Several solid-state lasers that operate in the eye-safe region of the spectrum could be employed in LIDAR applications. Although laser radar systems based on CO2 laser technology have successfully been employed in wind velocity LIDAR systems, several advantages would accrue if a shorter wavelength laser could be employed. Characteristics that normally improve as the wavelength decreases include: range resolution, velocity error, scattered signal strength, detector detectivity, and optics size. Disadvantage of operation at shorter wavelengths result primarily from the higher quantum-limited noise and increased susceptibility to atmospheric turbulence. However, if the wavelength becomes too short, eye safety problems may result, especially if coherent detection is required. Fortunately several solid-state lasers operate in the nominal eye-safe region, wavelengths longer than 1.5 μm. Solid-state lasers have the additional advantages of compact size and reliability. With the advent of diode pumping, these devices would also have the benefit of extremely long lifetime.

The development of a safe and easy-to-use DF laser for ranging applications is described in detail. The waveband of operation around 3.8 μm is optimum for atmospheric transmission in most weather conditions. The high peak powers in excess of 2 MW with a pulse length of less than 100 nsec, coupled with a beam divergence of approximately twice the diffraction limit make this an efficient source for long range applications.

The paper presents a review of tunable vibronic solid state lasers for DIAL measurements and provides new experimental results for tunable Ti:Sapphire lasers, materials development and spectral bandwidth narrowing through injection control. The Ti:Sapphire laser materials study indicates promise for reducing undesirable absorption in the lasing region to below 1% per cm. Pulsed injection control of a Ti:Sapphire laser with a 2.5 pm narrow band pulsed dye laser and with a Ti:Sapphire laser is demonstrated with near to complete energy extraction, indicating homogeneous line broadening. The review covers the status of tunable solid state lasers in the wavelength ranges around 1.6 and 2.3 μM for DIAL measurement of important trace gases such as CH4 and CO and development needs for lasers with reduced cryogenic cooling needs. Ti:Sapphire and Alexandrite lasers are compared as lasers for DIAL measurements of H2O vapor and pressure and temperature at ≈ 720, 940, and 760 nm. The effects of laser gain on optical damage, energy extraction and amplified spontaneous emission are indicated for several tunable lasers.

Measurements have been conducted to determine the utility of commercially available silicon avalanche photodiodes as detectors in single photon ranging systems. When cooled and operated in a gated Geiger mode these detectors offer an attractive alternative to photomultipliers. Seven different types of diodes were evaluated for Geiger mode operation. Characteristics such as dark noise and temporal response were used to select the best diode types. Response time studies were conducted on the selected diodes at very low light levels using a mode-locked frequency doubled Nd:YAG laser and a picosecond resolution streak camera system. Single photon response time distributions with standard deviations as small as 90 picoseconds were observed. Detection efficiency at the singles level was also studied. Using a parametric down conversion process to generate a source of correlated photon pairs, the absolute single photon detection efficiency was measured at 532 nanometers. Efficiencies of 28% were observed and changes in the detection efficiency with gating voltage were studied. Results from low and moderate intensity laser ranging with Geiger mode diodes are discussed. The ranging results acquired at the Goddard Optical Test Facility include both terrestrial targets and the Laser Geodynamics Satellite, LAGEOS.

We present results of an investigation into interdigital Schottky barrier photodetectors, using silicon-on-sapphire as an exemplary material. The steady-state and pulsed characteristics of the devices have been studied, both theoretically and experimentally, with good agreement being found in both regimes. In particular, the effective carrier lifetime in the material was found to be approximately 400 psec, and the response time of a small area device was found to be less than 30 psec. In addition, for purposes of comparison, some measurements are reported using this structure on bulk silicon.

In 2-D pulsed imager and 2-D Doppler imager laser radars, the intermediate frequency return signals are generally filtered, envelope detected, and peak detected in a pre-processor sub-system. Because the input to the peak detector is often thresholded, these systems are subject to dropouts. Because laser radar targets commonly exhibit speckle statistics, these systems are prone to detecting anomalous peaks. This paper presents results for the dropout, anomaly, and accuracy performance of peak detection pre-processors. The impact of the pre-processor on reflectivity determination is also analyzed. The essential features of the theory are verified through peak-detector reflectivity measurements made with the MIT Lincoln Laboratory 2-D pulsed imager laser radar test bed.

