status of an integrated receiver experiment based on a prototype Digicon detector is reported. Laboratory experiments on the magnification of the electron image to improve the spatial resolution of advanced Digicon detectors are also described.
A new, all-solid-state pulse generator has been fabricated to meet the SDIO requirement for an efficient, reliable pulsed power source to pump CO2 discharge lasers. This SCR-commutated nonlinear magnetic pulse compressor will be fully-compatible with present CO2 laser radar sources and will serve as a basis for future drivers designed to power lasers for airborne and spaceborne applications. By employing SCRs rather than thyratron commutators, this pulser provides a significant decrease in the weight and volume and a significant increase in reliability over the current generation pulsed power drivers employed on CO2 lasers. For example the 150 joule, 10 pps, CO2 laser driver (COLD- I) weighs 85 lbs which is 50% lighter than projected pulser weights having the same characteristics. Also pulsers using this innovative technology have already demonstrated reliabilities of 1010 shots. At this juncture a breadboard pulser has been constructed and tested. The results of these tests will be presented. Future options for design changes and their impact on weight and reliability will also be discussed.
This paper discusses the affect on probability of detection (Pd) and probability of false alarm (Pfa) of the illuminator laser beam width. Laser radar systems are required to optimize performance while minimizing the laser energy thereby requiring the illuminated target region to be optimized with regard to the uncertainty of the target location. A beam spread much larger than the uncertainty volume of the target can have a high Pd but would require excessive laser energy. A very narrow beam spread would minimize laser energy but not have a high Pd. The optimum beam spread for laser illumination of a point target with a normal distribution for its position uncertainty has been determined for both direct and heterodyne (coherent) detection. This work assumes a Gaussian laser illuminator beam spread and a Gaussian target uncertainty distribution. Values for Pd and Pfa have been computed for both direct and heterodyne detection. The results were then averaged over the intensity distribution expected for the illuminator beam and the target uncertainty. Contour plots were computed for fixed signal-to-noise ratio (SNR), defined as the on-axis SNR for the target. The minima for each contour can be plotted for various ratios of illuminator beam spread to target uncertainty. The results determine the optimum ratio for a given Pd, Pfa, SNR, and detection technique. The affect of laser speckle was also included. This method of optimum beam size determination proves extremely useful in many strategic laser radar systems.
This paper considers the measurement of amplitude modulation (AM) and phase modulation (PM) noise in a tunable CO2 laser source. Theoretical and experimental heterodyned output power spectrums are used to evaluate the quality of the acousto-optically tuned source.
A variety of enhancements are being made to the 1976-vintage LASERX computer code. These include: - Surface characterization with BDRF tabular data - Specular reflection from transparent surfaces - Generation of glint direction maps - Generation of relative range imagery - Interface to the LOWTRAN atmospheric transmission code - Interface to the LEOPS laser sensor code - User friendly menu prompting for easy setup Versions of VALUE have been written for both VAX/VMS and PC/DOS computer environments. Outputs have also been revised to be user friendly and include tables, plots, and images for (1) intensity, (2) cross section,(3) reflectance, (4) relative range, (5) region type, and (6) silhouette.
Target acquisition and tracking are crucial to the success of a range-Doppler imaging laser radar. When compared to a conventional microwave radar, the laser radar's very narrow beamwidth increases the difficulty of the both acquiring and tracking the target. This paper deals with some of the issues concerning both passive and active acquisition and tracking of an uncooperative target. Cooperative targets could make use of retro-reflector arrays or beacons to enhance the target.
Ladars have been designed for various applications but not adequately explored for airborne surveillance. A Moving Target Indicator (MTI) ladar concept is presented capable of detecting small aircraft at long ranges. Atmospheric attenuation for cloud-free lines of sight is controlled by operating wavelength selection and avoidance of long horizontal paths through the lower atmosphere. Atmospheric turbulence effects were found to be insignificant. The performance of the ladar is analyzed and subsystem requirements for lasers, detectors and spectrum analyzers are developed. A computer simulation capability was developed that allows prediction of the ladar signatures of aircraft.
Interest in laser radar for many strategic applications has been increasing. Depending on the application, the purpose of the laser radar can be accomplished by any one of a number of laser sources and wavelengths. The selection process for the active sensor should always be a comparison of system designs (wavelength, laser, detection scheme) which have been optimized for the important design criteria, (weight, power, efficiency, aperture, and volume). This paper discusses a design philosophy for systematically deriving subsystem requirements for an optimum system design. This resulting design process is dependent on the system requirements and operation concept. The design steps take into account optics, beam control, laser, receiver and signal processing issues.
