High Energy Laser Systems Research Articles

Precision Tracking Control Against Dynamic Targets for High-Energy Laser Systems

Precision control of a fast-steering mirror is coupled with an open-loop acquisition system to perform tracking control against a dynamic target. The integration of a high-bandwidth tracking control system with a much lowerbandwidth acquisition system is accomplished by offloading signals from the former to the latter. This technique precludes saturation of the inner loop when large-amplitude, unexpected changes occur in the target trajectory. Particular attention is given to the sensitivities of inner-loop transient response and steady-state error responses to design conditions and sample rate. Control design solutions are presented and demonstrated via experiments and simulations.

Thermal effects of a laser beam tube consisting of a window and nonflowing gas

High-energy laser systems include an optical train and a gas medium that must be capable of transporting and directing the beam. The optical train is termed a laser beam tube. Because of the finite absorption of laser energy, the thermal effects of the beam tube make the beam diffuse and cause degradation of the laser beam's quality. We study systematically a beam tube consisting of a laser window made from white bijou (i.e., Al2O3) or fused silica and a tube of nonflowing nitrogen or helium gas. The results show that the thermal effects of the window and the gas on the beam neutralize each other; in particular, in some cases they compensate for each other completely, such that the beam tube has no effect on the beam quality in spite of the fact that separately each has a severe effect. We explain how to acquire the specific cases.

High energy and narrow bandwidth mid IR nanosecond laser system

In this paper a high energy, tunable nanosecond laser system in the region of 6 μm is introduced. The laser contains three units. Via a difference frequency mixing IR-light at about 2.6 μm is generated, in a subsequent optical parametric amplifier process signal and idler at about 2.6 and 1.8 μm are produced, respectively. In the third unit the IR-light about 6 μm is produced by a difference frequency mixing of signal and idler in an AgGaSe2 crystal. The spectral bandwidth of the IR laser is better than 0.1 cm−1 and a laser energy of at least 1 mJ (at 10 ns pulse length) can be achieved. The laser is ideally suited to record IR spectra in the region of CO stretching and NH bending vibrations.

The national ignition facility high-energy ultraviolet laser system

The National Ignition Facility (NIF), currently under construction at the Lawrence Livermore National Laboratory, is a stadium-sized facility containing a 192-beam, 1 .&Megajoule, 500-Terawatt, ultraviolet laser system together with a 10-meter diameter target chamber with room for nearly 100 experimental diagnostics. When completed, NIF will be the world's largest and most energetic laser experimental system, providing an international center to study inertial confinement fusion and the physics of matter at extreme energy densities and pressures. NIF's 192 energetic laser beams will compress fusion targets to conditions required for thermonuclear burn, liberating more energy than required to initiate the fusion reactions. Other NIF experiments will allow the study of physical processes at temperatures approaching 10{sup 8} K and 10{sup 11} Bar, conditions that exist naturally only in the interior of stars, planets and in nuclear weapons. NIF is now entering the first phases of its laser commissioning program. The first four beams of the NIF laser system have generated 106 kilojoules of infrared light and over 10 kJ at the third harmonic (351 nm). NIF's target experimental systems are also being installed in preparation for experiments to begin in late 2003. This paper provides a detailed look the NIF laser systems,more » the significant laser and optical systems breakthroughs that were developed, the results of recent laser commissioning shots, and plans for commissioning diagnostics for experiments on NIF.« less

Fast ion emission from the plasma produced by the PALS laser system

This paper presents the results of studies of fast ion emission from the multiply charged high-Z number plasma generated using the PALS high-energy iodine laser system (⩽1.2 kJ, 0.4 ns) at the PALS Research Center in Prague. The properties of the emitted ion streams were investigated using ion collectors located at various angles with respect to the target normal and an electrostatic energy analyser. The x-ray emission from the plasma was measured using semiconductor detectors. Different groups of ions (slow, thermal and fast) were observed in the ion collector signals. Ion current densities higher than 80 mA cm−2 at ∼1 m from the target were demonstrated. The charge velocity distribution, ion current density and angular distribution of ion charge emission, as well as total charge and average ion energy were obtained from these signals. Using the electrostatic ion-energy analyser, the emission of highly charged heavy ions (Ta52+, Ag38+) with energies up to 7 MeV for Ta ions was demonstrated. The dependence of ion stream parameters on the experimental conditions is discussed. We also report the results of preliminary experiments on the direct implantation of laser-produced ions into various materials.

