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Petawatt laser approaches diffraction limit

29 Feb 2008

Adaptive optics and dynamic wavefront control shrink the focal spot to increase the intensity of a pulsed petawatt laser.

A French team has combined adaptive optics (AO) with an elaborate alignment system to effectively correct wavefront aberrations in a high peak-power laser, achieving focal spots close to the diffraction limit. "The optimization procedure produces a considerable improvement in focal spot quality with a Strehl ratio of 0.7 for full-energy kilojoule shots," Ji-Ping Zou of the LULI laboratory told optics.org. "The procedure, once integrated into our control system, is straightforward and there are no operational penalties." (Applied Optics 47 704.)

The LULI (Laboratoire pour l'Ulilisation des Lasers Intenses) laser delivers kilojoule pulses in the nanosecond range at 1053 nm, and is capable of reaching the petawatt regime through chirped pulse amplification.

Zou's group classified the wavefront aberrations in the source into three categories, and tackled each separately:
• Aberrations resulting from static imperfections of the optical elements and beam misalignment.
• Thermally induced aberrations appearing before, during and after a single shot caused by non-uniformity of heat deposition from the pump light.
• Cumulative thermal effects that build up in the LULI system during a sequence of shots and further degrade the laser wavefront through thermal relaxation during the following hours.

"We distinguish the thermally induced aberrations into two classes because their correction is different," noted Zou.

The first category of aberrations is minimized by precise beam realignment between two successive shots, combined with a closed-loop AO system employing a bimorph deformable mirror with 32 actuators (see Adaptive solution). An additional semi-automatic realignment of beam pointing and centring between shots controls the second category, while the AO system tackles the third group.

"This combination enables residual thermal tilt to be corrected before running the closed-loop AO system," said Zou. "Also the response matrices of the deformable mirror are always the same, so the closed-loop runs under good conditions."

The result has been reproducible focal spots close to the diffraction limit for full-energy kilojoule shots fired at one shot per hour. Zou's group has achieved a focal spot with a Strehl ratio - a measure of the fractional drop in light intensity as a function of wavefront error - of 0.7. The focal intensity can therefore reach 2.2 x 1018 W/cm2 in the kilojoule per nanosecond range, and intensities as high as 1021 W/cm2 are foreseen by Zou. Shot-to-shot reproducibility of the focal spot is said to be excellent, which is very important for laser-matter interaction experiments.

"I think this result is excellent in comparison with similar laser systems around the world," commented Zou. "There are some costs involved in the AO system and the semi-automatic alignment module, but this is a one-time investment." Once the system is in place the shot repetition rate, which is usually limited by the recovery time of the laser amplifiers, can be doubled with a clear economic advantage.

"Our next steps include studying more efficient pumping and cooling systems to minimize thermal effects, and investigating a hybrid AO system composed of a deformable mirror and a liquid-crystal spatial light modulator," said Zou. "Efforts will also be made to increase the laser pulse contrast, defined as the ratio between the main pulse and temporal noises around it."

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