Seung-Whan Bahk

A high-resolution, adaptive beam-shaping system for high-power lasers

A high-resolution, high-precision beam-shaping system for high-power-laser systems is demonstrated. A liquid-crystal-on-silicon spatial light modulator is run in closed-loop to shape laser-beam amplitude and wavefront. An unprecedented degree of convergence is demonstrated, and important practical issues are discussed. Wavefront shaping for the applications in OMEGA EP laser is demonstrated, and other interesting examples are presented.

A focal-spot diagnostic for on-shot characterization of high-energy petawatt lasers

An on-shot focal-spot diagnostic for characterizing high-energy, petawatt-class laser systems is presented. Accurate measurements at full energy are demonstrated using high-resolution wavefront sensing in combination with techniques to calibrate on-shot measurements with low-power sample beams. Results are shown for full-energy activation shots of the OMEGA EP Laser System.

Accurate target-plane focal-spot characterization in high-energy laser systems using phase retrieval

Target-plane intensities on the short-pulse beamlines of OMEGA EP, a petawatt-class laser, are characterized on-shot using the focal-spot diagnostic (FSD), an indirect wavefront-based measurement. Phase-retrieval methods are employed using on-shot and offline camera-based far-field measurements to improve the wavefront measurements and yield more-accurate, repeatable focal-spot predictions. Incorporation of these techniques has improved the mean cross-correlation between the FSD predictions and direct far-field fluence measurements in the target chamber from 0.78 to 0.94.

Band-limited wavefront reconstruction with unity frequency response from Shack-Hartmann slopes measurements

The analytic frequency responses of the traditional wavefront reconstructors of Hudgin, Fried, and Southwell are presented, which exhibit amplification or attenuation of the original signal at high spatial frequencies. To overcome this problem, a reconstructor with unity frequency response is developed based on a band-limited derivative calculation. The algorithm is both numerically and experimentally confirmed.

Highly accurate wavefront reconstruction algorithms over broad spatial-frequency bandwidth

New algorithms for reconstructing wavefront from slopes data are developed, which exhibit high accuracy over broad spatial-frequency bandwidth. Analyzing wavefront reconstructors in the frequency domain lends new insight into ways to improve frequency response and to understand noise propagation. The mathematical tools required to analyze the frequency domain are first developed for discrete band-limited signals. These tools are shown to improve frequency response in either spatial-or frequency-domain reconstruction algorithms. A new spatial-domain iterative reconstruction algorithm based on the Simpson rule is presented. The local phase estimate is averaged over 8 neighboring points whereas the traditional reconstructors use 4 points. Analytic results and numerical simulations show that the Simpson-rule-based reconstructor provides high accuracy up to 85% of the bandwidth. The previously developed rectangular-geometry band-limited algorithm in frequency domain is adapted to hexagonal geometry, which adds flexibility when applying frequency-domain algorithms. Finally, a generalized analytic expression for error propagation coefficient is found for different reconstructors and compared with numerical simulations.

Offner radial group delay compensator for ultra-broadband laser beam transport.

Conventional lens-based image relays introduce radial group delay that significantly reduces focal-spot intensity of ultra-broadband short pulses. A direct compensation scheme that can be used with conventional singlet image relays is proposed. The compensation scheme is based on using two negative lenses embedded within an Offner imaging system. The setup allows a single-pass configuration and eliminates the need for polarization switching optics. The performance of this setup is calculated using ray tracing. The compensation improves the spatiotemporal Strehl ratio from 0.02 to 0.97.

Spot-shadowing optimization to mitigate damage growth in a high-energy-laser amplifier chain

A spot-shadowing technique to mitigate damage growth in a high-energy laser is studied. Its goal is to minimize the energy loss and undesirable hot spots in intermediate planes of the laser. A nonlinear optimization algorithm solves for the complex fields required to mitigate damage growth in the National Ignition Facility amplifier chain. The method is generally applicable to any large fusion laser.

Application of phase retrieval for predicting a high-intensity focused laser field

Multiple-focal-plane spatial phase retrieval for a chirped-pulse-amplification laser is demonstrated for the first time. Advantages of this method are simplicity of setup, ability to measure angular dispersion, and potential integration into precision, on-shot focal-spot diagnostics.

Chromatic diversity: a new approach for characterizing spatiotemporal coupling of ultrashort pulses

Two-dimensional chromatic aberrations are characterized by a single-shot scheme based on a simultaneous measurement of chromatically diversified focal spots. The chromatic diversity is introduced by a 2-D grating with holographic defocus terms. The chromatic aberrations in the beam are either subtracted or added by the additional known chromatic aberrations in the grating, depending on the diffraction order. By analyzing the asymmetry in the size of diffracted focal spots, input beam chromatic aberrations can be deduced. Theoretical discussions and experimental results are presented.

A spectrally resolved lateral-shearing interferometer for measurement of relative group delay using a periodic entrance slit in a spectrometer

A concept for a spectrally resolved lateral-shearing interferometer is proposed to measure pulse front and radial group delay. A periodic slit in a spectrometer enables a simpler spatial interference scheme than a Mach-Zehnder interferometer.