Femtosecond laser technologies for linear collider designs

A highly stabilized high-energy femtosecond laser system was developed for Compton X-ray experiments. The laser system is based on the chirped pulse amplification, and each component is actively or passively stabilized. A master oscillator with less than 100fs timing jitter, two independent oscillators with 300fs relative timing lag, a new measurement technique of timing fluctuation of low-repetition amplified pulse, and a special designed regenerative amplifier with high quality beam were developed. New technical options for linear collider are proposed based on these expertises. The options are temporally square pulse for low emittance electron generation, a timing stabilized seeder for CO 2 amplifier, and multi-pulse high-energy lasers for γ–γ collision and for multi-bunch electron generation.

Acoustic scattering from laser damage spots

High energy laser systems sustain damage to critical optical components over their lifetime. This damage is manifested as localized pitting and cracking in the glass components. The damage pits have complicated morphology consisting of a hemispherical pit lined with crushed material, and surrounded by an outer zone with cracks. Monitoring of damage is necessary in some applications to prevent catastrophic failure of critical components. An acoustic system for monitoring optical damage spots was investigated to determine its applicability for the National Ignition Facility (NIF) at LLNL. Experiments and simulations of acoustic scattering from damage spots were performed to assess the ability to detect and size optical damage before failure becomes imminent. Pulses with a center frequency of 5 MHz were scattered from damage spots with nominal sizes of 0.5 mm to 7 mm. The amplitude of the measured return showed evidence of resonant scattering from the complicated morphology of the pits. This was confirmed with numerical simulations of scattering from simplified models of damage spots. This talk will emphasize the physics of scattering from damage spots and the comparison between experiments and numerical simulations.

Modelling of stress-induced birefringence in vacuum barrier lenses

Stress-induced birefringence arising from vacuum loading in the spatial filter lenses of high-energy laser systems is responsible for laser beam depolarization which may decrease the frequency conversion efficiency significantly. In order to predict these effects, we have carried out coupled mechanical-optical calculations. At first, we validated our predictions with experimental results. Then a parametric study allowed us to explain the influence of the geometrical characteristics of the lenses. Our main result is that symmetric lenses lead to minimum birefringence.

Spatial filter pinhole for high-energy pulsed lasers

Spatial filters are essential components for maintaining high beam quality in high-energy pulsed laser systems. The long-duration (21 ns) high-energy pulses envisioned for future inertial-confinement fusion drive systems, such as the U.S. National Ignition Facility (NIF), are likely to lead to increased plasma generation and closure effects within the pinholes in the spatial filters. The design goal for the pinhole spatial filter for the NIF design is to remove small-angle scatter in the beam to as little as a ?100-murad divergence. It is uncertain whether this design requirement can be met with a conventional pinhole design. We propose a new pinhole architecture that addresses these issues by incorporating features intended to reduce the rate of plasma generation. Initial experiments with this design have verified its performance improvement relative to a conventional pinhole design.

Materials for high-power laser optics: figures of merit for thermally induced beam distortions

High-power, high-energy laser (HEL) systems include an op- tical train consisting of mirrors and windows, which must be capable of transporting and directing the beam without seriously degrading the nominal performance. Since catastrophic failure modes are not a major threat at beam-power levels of current interest, the system's perfor- mance as measured in terms of achievable target irradiances may de- grade as a result of thermal lensing, that is, the wavefront distortion caused by thermally induced phase aberrations. Analytical investigations that address the problem of evaluating the nature of laser-driven mirror/ window distortions are reviewed and updated. In this context, a method is proposed for obtaining simple figures of merit (FoM) for rating the thermal lensing performance of mirror-substrate materials as well as window-material candidates. The performance of cooled HEL mirrors re- flects their ability to minimize irradiance-mapping wavefront distortions, which leads to defining a thermal distortion coefficientj that controls the out-of-plane growth of the faceplate. It is then straightforward to derive equations for characterizing the root mean squared surface deformation and to assess the merit of mirror-faceplate materials in a pulsed or a cw environment. Since state-of-the-art heat exchangers exhibit relatively modest Biot numbers, the thermal conduction is not a critical parameter but the modulus of elasticity must be properly factored into the FoM for cw operation. Window-induced wavefront deformations require special attention because they involve not only position-dependent variations of the window thickness but also position- and polarization-dependent variations of the refractive index. This situation leads to introducing sym- metric and antisymmetric distortion coefficients, which can be combined into an effective optical distortion coefficientx that specifies the relative weight of birefringence compared to all other sources of distortion and shows that zero distortion can be achieved only with stress- birefringence-free material having a negative dn/dT. As in the case of mirror-faceplate materials, FoMs for the prediffusion and the steady- state regimes emerge in a direct manner and demonstrate that fluoro- zirco-aluminate glass by far outperforms other window-material candi- dates in implementing the zero-distortion goal. © 1997 Society of Photo- Optical Instrumentation Engineers. (S0091-3286(97)00106-2)

