The purpose of the High Average Power Solid State Laser Program at the Lawrence Livermore National Laboratory is to develop new technologies for significantly increasing the average power capability, beam quality, efficiency, and wavelength agility of solid state lasers. The program emphasis is to advanced the state-of-the-art and is not directed toward a specific application. The performance objectives are more quantitatively defined in the following table.
Nova is a ten beam high power Nd: glass laser used for inertial confinement fusion research. It was operated in the high power high energy regime following the completion of construction in December 1984. During this period several interesting nonlinear optical phenomena were observed. These phenomena are discussed in the text.
After a brief overview of slab geometry work published to date we report on the zig-zag optical path slab laser work at Spectra Technology by describing a 100 Hz slab YAG system (CENTURION) and a 10 Hz dual slab glass system (GEMINI). We will also describe some of the major diagnostic work performed and in the process illustrate the major mechanisms responsible for beam distortion observed in these types of slab lasers.
High average power (HAP) solid state laser devices have been under development for over ten years at the General Electric-company and typically operate in the 10-100W HAP range. It has recently been recognized that such lasers may be scaled into the 1-100kW p9tcwr regime by taking advantage of favorable new materials properties2-4 i novel geometrcsh,', new techniques for increasing the fracture strength of thermally stressed surfaces, and by employing optical phased arrays 3. Here we review previously presented scaling laws for all HAP devices of current interest and indicate the modifications to those laws imposed by well-known scaling limitations such as amplified spontaneous emission and optically induced damage. In addition, we present for the first time a new limitation imposed upon HAP scaling due to fracture (or Wiebull) statistics and investigate its consequences for scaling and configuring constant probability of failure systems.
Isolation cooling of slab geometry solid state lasers potentially solves a number of problems with direct cooling. We have tested a static gas cooled Nd:Glass slab laser to determine the advantages and limitations of this type of cooling system. This paper reports on these tests and discusses approaches to improve the average power of such a laser. The zig-zag slab geometry laser is an established method of reducing thermo-optical effects in solid state lasers. Eggleston et.al.,1 built and tested a small test-bed Nd:Glass slab laser that confirmed computer model performance predictions but demonstrated the need to carefully control the heat distribution within the slab. Eggleston et.al.'s work also indicated problems with direct fluid cooling the slab. In addition to coolant sealing difficulties a degradation of the total internal reflection faces due to contamination and etching was apparent. Since the zig-zag geometry involves many reflections off this surface any increase in loss is extremely undesirable. This under-standing led to the design and construction of a second generation of slab lasers that avoided the difficulties of direct fluid cooling by using cooling across a thin static aas gap.2 Joseph Chernoch was awarded the patent for conduction cooled slab lasers in July 19723 but to our knowledge a slab geometry laser using this cooling approach was never constructed. The laser built to examine the merits of static gas conduction cooling is shown in cross-section in Fig. 1. A (150 x 45 x 6.3) mm slab of Hoya LHG 5 phosphate glass with 8% Nd doping was used for these tests. Pumping was provided by two 15 cm long xenon flashlamps surrounded by second surface silver reflectors. The inner cooling pane was a 1 mm thick pyrex window supported and sealed by flexible elastomer such that it was free to be pressed against teflon spacers over the slab by the water flow. This allowed a variable gas gap thickness and easy mounting for the glass slab. This sealed static environment avoids slab surface contamination and provides a 'soft' thermal boundary against sudden changes in coolant temperature. One disadvantage of the gas conduction cooling design is the reflection loss of pump radiation at the solid to gas interfaces. We measured a slope efficiency increase of 33% when the gas gap was filled with water. Antireflection coatings on the window and the slab would improve flashlamp coupling but cannot prevent the rejection of radiation outside the critical angle for this solid to gas boundary. It can be noted that a permanent conduction layer of static fluid would be acceptable if a seal around the slab face could be made and convection of the thin static fluid layer in the flat plane of the slab could be effectively baffled.
Recent performance of alexandrite single- and double-pass amplifiers is described. Amplification factors of 5x per pass are reported at pulse energies of several joules/pulse and pulse powers exceeding 100 MW.
Recent measurements of dn/dT in Nd:BEL have suggested that a judicious selection of crystallographic orientation can yield laser rods that exibit greatly reduced thermal lensing. Such "athermal" rods have been fabricated. Their optical properties and laser performance is reported.