Theoretical formulation of scattering for a layered atmosphere is presented in this paper. This theory is a generalization of previous formulation based on the small-angle approximation of the radiative transfer equation for laser beams propagating through spherical aerosols. Relative contribution to the beam irradiance from the various orders of scattering are presented for the case of laser beam propagating in successive layers of clouds. Numerical data on broadening as a function of optical depth is presented.

We present a summary of a theory for the ensemble average of the laser radar cross section of a target immersed in an atmosphere characterized by a random index of refraction. The theory is applied to the calculation of the effective cross section of a specular disk oriented normally to the incident field. Numerical results are presented for the effects of range and outer scale size on the effective laser radar cross section of the disk.

The interpulse coherence of the pulses within a pulse train from a mode-locked TEA CO2 laser was measured as the path delay of a Michelson interferometer was varied over two wavelengths about a nominal pulse spacing delay of 10.6 ns. An observed visibility of 70 percent was obtained, indicating a linewidth of about 5 MHz for the individual longi-tudinal modes in the gain profile.

A new method is described for reducing the length of the TEA-CO2 laser pulse tail. Experimental results are presented which demonstrate a reduction of the total pulse length, measured at a level of 0.5% of the peak intensity, from 2.5 μs to 0.32 μs. This is achieved with no concomitant losses in the peak intensity.

A modified Fabry-Perot interferometer along with a thin tilted etalon has been used successfully to obtain single longitudinal mode pulses from a 10 atmosphere pressure CO2 laser. Reliability of 85% was observed for cavity lengths up to 2.8 meters. Continuous tuning over a frequency range of 15 cm-1 was obtained on both the R and the P branches of the 9 μm and 10 μm transitions using a grating.

A laser distance and displacement measurement system is being developed to monitor small displacements in large space structures for strain analysis and structural control. The laser is amplitude modulated at a variable frequency by a voltage controlled oscillator which also serves as a reference oscillator in a mixer. The reflected laser beam is focused on a detector and the detected signal is mixed with the reference. The dc error voltage from the mixer is maintained at null by shifting the modulating frequency. Small displacements are indicated by a change in modulation frequency which is adjusted to maintain quadrature between the received signal and the reference signal from the voltage controlled oscillator in a phase-locked-loop. Measurement of absolute distance is accomplished by sweeping the modulation frequency from a quadrature lock point to an adjacent lock point. A breadboard system has been tested with a laser diode and a resolution of a few ppm has been demonstrated.

A class of portable, eye-safe, GaAs laser rangefinders for industrial applications are described. These units operate at high repetition rates and can achieve accuracies of a few centimeters over distances from less than 1 meter to over 500 meters. A range of applications and some of the features unique to these products are described in this paper.

In the design of military optical systems there frequently exist conflicts among requirements for high performance or precision, size and weight, and the ability to withstand harsh environments. The Mini Laser Rangefinder, a second-generation production system designed for the Army, accurately measures and displays the distance to battlefield targets. Designed to fit in a field jacket pocket, it is half the weight and 60 percent smaller than its predecessor, the AN/GVS-5. The Mini Laser Rangefinder was designed for complete interchangeability of modules without realignment or readjustment. It was designed to minimize production costs by eliminating unnecessary parts and precision interfaces, by incorporating a simpler, more easily made optical system and by using precision plastic molded parts wherever possible. This paper describes the design evolution, performances and packaging/material innovations of this new production laser rangefinder.

