The 12th Annual Symposium on Optical Materials for High Power Lasers was held in Boulder, Colorado, 30 September-1 October 1980. This Symposium has become one of the primary forums for reporting new results in the area of laser-related optical materials and optical component technology. In this most recent meeting, forty-five papers were presented in the general areas of Materials and Measurements, Surfaces and Mirrors, Thin Films, and Theory and Breakdown. Brief summaries of the principal conclusions reached in selected papers are reported.
This is a review article describing some of the techniques employed in meeting certain specifications for the fabrication of Antares 18" diameter NaC1 laser windows. Specifically, a pressure test for strength and window stability is described as well as a method for non-contact measurement of wedge angle and wedge direction utilizing a Fizeau 18" interferometer. Also the procedure followed at Harshaw in the mounting of windows is outlined.
Single layer sodium fluoride antireflection coatings have been successfully applied to the 16" diameter sodium chloride windows used in the Helios CO2 laser at Los Alamos Scientific Laboratory. This paper discusses some of the major problems encountered and reviews the results obtained.
As part of a program to develop highly reflective mirrors for applications with space-borne HF lasers, six candidate multilayer coating designs were subjected to a simulated space environment consisting of the combined effects of electron, proton, and solar UV radiation. Particle and UV rates were chosen to be only slightly accelerated over actual space conditions, and the particle energies were chosen to have appropriate penetration depths in the multilayer designs. The sample temperature was controlled and the radiations conducted in an oil-free, vacuum environment in order to minimize thermal and contamination effects. The results for two designs found to be stable - (Si,A1203)2Ag and (Si,Si0x)nAg - and a third relatively unstable design - (ZnS,A1203)4 - are discussed in this paper.
Single crystal silicon exhibits thermal, mechanical, optical, and fabrication properties appropriate for the requirements of HEL (High-Energy Laser), component technology. As a material, single crystal silicon offers low thermal expansion, low density, high thermal conductivity and high heat capacity which yield a thermal/mechanical high power deformation response that is superior to Mo, Be, WC and other mirror candidate materials. In addition, silicon's crystal structure can be used to produce periodic, sharply defined geometrical shapes in certain crystallographic planes through Preferential Chemical Etching (PCE). By properly combining this action with specific photoresist exposure and development techniques, coolant channels and other critical structures related to HEL component fabrication may be produced accurately. Silicon also can be polished to a low microroughness, yielding surfaces with low scatter and high damage resistance. These advantages apply to both CW and pulsed modes of operation, providing the potential for high quality operation at short as well as long wavelengths.
This is a review of the many advanced designs that have been proposed for large laser primary mirrors. Mirror designs are categorized and classified with the objective of suggesting a common nomenclature for identifying, describing, and evaluating the relative characteristics and merits of alternate designs. Examples are given of the construction and utility of each mirror type.
A new design is described for an unobscured, all-reflective laser beam expander. It has very high performance and can be designed for any desired beam expansion ratio. Since the components are just spherical mirrors, the cost of making this design is much less than the cost of making a confocal parabola beam expander, especially in large sizes. The key to the high performance and reasonable length of this new design is the use of a simple and very powerful design principle that can be applied to other types of beam expanders as well. An example is given of a 1.0 meter aperture 30X expander design.
Reactive etching at silicon and silicon-oxide surfaces is customarily carried out in a fluorocarbon plasma. Under such conditions, a large variety of reactive species is generated, making it extremely difficult to elucidate details of the etching mechanism; in addition, the charged species present in the plasma frequently produce undesirable radiation damage in the finished devices. We have found that dissociation of the parent fluorocarbons by multiple-infrared-photon excitation produces reactive neutral fragments which are capable of etching these surfaces. Etch gases such as CF3Br, CF2HC1, CF300CF3, and SF, may be used with poly-Si, Si02, Si3 Nd' and other substrates; SEM, ESCA, and Auger diagnostics are employed to characterize the reactions occurring at the surface. From these experiments we hope to develop a quantitative model for the reactive etching process. Possible commercial advantages of the laser-etching technique include reduction or elimination of radiation damage, increased etching rates, and improved Si02/Si specificity.
