There is increasing interest in the comparative roles of CO2 and the more recently developed eye-safe solid-state lasers for long-life efficient laser radar applications. This paper assesses recent technology advances in each area and their roles in laser radar and especially Doppler lidar and DIAL development. The key problems in eye-safe solid-state lasers are discussed relating to the energy transfer mechanisms between the complicated energy level manifolds of the Tm,Ho,Er ion dopants in hosts with decreasing crystal fields such as YAG or YLF. One concerns optimization of energy transfer for efficient lasing through choice of dopant concentration, power density, crystal field and temperature, with the highly practical goal of minimal cooling needs. Another key problem, specific to laser radar and lidar, involves tailoring of energy transfer times to provide efficient energy extraction for short, e.g., Q-switched pulses used in DIAL and Dopper lidar. Special emphasis is given to eye-safe lasers in the 2 μm range because of the high efficiency applications to DIAL and (windshear) Doppler lidar and because they are well suited for Optical Parametric Oscillator frequency conversion into the important ≈ 4 to 5 μm DIAL range. The discussion of CO2 lasers concerns recent advances in Pt/Sn02 oxide catalysts and other noble metal/metal oxide combinations. Emphasis is given to the dramatic effects of small quantities of H20 vapor for increasing the activity and lifetime of Pt/Sn02 catalysts and to increased lifetime operation with rare isotope 12C18O2 lasing mixtures; iL-the 12C18O2 laser wavelengths in the 9.1 μm range are of special interest for space-based Doppler lidar such as the proposed Laser Atmospheric Wind Sounder.
The presentation will focus on issues which relate directly to the generation of coherent CO2 laser pulses at energy levels of tens of joules and durations of tens of microseconds in 12C16O2 and 13C16O2 gas mixtures.
This paper describes two types of waveguide modulators designed for CO2 laser frequency shifting applications. The first one is a low optical loss ThF4 buffered GaAs waveguide, interfaced with a broadband microwave transmission line operating in the X- and Ku-band (8-18 GHz). The second one is a GaAs single mode waveguide modulator operating in the baseband (0-5 GHz). Device structures and performance will be detailed.
A tutorial review of the principles of single sideband (SSB) electro-optic modulators is presented. Recent developments for broadbanding such modulators by multisection techniques and dual tandem arrangement is described. The discovery and use of crystal rotational orientation effects on the performance of multicrystal configurations is considered. The techniques for the theoretical treatment using coupled-mode equations for both multicrystal arrangements are developed. The results of theoretical calculations are presented in graphical form. These demonstrate the enhanced capabilities in terms of efficient conversion and increased bandwidths. Wherever possible, comparison of theory and experiment will be presented. The enhanced performance at 10.6 μm and at 1.06 μm available high quality crystals will be reviewed. The choice of crystals for each wavelength region is based on the physical properties such as symmetry, dielectric constant, electro-optic coefficient, optical and microwave losses. These parameters are incorporated in the theory and practical considerations of availability and quality required for optimum performance.
The measured single sideband conversion efficiency of a 10.6 μm bulk-type CdTe electro-optic modulator over the 14-18 GHz modulation frequency range is shown to be in close agreement with the coupled-mode and segmented modulator theories. The paper addresses the effects of the rotational orientations of segmented crystals and indirectly proves that a broadband multisection modulator is feasible; it further shows that a modulator with crystals in rotatable segmented circular waveguides is spectrally widely tunable. The effects of mechanical pressure and off-axis beam propagation on conver-sion, mode purity, and beam quality are also discussed.
For 10.6 μm laser radar systems which utilize a double sideband modulator as a local oscillator frequency shifter in the receiver section, it is necessary to remove the carrier signal and the unwanted sideband signal prior to routing the frequency-shifted radiation to the heterodyne detector. This paper discusses the use of a reflective-mode Fabry-Perot etalon in conjunction with an optical duplexer to remove the carrier signal. The duplexer is a thin-film polarizer together with a quarter-wave plate. Since the sideband-to-carrier power out of the modulator is typically a few percent, it is imperative that mirror surface error and finite coating reflectivity be properly accounted for in order to achieve an adequate filter design. A rejection ratio corresponding to 20 dB between the laser carrier and the sideband was measured for the present design.