Since the time when heavy metal halide glass was produced by the creative work on fluorozirconate materials at the University of Rennes, there has been great interest in its use for light guides as a potential laser host and for optical display devices. The optical transparency of the glass from the ultraviolet to the infrared and the easy incorporation of foreign ions make this material especially promising for a number of future applications. However, in order for the material to be utilized in device technology a thorough characterization of the properties must be available. The characterization must include the optical, mechanical, magnetic and electrical properties. This paper will emphasize the optical properties of rare earth and 3d transition metal ion impurities in heavy metal fluoride glasses.
The original motivation for developing large high power lasers in the inertial confinement fusion (ICF) field was to provide a driver for fusion energy power plants. Consideration of power plant energy budgets leads to a practical requirement for the product of ICF pellet gain G and driver efficiency n to be greater than 10. When pellet gains of 103 were considered realistic, driver efficiencies of 1 to 2% would have been adequate. However, it is now well known that pellet gains are more likely to be in the range 100 to 200, which implies driver efficiencies greater than 5% for economic ICF power plants.
Recent experimental demonstrations of free-electron laser oscillators and amplifiers have verified the feasibility of these broadly tunable photon sources. We review the current status and their continuing evolution toward ever higher output power and shorter wavelengths.
The chemical laser has a record of steady, successful technology development culminating in the largest scale operating lasers. These existing lasers have been used to establish a first-order feasibility of strategic defense. They are currently being used to acquire the effects and vulnerability data to establish directed energy weapons requirements for robust systems. Furthermore, the chemical laser technology is quite promising as we project into the future and address defense requirements with advanced lasers.
A subpicosecond KrF laser system with peak power of the order 50 GW is described. Results of the short pulse amplification in an excimer laser amplifier have indicated that the stored energy can be efficiently extracted on a subpicosecond time scale.
The design and performance of several experimental configurations of alexandrite laser pumped intracavity raman lasers are described. These lasers combine the high energy storage of alexandrite with the high gain of the raman medium. They can produce short high-energy pulses of broadly tunable, raman shifted light.
In a high repetition rate excimer laser operation, the lifetime of the exciter is one of the most important problems. A thyratron is commonly used as a switch in the power supply but its lifetime limits laser operation time even if it is used with a magnetic assist which makes use of the nonlinearity of ferromagnetic material. To attain a nearly endless lifetime of the excimer laser exciter, we have developed an all solid state exciter which consists of a high-voltage transformer switched by a Silicon Controlled Rectifier (SCR), producing a pulse whose energy and duration are 11.2 J and 8 us, respectively, and a three stage magnetic compressor. With a 1.4-ohm dummy load, output peak power, energy/pulse, and pulse duration were 100 MW, 5.2 J, and 100 ns, respectively. The electrical efficiency of the exciter was 47%. The energy loss of 6 J in the exciter was due both to the core loss and the transfer loss. It should be noted that the time jitter between the SCR gate input pulse and the output voltage pulse was less than 12 ns. An KrF laser output of 33 mJ has been obtained after optimizing the total pressure, gas content (He buffer), output coupling, and the capacity of the resonant capacitor. With an Ne dilluent mixture, 42 mJ has been obtained (not optimized ). The design and the performance of the exciter and the laser operation are fully described. In addition, we have discussed the temperature rise of the core operating at a repetition rate of 1 kpps.
This paper is a survey of tunable solid state lasers operating with paramagnetic-ion doped crystals. Included in the discussion are near-infrared Cr3+-.- and V2+-doped lasers such as alexandrite, the broadly tunable Ti3+:Al203 laser, infrared systems based on the divalent ions Ni2+ and Co2+ and ultraviolet lasers using the Ce3+ ion.
Analysis of the absorption spectrum of Neodymium: Sodium Beta'' Alumina allows calculation of the theoretical radiative lifetime of the 4F3/2 level. Comparison with the experimentally measured radiative lifetime then yields the radiative quantum efficiency. Finally, the stimulated emission cross section can be obtained from the calculated Judd/Ofelt parameters and the fluorescent linewidth. Experimental measurement of single pass gain compares moderately well with that calculated from theory and with that measured in a previous platelet laser.
Tuning and narrowband performance of the Ti:Sapphire laser oscillator was investigated. Laser cross-sections and gain/loss ratios were determined in the spectral range from 720 to 1030 nm by laser measurements. Ti:Sapphire laser energy was shown to extract homogeneously, in a narrowband, using both oscillator and fluorescence sidelight experiments.
