Highly efficient relief diffraction gratings inscribed on a chalcogenide bulk glass by a femtosecond laser

Direct laser writing has been already demonstrated for the fabrication of under surface “buried” 3D mid-IR waveguides in chalcogenide glasses by employing a large photo-induced refractive index change in the features formed in the path of the focused beam from a short pulse laser. In this paper, we report on direct laser writing of relief diffraction gratings with periods of 6, 14 and 24 μm into the surface of Ge15Ga3Sb12S70 chalcogenide glass by using a 800 nm Ti:saphire femtosecond pulse laser. The first order diffraction efficiency of the fabricated gratings was over 60 % at 650 nm. We have also fabricated a “composite” grating composed of three relief diffraction gratings inscribed in the same position, but with a mutual tilt. Composite grating provided complex multidirectional diffraction of the light in the accordance with geometrical arrangement and grating period of all the gratings inscribed. The fabrication was implemented on a computer controlled stage employing surface-to-beam alignment, laser power and raster pattern control. Pulse energies of 1.5, 3.0 and 4.5 μJ were used, resulting in channel widths of around 4, 5 and 6 μm, respectively, and depths up to 1.7 μm. We propose practical applications including surface relief diffraction micro-gratings at the ends of multimode chalcogenide optical waveguides or on the surfaces of bare core optical fibers used for chemical sensing.