We report the first demonstration of uv phase conjugation. Using a 15 psec, 2660 Å pulse, 0.1% conjugate reflectivities were obtained via degenerate four-wave mixing in 1-mm samples of CS2 mixtures. While pure CS2 did not exhibit the effect, dilution in several uv transmitting solvents opened up a concentration-tunable (2450 Å - 2850 Å) spectral window, allowing the optical Kerr effect to be utilized. Weaker phase conjugation at 2660 Å was also observed in other Kerr media and in saturable absorber media.
Industrial acceptance of the high power CO2 laser as a metalworking production system has led to increased awareness of the potential of this powerful tool as a cost effective alternative to conventional manufacturing processes. Awareness of the laser's potential developed ten years ago and now, due to the availability of production-rated industrial systems, the laser is on the verge of becoming a widely used high technology product in the manufacturing plant for productivity improvement. Illustrated examples present an overview of the current status of high power lasers in applications, such as: welding, cutting, heat treating, cladding, and alloying. In these examples the unique advantage of the laser beam as a highly controllable heat source is shown to produce cost benefits unobtainable by conventional processes. Other laser beam advantages such as remote source location, long working distance, no-contact processing, and relative insensitivity to the workpiece provide attractive options in manufacturing operations. New applications in controlled surface melting and laser assist machining provide a glimpse into the future of high power laser technology.
Using a master oscillator-power amplifier (MOPA) configuration, 100J has been obtained from a 5 k optical volume KrF amplifier pumped at 300 kW/cm3 peak at the end of a 500 nsec pulse. A normalized input flux of 0.4 (I/Isat) was required to reduce amplified spontaneous emission (ASE) to one half the free running value. A peak extraction efficiency of 0.46 was obtained under these conditions at 1 atm total pressure. A kinetics model has been developed and refined to give excellent agreement with high and low pump regimes. An integrated kinetics and pulse propagation code has also been developed and agrees well with experiment.
A study program has been completed which assesses the feasibility of using rare gas-halide lasers as near term inertial confinement fusion (ICF) drivers. A prime objective of this program was to provide credible, design parameter maps to establish energy levels achievable with current and near term technologies. Key program elements include laser scaling projections, modular e-beam and pulsed power technology, optical angular multiplex configurations,optical structures, multibeam pointing and alignment, gas flow and acoustics issues, and trade-off analyses. Rare gas-halide lasers, in particular the KrF* laser, can be designed to meet ICF requirements. These lasers are scalable, emit at short wavelengths (KrF* 250 nm) and, through the use of optical angular multiplexing, can produce the required high energy (~1-5 MJ) in a short pulse (~10 ns) with projected overall efficiency in the range of 5-7%.
The free electron laser (FEL) is a source of tunable, high power, coherent light. In the FEL, the sinusoidally varying magnetic field of the wiggler, polarized at right angles to the beam, is backscattered by the beam electrons, resulting in a wave length shortening by a factor of approximately 2γ2 via a double Doppler shift. γ is the relativistic factor. The cross section of the interaction is enhanced by an axial beam bunching, effected by the ponderomotive force of the combined wiggler and scattered E.M. wave fields. Although the efficiency derived to date for high γ has been on the order of .2% (in terms of conversion from electron energy to output radiation), recent theoretical results suggest single pass efficiencies of 5 - 10% are achievable by tapering the magnetic wiggler fields. In this process electrons are trapped in ponderomotive potential wells which are decelerated, transfering the electrons' energy to output radiation. Free electron lasers which operate as both oscillators and amplifiers have been designed on this principle. This technology is potentially useful for a number of DOE programs, in particular, electron heating and inertial confinement fusion, as well as being an interesting scientific tool. The current status of research in the field and requirements for several applications will be discussed.
The Free Electron Laser (FEL) is shown to be a potentially attractive solution to the problem of finding a suitable short wavelength fusion driver. The design of a 3 MJ, 250 nm FEL fusion driver is discussed. An inertial confinement fusion driver must be an efficient, reliable, inexpensive source of high intensity radiation. The free electron laser has the potential for meeting these requirements, therefore we have investigated an approach for incorporating an FEL into a fusion reactor. The major components of an FEL system are a laser master oscillator, an electron accelerator, an FEL amplifier (wiggler magnet), and an electron beam dump for energy recovery. These are shown schematically in Fig. 1.
