The design of multilayer dielectric grating for laser frequency selection

Abstract

The metal grating has been widely used in the tunable laser to select frequency, the diffraction efficiency and damage threshold of the gratings are very important to the performance of high power tunable laser. However, because of the light absorption of metal material, the damage threshold of the metal grating is usually not high especially at short wavelength, it is hard to meet the requirements of high power laser. This paper presents the laser frequency selection with multilayer dielectric grating, in this method, the high diffraction efficiency is ensured, and the damage threshold of the grating is improved. Based on the rigorous coupled wave (RCWA) theory, the model of multi-layer dielectric film grating is established, and the theoretical design for the 473nm laser is given. After a series of optimal design, the following results are obtained. HfO2 and SiO2 are selected as multi-layer material, and the multi-layer structure is S(HL)^12HTA .The groove density is 3875 lines/mm. The profile of grating grooves is rectangular .The duty cycle of surface relief structure is between 0.31-0.35, the groove depth is between 270-310nm, the sum of residual thickness and groove depth is between 310-320nm.The -1st diffraction efficiency of the grating is over 98% (TE polarization) at the Littrow angle (66.4 degrees).The diffraction efficiency is higher than that of ordinary metal grating. At the same time, the electric field distribution of the grating is optimized, the peak electric field is avoid located at surface relief structure, and the laser induced damage threshold can be improved.