Short term laser frequency stability is an important parameter affecting target velocity resolution of a coherent laser radar (ladar) system. This paper presents CO2 laser design considerations which affect frequency stability, measures the frequency sensitivity of the laser to various stimuli, and evaluates performance of an actively stabilized, unmodulated output CO2 laser. A frequency modulated acousto-optic modulator is used to produce a control beam which is passed through a low pressure SF6 gas cell and the laser is locked to the resultant saturated absorption peak. This investigation demonstrated laser frequency instability for periods up to 50 msec of less than ±500 Hz when the piezoelectric transducer (PZT) drive voltage was off, and about ±900 Hz when the laser was free running with voltage applied to the PZT. When a 90 dB, 200 Hz acoustic disturbance was applied, the frequency instability was less than ±4.3 kHz and ±2.8 kHz when the laser was free running and locked, respectively.
The presentation will focus on issues which relate directly to the generation of coherent CO2, laser pulses at energy levels of up to~ 1003 and duration of tens of microseconds. Important factors which influence the beam coherence will be addressed in the context of E-beam and self sustained discharge technologies. An illustration of possible local and volumetric discharge non-uniformities will be provided. Diagnostic procedures being implemented at ARL will be reviewed and experimental data provided. Detailed computer modelling of chirp phenomena has been conducted at ARL. The results compare favorably with experimental data; the modelling and experimental data will be presented.
A KrF (250 nm) excimer laser radar system design has been developed with a transmitter waveform suitable for producing image, range and Doppler velocity measurements. The short wavelength results in desirable optic sizes (1/λ scaling) and high Doppler velocity sensitivity (A scaling). The several microsecond pulse length, in conjunction with frequency stable operation, allows measurements to be made with the narrow bandwidths necessary for cm/s velocity resolutions (1/τ scaling). Good image quality from rotating targets is produced by the coherent waveform due to temporal speckle reduction and the increased spatial resolution achievable with Doppler processing.
An effort is underway to develop a high acceleration beam steering system for rapid, sequential pointing to multiple targets. A generalized analysis is first presented relating mirror steering performance and residual vibration distortions. The analysis is further generalized to systems which use multiple, independently-targeted apertures to increase retargeting rates. Then the design of an agile, two-meter steering flat and drive system is discussed. Lightweight mirror structure designs are evaluated, drive system candidates are compared, and an experiment for characterizing hydraulic servovalves and actuators is described.
A new indoor SAL (synthetic aperture LADAR) test range is described that uses a pulsed CO2 TEA laser as the transmitter source. This monostatic range can be used to produce high-resolution 3-D synthetic aperture (range-Doppler) images of targets. The SAL is a homodyne based receiver system that uses a long path reference signal with a frequency offset generated via an acousto-optic cell. The TEA laser is a multicavity system that is stepped through a series of single frequencies to obtain the ranging information. The transmission frequencies are monitored and controlled by heterodyning part of the trans-mission signal against a stable low-pressure CW CO2 laser. Azimuth (Doppler) information is generated by stepping through a series of angles on a gimbaled transmit/receive mirror. The target and SAL are mounted on different vibration isolated tables. Motion monitoring interferometers between the tables furnish motion compensation data for the image processing stage while range variations are automatically corrected during data gathering. The TEA laser is run at up to 100 pulses per second. Both the received pulse and the trans-mitted pulse are recorded by a high speed wavefrom digitizer system. This data, along with motion compensation and reference data, is used to produce imagery via Fourier transform based processing.
Two-dimensional diode laser arrays are being developed for near-term application as efficient incoherent high-power pump sources for solid-state lasers and for uses in the long term as coherent primary sources. A technique for fabricating such arrays has been demonstrated which makes use of small deflecting mirrors integrated with diode laser elements that are formed on a semiconductor wafer surface by lithographic techniques. Monolithic arrays of over 100 elements have been fabricated with good uniformity and a power density over 40 W/cm2 of wafer area.
Considerable progress has been made in the technology of producing laser diode arrays for pumping of solid state lasers. Monolithic linear CW arrays with powers of more than 5 watts have been produced and used to pump rod geometry lasers. Area arrays with pulsed power densities of more than 2.5 kilowatts per square centimeter, have been demonstrated. By carefully designing and packaging the array, the same basic diode structure can be used to produce both types of arrays.
For the first time, continuous, moderate power operation of a Nd3+:glass fiber bundle laser is reported. The disordered fiber bundle laser consisted of 1450 fibers, each 100 pm in diameter, with 75 pm core, phosphate laser glass doped at 8 wt.% Nd203. The laser fiber bundle was energized by a xenon flashlamp or, a cw argon arc lamp. A maximum energy of 0.8 J at 4.9 kW peak power was obtained in the pulsed flashlamp experiments while the maximum cw power of 25 W was obtained with the arc lamp. The loss coefficient of 0.0266 cm-1 was inferred from the data. Neither the pulsed nor the cw systems were optimized. The beam profile was imaged with low spatial resolution and found to be lobed. Beam structure is indicative of coherence. Calculations for a would be ordered fiber array laser show that the beam far field approximates a Fraunhoffer diffraction pattern for the total aperture as the beamlets fill the apperture area.
