Interferometric adaptive optics testbed for laser pointing, wave-front control and phasing.

Implementing the capability to perform fast ignition experiments, as well as, radiography experiments on the National Ignition Facility (NIF) places stringent requirements on the control of each of the beam's pointing, intra-beam phasing and overall wave-front quality. In this article experimental results are presented which were taken on an interferometric adaptive optics testbed that was designed and built to test the capabilities of such a system to control phasing, pointing and higher order beam aberrations. These measurements included quantification of the reduction in Strehl ratio incurred when using the MEMS device to correct for pointing errors in the system. The interferometric adaptive optics system achieved a Strehl ratio of 0.83 when correcting for a piston, tip/tilt error between two adjacent rectangular apertures, the geometry expected for the National ignition Facility. The interferometric adaptive optics system also achieved a Strehl ratio of 0.66 when used to correct for a phase plate aberration of similar magnitude as expected from simulations of the ARC beam line. All of these corrections included measuring both the upstream and downstream aberrations in the testbed and applying the sum of these two measurements in open-loop to the MEMS deformable mirror.

A single-shot pixellated phase-shifting interferometer utilizing a liquid-crystal spatial light modulator.

We introduce a novel phase-shifting pixellated interferometer based on a liquid-crystal spatial light modulator and simulate the expected performance. The phase-shifted frames are captured simultaneously, which reduces problems arising from vibrations and air turbulence. The liquid-crystal spatial light modulator is flexible and can be configured to provide a large number of phase-shift levels and geometries to reduce measurement error.

High-speed horizontal-path atmospheric turbulence correction with a large-actuator-number microelectromechanical system spatial light modulator in an interferometric phase-conjugation engine.

Results of atmospheric propagation for a high-speed, large-actuator-number adaptive optics system are presented. The system uses a microelectromechanical system- (MEMS-) based spatial light modulator correction device with 1024 actuators. Tests over a 1.35-km path achieved correction speeds in excess of 800 Hz and Strehl ratios close to 0.5. The wave-front sensor was based on a quadrature interferometer that directly measures phase. This technique does not require global wave-front reconstruction, making it relatively insensitive to scintillation and phase residues. The results demonstrate the potential of large-actuator-number MEMS-based spatial light modulators to replace conventional deformable mirrors.

Performance of a phase-conjugate engine implementing a finite-bit phase correction.

The achievable Strehl ratio when a finite-bit correction to an aberrated wave front is implemented is examined. The phase-conjugate engine used to measure the aberrated wave front consists of a quadrature interferometric wave-front sensor, a liquid-crystal spatial light modulator, and computer hardware-software to calculate and apply the correction. A finite-bit approximation to the conjugate phase is calculated and applied to the spatial light modulator to remove the aberrations from the optical beam. The experimentally determined Strehl ratio of the corrected beam is compared with analytical expressions for the expected Strehl ratio and shown to be in good agreement with those predictions.

Quasi-phase-matched second-harmonic generation by use of a total-internal-reflection phase shift in gallium arsenide and zinc selenide plates.

We fabricated a novel quasi-phase-matched frequency converter, using a zigzag optical beam path in a thin, polished parallel plate. Second-harmonic generation experiments demonstrated angle-tuned output at 4.6 to 5.3mum in GaAs and 1.7 to 2.0mum in ZnSe crystals when pulsed infrared laser sources were used.

High-power wavelength converters with feedback.

We have shown theoretically and experimentally that very high conversion efficiencies can be obtained in high-peak-power wavelength converters utilizing small amounts of feedback, at fluences similar to the no-feedback case. Maximum efficiencies are obtained with radially varying reflectivity profiles. We have demonstrated the concept, using second-harmonic generation in a deuterated potassium dihydrogen phosphate crystal in which 70% energy conversion was obtained, in agreement with simulation results. Energy conversion efficiencies of 56% were also obtained in a beta-barium borate optical parametric oscillator.

Infrared laser heating for studies of cellulose degradation.

We describe a new technique for studying thermally induced chemical transformations in cellulose. The apparatus consists of a carbon dioxide laser for heating, an IR thermometer, and an optical reflectance spectrometer for tracking the progressive discoloration of the sample. To illustrate the technique, we present measurements from a single piece of sample linen along five isotherms in the 200-290 degrees C range. We derive an algebraic expression for the reflectivity of the sample as a function of the areal concentrations of the chromophoric states produced at temperature. The results are then explained in terms of first-order chemical rate theory and a four-step model. From the measurements we derive the activation energies, Arrhenius constants, and reflectivities of the chromophoric states.

High-efficiency joule-level Raman generation in Pb vapor.

We have obtained nearly 1 J of blue-green radiation in 60-nsec pulses by Raman shifting an injection-locked XeCl laser in Pb vapor. The measured 50% Raman energy conversion has been observed in both oscillator and oscillator-amplifier experiments. These results indicate that an XeCl/Pb blue-green laser efficiency of >1.4% should be possible.

Higher-Stokes-order Raman conversion of XeCl laser in hydrogen.

Efficient XeCl-laser Raman downconversion has been demonstrated with a novel oscillator-amplifier system based on higher-Stokes-order generation in H(2) gas. Simultaneous generation of the first (353-nm), second (414-nm), and third (499-nm) Stokes outputs has been observed in a Raman amplifier at energy-conversion efficiencies of 18, 22, and 14%, respectively, in good agreement with computer simulations.

Mode locking with a slow saturable absorber in a transverse resonator.

We report a new mode-locking technique, which greatly extends the number of dyes that can be used as saturable absorbers. When the absorber is enclosed in a high-Q transverse resonator its lifetime is reduced by stimulated emission to values that are sufficiently short to ensure complete recovery between passes. The technique has been demonstrated using rhodamine 6G for the amplifier and cresyl violet for the saturable absorber. In addition to its use for mode locking visible dye lasers, the technique also shows great promise for locking ultraviolet excimer lasers, such as krypton fluoride and xenon chloride lasers, using a visible dye for the slow saturable absorber.