High power laser mode conversion with volume phase elements recorded in PTR glass

Abstract

The manipulation of a laser’s transverse profile is of great interest for many applications. The most common and simple approach to beam shaping is by the use of optical phase masks. Conventional phase masks fabricated by surface profiling using spatially selective etching or deposition are easily damaged and limit high energy applications. We have shown in past work, that it is possible to create different phase profiles in the volume of photo-thermorefractive (PTR) glass, which unlike conventional phase mask materials has a high damage threshold that can withstand high peak and average powers. Here we present an approach for mode conversion of a high power fiber laser system using two different types of phase masks fabricated by encoding phase profiles into volume Bragg gratings. These holographic phase masks (HPMs) can successfully introduce wavefront change and achieve high diffraction efficiency (based on wavelength and the grating strength) for a broad range of wavelengths by tuning the element to the gratings Bragg condition. The first element had a grayscale vortex phase profile for HG1 conversion. The second had a binary four-sector phase profile, and was capable to perform fundamental mode conversion to a TEM01, TEM10, or TEM11. Mode conversion efficiency and thermal stress on each type of phase element were investigated using a 150 W of continuous wave power with a TEM00 profile Yb:fiber laser.