Wavelength-agile, single and multiline laser radiation has been obtained from a subsonic gas flow system which is optically pumped with a multiline chemical laser. This optical resonance transfer laser (ORTL) concept was first demonstrated on the 10.6μm DF/CO2 system in 1976. Since then, several IR laser pumped molecular lasers have been demonstrated. The pump laser is either a CW HF or DF chemical laser. Two classes of ORTL have been developed: inter- and intramolecular ORTLs. The demonstrated intermolecular systems include: 10.6μm DF/CO2, 10.8μm DF/N20, 4.1μm DF/HBr, 3.8μm HF/DF and 3.85μm HF/HCN. The intramolecular ORTLS include 2.9μm HF/HF and 3.9μm DF/DF. Demonstration experiments and the kinetics of ORTL systems will be described.
A methodology for advanced laser resonator alignment systems has been developed. The resonator alignment approach maintains acceptable phase quality of the output laser wavefront in the presence of optical element misalignment. The advanced laser resonator was of the Half Symmetric Unstable Resonator with Internal Axicon (HSURIA) configuration. The effects on wavefront quality of optical element misalignment in angle and translation were determined for this resonator configuration. The methodology for the alignment system was based on the misalignment sensitivity analysis. Perfect resonator alignment can be achieved only by optical element motion equal and opposite to the misalignment error. For resonator misalignments occuring at each optical element location, the requirements on the alignment system can become untenable. Adequate resonator alignment can be achieved by the introduction of wavefront aberrations which compensate those arising from resonator misalignment. The compensatory wavefront aberrations can be achieved by optical element motions which are different from the resonator misalignment. Residual wavefront error will be presented for a compensatory alignment system applied to the HSURIA resonator configuration. Acceptable residual errors can be achieved by an alignment system in which only two mirrors are controlled in angle.
A series of sixteen diagnostic experiments were developed for the Laser Scaling Evaluation Program (LSEP). These experiments, along with a data acquisition plan, provide the basic information required to scale from moderate power to high power CW chemical lasers, and represent a reasonably complete set of typical experiments for all CW chemical lasers. This paper describes the experiments, which include measurements of gain medium optical quality, small signal and saturated gain, output beam spatial and temporal polarization, beam centroid and beam quality, spatial phase map, near-field and far-field intensity profiles, homodyne spectroscopy, mode shape, spectral content, total power and power as a function of time with high temporal frequency resolution (P(t)), cavity pressure dynamics, and mirror accelerations. The plan for processing the various data and the expected processed data are also discussed.
Performance of a pulsed HF/DF chain laser was investigated for the case of transverse initiation by a magnetically confined electron beam. Laser energy and beam quality are presented as functions of electron-gun, magnetic field, gas mixture, and optical resonator parameters; results include 65 J/liter HF laser output.
As has been reported earlier in the literature, a cylindrical laser with a conical end reflector produces a higher order tangentially polarized output due to polarization effects at the cone. By applying a special dielectric coating which produced a 90° phase shift between the S and P polarizations, the polarization effect of the cone was eliminated, and a uniformly polarized, lowest order mode output was obtained.
System optical quality (SOQ) computer code is a diffraction-based optical system simulator. It has often been used as a tool for design, optimization, power management, trade-off analysis, and system end-to-end performance predictions for high power laser applications. The system approach to code development is briefly described. The code also simulates such performance-degrading contributors as the input beam irradiance and phase distributions, mirror thermal distortions and medium thermal blooming, aberrations and misalignments of the beam expander, absorptions, and jitter of the focused spot. Examples of the above uses include applications in optical systems for beam control and laser fusion.
The capability of a conventional inertial adaptive optical system in compensating for thermal distortion is examined. Since thermal irradiance mapping is a process that transfers anomalies in intensity to aberrations in phase, adaptive optics are particularly useful. Both linear spatial filtering and variance minimization methods of analysis will be discussed. The linear spatial filtering concept will be applied to the specific problem of thermal irradiance mapping and shown to be equivalent, within the system constraints, to a least-squares fit of the deformable mirror surface to the wavefront. The special cases of Gaussian beams, annular beams, and irradiance profiles which can be expanded into Zernike polynomials will be studied in detail.
