Noncritically phase-matched (NCPM) fourth harmonic generation (FHG) of Nd:glass laser radiation in partially deuterated dihydrogen phosphate (KD*P) crystals has been demonstrated. At an Nd:glass laser wavelength of 1053.0 nm, NCPM FHG is achieved in 70% deuterated KD*P at a crystal temperature of 18.5±0.1 °C. Tuning the fundamental laser wavelength from 1052.9 to 1053.2 nm, FHG in KD*P is NCPM by changing the crystal temperature from 17.9 °C to 20.5 °C. When driven with 2.4 J of second harmonic radiation in a 3 ns flat-top pulse, corresponding to 1 GW/cm(2) 2ω drive intensity, 1.9 J of fourth harmonic radiation was generated in a 6 mm long KD*P crystal, yielding a second to fourth harmonic energy conversion efficiency of 79%.

The stimulated Raman scattering gain coefficient in KDP/DKDP crystals for any orientation with respect to the crystal axis, propagation and polarization of the pump beam can be estimated using a) the spontaneous Raman scattering cross section of the material, b) the spectral profile of the Raman line and, c) the Raman scattering tensor. Of particular interest in ICF class laser systems are the parasitic Transverse Stimulated Raman Scattering effects. In this work we provide experimental results that help advance our ability to estimate these effects at operational conditions for second, third and fourth harmonic generation.

The spontaneous Raman scattering cross sections of the main peaks (related to the A1 vibrational mode) in rapid and conventional grown potassium dihydrogen phosphate and deuterated crystals are measured at 532 nm, 355 nm, and 266 nm. The measurement involves the use of the Raman line of water centered at 3400 cm-1 as a reference to obtain relative values of the cross sections which are subsequently normalized against the known absolute value for water as a function of excitation wavelength. This measurement enables the estimation of the transverse stimulated Raman scattering gain of these nonlinear optical materials in various configurations suitable for frequency conversion and beam control in high-power, large-aperture laser systems.

Thermally induced birefringence can degrade the beam quality in high-average-power laser systems with doped-glass substrates. In this work, we compare glass-laser slab amplifiers at either Brewster's angle or normal incidence and discuss trade-offs between both designs. Numerical simulations show the impact of thermally induced depolarization in both amplifier systems. A non-uniform temperature profile and the resultant mechanical stress leads to depolarization that worsens as the beam propagates through the slab-amplifier chain. Reflective losses for depolarized light at Brewster's angle cannot be compensated and degrade beam quality. This motivates the selection of normally incident slab amplifiers, which facilitates birefringence compensation.Tolerances for birefringence compensation of two matched normal-incidence glass-slab amplifiers balanced by a quartz rotator are also investigated. Imbalances in thermal load, relative amplifier position and beam magnification between amplifiers show the highest depolarization sensitivity and establish limits for manufacturing tolerances and amplifier design.

A diode-pumped injection-seeded Nd:YAG laser system with an average output power of 38 W is described. The laser operates at 300 Hz with pulse energies up to 130 mJ. The temporal pulse shape is nominally flat in time and the pulse width is user selectable from 350 to 600 ps. In addition, the spatial profile of the beam is near top hat with contrast <10%.

The effect of a change in the system parameters upon the one micron laser's power, energy and beam quality will be discussed. The parameters varied in the study were the optical losses, the gain and gain profile of the amplifiers. Additionally, the effect upon power, energy and beam quality as a function of slab count and position will be presented.

