The Lawrence Livermore National Laboratory has proposed construction of an upgrade to the Nova glass laser facility to give an output energy of 1.5-2 megajoules at 350 nanometers wavelength in a nominally 3-5 nanosecond shaped pulse. The Nova Upgrade will be suitable for driving inertial fusion targets to ignition. This paper reviews the design proposed for the laser.
Multi-segment disk amplifiers (MSA) are of particular interest in fusion laser systems because of the improved efficiency and compactness they offer compared with conventional disk amplifiers. A multisegment amplifier consists of two or more conventional disk amplifiers which share one or more common flash-lamp arrays. Each 'segment' amplifies one 'beamlet.' A 2 x 2 MSA can therefore amplify four beamlets. The shared flash-lamp arrays, lacking large reflectors, pump the disks more efficiently because of reduced lamp self-absorption. Furthermore, the reduction of reflector surface area per unit disk area reduces absorptive losses of pump light. The authors have constructed a 2 x 2 MSA with individual beamlet apertures of 24 cm. Each aperture contains five phosphate glass disks. This device is designed to be angularly double-passed by a 20-cm beam. Design details and rationale are presented. The energy transport performance of the amplifier was measured in both small- and large-signal regimes. A nominal small-signal gain of 11.8 was measured. The storage efficiency exceeded 3.5% at the nominal operating point. The amplifier was deployed as a booster for one beamline of the OMEGA laser system at the University of Rochester. In that configuration, 950 J of focusable energy was obtained from a single segment in a 800-ps pulse at 130-J drive. Details of these measurements are presented.
The authors have outfitted one beamline of Nova with random phase plates (RPPs) to spatially smooth the beam. The fabrication and characterization of large aperture (80-cm diameter) RPPs for use at 526.5 nm along with the performance of these optics on Nova are discussed. The present RPPs were fabricated by a conventional, although large scale vacuum deposition process. Fabrication of RPPs using a unique sol-gel coating process, which has been demonstrated at 15-cm diameter and can be scaled to the 80-cm diameter required for Nova, is also discussed.
Initial tests on experimental cesium amalgam flashlamps have yielded promising results. These lamps will pump Nd:YAG lasers with efficiencies that are comparable to those of conventional xenon and krypton flashlamps, but with substantially longer lamp life due to the vapor replenishment of the cesium cathode activator.
A high reflectivity, graded reflectivity mirror (GRM) design has been recently developed for use in low-gain lasers. A GRM with 63% central reflectivity was used as an output coupler in a low magnification (M equals 1.4) unstable resonator Nd:Glass laser to obtain a Q-switched energy of 2 J in a 25 ns pulse and over 3.7 J in fixed-Q operation. The beam has a diffraction limited divergence of 0.15 mrad. In an Alexandrite laser, a GRM having a central reflectivity of 78% was used in a M equals 1.2 unstable resonator. A fixed-Q output energy of over 500 mJ and a Q-switched energy of 400 mJ was obtained in a diffraction-limited beam. The second harmonic output was increased by a factor of ten from that obtained with a multimode stable resonator.
The continuing effort to develop, design, construct, and evaluate the performance of an airborne autonomous wavemeter for tuning solid state lasers is summarized. The wavemeter supports tunable solid state lasers that are used for an atmospheric remote sensing technique. One atmospheric species, water vapor, is measured by tuning one laser to precisely the line center of a water vapor line and by tuning another laser off the line. The two sets of received backscattered radiation are ratioed and corrected to determine the vertical profiles of water vapor. On a spacecraft platform, an advanced system could monitor the vertical water vapor profiles. This would provide a technology improvement for meteorological forecasting.
Transparent conductive coatings of indium tin oxide have been deposited onto KD*P crystals using an ion-assisted deposition process. Three devices have been fabricated with gold rings for electrical contacting. The devices have been tested optically and electrically, and the damage thresholds of the coatings have been measured. A device was used as the Q-switch in a pulsed Nd:YLF laser.
Recent progress in rapid growth of high optical quality laser crystals is reported. Small rods of 1 to 3 mm diameter of singly or multiply doped garnets and fluorides can be grown at low cost for material evaluation and for use in diode pumping.
The authors have stabilized the frequency of a cw, lamp-pumped, mode-locked Nd:YLF oscillator. The output pulse duration is 50 ps and, simultaneously, the mode jitter is less than 25 kHz relative to a stable, high-finesse Fabry-Perot. The frequency shifts and broadening of the oscillator modes caused by pulsed amplification in a flashlamp-pumped Nd:glass rod with a gain of 100 have been measured. Broadening and frequency shifts on the order of 10 kHz are induced by the amplifier. The changing population of the active transition in the amplifier is the dominant source of the frequency shifts. For frequency-stable amplification, significant saturation of the gain medium must be avoided.