The Tracking Techniques Branch of the Johnson Space Center has had a five-year effort to develop advanced rendezvous, stationkeeping, docking, and robotic tracking sensors. This effort was in response to an identified NASA need for a development program to address the long term tracking needs of Shuttle, Space Station, Manned Maneuvering Units, Orbital Transfer Vehicles, Orbital Maneuvering Vehicles, Tethered Satellites, Satellite Servicing, and other free flying experiments and payloads. One result of this effort at JSC has been the design, development, and breadboarding of a laboratory model of a Laser Docking System Radar. This system uses a solid-state laser to measure target position and attitude for a large variety of vehicles and payloads. This laboratory prototype has been software modeled and used in an active simulation to verify performance. Also, hardware testing is in progress on the laboratory model in the tracking test bed at NASA JSC. The next step in the verification of the Laser Docking System Radar is its use in flight experiments aboard the Orbiter. Experiments are planned which will allow flight experience to be gained for the Laser Docking System Radar with minimum impact to the Orbiter. The Radar would be mounted in the payload bay of the Orbiter in a location which would allow a view of the -Z docking axis. A Grid Computer would be used inside the cabin to provide control to and display data from the radar. The connection between the radar and the Grid will be a fiber optics link that looks through a very small part of the aft Orbiter window. This method will eliminate the need for complex Orbiter interfaces or modifications. The Laser Docking System Radar will be manifested as a piggyback experiment on normal retrieval missions. The Radar will operate in a data taking and displaying mode while the crew carries out manual stationkeeping and docking maneuvers. The data from the flight experiments will then be used to design an operational sensor that is small, lightweight, and capable of meeting the long term tracking needs of NASA and industry in rendezvous stationkeeping, docking, berthing, and robotic control.

A pulsed laser diode ranging system is described that contains glass fibre reference paths. By repetitively switching between target measurements and calibration measurements via the fibres the target range data are directly related to the fixed fibre calibration paths. Since long term stability of the rangefinder time base is no longer mandatory, a very high frequency digitizer clock can be used. Within a limited distance range of some meters accurate and fast range measurements are possible.

It is theoretically and experimentally demonstrated that Gaussian reflectivity mirrors (GRMs) can improve the performances of lasers for radar applications. The effects of misalignment and hard apertures are investigated. The optimum design parameters are given. Single mode operation of a TE-CO2 laser was obtained in a Cassegrain resonator made of a hard concave mirror and a convex mirror with a Gaussian reflectivity profile. The 70-nsec pulses (FWHM) had an energy of 175 mJ, a peak power of ≈ 2.0 MW, a near diffraction limited far field and a chirp rate smaller than .060 MHz/μsec2.

Most applications of TEA CO2 lasers in heterodyne laser radars require that the transmitter have a high degree of frequency stability. This ensures good Doppler resolution and maximises receiver sensitivity. However the environment within the device is far from benign with fast acoustic and electrical transients being present. Consequently the phenomena which govern the frequency stability of pulsed lasers are quite different from those operative in their cw counterparts. This review concentrates on the mechanisms of chirping within the output pulse; pulse to pulse frequency drift may be eliminated by frequency measurement and correction on successive pulses. Experimental evidence for laser-induced, plasma, acoustic, and anomalous dispersive effects is examined, and it is demonstrated that normally only the first two effects are of any significance. It emerges that good stability hinges on correct cavity design. The energy-dependent laser-induced frequency sweep falls dramatically as mode diameter is increased. Thus it is necessary to construct resonators with good selectivity for single mode operation while having a large spot size. Various approaches to this requirement are described including recently developed novel techniques. Finally the future for compact frequency-stable devices is investigated.

This paper reports results on recombination of pulsed CO2 laser dissociation products with Pt/Sn02 catalysts, and supporting studies in a surrogate laboratory catalyst reactor. The closed-cycle, pulsed CO2 laser has been continuously operated for 106 pulses with an overall power degradation of less than five percent. This operation was achieved by flowing the laser gas mixture through a 2% Pt/Sn02 catalyst bed. In the surrogate laboratory reactor experiments have been conducted to determine isotopic exchange with the catalyst when using rare-isotope gases. Some isotope exchange has been observed but a technique for minimizing such exchange has been demonstrated. The effects of catalyst pretreatment, catalyst sample weight, and catalyst composition and temperature on catalyst efficiency have also been determined.