The design of the National Ignition Facility (NIF) is the result of optimization studies that maximized laser performance and reliability within a restricted cost budget. We modeled the laser using a suite of tools that included a 1D propagation code, a frequency conversion code, a 2D ray trace code for calculating the gain profile, thermo- mechanical codes for calculating the pump-induced distortions in the slabs, a database giving estimates of optics bulk/finish quality, and costing models of the laser/building. By exploiting parallel processing, we were able to consider approximately 750 possible designs per hour using a cluster of 28 workstations. For our optimization studies, we used a temporally shaped (ICF indirect drive) pulse producing at least 2.2 MJ and 600 TW in a 600 micron diameter hole at the target entrance plane. We varied as many as 20 design variables (e.g., slab counts, slab thickness, Nd concentration, amplifier pulse length) and applied as many as 40 constants (e.g., flashlamp voltage and fluence damage/filamentation at various points in the chain). We did not vary the number of beamlets (fixed at 192 or the aperture (fixed at 40 cm). We used three different optimization approaches: a variable metric algorithm, an exhaustive grid search of more than 50,000 candidate designs, and a parabolic interpolation scheme. All three approaches gave similar results. Moreover, a graphical analysis of the parameter scan data (analogous to sorting and pruning designs using a spreadsheet) has allowed us to understand why the optimizers eliminated alternate designs. The most inexpensive main-switch-boot slab configuration meeting the mission requirements and satisfying all constraints was 9-5-3. The cost of this configuration is approximately $DOL10M less than the 9-5-5 conceptual design. However, the NIF Project has chosen a slightly more expensive 11-0-7 configuration for continued Title I engineering because of its similarity to the Beamlet 11-0-5 design and a lower B-integral.

We have tested high-threshold KDP in a type II/type II polarization mismatch THG configuration at fluences and intensities in a 3-cm aperture which approach Nova Upgrade levels. The purpose of this work was to demonstrate high efficiency, damage-free third-harmonic generation in the proposed Upgrade operating regime of 9 to 12 J/cm{sup 2} at a 3-ns pulse duration.

In the course of our frequency conversion work on Nova we have had to examine the effect of non-uniform near-field irradiance on conversion efficiency. To this end we have developed a convenient statistical model for describing the irradiance of apodized noisy'' near-field beams. Our model is based on Rician intensity statistics, suitable for noisy laser fields with no boundaries, that we apply over a supergaussian profile describing the locally average intensity of a finite-in-space beam. The model is mathematically rigorous. The superrician'' distribution functions generated by this model can successfully simulate data obtained from small-aperture (3 cm) beam as well as the full-aperture intensity distributions measured on Nova. In the following discussion, we review the Rician probability density function which describes the intensity of flat-in-space laser fields, relative to the average intensity. Then, after deriving the intensity probability density function for a smooth supergaussian envelope, we combine the two distributions using the concept of conditional probabilities. We conclude by using our results to model some actual data.

We have broadened the output spectrum of a single-mode, Q-switched Nd: YLF oscillator (lambda = 1.053..mu..m) by co-propagating it with a noisy, broadband pulse in a polarization-preserving fiber. The intensity as a function of time remains smooth, despite the 10/sup 4/ increase in bandwidth. 4 refs., 3 figs.

We describe the results of experiments designed to study the spectral and temporal dynamics of stimulated rotational Raman scattering (SRRS) in air as a function of intensity, pulse shape, and pulse length.1 Our experiments were performed using one arm of the Nova laser at Lawrence Livermore National Laboratory at a wavelength of 1053 nm. A collimated spatially uniform beam (11.5-cm diameter) was propagated over a 75-m air path to a diagnostic station. Data were obtained for nominally square and ramped temporal pulse shapes, 1 and 2 ns in length, with intensities ranging from 1.7 to 3.3 GW/cm2. Time-resolved spectra were recorded using a 1-m spectrometer close-coupled to a film-backed streak camera. Experimental results indicate that the initial pulse shape is most influential in the evolution of SRRS. Differences of up to 50% in the temporal onset and intensity of SRRS for ramped and square pulses of equivalent average intensity have been observed. Numerical simulations of the laser and Stokes pulse evolution have been performed including pump depletion and multiple rotational transitions. These calculations are found to be in good agreement with experiment. Experimental results and additional modeling are discussed.

Type I/Type II third harmonic conversion has been implemented at the 74 cm aperture of the Nova laser system. We discuss the performance capabilities and alignment issues of this scheme for Nova relative to conventional Type II/Type II conversion. 3 refs., 2 figs.

We report here our recent observations of low J-value Stokes and anti-Stokes lines from stimulated rotational Raman scattering (SRRS) in long air paths with a large Fresnel number beam at a pump wavelength of 1053 nm. The spectral and temporal dynamics of SRRS is important to our understanding of SRRS as applied to the propagation of large-aperture beams in laser systems for fusion research and in defense applications.1,2 An example of this is the proposed implementation of broad bandwidth and spatial incoherence for target irradiation experiments in the field of laser-plasma interactions.