ULTRAFAST LASERS: Improved gratings have high efficiency

Aug. 1, 2000
To create dense and hot plasmas—notably for laser-based inertial fusion, x-ray sources, and particle acceleration—ultrabright lasers are currently being developed at the National Ignition Facility in the USA and the Laser Megajoule in France

To create dense and hot plasmas—notably for laser-based inertial fusion, x-ray sources, and particle acceleration—ultrabright lasers are currently being developed at the National Ignition Facility in the USA and the Laser Megajoule in France. Other ongoing projects are being upgraded, such as at Phelix at GSI (Germany), Gekko XII (Japan), Vulcan (England), and Pico 2000 (France).

These laser systems incorporate chirped-pulse amplification (CPA), which allows the systems to achieve very high peak beam power. A hindrance to further increases in the peak power relates to the process of temporally compressing the pulses. The temporal compressors are based on large gratings that have a limited (although not poor) diffraction efficiency and an insufficiently high damage threshold; in addition, they are difficult to produce in large sizes with the required flatness.

These limitations are largely due to the metallic reflecting coatings used for the gratings. The use of alternative coatings is prevented by the weakness of the underlying photoresist layer into which the grooves are formed.

The limitations are important to overcome because any improvement in these gratings would lead to a performance increase in terms of energy and peak power delivered to the target. An order-of-magnitude improvement in the focused intensity—that is, to well above 1020 W/cm2—should be reachable with improved gratings.

A concerted approach has been initiated by the European Community. Organizations in three European countries are participating in the project, including CEA-CESTA (Bordeaux, France), which is also in charge of the coordination; LULI (Ecole Polytechnique, Palaiseau, France); Rutherford Appleton Laboratory (England); University of Jena (Germany); and commercial firms such as Thomson-CSF, Jobin Yvon, and Carl Zeiss. The work is supported by the European Commission through a Research Technology Development contract, "Gratings for Ultrabright Lasers."

Two main technologies are being evaluated and classified according to whether the diffraction is achieved on the surface of the grating or within its volume. The conventional realization of surface diffraction consists of a metallic layer deposited on a photosensitive layer (photoresist) whose profile is recorded using photographic methods. Unfortunately, these gratings have a damage threshold limited by the heating of the metal coating and the effects of the photoresist.

In a first attempt to improve the gratings, the photosensitive layer is replaced by solid gold or aluminum into which the grooves are directly engraved by an ion gun. Although the improvement is limited, the subsequent deposition of dielectric multilayers can avoid large variations of the electric field on the grating surface, leading to enhanced efficiency.

Nevertheless, the most promising solution consists of engraving grooves directly into the top layers of a multilayer dielectric. This ionic engraving permits profile control that avoids point effects due to sharp edges. In addition, the engraving seems to eliminate surface impurities.

In volume diffraction, the whole material volume is used to diffract the beam, leading to a higher damage threshold. Holographic gratings made by Thomson LCR and Sextant consist of a plastic material containing a polymer (Thomson LCR) or a dichromated gelatin (Sextant) into which a periodic variation in refractive index is recorded. A vertical sinusoidal variation in refractive index produces a Bragg mirror. In a version with tilted Bragg planes, the optical layer produced a volume beam diffraction reaching an efficiency of close to 100%.

The samples fabricated by Jobin-Yvon, Thomson LCR, and Sextant have been tested by the Département Lasers de Puissance du CEA at the Centre d'Etude de Limeil (Valenton, France). A high-energy laser delivered 250- to

300-fs pulses at 1.05-µm wavelength to a test sample—pulses representative of a CPA system used for pulse compression. Diffraction efficiency reached 98%.

Roland Roux

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