The use of Mercury Cadmium Tellurium photodetectors in CO2 laser radar systems has been of interest for many years. In this paper we will describe data on our long wave infrared photodiode detectors for use in 10.6 μm heterodyne laser receivers which operate at 78 K with bandwidths up to 1 GHz. Performance data on these diodes at 78 K will be presented. Typical measured dc characteristics are quantum efficiencies greater than 60% at 10.6 μm a spectral response optimized for CO2 laser radiation, RA products greater than 120 Ω-cm2 at 50 mV reverse bias and breakdown voltages greater than 1.5 volt. The frequency response of these devices is limited by the dewar and preamp characteristics and is typically greater than 1 GHz. The heterodyne NEP is within a factor 1.5 of the quantum limit. Heterodyne NEP data as a function of local oscillator power, bias voltage and frequency are presented. These data will be compared with a heterodyne device/equivalent circuit model. Device model predictions of junction resistance and capacitance are made and the relevance to heterodyne operation is discussed. Theoretical predictions are compared to measured data and the implied device physics is discussed.

CO2 laser rangefinders have been used for some years to complement the operation of 8 to 12um thermal imaging systems. Being wavelength compatible they can range on to any point in the thermal scene, and have the added advantage of being eye safe.' Using coherent detection and operating the laser in a CW mode the rangefinder can also provide information on target movement in the form of Doppler shifts which produce frequency modulation of the carrier. This enables acquisition of velocity and vibration data. Other options can include active imaging by use of an add on scanner and anemometry for air turbulence studies.2 Systems of this type normally use a low power, (up to 10 watts) frequency stable CO2 laser, a modulator to superimpose any carrier frequency code on the laser beam and a cooled detector, normally a Cadmium Mercury Telluride photo-diode operating at 77K. These detectors have a high quantum efficiency, and a frequency response which can be well in excess of 1GHz. One disadvantage is that the detectors require Joule-Thompson or engine cooling; this results in added bulk due to high pressure gas cylinders or compressor, with consequent extra capital and running costs. These systems would be more attractive and versatile if this inconvenience could be avoided by running at higher temperatures. With direct detection, higher temperatures result in a rapid degradation of performance due to reduction in detectivity. Heterodyne detectors, thermoelectrically cooled to about 190K, offer convenience without as large a penalty, due to their greater dependance on quantum efficiency. This paper describes the design of a Cadmium, Mercury Telluride p-type photoconductive detector with its associated thermo-electric cooler, bias supply and amplifier in a complete package. Performance under both laboratory conditions and in a laser rangefinder is discussed and results are compared, with a photo-voltaic detector,operating at 77K using a joule-Thompson cooler.

Acousto-optic spectrum analysis is probably the simplest optical signal processing technique with real practical value. A signal applied to an acousto-optic Bragg cell deflects a beam of light onto an array of photodetectors. The detector which is illuminated indicates the signal frequency. The range of bandwidths and resolutions of currently available Bragg cells is well matched to the requirements of laser radar systems, making the technique of interest for laser radar applications. In this paper the acousto-optic spectrum analyser is described and the performance of such a processor is presented.

In the present paper some investigations on technology and applications of room-temperature 10.6 µm HgCdTe detectors are described. They have intermediate parameters between cooled photondetectors and uncooled thermal ones at high frequency. The limit performance, theoretical possibility of obtaining near quantum limit of NEP and some experimental re-sults are shown.

The laser cloud mapper is a rapidly scanning laser radar system for transmission and concentration mapping of aerosol clouds in 3-dimensions and real-time dynamic information. It has been applied to the evaluation of IR screening clouds, forestry spraying and rarified aerosols.

GEC Avionics have supplied a number of long range CO2 Laser Doppler Velocimeter (LDV) systems for specific research applications. This paper describes an experimental laboratory based Laser Doppler Velocimeter system, which has been constructed as part of a programme of work aimed at reducing the size and cost of ground based LDVs. The system incorporates an acousto-optic Bragg cell to provide an offset local oscillator, which enables target speed and direction to be resolved. An RF waveguide CO2 laser is e,Liployed to provide both the transmitter and local oscillator beams. The system has been used to measure the instantaneous velocity of spinning wheel targets and laboratory generated aerosol. Some examples of the processed returns from a spinning wheel target are presented.