Bulk acousto-optic modulation was investigated as a technique for generating high bandwidth linear chirp waveforms for ladar applications. The high modulation bandwidth was achieved by cascading the Bragg cells, in which each cell produces only a fraction of the total bandwidth, and large optical beam diameters and high optical energies can be used. Both germanium and thallium arsenic selenide (TAS) were investigated as acousto-optic materials, and comparisons were made between the two materials. Cooled germanium was also studied as a means of lowering the acoustic attenuation, and the thermal conductivity of cryogenically cooled Ge was measured.
Shorter wavelength lasers have advantages over longer wavelength lasers due to their narrower beam divergence angles and larger Doppler frequency shifts. However these potential advantages may not always be useable since they pose stringent requirements on beam pointer/tracker accuracies and on the laser gain medium optical distortions (spatially and temporally). Wavelength scaling equations are discussed which numerically show these advantages and limitations. Both coherent and non-coherent systems are evaluated, but the main emphasis is on coherent systems. The advantages of shorter wavelengths are often not as great as initially perceived. The wavelength variation of target cross section and the reflected laser speckle pattern are discussed. Single speckle-lobe detection imposes wavelength dependent limits on the receiver aperture. Speckle pattern rotation and translation puts limits on coherent detection times vs. wavelength, but velocity resolution is unchanged with wavelength. Laser propagation through the atmosphere is briefly reviewed for various laser wavelengths. The "maturity" and "applicability" of laser technology is discussed as a function of laser wavelength.
Spaceborne optical pointing and tracking systems have historically used passive sensors. With the advent of space based laser programs ,such as submarine laser communication ( SLC SAT ) and laser atmospheric wind sounder ( LAWS ), it is now possible to conceive of laser based active pointing and tracking systems. In this paper we present some the advantages of going to an active pointer-tracker, the advantages of going to a short wavelength system, and what the performance of this system would be based on current technology.
This paper describes the breadboard of a coherent imaging infrared radar, which is based upon the use of a continuous wave CO2 laser and can operate in three different modes invol-ving frequency modulation/demodulation processing techniques, i.e. Linear Frequency Modulation (or chirp) pulse compression, FMCW (triangular Frequency modulation) and CW (or offset homodyne). In each mode of operation, this laser radar delivers two dimensional images of a scene, coded in range and/or Doppler velocity, and/or reflectance. After recalling the basic principles of these techniques, one describes their experimental implementation and compares their performance.
A microwave synthetic aperture radar (SAR) exploits coherent target-return processing to achieve an along-track spatial resolution better than its antenna's diffraction limit. It also uses its range resolution capability to enhance its across-track spatial resolution. This paper considers the application of SAR techniques to the optical wavelength-coherent laser radar-regime. It presents performance analysis for optical SARs, focusing on their along-track and across-track spatial resolutions, their carrier-to-noise ratios (CNRs), and their signal-to-noise ratios (SNRs) in a variety of situations. Results are first obtained for performance under ideal operating conditions. Then the effects of laser frequency instability, atmospheric turbulence, and radar-aim errors are factored into the theory. The analysis is illustrated by system performance calculations for optical SARs based on reasonable technology parameters.
When the aperture size of a coherent laser radar is much. larger than the turbulence coherence length, the radar's performance is significantly degraded by turbulence-induced beam distortions. This paper addresses the correction of turbulence effects on such large aperture radars by means of adaptive optics. The radar system model is of a ground-based monostatic radar doing angle-angle imaging of a resolved speckle target located high above the earth's atmosphere. The degradation in performance due to turbulence is quantified. A system for correcting the turbulence distortions is then described and its performance analyzed. The turbulence information for wavefront correction is obtained from the target return. Separation of the speckle and turbulence distortions in the target return is necessary in order to improve the spatial resolution. The system transmits multiple wavelengths to achieve this separation, and gives a spatial resolution within 1.2 times diffraction limited.
The design of imaging waveforms for a heterodyne detection range-Doppler laser radar is dependent upon the target dynamics as well as the laser radar transmitter and receiver component technology constraints. This paper deals with the receiver and signal processing issues associated with three specific coherent waveform types. The waveform types are simple unmodulated pulse trains, linear frequency modulated (LFM) chirp pulse trains and bi-phase shift keyed (BPSK) pulse trains. The pulse trains of each type are parametric and can be used to produce a variety of range-Doppler imaging, range only, or Doppler only measurement waveforms.