Two microns is a desirable wavelength for many medical applications because of its good tissue interaction and excellent transmission through silica fibers. High repetition rate, required in most of the cases, is difficult to achieve in Tm;Cr:YAG because of the quasi 3-level nature of this transition. This paper discusses possibilities of improvement through optimization of doping concentration.
The 2.8-micrometers Er3+:YLF4I11/2yields4I13/2 laser is shown to have higher efficiency for resonant pumping into the 970-nm absorption band than for the 800-nm pump band. The improved efficiency is due to the lower energy of the 970-nm pump radiation, and the avoidance of the self-quenching process 4I9/2+4I15/2yields4I13/2+4I13/2 which partially bypasses the upper laser state for 800-nm pumping. These advantages result from the direct pumping of the 4I11/2 upper laser state in the 970-nm pumping scheme.
Optical properties and upconversion-pumped lasing on the 4I11/24I13/2 transition of the Er3+ ion were studied in two garnet crystals, (33% Er) Y3Al5O12 and (25%) Y3Sc2Ga3O12. The (Er) YSGG crystal was grown at Union Carbide. Absorption and emission spectra of the crystals were studied at 300K, 77K and 4K. Stark-splittings were determined from the 4K absorption spectra for the five lowest excited states. Fluorescence lifetimes for several excited states were measured at 300K and 77K using 355 nm 10 ns Q-switch pulses of tripled Nd:YAG; and 1.53 micrometers pulses of an Er:glass laser. In the latter case, fluorescence was excited via ion-pair upconversion. In (Er)YAG at 300K under UV excitation, the lifetime of the 4I13/2 state is almost two orders of magnitude longer than the lifetime of the 4I13/2 in Er:YSGG. The lifetime of the 4I13/2 state is only a factor of 5 longer than the 4I11/2 state. The upconversion lasing characteristics of Er:YSGG and Er:YAG are described and related to the measured spectral and lifetime measurements.
The authors describe room temperature, flash-pumped laser operation of several Cr3+-sensitized, Tm3+-activated garnet crystal hosts. The hosts which have been investigated include YAG, YSGG, and GSGG. Laser action for these systems is based on the 2.0 micrometers, 3F4yields3H6 transition in Tm3+. Slope efficiencies for the 2 micrometers transition exceeded 4% in some of the hosts investigated. Free-running, multimode output energies have exceeded 2 J at 1 Hz repetition rates, while TEM00 output energies of greater than 1 J have also been reported.
The output of most pulsed solid state lasers operating in a normal mode (long-pulse) condition typically exhibits a transient spiking behavior, characteristic of relaxation oscillations. Suppression of this spiking behavior can be achieved using a number of passive techniques. In this paper, the use of intracavity second harmonic generation to achieve smooth, long-pulse operation of a flashpumped 2 micrometers Cr;Tm:YAG laser is described.
Both Cr:LiCaAlF6 (Cr:LiCAF) and Cr:LiSrAlF6 (Cr:LiSAF) lasers have been pumped using visible laser diodes. Two commercial 10 mW laser diodes were polarization combined to demonstrate lasing in a low loss resonator. In addition, a higher power 665 nm laser diode was used to pump each laser, producing 15.9 mW cw and 51.8 mW pulsed at 10 Hz for Cr:LiCAF and 19.9 mW cw and 78 mW pulsed for Cr:LiSAF. Optical characterization of the gain media was performed using a dye laser. Gain, loss, slope efficiency and the dependence of the threshold on pump wavelength for each laser are reported.
The absorption band of titanium sapphire is excellently matched to the 510.6 nm and 578.2 nm emission lines of the copper vapor laser (CVL). The availability of high power and high repetition rate output from CVLs allows the generation of high average power tunable radiation from these crystals at the repetition rate of the pump. Such lasers fulfill the rapid data collection requirement of many spectroscopic applications such as resonance ionization mass spectrometry. A computer model of the absorption of CVL radiation in the crystal which incorporates the polarization and beam quality of the pump laser is used to map the gain distribution in the crystal so enabling parameters such as crystal doping level and length to be optimized. Gain switched analysis of the titanium sapphire laser predicts threshold, slope efficiency, pulse width and pulse build up time which are in good agreement with observed values. Thermal effects have also been computed. The experimental pump energy for threshold is typically 25 (mu) J and the slope efficiency for broadband lasing is greater than 20% with output powers up to 750 mW. The shortest pulse width and build-up time observed are 24 ns and 60 ns, respectively, for a resonator with a 0.85 m round-trip length. Tuning methods and resonator designs are also reviewed.