A scanning lidar bathymeter has been developed for airborne hydrographic surveying. The system has depth penetration capability of four optical diffuse attenuation lengths, with an accuracy of ±0.3 m. From an altitude of 500 m the system generates a swath 270 m wide, and a uniform sounding density on a 35 m grid spacing, with a positioning accuracy of approximately 15 m. A description of the system is given, with emphasis on the design and performance characteristics of the optical transceiver. Results from system flight tests are summarized, including those from a recent operational hydrographic survey in the Canadian Arctic.

The power of lidar to measure vertical profiles of stratospheric clouds and aerosols with fine vertical resolution (< 1 km) and a narrow field of view (approximately 1.5 milliradians) has been demonstrated through a long history of ground-based and aircraft experiments l. Additionally, lidar techniques have been used to study volcanic eruptions, to measure parameters such as cloud top heights, height of planetary boundary layer, and the distribution of cirrus and sub-cirrus clouds2. Additionally, performance criteria of a lidar instrument in space to measure atmospheric parameters have been defined through extensive development of mathematical simulations.3,4 The purpose of this experiment will be to provide experimental data of atmospheric backscatter at three wavelengths (1.06, .532, and .355 microns) for validation and verification of key parameters in the mathematical simulations of future space-based lidar experiments. Additionally, the performance of the lidar electro-optical system and mechanical, thermal, and structural configurations will be evaluated. In this paper, plans to conduct a technology experiment with a Shuttle-based lidar instrument will be presented. Design of the LITE instrument will also be given, and will include performance goals for the laser transmitter, telescope, optical receiver and associated electronics. Experimental results from space flight of the LITE instrument will be used to define performance criteria and designs for future atmospheric sounding experiments planned for the Space Station, Earth Observing System (EOS) platform and for more complex scientific lidar experiments.

The ability of lidar to measure vertical profiles of water vapor using the Differential Absorption Lidar (DIAL) technique has been demonstrated for several years using ground- 3Fasedi- ana airborne experiments. 4,5 Tunable dye laser transmitters have been used with real-time experimenter control in tuning of the laser wavelength to locate and lock onto the atmospheric absorption spectrum lines for DIAL measurements. The LIDAR Atmospheric Sensing Experiment (LASE) program is the first step in the overall NASA effort to develop and demonstrate an autonomous tunable DIAL laser instrument for airborne and spaceborne flight experiments. Performance criteria of a DIAL instrument to measure water vapor and aerosol vertical profiles in the atmosphere have been defined through extensive development of mathematical simulations. One of the objectives of the LASE program is to verify and validate these mathematical simulations, and conduct scientific investigations of lower tropospheric water vapor and aerosols on a broad spatial scale. In this paper, plans to develop an instrument to conduct scientific experiments aboard a NASA U-2 (ER-2) aircraft will be presented. The scope of the instrument development is to design a modular instrument, to measure water vapor and aerosol in the atmosphere with a fine vertical resolution, ranging from 0 to 17 km altitude during daytime and nighttime. Design of the LASE instrument will be discussed including performance criteria for the laser transmitter, wavemeter, telescope, optical receiver and associated electronics. Two tunable Alexandrite lasers will be used as the laser sources in the LASE instrument. The modular design is intended to permit future experiments to be performed with new laser technology as it evolves from the laboratory stage. Ultimately, experimental data from the LASE program will be processed by the Experiment Scientist and made available to the scientific community for further interpretation.

Researches on the application of fluorescence LIDARs to the remote-sensing of land and sea surfaces were carried out at IROE and IEQ of the National Research Council (CNR) during last years. The researches involved the use of laboratory and computer simulations and they were mainly devoted to the selection of laser sources, to the detection and characterization of oil spills, and to the analysis of vegetation stresses. As a result a prototype of fluorescence LIDAR was designed and built with a particular attention to the problems connected with the operation on board of trucks, ships, and aircrafts. The system is therefore a rugged, light-weight, small-dimension one, having a low power consumption. The LIDAR was called F-LIDAR IROE-2. It is able to record high-resolution fluorescence spectra and most of the data-processing can be done on board of the moving platform.