Multilayer Dielectric Gratings Research Articles

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)9H 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° for 413nm light and a high reflectance of 99.61% under air at the use angle of 51.2° for 1053nm 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.

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.

New MLD gratings adapted for tiling in petawatt-class lasers

Laser-induced damage threshold of gratings remains the main limiting factor towards generating higher-energy compressed pulses (terawatt and petawatt). HORIBA Jobin Yvon has first developed the manufacturing of large gold coated gratings, dimensions up to half a meter. Then we have developed large multilayer dielectric (MLD) gratings for working at 1.053 microns with 2 to 3 times higher damage threshold than gold coated gratings for ps pulses, and higher efficiency (up to 96% averaged over the full aperture). In addition these large MLD gratings are optimised for mosaic (tiling) configurations, in which the total useful grating length may reach 1 meter or more.

Near-field optical property of multi-layer dielectric gratings for pulse compressor

Multi-layer dielectric gratings for pulse compressor in high-energy laser system must provide high diffraction efficiency. In addition, its laser induced damage property is critical for the system. Nonuniform optical near-field distribution of multi-layer dielectric gratings is one of the important factors to limit its laser induced damage threshold. Electric field distributions in the grating region and multi-layer film region are analyzed by using Fourier modal method. Effects of grating structure on peak magnitude of electric field in grating ridge are analyzed when the top layer material is HfO2 and SiO2, respectively. The results show that there exists an optimum top layer thickness, at which the peak magnitude of electric field within grating ridge is a minimum. And the peak electric field in the grating ridge can be reduced by designing top gratings with high aspect ratio structure when top layer thickness is increasing. Finally, the peak electric field in the grating ridge can also be reduced when the multi-layer dielectric grating is used at big incident angle.

Design of multi-layer dielectric grating for femtosecond laser

Based on optical interference theory and Fourier modal theory, a new multi-layer dielectric gratings is designed for use in femtosecond laser system at wavelength 0.8μm. A multi-layer dielectric with stack of H3L(HL)^9H0.5L2.4H is used as a substrate to be etched with corrugated structure. Numerical computation indicates that the diffraction efficiency of -1 order can be more than 95% when the parameters of multi-layer dielectric grating are the following: laser is incident on the grating at 36.7° in TE mode, the duty cycle is 0.35 and the line density is 1480/mm, the groove depth is 0.2μm and the top residual HfO2 is 0.15μm.

Development of a mirror backed volume phase grating with potential for large aperture and high damage threshold

The present paper proposes a new grating concept by which a volume phase grating is placed on top of a dielectric mirror stack. Instead of using a multilayer dielectric grating, wherein the uppermost layer of a thin film mirror is etched to create the desired binary phase grating we suggest to have an upper grating layer of a lower density gelatin-based volume phase grating in either sol–gel or dichromated gelatin. The key benefit is the elimination of the etching step which is the limiting factor in the production of large aperture gratings. We have investigated designs for s- and p-polarization as well as the necessary chemistry that is needed to achieve the required index modulations. Damage testing on dichromated gelatin and aerogel was performed and diffraction efficiencies for various prototypes were measured.

Investigation of Laser-Induced Damage on Multi-LayerDielectric Gratings

We investigate mechanisms of laser induced damage thresholds (LIDTs) ofmulti-layer dielectric gratings (MDGs). It is found that the laserdamage thresholds of MDGs and unstructured dielectric multi-layercoatings (the substrate of MDG) are 3.15 J/cm2 and9.32 J/cm2, respectively, at 1064 nm (12 ns) with the Littrow angle51.2 degrees and the TEM00 mode. The laser-induced damage mechanism ofmulti-layer dielectric is presented with the analysis of the followingfactors: The dominant factor is the pollution on the corrugatedsurface, which is induced by the complex manufacture process ofmulti-layer dielectric gratings; another is the electric fielddistribution along the corrugated surface. The third reason is due tothe reduction in stoichiometry of oxide films, resulting from themanufacture process of etching.

Precision short-pulse damage test station utilizing optical parametric chirped-pulse amplification

The next generation of high-energy petawatt (HEPW)-class lasers will utilize multilayer dielectric diffraction gratings for pulse compression, due to their high efficiency and high damage threshold for picosecond pulses. The peak power of HEPW lasers will be determined by the aperture and damage threshold of the final dielectric grating in the pulse compressor and final focusing optics. We have developed a short-pulse damage test station for accurate determination of the damage threshold of the optics used on future HEPW lasers. Our damage test station is based on a highly stable, high-beam-quality optical parametric chirped-pulse amplifier (OPCPA) operating at 1053 nm at a repetition rate of 10 Hz. We present the design of our OPCPA system pumped by a commercial Q-switched pump laser and the results of the full system characterization. Initial short-pulse damage experiments in the far field using our system have been performed.

The Effect of Radiation on Transmission of Quasi-Periodic Multi-Layer Planar Dielectric Gratings

The transmission characteristics of quasi-periodic multi-layer planar dielectric gratings are systematically investigated by a method that combines the multimode network theory with rigorous mode matching method. The effect of radiation on the transmission characteristics of the multi-layer dielectric gratings is discussed in details, and some guidelines are given for accurate design of the gratings.