The association of F-centers to CN defects in alkali halide crystals forms "FH(CN)" complexes which under excitation of their electronic absorption transitions in the visible produce optical emission from vibrationally excited CN- molecular states around 5 μm. In the host CsCl, with the most efficient F-center/CN- molecule coupling, laser oscillation has been obtained at temperatures up to 77 K on the three strongest vibrational emission transitions at 2000, 2025 and 2050 cm -1. Threshold pump powers are in the mW range.
The Laboratory for Laser Energetics (LLE) of the University of Rochester has operated a single beam, 351 nm laser irradiation facility for several years.l This facility has been based on the Glass Development Laser (GDL); a Nd:phosphate glass laser system that is capable of generating more than 150 joules in 1 nsec pulses at 1054 nm.2 With an overall frequency tripling efficiency of greater than 50%, this facility has also been capable of generating more than 75 joules in 1 nsec at 351 nm. We now report on the upgraded GDL facility that consists of a frequency tripled, active-mirror boosted GDL laser system which produces in excess of 600 joules in 500 ps and 150 joules in 100 ps for a substantial increase in third harmonic radiation.
The technology of frequency converting intense laser light was demonstrated during the early 1960s1. In the initial experiments, the deep red wavelength of a ruby laser produced ultraviolet light, a shorter wavelength harmonic of the fundamental laser radiation, as it was passed through a quartz crystal. Shortly afterwards the Nd:YAG infrared laser was used to produce visible or ultraviolet light as it passed through a crystal of potassium dihydrogen phosphate (KDP).2.3 Today frequency conversion is a well characterized and widely used method of producing intense, coherent light at wavelengths unavailable from the source laser medium.
This paper presents a selective review of the use of nonlinear phase conjugation for wavefront control in laser systems. Nonlinear methods for producing a phase conjugate beam are described, and a selection of laser system concepts utilizing nonlinear phase conjugation are discussed. The key issues affecting beam quality and extraction efficiency in phase conjugated systems include phase conjugation fidelity, phase conjugate mirror efficiency and threshold, suppression of amplified spontaneous emission, the interaction of amplifier gain saturation with phase conjugation, and birefringence.
We discuss material properties relevant for high power harmonic conversion and present information about the linear and non-linear optical characteristics of some new classes of materials. We give an assessment of the possible usefulness of 1-arginine phosphate monohydrate (LAP) as a harmonic generator for high power lasers.
A flux procedure has been developed to grow cm size single crystals of 8"-alumina with composition Na1.67Mg 0.67A110.33017. The melt containers were iridium crucibles of 9 cm diameter x 9 cm height. The heating and flux evaporation were conducted at 1800-1900°C by means of 450 KHz radio frequency power. Single crystals up to 3-5mm thick and 5-10cm2 in area were obtained within a few days. In order to obtain larger single crystals, variations of the seeded solution method were used since 13"-alumina crystals melt incongruently. This procedure is complicated by high Na2O solvent vapor pressure at 1800°C growth temperature. Radial thermal gradients must also be minimized for best results. The quality of our crystals is good as judged by passive tests usually applied to laser rods. Doped runs of Vi-alumina were prepared for investigation as laser materials. These included rare earths and first transition elements grown in, diffused in, or both. Experiments on Ce34", Cr3+-Nd3+, and Ti3+ are described. The implications of the structural site where the dopant ion enters is discussed in reference to energy transfer and possible tunable laser action.
The emission of Nd:YLF to unpopulated terminal levels includes prominent lines at 1.047, 1.053, 1.313, 1.321 microns and continuous emission over the band between 1.362 to 1.374 microns. The properties of this system permit Q-switched operation on all these lines in spite of very high gain competition from the strongest (1.047) peak. Several schemes for shifting these lines to the Cs resonance lines are described.
A novel method has been developed for measuring surface absorption of optical coatings used in high power laser systems. Advantages of the method, which uses an infrared thermal camera to measure surface temperature and a detailed computer model of heat loss mechanisms to calculate surface absorption, include real time, non-contact measurement and in situ monitoring of optics under operational conditions. Surface absorptions ranging as low as 10-5 of the incident laser power can be measured with a 50 W test beam. The technique is used routinely to easily and quickly acquire data for coating performance sensitivity testing, coating development and for quality assessment of purchased optics. More importantly, it can identify hot absorbing coatings that are likely to fail in service or to become optically distorted in an operating laser system.