Abstract

Due to the rapid progress in high-power laser systems, there is an increasing demand for highly efficient damage resistant components. One decisive role in the power limitation of such systems, e.g. chirped pulse amplification setups [1], is played by the grating pair, which is used to recompress the amplified optical pulses. In order to achieve pulse peak powers in the megawatts range, these gratings have to be dielectric, where they can be optimized either for reflection or transmission. Up to now, one of the main disadvantages of transmission gratings was the problem of significant reflection losses in the case of high dispersion and grazing incidence. These losses are typically in the range above 10% for grating periods near half of the wavelength (Fig. la). However, we have developed a simple grating layout, which can be optimized to avoid any reflection losses, thus achieving theoretically 100% diffraction efficiency in the highly-dispersive -1^st diffraction order, when the grating is illuminated in Littrow-mounting (Fig. lb). In this layout a rectangular surface relief grating is etched into a fused silica substrate and afterwards covered by a second silica substrate. By optimizing the geometrical parameters of this embedded grating, the reflection can be suppressed completely, while simultaneously all the transmitted light is deflected to the -1^st order. This remarkable effect can easily be explained by investigating the field inside the grating, which can be described by discrete modes. This modal vision, which we have presented in earlier papers ([2] and references herein), implies that the diffraction of such an embedded grating is mainly determined by two grating modes, which are excited by the incident wave and propagate through the grating with different propagation constants. At the grating bottom they couple their energy to the transmitted diffraction orders, while the intensities in the two orders are a result of the modal interference. It can be shown that the reflection of this device can be represented by a symmetrical Fabry-Perot-Interferometer, which allows for the complete suppression of reflection, if the grating groove depth is optimized. Based on this intelligible investigation of how the diffraction efficiency is influenced by the grating parameters, fill factor and groove depth, the diffraction efficiency can be calculated without complicated grating solver software. This simplified modal approach teaches that even theoretically the transmitted diffraction efficiency of a conventional surface relief rectangular grating can never reach 100%, while this problem is easily solved by covering the grating.

© 2007 IEEE