An essential requirement of high-energy laser resonator configurations is the capability of efficiently extracting power in a near-diffraction limited beam. Mode discrimination as well as optical beam quality are often compromised as a result of temporarily varying refractive index perturbations within the gain medium, optical cavity misalignment, mirror figure errors resulting from manufacturing tolerances, and thermally induced mirror distortions. Various design approaches have emerged in recent years to optimize the far-field irradiance by utilizing different resonator concepts for the purpose of obtaining the best mode control and near-field beam quality. The different concepts utilize annular and compacted gain mediums, spatial filtering techniques, novel optical components and adaptive optics. This paper will describe recent results using intracavity adaptive optical techniques versus extracavity adaptive optical techniques. Experimental results will be provided which show the correction capability for controlled low order aberrations (tilt and astigmatism) commonly found in high energy lasers. A comparison of two optimization control techniques using a multidither zonal COAT system will be discussed describing the effects of hardware limitations and considerations on the performance of the system.
This paper describes an approach for computing the illumnator power requirements of an active return wave adaptive optics system such as would be used in a long-range laser system. The illuminator power is related to the number of degrees of freedom in the adaptive optics (e.g., number of subapertures) and to the laser system beam quality requirements.
Proc. SPIE 0270, Experimental And Analytical Investigations Of The Effect Of Wall Roughness And A Curved Elbow Inlet On Thermal Blooming In Axial Pipe Flow, 0000 (15 September 1981); doi: 10.1117/12.931751
An experimental and theoretical study of thermal blooming phenomena in a laser beam propagating in an axial turbulent pipe flow is presented. The experiments were conducted by using a 250 watt CW CO2 laser as the heat source and a mixture of CO2 and N2 as the absorbing medium. The optical path difference (OPD) caused by the heating of the gas by the beam were measured by holographic interferometry, using a krypton laser at 0.5309 μM. The pipe wall roughness was varied by inserting an inner lining made of porous material. Two different inlet conditions to the pipe were considered in the experiments, namely, a straight inlet and a curved elbow. The theory was developed by modifying an existing numerical code in order to include the volumetric heating due to absorption of the laser radiation by the flowing gas. The theoretical OPDs were calculated directly from the temperature profiles, and comparison with the experimental values for the case with a straight inlet showed good agreement. The effects of a curved elbow inlet and pipe wall roughness on OPD were determined by comparison of the results with those for a straight inlet and smooth wall respectively.
A theoretical model of the photolytically initiated, pulsed DF chemical laser medium has been developed and validated against experimental data obtained on the PHOCL-50 laser device. The model employs a finite rate kinetic mechanism with all the relevant chemistry and vibrational energy transfer processes. A major assumption of the model is the use of rotational equilibrium. However, this approximation is considered adequate for the operating pressure range of the laser, near atmospheric. The PHOCL-50 data base provides a wide range of well documented experimental parameters, including the first simultaneous determination of laser pulse energy and initiation strength. The model predicts with good accuracy the experimental laser performance for a wide range of variations in mixture stoichiometry, initiation strength, added 02, and initial DF due to prereaction. An interesting area of disagreement concerns the inability of the theory to predict the laser pulse shape. This disagreement appears to be due to the details of the chemistry in the higher vibrational levels of DF. The present model has been successfully used to develop scaling algorithms and in the design of laser devices.
Thorough documentation of an object's microstructure utilizing standard photo-micrographic techniques is lengthy and laborious, especially when the object is transparent as are many optical components. Full-field, high-resolution documentation of surfaces and volumes is accomplished through holography. The holographic documentation system presented records and reconstructs objects with better than 4-μm resolution over full fields 2" in diameter. Using an Argon laser, this system is designed to document optical components such as mirrors and windows. However, it should be useful whenever high-resolution initial state documentation of any object is required.
High-power lasers require beam directing optics with high damage thresholds. To achieve this, coatings with very high reflectivity and low absorption are being developed by many investigators. This paper describes a reflectometer for measuring these high reflectances to a very high precision. Parameters investigated for this design are source stability, detector limitations, data collection schemes, and the basic optical configuration. The most promising optical configurations investigated include a single-bounce system and a goniometric type double-bounce system. Data collection was best accomplished using a noise-eliminating sample and hold system in conjunction with a gain compensated, AC coupled amplifier referenced to a duplicate system sampling the source output. Conventional lock-in and differential amplifiers were not found acceptable for the high precision sought.