Treatment of facial rhytides with a high-energy pulsed carbon dioxide laser.

Facial rhytides are a common problem that often presents for treatment. Because of the tendency for rhytides to involve delicate tissue areas, traditional treatment modalities have been limited by complications of scarring or pigmentary alterations. With the advent of the new-generation high-energy pulsed carbon dioxide lasers, skin requiring resurfacing can now be treated successfully with minimal risk and side effects. This study included 259 patients with facial rhytides (104 perioral, 83 periorbital, 53 glabellar, and 17 forehead). Each patient received treatment with a high-energy pulsed CO2 laser system, and individual responses were evaluated independently by two blinded assessors at 1, 4, 8, 12, and 24 weeks postoperatively. While clinical response rates were uniformly excellent, they varied in different locations. On average, there was a 90 percent improvement in all areas under study. The periorbital regions responded best, an average 93 percent improvement being seen. The facial rhytides with the lowest response rates (86.8 percent) were those which were most severe and those caused by excessive muscle movement (i.e., frown lines at the glabella). No incidences of scarring were noted, but erythema persisting 1 to 3 months (mean 2.2 months) following laser irradiation was typical. Transient postinflammatory hyperpigmentation of 3 months' average duration was seen in 30 percent of patients and was not limited to those individuals with darker skin tones. The high-energy pulsed carbon dioxide laser is a safe and effective treatment for facial rhytides. Treatment can be delivered in an outpatient setting without the need for general anesthesia, and preliminary evidence suggests long-lasting results.

Comparison of Two High-Energy, Pulsed Carbon Dioxide Lasers in the Treatment of Periorbital Rhytides

Carbon dioxide (CO2) laser technology has expanded over the past 20 years from the production of precise excisional and cutting instruments to the recent development of high-energy, pulsed systems that allow for controlled tissue vaporization. These newest lasers permit removal of thin layers of skin with minimal damage to normal adjacent skin structures. The precise and reproducible nature of tissue destruction has led to a resurgence of interest in cutaneous resurfacing. To compare the clinical effectiveness, side effect profile, and postoperative course of two different high-energy, pulsed CO2 lasers (ultrapulse and surgipulse) in the treatment of periorbital rhytides. Ten patients with moderate to severe periorbital rhytides received laser treatment using the surgipulse CO2 laser on one side and the ultrapulse CO2 laser on the opposite periorbital region. Equivalent laser parameters and treatment conditions were used with both systems. Sequential clinical analyses were performed independently by two blinded assessors. In addition, optical profilometry measurements of silicone rubber skin surface casts were obtained before and after laser irradiation to determine skin texture changes. There was a 63% average clinical improvement of periorbital rhytides following surgipulse laser treatment and a mean improvement of 82% after ultrapulse laser irradiation. Skin surface texture in all laser-treated rhytides approximated those found in normal skin. An increased number of laser passes were required using the surgipulse system to effect the same clinical endpoints as the ultrapulse system. While both the surgipulse and ultrapulse high-energy CO2 laser systems can effectively improve periorbital rhytides, the ultrapulse system provides a slightly enhanced clinical response with fewer passes, presumably due to improved tissue vaporization.

Polishing high heat load mirrors

The need for high heat load optics has its origin in high-energy laser systems designed in the 1970s and 1980s. The advancement of these systems to shorter wavelengths and higher flux levels and the inclusion of aspheric surfaces provided the early impetus for developing optics with advanced internal heat exchanger designs and for improved processes for polishing and testing associated optical surfaces. Today this technology continues to evolve for use in third-generation synchrotron beamlines, an emerging scientific application offering far-reaching implications for materials research , pharmaceutical development, micro-machining, and integrated circuits. Components for these applications require grazing incidence optical prescriptions and special polishing techniques, often leading to the construction of new equipment and the development of suitable polishing processes and optical test procedures. An overview is provided of the advancing state of polishing and testing techniques supporting a developing class of high heat load optical components characterized by rigorous slope error tolerances and demanding surface roughness specifications.

