This paper presents a brief overview of excimer laser research and development performed at the Avco Research Laboratory during the past decade from 1975 to the present day. Progress and highlights are depicted against a backdrop of parallel developmental paths connecting the original search and discovery activities of the mid-seventies to the development of high power repetitively-pulsed Raman shifted excimer devices in progress today.
The Laser Physics Branch at the Naval Research Laboratory has been involved in research on excimer lasers since their inception, and has conducted research in both electron beam and discharge-pumped systems. For the past few years, a major interest has been an investigation of the scalability of discharge-pumped excimer lasers, which have advantages over electron beam systems in their demonstrated overall efficiency and operability at high repetition rates. With the use of x-ray preionization and long pulse driving circuitry, it became evident that discharge-pumped lasers had the potential for volumetric scaling well above the small aperture common to laboratory and commercial devices. Experimental demonstrations were necessary to confirm that appropriate discharges could be sustained in high pressure excimer gas mixtures over large apertures. This was done with the construction and operation of xenon chloride lasers with 10,15, and 20 cm electrode separations. Investigations were made into the feasibility of further scaling, resulting in new techniques to minimize the inductance while eliminating surface discharges.
The development of short wavelength chemical lasers is an extremely complex problem. A commonly used approach is to study chemiluminescence and measure reaction rate coefficients. In the paper we briefly describe how optically pumped lasers can be used to address critical questions concerning a candidate system's viability as a chemical laser candidate.
We describe the use of Raman beam cleanup for reducing the divergence of laser beams. Distortion-free amplification of a diffraction-limited Stokes beam with a pump beam aberrated to 120 times its diffraction limit is demonstrated. With a power conversion efficiency of 30% the resulting increase in brightness was 5000 times. We also demonstrate the combination of phase conjugation and Raman beam cleanup to remove distortions on the Stokes beam. Results are compared with theory.
Stimulated Raman scattering (SRS) in hydrogen has been used to efficiently convert ultraviolet XeF laser energy at 353 nm to both the first Stokes at 414 nm and to the second Stokes at 500 nm. Experiments with visible dye lasers have shown that the wavefront of a low intensity input beam at the first Stokes wavelength can be preserved as the beam is amplified. These results indicate that SRS techniques can be used to correct for some laser system and propagation path phase aberrations while shifting the output wavelength to regions with lower propagation losses.
This paper reviews the spectral purity, frequency stability and long-term stabilization of CO2, isotope lasers developed at MIT Lincoln Laboratory. Extremely high spectral purity and short-term stability of less than 1.5 x 10-13 have been achieved and will be discussed. A long-term stabilization technique, which was used to line-center lock any regular or hot-band CO, isotope laser transition, is described. A brief description on using CO, lasers as secondary frequency standards is also given.
Laser radar oscillators are necessarily low output power devices since (1) short optical cavities and low pressure operating conditions are required for minimum fluctuations, (2) short laser cavities and low pressures are required for single frequency, single longitudinal mode operation, and (3) small optical beam cross-sections are required for single transverse mode operation. All of these combined restrictions severely limit the attainable output power and stability of oscillators. Amplifiers, on the other hand, have no such restrictions in discharge length, gas pressure or optical cross-section. With regard to frequency stability, amplifiers are more forgiving than oscillators since only the time variation of the electron and gas density gradients affects the frequency stability. TETIi oscillator-amplifier configurations are used whenever moderate to high power (greater than about 50 watts) applications are desired. In this paper the design of amplifiers for CO, laser radar applications will be presented. This discussion will examine the issues of discharge technique selection, gain requirements, frequency stability requirements/achievability, optical folding schemes to obtain high gain-length products, backward wave feed-back minimization/suppression, and flow requirements/achievability for high average power operation. These are some of the issues that must be considered in the design of moderate power CW and pulsed laser radar amplifiers for short and long range applications.
Two instrumentation facilities for laser-related research in physical sciences and biomedicine are in operation at the MIT Spectroscopy Laboratory. This article describes their organization, resources and performance.
