Diffraction gratings for high intensity lasers

A diffraction grating is an optical component obtained by periodically structuring an interface separating two materials. This structuring results in a modulation of the refractive index along one or two directions in the plane of the interface. This periodic modulation of the interface is at the origin of the most remarkable optical property of diffraction gratings, which can be summed up very simply: a light beam illuminating the grating at a given incidence is diffracted into secondary beams reflected and/or transmitted at several precise angles. Each of these diffracted beams corresponds to a diffraction order. The number of reflected and/or transmitted beams, and their associated angles, can be predicted very simply using the grating law. This law predicts diffraction angles as a function of modulation period, angle of incidence and wavelength. And it's precisely this last dependency - the diffraction angle as a function of wavelength - that is behind the many applications of gratings. This characteristic leads to dispersion of the diffracted beams. In particular, a light beam with a given spectral content will be diffracted, for each given diffraction order, at different angles as a function of wavelength. The spectral intensity of the beam can then be measured. This dispersive property of diffraction gratings has led to their widespread use in spectroscopy for applications in space, biosensors and sensors.

Since the grating period is of the order of a wavelength, the manufacture of these optical components requires sub-micrometric control of the engraving. The first grating was manufactured in the 18th century, and manufacturing techniques have never ceased to progress since. In the 1960s, these techniques benefited from the invention of the laser. The shaping of coherent monochromatic beams led to the development of photolithography. This interferometric technique enabled considerable progress to be made in controlling the etching period over large surfaces. If the advent of laser photolithography marked a technological turning point for diffraction gratings, history shows that diffraction gratings in turn revolutionized the field of lasers from the 1980s onwards, more specifically the field of high-intensity lasers.

These lasers use frequency-drift amplification, a technique proposed in 1984 to overcome the problem of pulse damage to amplifier crystals. The pulse is first temporally stretched before being compressed by one or more diffraction gratings. The dimensioning of these high-intensity laser beams leads to the manufacture of very large gratings, i.e. gratings manufactured on a decimetre or even metre scale. These sizes are very important, especially if we refer to the sub-micrometer scale of the modulation period. However, these laser beams also...