The optimal single-pixel processor for speckle-target detection using laser-radar intensity measure-ments has long been known, along with its performance-its receiver operating characteristic (ROC). Most laser radar target-detection problems, however, involve multidimensional measurements-e.g., range and intensity measurements-of multipixel speckle objects. Recently, Mark applied statistical decision theory to the multipixel, multidimensional problem; he derived quasi-optimal intensity-only and range-only target detection processors, and obtained quantitative results for their ROCs. This paper extends Mark's work by presenting a quasi-optimal joint-range intensity processor which can be easily implemented in existing peak-detection systems, and developing-through computer simulation-its ROC behavior. The simulation routine is based on the previously-verified pixel statistics of 2-D pulsed-imager operation, and it reproduces Mark's theoretical intensity-only and range-only ROC predictions. The simulation shows that joint range-intensity processing offers significant performance benefits over intensity-only and range-only systems.
The performance of a coherent range-Doppler imaging laser radar is dependent upon the quality of the waveform generation and processing techniques. This paper deals with the performance of wideband waveforms in the presence of systematic and random amplitude and phase errors which occur during the generation and processing of these waveforms.
Previous work in the theory of laser radar operation has addressed the problems of detection, range and Doppler estimation for multipixel speckle targets. In this paper, the theory of automatic tracking for such targets is developed. A track-while-image approach is adopted in which the laser radar produces a sequence of raster scans, called frames, across the region containing the target. From the present frame, an estimate of the current target location is generated and fed to a tracking filter, which both combines this information with the evolving sequence of location estimates, and determines the radar's line of sight for the next frame. Within this structure, various image-centroid, hot-spot, and template-matching trackers for multipixel speckle targets are described. These estimators all exhibit signal-dependent noise, i.e., their covariances consist of a constant term plus one or more terms that are outer-quadratic in target displacement from frame center. Nevertheless, they have been successfully incorporated into a generalization of the Kalman-linear-least-squares tracking-filter that can use pre-computed gains. Specific system examples are presented to illustrate the transient and steady-state behavior of several of these trackers.
A new method for inverting single wavelength lidar returns, has been elaborated. The method is considered to be, in particular, of practical use for determinations particulate emissions from industrial stacks. The advantageskof the method are ilustrated by comparison of its solutions with solutions that one obtains from the other methods that have been receantly published.
Recent work in optical autodyne techniques have presented the underlying theory and have investigated both S/N capabilities and measurement sensitivities for different target scenarios. Here we examine time-varying Doppler derived autodyne signatures of rotating targets. In contrast to heterodyne measurements of Doppler signatures, which may be related to projections of cross-range target scattering centers, autodyne measurements may be related to the autocorrelation of projected target scattering centers. It is shown that, under certain circumstances, application of reconstruction from projection techniques (e.g., backprojection) to time-varying autodyne signatures of rotating targets reconstructs the autocorrelation function of target scattering centers. For simple targets, the reconstructed auto-correlation function may aid in the determination of maximum target extents and target symmetries. Reconstruction techniques have been applied to both simulated and experimental autodyne data.
Coherent ultraviolet light has been generated by frequency doubling of visible wavelength mode-locked pulse trains derived from krypton-ion pumped dye lasers and flashlamp pumped dye lasers operating in a master oscillator/power amplifier configuration. Measurements of the coherence of the resulting ultraviolet pulse trains have shown them to have transform limited frequency spread. Optical heterodyne measurements were made on spinning objects in the visible and translating objects in the ultraviolet at resolutions of sub-cm/s and about 2 cm/s accuracy, respectively.
The frequency chirping of high pressure CO2 lasers operated at 10 atmospheres was investigated experimentally. When the high pressure lasers were not placed in optical cavities, significant changes in the index of refraction of the laser medium were observed. A rapid phase perturbation associated to the discharge was then detected; the phase shift could attain 100 degrees. This rapid phase perturbation was followed by a much slower phase drift. The insertion of the high pressure CO2 lasers in optical cavities led to single mode emission when seeded with continuous CO2 lasers. Absolute chirp rates reaching 200 MHZ/μs were measured during the 100 nanosecond pulses.
