Spectral broadening of single-frequency laser pulses by optical cross-phase modulation (XPM) with chaotic laser pulses in birefringent single-mode optical fibers is investigated numerically and results are compared with experiments. By this process we have generated laser pulses of variable bandwidth (1 - 25 angstrom) at the fundamental wavelength (1053 nm) for amplification in high power solid-state Nd:glass lasers used for inertial confinement fusion research. Simulations indicate that a temporally smooth XPM pulse can be generated with intensity fluctuations of less than 10% and spectral width greater than 50 angstrom using a short length (approximately 5 m) of special low dispersion and low birefringence fiber, e.g., D equals 10 ps/nm-km (normal dispersion) and (Delta) n equals 2 X 10^-5. Readily available fibers of similar length, with parameters of D equals 40 ps/nm-km and (Delta) n equals 6 X 10^-5, can give spectral widths exceeding 25 angstroms, but the noise will range from 25 to 60%. Broadband laser pulses generated by XPM are now routinely used at Lawrence Livermore National Laboratory for active smoothing of the laser irradiance on targets by the technique of smoothing-by-spectral dispersion.

In the OMEGA Upgrade laser system 60 beams are used to symmetrically irradiate the target. To achieve optimum fuel compression, the beams must irradiate the target uniformly, requiring excellent beam quality in each beam and nearly identical temporal waveforms so that target irradiation remains balanced throughout the course of the implosion process. Propagation of high intensity, pulse-shaped IR and UV laser beams through long air paths leads to losses and decreased beam quality due to stimulated rotational Raman scattering (SRRS). With short pulses, it is necessary to use transient Raman theory to include build up and relaxation of the medium. Imperfect beam quality leads to intensity not spots that can lower the effective threshold for Raman scattering. A detailed analysis of the system to determine energy efficiency and beam quality necessitates a four-dimensional (4-D) treatment of both the optical beam and the air path. In this paper we outline the 4-D modeling of the OMEGA Upgrade laser and describe full-scale experimental investigations of SRRS in air.

For some kinds of industrial applications like laser photochemistry, high pulse power and fine tunability is required for a laser system. A cw oscillator and pulse dye amplifiers system is one method to generate high power laser pulses with narrow spectral bandwidth enough for laser photochemistry. In this paper, we report that the spectral sidebands occur in the process of amplification pumped by CVLs of which the intensity fluctuated. The spectral profile fluctuations and mode beats are deduced to cause these fluctuations. According to the rate equation analysis, the DL waveform is modulated by the CVLs, and the frequency sidebands are generated, and its frequency bandwidth is several GHz. Since the spectral bandwidth of the DL is usually a few 100 MHz, the bandwidth and the level of amplitude modulation of the pumping laser are very important. This modulation may be reduced by multi pumping beam irradiation or other methods.

We report on the shaping and stretching of nanosecond, linearly chirped pulses. The temporal response of linearly chirped pulses shaped by a unit-step spectral mask is examined. Schemes for the shaping of narrow-band nanosecond pulses are also introduced. Experimental and theoretical results show that spectral pulse-shaping techniques provide the required stability, flexibility, fast rise time, and high contrast for laser-fusion research.

An inherently fringe stable interferometer, with uniform recording-plane exposure, produces transmission gratings with uniformly high efficiency. The surface-relief structures recorded in positive photoresist are ideal for their transfer into specialized candidate materials. These structures have many applications in high-power lasers including beam smoothing, pulse compression, and low-reflection surfaces. Also, accurate modeling of surface-relief grating efficiencies, using the actual groove profile, allows optimization of grating parameters.