High power laser radar amplifier design issues are addressed. Laser radar oscillators must be short length and/or low pressure to obtain single longitudinal and single transverse mode (hence, single frequency) operation. Injection initiated oscillators can be used to obtain higher powers, but waveform diversity and frequency stability are limited. Master oscillator power amplifier (MOPA) configurations overcome these limitations. This paper discusses frequency stability issues and overall design alternatives for these power amplifiers. The discussion includes frequency stability calculations, allowable gas density power spectral density fluctuations, allowable electron density fluctuations, intrapulse expansion wave effects, discharge technique selection, alternative flow systems, and laser induced medium perturbation effects.
In this paper, we present an overview of different laser resonator configurations leading to smooth single transverse mode of large cross sectional area. Particular attention is given to Cassegranian resonators in which one of the mirror has a radially varying reflectivity profile. Experimental results obtained with TE-CO2 lasers are discussed. It is shown that graded reflectivity mirrors are particularly well suited to frequency stable laser emitters in coherent lidar applications.
Test data is provided on a solid state infrared source developed by High Technology Sensors.* Modulated infrared output has been obtained by injecting free carriers into a heated semiconductor. The source emits radiation from 3 microns to 12 microns. Modulation to 20kHz has been demonstrated. A qualitative source emittance model is presented and the utility of the source for gas sensing applications is discussed.
The combination of contractive solid geometry (CSG) and ray casting provides a number of advantages for modeling target signatures obtained using active laser radar systems. These include ease of modeling complex targets and a methodology well-suited for the incorporation of most important laser radar phenomenology. For applications requiring image understanding or discrimination, the use of multiple types of sensors may be required. Fusion of information from several types of sensor may provide discrimination or identification capability not achievable using a single sensor. Sensor fusion studies require a consistent set of signatures for a given target, and such sets are not currently available from measurements. The use of separately simulated signatures generated using different target models may introduce artificial signature differences due to differences in the target model which would not be present in measurements made with real systems. Our approach has been to retain the advantages of the ray casting and CSG approach, which is well-suited to active systems, and to make use of mapping techniques to include the effects of surface temperature and emissivity variations, permitting the calculation of infrared signatures. This paper discusses high-resolution signature generation for both active and passive scenes. Phenomenology addressed includes the illumination beam profile, material bidirectional reflectance effects, glint insertion, and bistatic illumination for active images, and incorporation of temperature and emissivity information for passive scenes. Simula-tion of receiver optics and detector effects are discussed for both types of sensors. In the final section, examples of multimode imagery will be presented.
Factors affecting the selection of laser rangefinder design for long-range surface-to-air and airborne ranging applications are discussed. Several candidate designs (including Nd:YAG, Raman-shifted Nd:YAG, and CO2 direct detection and CO2, 13C isotopic CO2 and frequency-doubled CO2 heterodyne detection systems) are summarized and their performance is analyzed. The 13C isotopic CO2 and frequency-doubled CO2 heterodyne detection systems significantly outperform all other candidates.
Carbon dioxide (CO2) lasers can be used in coherent optical radars. Some CO2 radars will operate through the Earth's atmosphere. Atmospheric CO2 and other gases will absorb the laser radiation. Because this is resonant absorption, dispersion will also occur. The combined effects of absorption and dispersion can significantly degrade the resolution of the radar. These effects are calculated in this paper. The particular example chosen for calculation is for a radar that is used to determine the precise range to a satellite in Earth orbit. This example was chosen to show the effects of CO2 absorption both at low and high altitudes in the atmosphere.
The superb angular, range, and Doppler resolutions of coherent laser radars have led developers to design imaging radars in multiple measurement dimensions. Designing processors to detect targets in the images generally proceeds in an ad hoc fashion and it is difficult to predict the performance of the resulting processors. This paper proposes simplified statistical models for the target, radar, and signals then uses classical detection theory to derive quasi-optimal processors which take advantage of the multipixel, multidimensional nature of the image. The target model is of a radar looking down at a vertical target against a uniform, sloping background. The paper also presents the receiver operating characteristics (ROCs) for the resulting generalized likelihood ratio test (GLRT) processors. The receivers may use any combination of intensity, range, and Doppler measurements. The target reflectivity, range, and angular location are unknown and the background reflectivity is also unknown. The forms of the quasi-optimal receivers provide analytical confirmation of the principles used in many ad hoc processors. The ROCs not only give bounds on the performance of any ad hoc processors and prove the range-only processors are usually superior to the intensity-only processors, but go on to predict how much better and under what conditions. The ROCs also predict how performance changes as a function of resolution in one or several measurement dimensions.
