The Solid-State, Heat-Capacity Laser (SSHCL), under development at Lawrence Livermore National Laboratory (LLNL) is a large aperture (100 cm2), confocal, unstable resonator requiring near-diffraction-limited beam quality. There are two primary sources of the aberrations in the system: residual, static aberrations from the fabrication of the optical components and predictable, time-dependent, thermally-induced index gradients within the gain medium. A deformable mirror placed within the cavity is used to correct the aberrations that are sensed with a Shack-Hartmann wavefront sensor. Although it is more challenging than external correction, intracavity correction enables control of the mode growth within the resonator, resulting in the ability to correct a more aberrated system longer. The overall system design, measurement techniques and correction algorithms are discussed. Experimental results from initial correction of the static aberrations and dynamic correction of the time-dependent aberrations are presented.
Laser shot peening, a surface treatment for metals, is known to induce residual compressive stresses to depths over 1 mm providing improved component resistance to various forms of failure. Recent information also suggests that thermal relaxation of the laser induced stress is significantly less than that experienced by other forms of surface stressing that involve significantly higher levels of cold work. We have developed a unique solid state laser technology employing Nd:glass amplifier slabs and SBS phase conjugation that enables this process to move into high throughput production processing.
We describe the Optical Pulse Generation (OPG) testbed, which is the integration of the MOD and Preamplifier Development Laboratories. We use this OPG testbed to develop and demonstrates the overall capabilities of the NIF laser system front end. We will present the measured energy and power output, temporal and spatial pulse shaping capability, FM bandwidth and dispersion for beam smoothing, and measurements of the pulse-to-pulse power variation o the OPG system and compare these results with the required system performance specifications. We will discus the models that are used to predict the system performance and how the OPG output requirements flowdown to the subordinate subsystems within the OPG system.
The multi-pass amplifier (MPA) is the last subsystem of the NIF preamplifier, which feeds the main amplification stages of the NIF beamline. The MPA is based on a flashlamp pumped 5-cm diameter by 48 cm long Nd:glass rod amplifier operated at a single pass small signal gain of 15 to 17. The MPA is an off-axis multi-pass image relayed system, which uses two gain isolating image relaying telescopes and passive polarization switching using a Faraday rotator to output the pulse. We describe the MPA system, techniques used to avoid parasitic oscillation at high gain, and suppression of pencil beams. The system is used to generate a well- conditioned 22-joule output from one millijoule input. The output pulse requirements include 22 joules in a square, flat topped beam, and with near field spatial contrast of <5% RMS, square pulse temporal distortion <2.3, and an RMS energy stability of <3%. All of these requirements have been exceeded. The largest impediment to successful operation was overcoming parasitic oscillation. Sources of oscillation could be generally divided into two categories: those due to birefringence, which compromised the polarization contrast of the system; and those due to unwanted reflections from optical surfaces. Baffling in the vacuum spatial filters helps to control the system sensitivity to unwanted stray reflections from flat AR coated surfaces. Stress birefringence in the rather large glass volume of the rod (942 cm3) and the four vacuum loaded lenses are significant, as each of these elements is double passed between each polarizing beam splitter pass. This lowers the polarization contrast of the system, which can prevent the system from operating at sufficient gain. Careful analysis and layout of the MPA architecture has allowed us to address the challenges posed by a system small signal gain of ≈ 33000 and with an output pulse of as high as 27 joules.
