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

Abstract

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.

Wavefront measurement techniques used in high power lasers

“…This diagnostic method was evaluated on the OMEGA EP laser beam over a population of 175 shots to illustrate its reliability and stability; the evaluation was conducted for approximately 18 months from 2010 to 2012 [44] . Figure 13(a) shows the frequency of cross-correlation values between the FSD and the far-field CCD.…”

Channeling of multikilojoule high-intensity laser beams in an inhomogeneous plasma

“…The intensity distribution in the experimental laser focus was not diffraction limited and contained some spatial inhomogeneity [18], which probably seeded the filamentation instability and self-focusing in the plasma driven by ponderomotive and relativistic effects. Filamentation was predicted by a split-step paraxial wave equation calculation taking the ponderomotive and relativistic effects into account and using the measured wavefront map of the channeling beam and the refractive index of the plasma.…”

“…The propagation of a laser beam at relativistic intensities (>10 18 W/cm 2 ) through plasma with a large density scale length is dominated by highly nonlinear interactions including ponderomotive expulsion of electrons [1], channeling [2][3][4], and the development of hosing and bifurcation instabilities [3,4]. These effects are important for both fundamental aspects of relativistic laser-plasma interaction physics and applications such as fast ignition in inertial confinement fusion [5].…”

“…An intensity comparison was performed by using 100-ps and 10-ps pulse widths, respectively. The wavefront of the channeling beam was measured and the focal-spot irradiance map was inferred for each shot [18]. The vacuum focal spot contained 80% of the laser energy in a 20-µm-radius spot with peak intensities of ∼ 1 × 10 19 and ∼ 4 × 10 19 W/cm 2 for 2.6-kJ, 100-ps and 1-kJ, 10-ps pulses, respectively.…”

Wavefront Shaping by a Small-Aperture Deformable Mirror in the Front Stage for High-Power Laser Systems

“…As an important factor related to the beam quality, a uniform wavefront is conducive to maintaining the high quality spatial distribution in propagation. The wavefront is directly related to the far-field distribution of the output laser that influences the focused intensity as well as the alignment of the beam path in a high-power laser system (like spatial filter) [13]. Additionally, in a frequency conversion module, the frequency conversion efficiency decreases sharply with the increase of the wavefront error [14,15], due to the angle matching of conversion cannot be achieved perfectly in the full aperture range.…”