Aerodyne Research, Inc. is pursuing an active research program aimed at the discovery, development and demonstration of new laser based techniques for the measurement of trace molecular species of interest in environmental and industrial processes. Laser technology exploited in these efforts include advanced lead salt tunable diode, IR HeNe, dye and excimer systems. Specific examples of environmental applications include atmospheric boundary layer trace flux measurements for biogenic, photochemical and combustion produced trace species such as CH4, N20, OCS, 03, NO2, HNO3, and SO2 important in stratospheric ozone layer chemistry, acid dry deposition and atmospheric greenhouse studies. Methods for probing the trace species content of natural water samples will also be discussed. Laser based monitoring systems for industrial processes include detectors for key reactive species in microcircuit fabrication techniques such as plasma etching and chemical vapor deposition and trace species monitoring in molten foundry metals and electronic chip materials.
As part of its research mission - to investigate the interaction of intense radiation with matter -the Laboratory for Laser Energetics (LLE) of the University of Rochester is developing a number of high-peak power and high-average-power laser systems. In this paper we highlight some of the LLE work on solid-state laser research, development and applications. Specifically, we discuss the performance and operating characteristics of Omega, a twenty-four beam, 4,000 Joule, Nd:glass laser system which is frequency tripled using the polarization mismatch scheme. We also discuss progress in efforts to develop high-average-power solid-state laser systems with active-mirror and slab geometries and to implement liquid-crystal devices in high-power Nd:glass lasers. Finally we present results from a program to develop a compact, ultrahigh-peak-power solid-state laser using the concept of frequency chirped pulse amplification.
A. Linear Frequency Converters
1. Erbium 1.73 μm-pumped KI Laser between 2-4 μm
Previously,1 we reported laser action for the lithium (F2+)A center in KI in which tuning was achieved from 2.59 to 3.65 μm. With more experimental improvements, we reported (F2+)A-center tunability ranging from 2.38 to 3.99 μm. Furthermore, with the addition of F2+ centers coexisting with (F2+)A centers, the combined range of continuous tuning extends below 2 μm, spanning almost an entire octave. The 1.73-μm line from a pulsed Er:LiYF4 (YIE) laser was used as a pump source for both centers. The F2+ center consists of an electron trapped by an adjacent pair,2-5 and the (F2+)A center consists of an F2+ center next to a substitutional cation impurity, such as Li+.6-7
Alexandrite lasers have been developed to meet the requirements of several important applications. A number of these lasers are described which show capabilities including high average power, high energy/pulse, narrow linewidth, and tunability. Other lasers with especially simple, conduction cooling are also described.
Vibronic lasers are continuously tunable because of the interaction of lattice vibrations with transition metal electronic energy levels. Their tunability makes them attractive for a number of applications. The history, practical limitations on performance and basic theory of operation are described. Investigations of Ti:YA103 and Ti:GSGG, two promising new materials, are discussed.
A technique for using two lasers operating in tandem to remotely ignite oil pools in the harsh Arctic environment has been developed. A CW CO2 laser "preheats" a localized area of the oil pool for several seconds, thereby raising the surface temperature above the fire-point. A fire is then ignited by a focused high-power pulse from a second laser. The fire sustains itself and spreads when the absorbed laser energy is distributed, by convection in a thin surface layer, over a sufficiently large area. An optical system has been designed which allows the laser beams to be focused and directed onto oil pools from a helicopter.
Two new capabilities offered by the application of laser rangers to submunitions are (1) range to the target and (2) target height profiles. This additional information enhances submunition performance by improving the probability of detection and reducing false alarms. Laser rangers using new semiconductor laser diodes offer the potential to meet the severe size, weight, and power constraints of advanced submunition concepts. The feasibility of using laser rangers in a submunition application is explored and trade-offs identified.
Cambridge Research and Instrumentation, Inc. (CRI) has been involved in a program of calibrations and detector characterization using intensity-stabilized lasers as light sources. The intensity stabilizers have proved widely useful in calibrations applications, several of which are discussed here. A commercial version of this instrument is currently being manufactured for use in calibrations and other applications in the visible and near-IR; a version for use at up to 1.6 microns is being developed.
A new kind of active atomic line filter (ALF) using the thallium metastable state as an absorption medium is discussed. The basic principle underlying the operatiqn of a Tℓ-ALF is based on photodissociative excitation of TℓCℓ, or transfer excitation of Tℓ(2P3/2) in Tℓ-Cs mixtures. The operating wavelength of a Tℓ-ALF is at 535 nm which matches the wavelength of a frequency-doubled Nd-Bel laser. The results of modeling calculations and related experimental studies are presented.