In order to demonstrate new technology for the proposed National Ignition Facility (NIF), we are currently building a 5-kilojoule laser called Beamlet. The oscillator and pulse shaping system for Beamlet represents a major technological improvement over previous designs. Using integrated optics, fiber optics, and diode-pumped lasers instead of bulk optics and flashlamp-pumped lasers, this new master oscillator takes advantage of current technology to make a system with numerous advantages. The requirements for a NIF for greater flexibility and reliability necessitate this new approach; the pulse-forming system for the Beamlet demonstrates a subset of the capabilities required for a NIF. For the Beamlet, we must produce a single 1 - 10 ns, shaped- and frequency-modulated pulse. The Beamlet needs only to generate square output pulses for technology demonstration purposes, but the input pulses must be shaped to compensate for gain saturation in the power amplifier. To prevent stimulated Brillouin scattering (SBS) from damaging the output optics, the output pulse must have some bandwidth, and thus the pulse-forming system phase modulates the input pulse. These requirements are very similar to those for the Nova master oscillator system, but Nova technology is not the best choice for the Beamlet. In developing an oscillator design for a fusion laser system, the system requirements are defined by the oscillator's place in the overall laser architecture. Both Nova and Beamlet use a master oscillator-power amplifier (MOPA) architecture. In a MOPA-laser architecture, a low-power oscillator is followed by a high-gain, high-power amplifier. If the output signal is to have a high signal-to-noise ratio (SNR), the oscillator-signal power must be high above the amplifier noise power.

This paper describes the amplifier and beam shaping section of a new pulse generation system that will drive the Beamlet laser at LLNL. The master oscillator and pulse shaping system are described in an accompanying contribution [R. B. Wilcox ea., `Fusion Laser oscillator and pulse-forming system using integrated optics.', these proceedings]. A modified regenerative amplifier produces a gain of 10⁹ to bring the oscillator pulses to the mJ- level. A serrated aperture and birefringent beam shaper produce a flat-topped square beam with high fill factor. A single four-passed Nd:glass rod amplifier provides sufficient gain to generate the desired 12 J output energy in a 3 nsec pulse with very small beam profile, wavefront and pulse shape distortion. We present a description of the system components, followed by a discussion of its performance, based upon over 150 full front end shots being completed since its assembly.

The smoothing of focal plane distributions of high-power lasers is of particular importance to laser-plasma interaction studies. There are many techniques used to achieve the smoothing and this paper presents an overview of the methods as applied on the VULCAN high power Nd:glass laser at the Central Laser Facility, Rutherford Appleton Laboratory. These have played an important part in high-power laser-plasma interaction experiments into fusion studies, energy transport, filamentation etc. These smoothing techniques all involve placing an optical component in the laser beam to produce either a spatial, temporal, or spectral modification of the laser pulse and these are discussed under the topics of Random Phase Plates (RPP), Induced Spatial Incoherence (ISI), and Smoothing by Spectral Dispersion (SSD). We also describe techniques for tailoring the focal profile using RPPs.

We report on the design and fabrication of continuous contour (kinoform) phase plates for homogenizing the focal plane irradiance of high-power, inertial confinement fusion laser systems. These kinoform phase plates are designed using an iterative algorithm. They offer the flexibility of controlling the overall shape of the far-field irradiance profile and the ability to concentrate the energy within a central region of the focal profile. These properties make kinoforms superior to the conventional, binary random phase plates for many applications. Potential methods for fabrication of such kinoform phase plates are discussed.

Efficient phase conversion of laser light can be achieved using continuous distributed phase plates (DPPs) for applications in laser-beam shaping and laser-beam smoothing. These DPPs are deep, surface-relief, continuous phase plates that exhibit near-unity energy efficiency, envelope and power spectrum flexibility, and reduced near-field intensity modulation as compared to stepped diffractive optics. Specific DPP designs for inertial-confinement-fusion laser systems, used to irradiate fuel capsules, is presented.

An aspherical lens for the control of the intensity distribution on the target has been designed and fabricated. The diffraction from the edge of the element lens was eliminated with an edge- shaped plate. The beam profile on the target was measured, and it was in good agreement with the calculated profile by the diffraction code with the aspherical lens shape.

Phase plates have been used in holography and laser fusion to produce a well-defined far-field intensity envelope that is relatively independent of the input-beam profile. However, superposed on this envelope is unwanted, highly modulated speckle from the interference between different phase-plate elements. We have developed a technique for choosing the phase-plate elements (in combination with polarization rotation) such that these intensity fluctuations approach zero in the limit of plane-wave near-field irradiation. Strategies are discussed for reducing the sensitivity of these zero-correlation phase masks to phase aberrations and other near-field imperfections, with particular emphasis on laser-fusion applications.

Diffractive optical elements can be designed to shape and, hence, optimize a laser beam intensity distribution for a specific application. The elimination of spherical and chromatic aberration in CO₂ industrial and medical diffractive focusing elements, respectively, is discussed.