Plasmas produced from laser-irradiated gas puff xenon targets, created by pulsed injection of xenon with high-pressure solenoid valve, offer the possibility of realizing a debrisless x-ray point source for the x-ray lithography applications. In this paper we present results of the experimental investigations on the x-ray generation from a gas puff xenon target irradiated with nanosecond high-power laser pulses produced using two different laser facilities; a Nd:glass laser operating at 1.06 micrometers , which generated 10-15 J pulses in 1 ns FWHM, and Nd:glass slab laser, producing pulses of 10 ns duration with energy reaching 12 J for a 0.53 micrometers wavelength or 20 J for 1.05 micrometers . To study the x-ray emission different x-ray diagnostic methods have been used. X-ray spectra were registered using a flat CsAP crystal spectrograph with an x-ray film or a curved KAP crystal spectrograph with a convex curvature coupled to an x-ray CCD readout detector. X-ray images have been taken using pinhole cameras with an x-ray film or a CCD array. X-ray yield was measured with the use of semiconductor detectors (silicon photodiodes or diamond photoconductors).
A diode-pumped Nd:YAG laser for use as a driver for a soft x-ray projection lithography system is described. The laser will output 0.5 to 1 J per pulse with about 5 ns pulse width at up to 1.5 kHz repetition frequency. The design employs microchannel-cooled diode laser arrays for optical pumping, zigzag slab energy storage, and a single frequency oscillator injected regenerative amplifier cavity using phase conjugator beam correction for near diffraction limited beam quality. The design and initial results of this laser's activation experiments are presented.
The shot-to-shot phase fidelity of an SBS phase conjugator operated many times above threshold has been found to be very sensitive to the slope of the leading edge of the input pulse. For a pulse with a rising edge that is short relative to the acoustic lifetime of the SBS medium, strong random fluctuations in the fidelity of the wavefront reversal are observed. However, by tailoring the leading edge of the pulse relative to the acoustic response time of the medium, good phase reproduction can be achieved. No increase in shot-to-shot fidelity fluctuation was observed using a carbon tetrachloride SBS cell at input energies up to 100X threshold, resulting in reflectivities of 90%. Conclusions are made about the source of the observed random fluctuations which are supported both by experimental measurements and numerical modeling.
A two-cell stimulated Brillouin scattering (SBS) pulse compressor design is presented that can be scaled to large laser pulse energies and a numerical model has been developed which accurately predicts the performance of this pulse compressor system over a wide range of operating parameters. The compression of a 2.5 J input pulse from a width of 15.8 ns to 1.7 ns is demonstrated with 80% energy efficiency.
The electron-beam pumped XeF(C yields A) excimer has been investigated as a novel gain medium for ultrashort pulse, high power amplification in the visible spectral region over a wide range of laser pulse durations. Gain measurements for 100 ps and 800 fs pulses resulted in a small signal gain coefficient of 3%/cm and a saturation energy density of 80 mJ/cm2. For 250 fs pulses, a saturation energy density of 50 mJ/cm2 was observed. Narrowband absorbers in the XeF(C yields A) spectrum could be bleached out, yielding a smooth gain profile of 60 nm bandwidth. An unstable resonator was designed with particular consideration of the small XeF(C yields A) gain coefficient and optimized energy extraction in a single pulse output. A maximum pulse energy of 275 mJ was obtained by amplification of 250 fs pulses at 490 nm wavelength, generating laser powers in the terawatt range. The beam quality of the amplified pulses was within 1.3 times the diffraction limit, making possible focused intensities in the 1018 W/cm2 range.
The development of scalable high power lasers in the UV-visible range and ultrashort high brightness laser sources will have significant impact in a number of key technologies. Experiments of scaling the e-beam pumped
XeF(C—>A) laser system to the 1 Joule/pulse output level at a 1 Hz repetition rate are described. Recent progress in the amplification of tunable ultrashort laser pulses in the visible spectrum, utilizing the broadband XeF(C—>A) excimer transition, is also reported.
The performance of a scaled, repetitively pulsed injection controlled XeF (C-A) laser system is reported. A 1 Hz electron beam pumped XeF (C-A) laser system produced 0.8 J per pulse with an intrinsic efficiency of 1.5 percent. Details of a compact halogen compatible flow loop is reported. Various unstable resonator geometries were evaluated. A minimum beam divergence of 150 microrad (full angle) corresponding to a three times diffraction-limited beam was determined from far field measurements.