This paper describes an experimental imaging CO2 laser radar breadboard that is capable of producing simultaneous intensity, Doppler, and frequency modulated range images. The uniqueness of this system is that three-frequency homodyne detection, combined with FM chirp pulse compression, allows for wide frequency shifts in the intermediate frequency signal while retaining the signal-to-noise advantages of FM pulse compression. Three-frequency detection gives simultaneous Doppler-independent range and Doppler information. Higher signal-to-noise ratio gives increased system sensitivity and/or range capability. The technique for generating the two transmitted beams necessary for implementing the three-frequency detection is discussed. These beams must be parallel with spatial overlapping wavefronts. The methods for processing the return signal for obtaining intensity, Doppler, and range information are also described. The technique for obtaining Doppler-independent FM range information is discussed. Since the Doppler shift is eliminated from the signal for range processing, only one side of the FM chirp is needed to process the range and thus only one surface acoustic wave (SAW) device is needed. Moreover, the ambiguity range can be varied by fixed increments without changing any hardware and hence without changing the range resolution. Finally, some resulting images are presented to illustrate the system capability.
When a non-cooperative target is at a distance within the coherence length of a laser, the backscattered light induces modulation signals in the laser power output that can be used for velocimetry and ranging. Backscatter modulation in semiconductor laser diodes has had little practical impact in laser radar because the coherence length of these devices has restricted their operational range to about 2 m. In the present work, we report experimental measurements of absolute range using a backscatter-modulation laser radar in an external cavity whose length is varied by piezo-electric transducers. The laboratory demonstration system operates at 0.5 mW, has an operational range of 40 m and is sensitive to backscattered light levels of less than 10 pW. The extended operational range and high sensitivity of the external-cavity geometry make this simple, compact, low-power laser radar a practical alternative to heterodyne detection for many applications.
We describe results from an optical detector capable of producing three-dimensional images using single laser pulses. The method consists of detecting reflected light from an object illuminated by a short pulse from a laser with a detector that resolves many pixels in the object's image into fine time bins equivalent to range resolution of 4 cm. The detector utilizes a fiber optic image converter to transform a square focal plane into a line array that is input to a streak camera to obtain high time resolution in all the pixels. We show data from simple objects, like posts and cones, as well as more complicated objects. This work builds upon our results reported in September 1988 at the Laser Radar III SPIE conference. In that work we imaged the entire object to a point that was a single input to the streak camera. In addition we viewed the object from many aspect angles and used the range measurements to produce a two-dimensional projection image of the object. That method of reflective tomography requires gathering data from many aspect angles while the method reported here, which we call angle-angle-range, provides a three-dimensional image with data from a single aspect angle using a single laser pulse. Here we will compare the methods in detail.
The design and manufacture of the opto-mechanical sub-system for a laser radar scanner is presented, with a discussion of the initial system design considerations leading to a description of the detailed optical design. The scanner offers high spatial resolution in three dimensions, a characteristic that imposed severe limitations on the optical configuration. In addition, the requirement that it should be capable of operation in a factory environment, and hence safe to the eyes of operators, demands an extremely high signal-to-noise level to permit the use of a relatively low power laser. This requirement also dictates the need for accurate alignment, details of which are presented along with results of the optical testing.
The results of a computer model calculating the coherent LADAR vibrational signatures of a flat diffuse plate rotating about its axis of symmetry are presented. Descriptions of the target and the program structure are followed by a discussion of some specific results.
Laboratory targets have been imaged by a multi-sub-aperture, coherent receiver technique in which a common local oscillator illuminates the sub-aperture array to preserve both phase and intensity information. The target, receiver and range dimensions were chosen such that each sub-aperture was smaller than the speckle size. Various targets were illuminated by microsecond pulses from an e-beam pumped XeF power amplifier, which was seeded by a coherent ultraviolet beam generated with a frequency doubled visible dye laser. Data is presented showing comparisons between the coherent multi-sub-aperture approach and conventional, full aperture photography of the same target(s).
An optimal waveform for range-Doppler resolution is the symmetric quadratic fm. When the transmitter signal processing is ineffective in generating this waveform, due perhaps to limitations in modifying the natural chirp response of the oscillator, the resultant waveform has amplitude modulation distortion. This distortion will degrade resolution both in range and Doppler. A simple step approximation to this am distortion displaying the cogent properties is considered, and the resul-tant degradation in performance is analyzed in terms of the ambiguity function output. The am distorted waveform is less efficient in resolution than the purely fm pulse. This is because the pulse-like components have lower bandwidths, degrading compressed pulse range resolution; have shorter durations, degrading Doppler resolution; and contribute to the wave-form asymmetry, enhancing range-Doppler ridge ambiguities. Parametric analytical results, a specific example, and associated graphic illustrations are described.