Investigation of laser-induced damage on multi-layer dielectric gratings

We investigate mechanisms of laser induced damage thresholds (LIDTs) of multi-layer dielectric gratings (AIDG,). It is found that the laser damage thresholds of MDGs and unstructured dielectric multi-layer coatings (the substrate of MDG) are 3.15J/cm(2) and 9.32 J/cm(2), respectively, at 1064nm (12ns) with the Littrow angle 51.2 degrees and the TEM00 mode. The laser-induced damage mechanism of multi-layer dielectric is presented with the analysis of the following factors: The dominant factor is the pollution on the corrugated surface, which is induced by the complex manufacture process of multi-layer dielectric gratings; another is the electric field distribution along the corrugated surface. The third reason is due to the reduction in stoichiometry of oxide films, resulting from the manufacture process of etching.

Laser induced damage of multi-layer dielectric used in pulse compressor gratings

Laser induced damage threshold (LIDT) of multi-layer dielectric used in pulse compressor gratings (PCG) was investigated. The sample was prepared by e-beam evaporation (EBE). LIDT was detected following ISO standard 11254-1.2. It was found that LIDTs of normal and 51.2 deg. incidence (transverse electric (TE) mode) were 14.14 and 9.31 J/cm2, respectively. A Nomarski microscope was employed to map the damage morphology, and it was found that the damage behavior was pit-concave-plat structure for normal incidence, while it was pit structure for 51.2 deg. incidence with TE mode. The electric field distribution was calculated to illuminate the difference of LIDT between the two incident cases.

Optimization of near-field optical field of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric (MLD) gratings for pulse compressors in high-energy laser systems should provide high diffraction efficiency as well as high laser induced damage thresholds (LIDT). Nonuniform optical near-field distribution is one of the important factors to limit their damage resistant capabilities. Electric field distributions in the gratings and multi-layer film region are analyzed by using Fourier modal method. Optimization of peak electric field in the gratings ridge is performed with a merit function, including both diffraction efficiency and electric field enhancement when the top layer material is HfO2 and SiO2, respectively. A set of optimized gratings parameters is obtained for each structure, which reduce the peak electric field within the gratings ridge to being respective 1.39 and 1.84 times the value of incident light respectively. Finally, we also discuss the effects of gratings refractive index, gratings sidewall angle and incident angle on peak electric field in the gratings ridge. (c) 2006 Elsevier B.V. All rights reserved.

Optimization of thin-film design for multi-layer dielectric grating

Thin-film design used to fabricate multi-layer dielectric (MLD) gratings should provide high transmittance during holography exposure, high reflectance at use wavelength and sufficient manufacturing latitude of the grating design making the MLD grating achieve both high diffraction efficiency and low electric field enhancement. Based on a (HLL)H-9 design comprising of quarter-waves of high-index material and half-waves of low-index material, we obtain an optimum MLD coating meeting these requirements by inserting a matching layer being half a quarter-wave of Al2O3 between the initial design and an optimized HfO2 top layer. The optimized MLD coatings exhibits a low reflectance of 0.017% under photoresist at the exposure angle of 17.8 degrees for 413 nm light and a high reflectance of 99.61% under air at the use angle of 51.2 degrees for 1053 nm light. Numerical calculation of intensity distribution in the photoresist coated on the MLD film during exposure shows that standing-wave patterns are greatly minimized and thus simulation profile of photoresist gratings after development demonstrates smoother shapes with lower roughness. Furthermore, a MLD gratings with grooves etched into the top layer of this MLD coating provides a high diffraction efficiency of 99.5% and a low electric field enhancement ratio of 1.53. This thin-film design shows perfect performances and can be easily fabricated by e-beam evaporation. (c) 2006 Elsevier B.V. All rights reserved.