HIGH-ENERGY LASERS

Cascaded amplifiers produce 100 TWA 100-TW Nd:glass laser system is being tested at the Laboratoire pour l`Utilisation des Lasers Intenses, Ecole Polytechnique Palaiseau (LULI; Palaiseau, France). This laboratory is one of the primary European and worldwide research centers investigating the interaction and applications of high-energy lasers with matter, including, for example, inertial fusion.

HIGH-ENERGY LASERS

Cascaded amplifiers produce 100 TWA 100-TW Nd:glass laser system is being tested at the Laboratoire pour l`Utilisation des Lasers Intenses, Ecole Polytechnique Palaiseau (LULI; Palaiseau, France). This laboratory is one of the primary European and worldwide research centers investigating the interaction and applications of high-energy lasers with matter, including, for example, inertial fusion.

To address the needs of such research, LULI scientists designed a new Nd:glass laser capable of delivering 100-TW pulses, that is, 25 J in a 250-fs pulse. The design program was launched in 1996, and the system was first operational last year. Initial results of experiments with the new laser were published in the LULI annual scientific report.1 The report describes the Ti:sapphire Nd:glass laser, which is based on chirped pulse amplification.

Laser setup

In operation, an 80-fs pulse is produced at 1057 nm by a modelocked Ti:sapphire oscillator pumped by an argon-ion laser. Passing the pulse four times through a diffraction grating--1740 grooves/ mm--enables the pulse to be stretched to 2 ns without any change of the spectral width. A frequency-doubled Nd:YAG-laser-pumped Ti:sapphire regenerative amplifier provides an energy gain of more than a factor of 1 million, producing millijoule pulses at the amplifier output. Subsequent amplification is based on cascaded neodymium-doped silicate and phosphate glasses, which together reduce any spectral narrowing. The cascade begins with a small-diameter amplifier rod and is terminated with 108-mm disk amplifiers (see photo).2

The purpose of the amplifier cascade is to amplify laser pulses with a large spectral width while maintaining that width. The width is important to obtaining a very short pulse after the pulse has been recompressed. The use of both silicate and phosphate glasses allows the amplifier gain to be relatively uniform over the entire spectral region of interest.

The multiple Nd:glass laser amplifier cascade is comprised of three main elements. First are three amplification stages using Nd:glass rods with increasing diameter--16, 25, and 45 mm--followed by a 108-mm-diameter amplifier disk that is used in a double-pass arrangement. Each rod amplifier stage includes two laser heads--one uses silicate glass and the other phosphate glass.

The 108-mm disk amplifier, designed and constructed by the Rutherford Appleton Laboratory (England), contains six phosphate glass disks tilted at Brewster`s angle. This amplifier has a very uniform radial gain across the large surface and avoids the intensity variations typically found at the edge of large amplifier rods.

The second main element is made u¥of four spatial filters--three are under vacuum--that are used between the different amplification stages. Their purpose is not only spatial filtering of the beam but also adjusting for changes in the laser-beam dimension on the various optical components caused by any image shift. The third element is an antireturn device based on a 25-mm-diameter Pockels cell and a 140-mm-diameter Faraday rotator. Antireturn devices are installed after the 25-mm amplifiers and at the system end.

The fill factor of the rods is kept low to limit thermal effects. For the last two amplifier stages, however, the fill factor is 0.6 or higher, which is a compromise that takes into account the thermal-lens effect and the possibility of threshold damage to the optical components, because the laser-beam intensity is very high at the system output.

The researchers emphasize the role played by silicate and phosphate glass rods to minimize spectral narrowing during the amplification process.3 To obtain a very short pulse after temporal compression, the gain of the different glasses must be balanced because each of these materials has a relatively narrow fluorescence spectrum with respect to the incident beam. Despite this technical contribution, however, the spectral width actually depends on total gain amplification, and, when this does not exceed 50, a 10% reduction in spectral width is measured, which shows that it is difficult to master this phenomenon.

The laser system will be available to both the French scientific community--within a users association of 12 French laboratories--and to 11 laboratories of the European Union. LULI plays an important role coordinating work done with the laser. Other laboratories working with LULI include the Laboratoire d`Optique Applique, CEA DAM (megajoule laser), the Rutherford Appleton Laboratory, and the Max Planck Institut Garching (Germany).

The system has been recently tested to accelerate electrons in a plasma by the wake method. The results are very encouraging and should be published shortly.

Roland Roux

REFERENCES

1. LULI, CNRS, Ecole Polytechnique, Palaiseau, France.

2. J. P. Zou et al., Chaine amplificatrice multi-verres. LULI, CNRS, Ecole Polytechnique, Palaiseau, France.

3. J. P. Zou et al., Etude de l`amplification et du spectre dans la chaine à barreaux (16 et 25 mm, en silicate et en phosphate). LULI, CNRS, Ecole Polytechnique, Paliseau, France.

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