Phase-shifting masks, which have many similarities to the phase plates used to smooth laser beams, are being developed to project high-contrast patterns for semiconductor manufacturing.

One of the key requirements for direct-drive laser fusion is a laser whose focal profile is sufficiently smooth and controllable to produce highly symmetric implosions. To achieve this, the NIKE laser will implement the echelon-free ISI technique, in which the desired focal profile is formed at the output of a spatially and temporally incoherent oscillator, then imaged through the laser amplifiers onto the target. Because the amplifiers are located in the quasi far- field of the oscillator, their large scalelength gain and phase nonuniformities will have little effect on the image as long as the light remains highly incoherent. However, small scalelength phase aberrations and nonlinear optical processes must be minimized to maintain good control over the image. After a brief description of the NIKE system, this paper reports on numerical simulations and measurements of profile distortion due to random amplitude and phase nonuniformities, nonlinear refraction, and self-seeded stimulated rotational Raman scattering, and describes the steps being taken to control these effects.

The Nd-glass Phebus laser has the capability of delivering 2 X 10 kJ in a 1.3 ns pulse when using the usual narrow band oscillator. For the purpose of focal spot smoothing the coherence of the pulse is reduced by injecting the beam of a 1 nm bandwidth oscillator into a step index optical fiber. The use of the optical fiber at the laser front end changes the common point laser source into an extended source. The required optical arrangement taking into account the divergence of the extended source beam is described. The smoothing efficiency is measured by the reduction of one order of magnitude of the contrast of the energy modulations in the focal spot. It is experimentally established that non linear effects limit the power density in the amplifying medium to a value depending on the geometrical extension of the source. The identification of the relevant non linear effects is discussed.

In this paper we report the characteristics of the partially coherent light sources used in the high-power Nd-phosphate glass laser system, Gekko XII. One is an amplified spontaneous emission (ASE) from Nd:glass rod and the other is a spatial mode mixing by an optical multimode fiber of a frequency chirped and pulse compressed laser light. We also introduced the angular dispersion of spectrum (AD) with a large beam divergence which is very effective for the beam smoothing without sacrificing harmonic conversion efficiency. The effect of AD gave an additional spatial and also temporal speckle smoothing at a farfield due to movement of the interference pattern. The measured temporal evolution of the speckle structure showed the rapid smoothing of the speckle with a larger beam divergence. The focused beam patterns at a target surface at Gekko XII were measured at the equivalent target plane by changing the beam divergence. A statistical model is applied for analyzing the speckle pattern in order to treat a beam smoothing effect.

We present the results of experiments performed on the Nova laser system to determine the effect of bandwidth on third harmonic (3(omega) ) frequency conversion and beam smoothing by spectral dispersion (SSD). Our experiments utilized a wide bandwidth fiber optic cross- phase modulated (XPM) source and a narrower bandwidth microwave modulated (FM) source, each centered at 1053 nm (1 (omega) ). A significant fraction (> 50%) of the 1(omega) XPM bandwidth was transferred to the 3(omega) beam (22 cm^-1 yields 36 cm^-1), yielding 0.13% bandwidth at 3(omega) . The maximum intrinsic narrowband 3(omega) frequency conversion obtained using a type-II/type-II KDP crystal array was 62%. The intrinsic efficiency obtained at the Nova 10-beam chamber is typically > 65%. Frequency conversion was essentially unaffected by the 2 cm^-1 bandwidth obtained from FM source. However, the 5 - 16 cm^-1 of bandwidth from the XPM source reduced the conversion efficiency to approximately 24%. We have developed broadband frequency conversion codes and broadband pulse simulations to model our results, and have obtained good agreement with experiment.

In order to improve an irradiation uniformity on a spherical target, an amplified spontaneous emission (ASE) with an angular dispersion of spectrum is introduced in Gekko XII laser system. The irradiation nonuniformity was estimated from a measured beam pattern at the equivalent target plane. The ASE light substantially reduces the irradiation nonuniformity (rms value) from 19.7% to 3.8%. We also theoretically analyzed smoothing characteristics of the ASE light, and compared them with the experimental results. The Legendre-mode-dependent reduction rates of irradiation nonuniformities which are determined by the ratio of mode amplitudes with and without introduction of ASE, were very consistent with theoretical predictions.