Serrated-aperture apodizers for high-energy laser systems

Spatial beam apodization is a critical part of the design of high-energy solid-state laser systems. Standard methods of making apodizers include photographic and metal-vapor-deposition techniques. Apodizers fabricated with these methods are subject to damage and deterioration from high-intensity laser pulses. An alternative approach is to use a serrated-edge aperture in conjunction with the spatial filter. This system can produce beams with smooth edge profiles. We present the theory of operation of the serrated aperture along with some useful design rules and describe the successful application of a serrated-aperture apodizer in the Beamlet laser system.

Continuous facesheet low voltage deformable mirrors

Litton/Itek Optical Systems has been a leader in the development of deformable mirrors for use in high quality optical systems since 1973. The monolithic piezoelectric mirror (MPM), which was introduced in 1974, has been the quality product against which all other visible wavelength compensators have been compared. The stacked actuator deformable mirror (SADM), first delivered in 1980, set new performance standards for infrared systems. These devices continue to maintain high quality large stroke performance in the field. In 1981 Itek began the development of a new concept in deformable mirrors, the low voltage electrodisplacive mirror (LVEM). Itek teamed with Bell Aerospace in 1984 to develop a cooled silicon electrodisplacive mirror (CSEM) technology. The CSEM development emphasized extending the LVEM technology to high energy laser systems applications by incorporating a silicon multiport pin-fin heat exchanger into the mirror structure. Recently, in 1988, Itek began developing a cost efficient cooled mirror technology compatible with moderate flux levels and quantity production. The LVEM uncooled technology has matured and represents the current state of the art in deformable mirror performance. Upon its maturity, the CSEM technology will provide both high performance and cost efficient cooled deformable mirrors for the needs of the high energy laser community.

Modifiziertes Lasersystem für die Laserangioplastie mit Saphirspitzen

This study deals with 26 patients on whom successful laser angioplasty of peripheral vascular occlusions had been carried out, using an Nd-YAG high energy laser system (100 watts) with a sapphire point. The technical requirements are presented and the primary success and complication rates in our material are discussed.

Large-aperture electrooptical switches with plasma electrodes

Longitudinal Pockels' cells with large apertures and high aspect ratios can be designed utilizing a low-pressure glow discharge as a transparent electrode medium. With such plasma electrodes, we have constructed a Pockels' cell with a clear aperture of 26 × 26 cm. We present here the results of a parametric study of this device, a detailed engineering model for the optical performance with plasma electrodes, and considerations for scaling the device to the large apertures (1-3 m) which are of interest to high-energy laser systems for inertial confinement fusion.

A long-pulse amplifier for solar-pumped iodine lasers

The characteristics of a long-pulse iodine photodissociation laser amplifier were investigated in order to evaluate the adaptability of an oscillator-amplifier chain as a high-energy solar-pumped laser system. The long storage time and the substantial gain observed experimentally confirm that developing such a system is indeed feasible.

KrF-laser-triggered switching of a multi-line pulsed power system

Describes the switching aspects of a 200 GW pulsed power generator which is used to power a high-energy gas laser system. Four 5 Omega water-filled pulsed-forming lines are synchronously switched with a jitter of approximately 1 ns using a few mJ from a high-brightness KrF laser. The switches are high-pressure SF6-insulated spark gaps which achieve a hold-off voltage of 1 MV, a peak current of 105 A and a peak di/dt of 1013 A s-1.

Multiple aperture laser systems for thermal blooming environments

An approach to improving power densities on target for high-energy laser systems operating in thermal blooming environments is presented. The concept employs small discrete subapertures in place of a large single aperture to reduce thermal blooming effects in the target region. Preliminary calculations indicate that subunit phase control is not essential to achieving adequate performance. Wave-optics computer codes were used to study key performance parameters in addition to predicting target irradiances for several simple (two- and four-subaperture) system models. Improvements of the order of 30% in peak irradiances were found, compared with equal total area single apertures. A preliminary evaluation implies that a multiaperture system can be configured to achieve at least comparable performance as a monolithic system but with smaller less costly optical elements.