Differential Absorption Lidar (DIAL) is an active remote sensing technique that can be used for the measurement of atmospheric constituents like water vapor (H2O) from airborne and spaceborne platforms. An analysis of the random and systematic error sources associ-ated with the measurement of H2O are presented. The DIAL measurement analysis is given for the H2O absorption bands around 720 nm but the approach also applies to other absorp-tion bands of H2O and to other molecules. The random errors due to the uncertainties in the measurement of the lidar signal and noise associated with the background and detector dark current are discussed. Systematic errors related to the atmosphere including Doppler broadening due to backscattering from air molecules, pressure shift of H2O lines, laser line distortion and systematic errors associated with the DIAL system including spectral impurity, laser tuning error, and stability are also discussed. The results of the anal-ysis are used to evaluate the performance of the airborne Lidar Atmospheric Sensing Exper-iment (LASE) H2O DIAL system presently under development. This analysis shows that a 10 percent H2O profile measurement accuracy is possible with spatial resolutions of 200 m vertical by 10 km horizontal during night and 250 m vertical by 20 km horizontal during day. Global measurements of H2O profiles from spaceborne DIAL systems can be achieved with similar accuracy with spatial resolutions of 500 m vertical by 100 km horizontal.
The first GaAs acousto-optic modulators, recently developed, have center frequencies ranging from 1 GHz to 1.5 GHz with 300 MHz bandwidth and diffraction efficiencies reported between 28% to 12% per RF watt respectively. Applications of such devices at IR wavelengths from 1.0 to 4.0 microns seem to be ideal for uses including fiber optics.
This paper describes the development and fabrication of a low cost ranging and detection system. The system employs a pulsed GaAlAs semiconductor laser diode transmitter operating at 820 nm wavelength, where the atmospheric attenuation is reasonably low. A high sensitivity PIN diode receiver captures the reflected laser beam from the target and provides digital output directly. An ECL counter triggered by a high frequency crystal oscillator measures the time of flight of laser pulse and consequently range is calculated. An IBM (AT) compatible personal computer performs the necessary computations and displays the range, velocity and position of the target. The use of personal computer provides hardware and software flexibility to the user and minor system modifications to suit any particular application can be achieved at low cost.
Advanced, three-dimensional laser ranging systems have taken robotic applications such as bin picking, robotic guidance, manipulative tasks, and inspection processes to a new level of technological sophistication. This optical radar provides three-dimensional vision for a robotic system. This then allows tasks to be completed that were not previously possible.
Long-life, closed-cycle operation of pulsed CO2 lasers requires catalytic CO-02 recombination both to remove 02, which is formed by discharge-induced CO2 decomposition, and to regenerate CO2. Platinum metal on a tin-oxide substrate (Pt/Sn02) has been found to be an effective catalyst for such recombination in the desired temperature range of 25°C to 100°C. This paper presents a description of ongoing research at NASA-LaRC on Pt/Sn02 catalyzed CO-02 recombination. Included are studies with rare-isotope gases since rare-isotope CO2 is desirable as a laser gas for enhanced atmospheric transmission. Results presented include 1) the effects of various catalyst pretreatment techniques on catalyst efficiency 2) development of a technique, verified in a 30-hour test, to prevent isotopic scrambling when C180 and 1802 are reacted in the presence of a common-isotope Pt/Sn'02 catalyst and 3) development of a mathematical model of a laser discharge prior to catalyst introduction.
This paper describes the design of a coherent imaging CO2 laser radar, based upon the use of a continuous wave, single mode CO2 laser and the "chirp" pulse compression technique. This experimental breadboard generates three independent images of a given scene, respectively representative of the range, radial velocity and reflectance (at 10,6 μ) for each of its resolution spots. It has been tested outside and some typical images are pre-sented here, demonstrating the high Doppler sensitivity and range accuracy of pulse com-pression techniques applied to coherent CO2 laser systems.
The performance of laser signature analysis requires a diverse set of simulations and analysis tools. A key computer program for calculating target laser signatures is described as one component of the required set of tools. The program, referred to as the Laser Signature Code, version 2 (LSC-2), computes resolved and unresolved cross section, scattered field quantities, and can account for a number of observable effects, such as beam illumination, aperture diffraction, speckle noise effects, encounter dynamics, and near-field measurement geometry. A specific example is presented to demonstrate the code's utility.
In this paper we would like to summarize the United States Army Chemical, Research, Development and Engineering Center (CRDEC) development of an advanced laser standoff (remote) chemical detection system for multiple use in the modern battlefield. We will first discuss the overall program including its goals, purpose, objectives and capabilities. Next we will review the basic detection principles on which this program is based. This includes Differential Absorption Lidar (called DIAL), Differential Scattered Lidar (called DISC), Topographic Reflection for surface detection and Range Resolution for DIAL and DISC. The third and fourth areas of the paper will be a brief discussion of the existing breadboard Lidar system and its field test goals and accomplishments. To conclude we will say a few words about our future program direction.