A digital signal processing sub-system has been developed for a coherent carbon dioxide laser radar system at Lincoln Laboratory's Firepond Research Facility. This high-resolution radar is capable of operating with a variety of waveforms; hence, the signal processing requirements of the sub-system vary from one application to the next, and require a sub-system with a high degree of flexibility. The primary function of the Data Acquisition sub-system is to provide range-Doppler images in real-time. Based on this objective, the sub-system must have the ability to route large amounts of digitized data at high rates between specialized processors performing the functions of data acquisition, digital signal processing, archiving, and image processing. A distributed processing design approach was used and the hardware design implemented was configured using all off-the-shelf commercially available products. The sub-system uses a high speed 24 MB/sec central bus and associated processor acting as the hub of the system. Attached to the bus is a large RAM memory buffer. Also attached to the central bus are individual processors which interface to specialized peripherals, performing the tasks of digitizing, vector processing, imaging, and archiving. The software for the complete Data Acquisition and Signal Processing sub-system was developed on a Digital Equipment MicroVAX IITM computer. Software developed for the completed system is coded mostly in a high level language to promote flexibility, modularity, and reducing development time. Some microcode had to be used where speed is essential. All Software design, development, and testing was done under VMSTM.
Image reconstruction from projections has been extensively developed in the medical field. For example, Computer Assisted Tomography (CAT) scanners measures the absorption of X-rays along ray-projections through a slice of a body. Applying reconstruction algorithms to these projection measurements leads to two or three-dimensional distribution of the mass density. We have applied similar methods which demonstrate image reconstruction from reflective projections obtained with laser radars. These baseband SAR techniques can be applied to any N-dimensional measurements resulting in reconstructions of N 1-dimensional images. For example, two-dimensional range-Doppler or angle-angle images taken from several views can be reconstructed into three-dimensional images. Shown in this paper, two-dimensional images have been reconstructed from one-dimensional range-time-intensity (RTI) data, Doppler-time-intensity (DTI) data, or a combination of both types of laser radar measurements. Typical RTI measurements can be obtained with either coherent linear FM waveforms, or with incoherent short pulse waveforms. In the DTI case, a cw waveform is sufficient and leads to true narrow-band imaging. We have applied these methods to both computer simulated data and field measurements, at both 10.6 μm and 0.53 μm wavelenghts, using various test targets.
We demonstrate a method of constructing two-dimensional projection images of objects using reflective tomography. The method consists of detecting reflected light from an object illuminated by short pulses (FWHM ≈ 100 ps) from a laser with high time resolution (FWHM ≈ 250 ps) to produce one-dimensional range-resolved data. Repetition at many aspect angles provides input to a filtered back projection algorithm that produces a 2D projection image of the object. The receiver consists of 1) a lens to image the object to a point, 2) a streak camera to provide time resolution, 3) a tv detector to record the streaked light, and 4) electronics to control the system and to store data. This paper describes the concept on which the receiver is based, the details of the prototype receiver, and the characteristics of images of many objects.
Laboratory simulation of atmospheric laser scintillation has been accomplished through a Reflective Membrane Optical Scintillator (RMOS). RMOS incorporates a novel vibrating membrane design that is virtually wavelength independent and statistically programmable in real time. The device has application in the context of wavefront simulation as it exists in turbulent medium. Wavefront disturbances are created by reflecting collimated light off of the membrane surface. The reflected light undergoes spatial modulation and angular redistributions which mimic the effects observed over long path propagation through the atmosphere. Acoustic energy, coupled into the membrane through an electro-mechanical transducer, sets-up nodal vibrational modes in the bounded membrane, creating angular distortion areas that redistribute energy in the reflected beam. The statistics of the energy redistribution is a function of the applied acoustical spectrum, rms transducer voltage, and distance from the membrane surface, as well as the shape and acoustic impedance of the membrane boundary. Laboratory testing of a prototype device indicate spatial irradiance variations in the reflected beam exhibit probability distributions having log-normal symmetry with angle-of-arrival deviations up to 25 milliradians and variances from .01 to 1.0.