Upconversion-induced heat generation and thermal lensing in Nd:YLF and Nd:YAG

We investigate the influence of interionic upconversion between neighboring ions in the upper laser level of Nd:YLF and Nd:YAG on population dynamics, heat generation, and thermal lensing under lasing and non-lasing conditions. It is shown that cascaded multiphonon relaxations following each upconversion process generate significant extra heat dissipation in the crystal under non-lasing compared to lasing conditions. Owing to the unfavorable temperature dependence of thermal and thermo-optical parameters, this leads, firstly, to a significant temperature increase in the rod, secondly, to strong thermal lensing with pronounced spherical aberrations and, ultimately, to rod fracture in a high-power end-pumped system. In a three-dimensional finite-element calculation, excitation densities, upconversion rates, heat generation temperature profiles, and thermal lensing are calculated. Differences in thermal lens power between non-lasing and lasing conditions up to a factor of six in Nd:YLF and up to a factor of two in Nd:YAG are experimentally observed and explained by the calculation. This results in a strong deterioration in performance when operating these systems in a Q-switched regime, as an amplifier, or on a low-gain transition. Methods to decrease the influence of interionic upconversion are discussed. It is shown that tuning of the pump wavelength can significantly alter the rod temperature

LLNL medical and industrial laser isotope separation: large volume, low cost production through advanced laser technologies

The goal of this LDRD project was to demonstrate the technical and economical feasibility of applying laser isotope separation technology to the commercial enrichment (>lkg/y) of stable isotopes. A successful demonstration would well position the laboratory to make a credible case for the creation of an ongoing medical and industrial isotope production and development program at LLNL. Such a program would establish LLNL as a center for advanced medical isotope production, successfully leveraging previous LLNL Research and Development hardware, facilities, and knowledge

Production of high-value isotopically separated materials

The purpose of this project was to complete the development of the laser systems and separator systems needed to investigate the potential for the economical separation of high value isotopes used in medical and industrial applications, then demonstrate this separation capability. The project was to focus on the isotopic purification of lead for use as solder in high- end electronics, and on the isotopic enrichment of thallium for medical applications. Ultimately the goal is to demonstrate the economical and technical viability of the technology for lead and thallium and to develop a more general capability for other possible isotope separation missions. Both lead and thallium are usefulapplications in this context because they require- dye lasers, solid- state lasers, and a frequency doubling capability of some of the lasers. This later capability allows access to the wavelength range 250 to 450 mn, with tunable, high- power and high repetition frequency lasers. Until recently, these wavelengths have been largely inaccessible in combination with these other laser characteristics. In addition, up to two new potential laser- isotope separation applications would be conceptually developed through a process of needs analysis and technical feasibility studies. Because of an unanticipated reduction in the size of the LDRD pool of funds, it was decided not to fully fund this project. However, enough funds were allocated to allow an orderly closeout of the project activities. Two key technical steps in the laser development were accomplished during the closeout phase, both of which are required for lead and thallium isotope separation. The first accomplishment successfully demonstrated the power scalability of a master- oscillator, power amplifier (MOPA) approach to a high- power Ti- sapphire laser host material. The net result output power produced, after a series of amplification stages, was about 50 W of tunable (red) laser light. In addition, the output power showed no signs of power saturation as the input power was varied. The demonstration verified the design intent of the amplifiers as well as that of the host material. This result is suggestive that greater than 100 W should easily be achievable from this laser architecture. Such powers are likely required for many applications such as efficient frequency doubling of red radiation to obtain blue, or near UV radiation, and for medium to large- scale isotope separation. The second accomplishment was the successful demonstration of a dye master oscillator that pulse- amplified, at pulse repetition frequencies greater than 10,000 pulses per second, an injected continuous- wave input signal with low- signal gains greater than 500. This is important to potential isotope separation missions (and other applications) that require narrowly- tuned, high pulse repetition frequency dye laser light and would normally be the first step in a dye MOPA chain to achieve the required high average power. As part of this demonstration the low signal gain was measured as a function of pump pulse power and injected signal strength. Both measurements are required in order to specify the requirements for an eventual system that uses such a master oscillator

