Carl W. Wilson

High-speed random access laser tuning

We have developed a technique for laser tuning at rates of 100 kHz or more using a pair of acousto-optic modulators. In addition to all-electronic wavelength control, the same modulators also can provide electronically variable Q-switching, cavity length and power stabilization, chirp and linewidth control, and variable output coupling, all at rates far beyond what is possible with conventional mechanically tuned components. Tuning rates of 70 kHz have been demonstrated on a radio-frequency-pumped CO2 laser, with random access to over 50 laser lines spanning a 17% range in wavelength and with wavelength discrimination better than 1 part in 1000. A compact tuner and Q-switch has been deployed in a 5-10-kHz pulsed lidar system. The modulators each operate at a fixed Bragg angle, with the acoustic frequency determining the selected wavelength. This arrangement doubles the wavelength resolution without introducing an undesirable frequency shift.

Remote mapping of vegetation and geological features by lidar in the 9–11-µm region

We report examples of the use of a scanning tunable CO(2) laser lidar system in the 9-11-mum region to construct images of vegetation and rocks at ranges as far as 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is based on reflectance spectra, which are sufficiently distinct to allow discrimination between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Limitations imposed by laser speckle noise are discussed.

Acousto-optically tuned isotopic CO2 lasers for long-range differential absorption lidar

We are developing 2-100 kHz repetition rate CO2 lasers with milliJoule pulse energies, rapid acousto-optic tuning and isotopic gas mixes, for differential absorption LIDAR applications. We explain the tuning method, which uses a pair of acousto-optic modulators and is capable of random access to CO2 laser lines at rates of 100 kHz or more. The laser system is also described, and we report on performance with both normal and isotopic gas mixes.

Target characterization in 3D using infrared lidar

We report examples of the use of a scanning tunable CO2 laser lidar system in the 9-11 micrometers region to construct images of vegetation and rocks at ranges of up to 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Object classification is made possible by the distinct spectral signatures of both natural and man-made objects. Several multivariate statistical methods are used to illustrate the degree of discrimination possible among the natural variability of objects in both spectral shape and amplitude.

Rapidly tunable acousto-optic spectrometer for a space environment

As a complement to our work developing rapidly-tunable (approximately 10 - 100 kHz) CO2 lasers for differential absorption lidar (DIAL) applications, we have developed a rapidly-tunable spectrometer. A rapid spectral diagnostic is critical for a high speed DIAL system, since analysis of the return signals depend on knowing the spectral purity of the transmitted beam. The spectrometer developed for our lidar system is based on a double-passed large- (75 mm) aperture acousto-optic deflector, a grating, and a fast single-element room temperature mercury-cadmium-telluride detector. The spectrometer has a resolution of approximately 0.5 cm-1, a tuning range of 9.0 - 11.4 micrometers , a random-access tuning speed of greater than 80 kHz and a S/N ratio of greater than 100:1. We describe the design and performance of this device, as well as of future devices featuring improved resolution, higher speed and easier and more robust alignment. We will also briefly discuss the applications and limitations of the technique in a space environment.

Heterodyne lidar for chemical sensing

The overall objective is to assess the detection performance of LWIR (long wavelength infrared) coherent Lidar systems that potentially possess enhanced effluent detection capabilities. Previous work conducted by Los Alamos has demonstrated that infrared DIfferential Absorption Lidar (DIAL) is capable of detecting chemicals in plumes from long standoff ranges. Our DIAL approach relied on the reflectivity of topographical targets to provide a strong return signal. With the inherent advantage of applying heterodyne transceivers to approach single-photon detection in LWIR, it is projected that marked improvements in detection range or in spatial coverage can be attained. In some cases, the added photon detection sensitivity could be utilized for sensing “soft targets”, such as atmospheric and threat aerosols where return signal strength is drastically reduced, as opposed to topographical targets. This would allow range resolved measurements and could lead to the mitigation of the limiting source of noise due to spectral/spatial/temporal variability of the ground scene. The ability to distinguish normal variations in the background from true chemical signatures is crucial to the further development of sensitive remote chemical sensing technologies. One main difficulty in demonstrating coherent DIAL detection is the development of suitable heterodyne transceivers that can achieve rapid multi-wavelength tuning required for obtaining spectral signature information. LANL has recently devised a novel multi-wavelength heterodyne transceiver concept that addresses this issue. A 5-KHz prototype coherent CO2 transceiver has been constructed and is being now used to help address important issues in remote CBW agent standoff detection. Laboratory measurements of signal-to-noise ratio (SNR) will be reported. Since the heterodyne detection scheme fundamentally has poor shot-to-shot signal statistics, in order to achieve sensitive detection limits, favorable averaging statistics have to be validated. The baseline coherent DIAL detection sensitivity that can be achieved averaging multiple laser pulses and by comparisons of different wavelengths will be demonstrated. Factors that are presently limiting performance and attempts to circumvent these issues will be discussed.

GROUND COVER IDENTIFICATION AND MAPPING BY CO2 LIDAR IMAGING.

We report examples of the use of a scanning tunable CO2 laser lidar system in the 9-11 mm region to construct images of vegetation and rocks at ranges of up to 5 km from the instrument. Range information is combined with horizontal and vertical distances to yield an image with three spatial dimensions simultaneous with the classification of target type. Reflectance spectra in this region are sufficiently distinct to discriminate between several tree species, between trees and scrub vegetation, and between natural and artificial targets. Limitations imposed by laser speckle noise are discussed.

100-kHz randomly-tuned Q-switched CO/sub 2/ laser for remote sensing

Summary form only given. LIDAR return signals from targets inherently suffer from noise caused by atmospheric variations. Increasing laser energy and improving receiver photon sensitivity enable longer-range LIDAR capability; however this type of noise is multiplicative, and thus higher laser energy will not always improve the LIDAR system performance once this kind of propagation noise becomes the dominant noise source. In the latter case the system performance can be improved by increasing the number of measurements at many different transition lines before the atmosphere changes. Considering the correlation time of atmospheric turbulence of -1 ms, as well as number of transition lines of CO/sub 2/ laser, a laser transmitter with pulse repetition and tuning rate over 50 kHz should improve the LIDAR system due to the rapid collection of data before significant environmental changes occur. For this reason, we are developing a high repetition-rate rapidly-tuned Q-switched CO/sub 2/ laser transmitter. The laser consists of an RF excited CW wave-guide gain cell and an acoustooptic tuner. The tuner consists of two acoustooptic modulators (AOMs) and a beam expander, for rapid tuning and Q-switching. Laser lines are tuned by adjusting the RF frequency to the AOMs.

A rapidly-tunable acousto-optic spectrometer for tunable lasers

Summary form only given. As a complement to our work developing rapidly-tunable (/spl sim/10-100 kHz) CO/sub 2/ lasers for differential absorption lidar (DIAL) applications, we have developed a rapidly-tunable spectrometer. A rapid spectral diagnostic is critical for a high speed DIAL system, since analysis of the return signals depends on knowing the spectral purity of the transmitted beam.

Acousto-optically tuned isotopic CO/sub 2/ lasers for long-range differential absorption LIDAR

The authors are developing 2--100 kHz repetition rate CO{sub 2} lasers with milliJoule pulse energies, rapid acousto-optic tuning and isotopic gas mixes, for Differential Absorption LIDAR (DIAL) applications. The authors explain the tuning method, which uses a pair of acousto-optic modulators and is capable of random access to CO{sub 2} laser lines at rates of 100 kHz or more. The laser system is also described, and they report on performance with both normal and isotopic gas mixes.