Transparent dielectric material modification with ultrafast laser pulses has seen enormous interest and implementation in both scientific and industrial communities. Formation of well-defined homogenous regions of modified refractive index is possible by keeping the intensity of femtosecond laser pulses slightly above the given material's damage threshold [1]. This method allows us to fabricate many structures in a number of wide band-gap transparent materials, which expanded the choice of glasses suitable for volume Bragg grating (VBG) fabrication since photosensitive materials are no longer required. VBGs are phase gratings that exhibit outstanding angle and wavelength selectivity [2].
The diffraction efficiency of VBGs at Bragg condition is determined by the product of refractive index modulation (RIM) and grating thickness. Moreover, the value of RIM is determined by the structure of the investigated material and laser parameters [3]. In order to obtain the most cost-effective VBG, we fabricated VBGs with the same thickness but different RIM, which is achieved by varying the laser fluence. In this work, we compared the first-order selective diffraction efficiency of volume Bragg gratings obtained by direct laser writing (DLW) using Gauss-Bessel beam in three different types of glass: silica, borosilicate, and gorilla glass.
In this work, we first theoretically calculated the maximum diffraction efficiency for each type of glass using Kogelnik's coupled wave theory [4]. Then we used DLW to fabricate multiple VBGs with different laser fluence in all given samples. Finally, we compared first-order selective diffraction efficiencies of VBGs in different glasses using a CCD camera, which allowed us to capture the intensity profile of the diffracted beam.