Spatial distributions of photochemically prepared XeF molecules in Ar crystals of 1 cm3 volume are reported for several illumination and annealing conditions and absolute XeF concentrations are derived. Small signal gain coefficients for the C-A (536 nm) transition in addition to the D-X (286 nm), B-X (411 nm) transitions are determined from the XeF concentrations and from measurements of the amplified spontaneous emission intensity versus amplification length and from line narrowing. High gain values of the order of 10 cm-1 are obtained for rather low pump energies. The threshold for laser oscillation and absolute efficiencies are determined.
This paper reports the experimental results on an active-passive colliding pulse mode-locked (APCPM) Nd:YAG laser with variable pulsewidths of 15-20ps and 0.15-2ns. The amplitude fluctuations of the pulse train were less than ± 4%.
Pulse train timing jitter, or phase noise, is an important problem for systems which require multiple synchronized lasers. The authors report on sources observed in their system and show the results of a technique used to eliminate one source of timing jitter.
Transverse effects in nonlinear optical devices are being widely investigated. Recently, synchronization of a laser set by means of the Talbot effect has been demonstrated experimentally. This paper considers a Talbot cavity formed by a solid-state amplifying laser separated from the output mirror by a free space interval. This approach involves the approximation of the nonlinear medium as a thin layer, within which the diffraction is negligible. The other part of a resonator is empty, and the wave field is transformed by the Fresnel-Kirchoff integral. As a result, the dynamics of the transverse (and temporal) structure is computed by a successively iterated nonlinear local map (one- or two-dimensional) and a linear nonlocal map (generally speaking, infinitely dimensional).
Thermally induced index of refraction gradients and their effect on the performance of diode-laser-pumped monolithic Nd:glass lasers are discussed. A simple technique for modeling thermal effects in these lasers is described. This model is useful in optimzing the cavity design and selecting the best glass for the monolithic ring laser.
Acousto-optically mode-locked cw Nd:YAG and Nd:YLF lasers have been efficiently frequency doubled with noncritically phase-matched lithium triborate. LiB3O5 crystals from 6 mm to 15 mm in length were obtained from Castech, People's Republic of China. These were polished and coated for antireflection at both 1064 nm and 532 nm by Coherent Components Group, Auburn, Calif. The coating has a damage threshold in excess of 1 GW/cm2 for mode-locked pulses. More than 11 W of average power at 532 nm has been generated by single pass conversion for a 25 W input at 1064 nm, a conversion efficiency of greater than 45%. Second harmonic generation dependence on laser power and focusing, and on crystal length and temperature have been measured and modeled. Stable longterm operation and applications for high power modelocked 532 nm laser pulses are discussed.
The authors review the motivations for the rapid progress in ultra-high intensity, short-pulse lasers. The advantages, advances and limitations of current laser approaches are discussed. The potential impact on this field of new laser materials and advanced laser engineering is also appraised. Some of the new sciences and applications which will benefit from future advances in this field are identified.
The review of research on generation and amplification of single frequency radiation in solid state lasers carried out at the All-Union Research Centre at S.I. Vavilov State Optical Institute is presented. The principal limitations in frequency and amplitude stabilization are considered. Expressions describing amplitude and frequency stability characteristics versus parameters of active medium, cavity and pumping were obtained. Experiments were carried out with YAG:Nd lasers and amplifiers both with lamp and laser-diode pumping. The results of these experiments confirmed the authors' theoretical considerations.
Practical use of laser tecnique surgery demonstrated the true choice of lasers generating in the long wavelength spectrum. In this field the pigment content of biologic tissues is not noted at absorption spectrum. The latter is generally defined by the percentage of water in tissues. Among the known laser types the nost proved and developed are the lasers generating at 1060 nm, 2090 nm, 2900 nm and 10600 nm. Ho-lasers of 2090 nm radiation wavelength are perspective instruments for surgery purposes due to the high radiation absorption at biologic tissues: 30 cm-1. This value is much higher than the absoptance at 1060 nm and a little bit lower at 1060 nni. An important feature that distinguishes Ho-laser among other types is the possibility of its use with fiber channel made of fused silica for abdominal operations.
The use of high energy solid state lasers for the treatment
of human skin neoplasia was based on the experiments and clinic
studies by Helsper and Goldman (1964), McGuff (1966). The heat
of precise local volume is emitted due to the pulse laser
radiation. The thermal effect results in the superficial necrosis
of tissues with their integrity destruction, vascular repture
accompanied by bloodstoke in some cases and by capillary embolism
Obvious tumour destruction is note only in case of high density
irradiation. General tumour destruction depends on biological
neoplasia features as well as the laser type.