Highly Efficient Tabletop Optical Parametric Chirped Pulse Amplifier at 1 (micron)m

Optical parametric chirped pulse amplification (OPCPA) is a scalable technology, for ultrashort pulse amplification. Its major advantages include design simplicity, broad bandwidth, tunability, low B-integral, high contrast, and high beam quality. OPCPA is suitable both for scaling to high peak power as well as high average power. We describe the amplification of stretched 100 fs oscillator pulses in a three-stage OPCPA system pumped by a commercial, single-longitudinal-mode, Q-switched Nd:YAG laser. The stretched pulses were centered around 1054 nm with a FWHM bandwidth of 16.5 nm and had an energy of 0.5 nJ. Using our OPCPA system, we obtained an amplified pulse energy of up to 31 mJ at a 10 Hz repetition rate. The overall conversion efficiency from pump to signal is 6%, which is the highest efficiency obtained With a commercial tabletop pump laser to date. The overall conversion efficiency is limited due to the finite temporal overlap of the seed (3 ns) with respect to the duration of the pump (8.5 ns). Within the temporal window of the seed pulse the pump to signal conversion efficiency exceeds 20%. Recompression of the amplified signal was demonstrated to 310 fs, limited by the aberrations initially present in the low energy seed imparted by the pulse stretcher. The maximum gain in our OPCPA system is 6 x 10{sup 7}, obtained through single passing of 40 mm of beta-barium borate. We present data on the beam quality obtained from our system (M{sup 2}=1.1). This relatively simple system replaces a significantly more complex Ti:sapphire regenerative amplifier based CPA system used in the front end of a high energy short pulse laser. Future improvement will include obtaining shorter amplified pulses and higher average power

RadTracker: Optical Imaging of High Energy Radiation Tracks

This project examined the possibility of extending the recently demonstrated radoptic detection approach to gamma imaging. Model simulations of the light scattering process predicted that expected signal levels were small and likely below the detection limit of large area, room-temperature detectors. A series of experiments using pulsed x-ray excitation, modulated gamma excitation and optical pump-probe methods confirmed those theoretical predictions. At present the technique does not appear to provide a viable approach to volumetric radiation detection; however, in principal, orders of magnitude improvement in the SNR can result by using designer materials to concentrate and localize the radiation-absorption induced charge, simultaneously confining the optical mode to increase 'fill' factor and overlap of the probe beam with the affected regions, and employing high speed gated imaging detectors to measure the scattered signal

RadTracker: Optical Imaging of High Energy Radiation Tracks

This project examined the possibility of extending the recently demonstrated radoptic detection approach to gamma imaging. Model simulations of the light scattering process predicted that expected signal levels were small and likely below the detection limit of large area, room-temperature detectors. A series of experiments using pulsed x-ray excitation, modulated gamma excitation and optical pump-probe methods confirmed those theoretical predictions. At present the technique does not appear to provide a viable approach to volumetric radiation detection; however, in principal, orders of magnitude improvement in the SNR can result by using designer materials to concentrate and localize the radiation-absorption induced charge, simultaneously confining the optical mode to increase 'fill' factor and overlap of the probe beam with the affected regions, and employing high speed gated imaging detectors to measure the scattered signal

Atomic vapor laser isotope separation using resonance ionization

Atomic vapor laser isotope separation (AVLIS) is a general and powerful technique. A major present application to the enrichment of uranium for light-water power-reactor fuel has been under development for over 10 years. In June 1985, the Department of Energy announced the selection of AVLIS as the technology to meet the nation's future need for enriched uranium. Resonance photoionization is the heart of the AVLIS process. We discuss those fundamental atomic parameters that are necessary for describing isotope-selective resonant multistep photoionization along with the measurement techniques that we use. We illustrate the methodology adopted with examples of other elements that are under study in our program