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

摘要

The manipulation of a laser’s transverse profile is of great interest for many applications. The most commonand simple approach to beam shaping is by the use of optical phase masks. Conventional phase masks fabricated bysurface 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 withstandhigh peak and average powers.Here we present an approach for mode conversion of a high power fiber laser system using two different types of phasemasks fabricated by encoding phase profiles into volume Bragg gratings. These holographic phase masks (HPMs) cansuccessfully introduce wavefront change and achieve high diffraction efficiency (based on wavelength and the gratingstrength) for a broad range of wavelengths by tuning the element to the gratings Bragg condition. The first element had agrayscale vortex phase profile for HG1 conversion. The second had a binary four-sector phase profile, and was capable toperform fundamental mode conversion to a TEM01, TEM10, or TEM11. Mode conversion efficiency and thermal stress oneach type of phase element were investigated using a 150 W of continuous wave power with a TEM00 profile Yb:fiberlaser.