Beam shaping by volume phase structures in photo-thermo-refractive glass

Abstract

We demonstrate the recording of volume phase masks in the bulk of photo-thermo-refractive glass. Recording was produced by exposing the glass to UV radiation through binary amplitude masks. Depending on the profile of the amplitude mask either a binary volume phase mask or a grayscale phase mask may be produced. Volume phase masks have been used to generate Fresnel lenses, convert a Gaussian beam into higher order Hermite-Gauss and LaguerreGauss modes, to produce optical vortices, and to create aberration-correcting optical components.Phase masks have been used for decades for a variety of applications, including improving the depth of field [1-3], manufacture of electronics [4], encryption [5][6][7][8], and coronagraphy [9][10][11]. Conventional fixed phase masks are generally produced by either sculpting the surface of a thin film such as PMMA or by recording it in the bulk of a photosensitive material such as DCG or lithium niobate. Active phase masks may also be produced using spatial light modulators. In either case these masks may be used effectively in low power systems as they can be designed to have nearly any phase profile. However, such elements have several drawbacks as well. Absorption in these materials prevents them from being used in high power systems, and thin film and SLM phase masks may also be damaged from mishandling or by placement in high temperature conditions. In order to have a robust phase mask suitable for use in such systems it is therefore necessary to utilize a low-absorption substrate with the phase mask recorded in the bulk to prevent damage via mishandling. Here we present a method for recording volume phase masks (VPMs) in the bulk of photo-thermo-refractive (PTR) glass.PTR glass is a sodium-potassium-zinc-aluminum-fluorine-bromine-silicate glass doped with cerium, antimony, tin, and silver, with a region of transparency from 350 nm to 2700 nm and a damage threshold of 40 J/cm 2 [12,13]. Due to this wide transparency window, PTR glass is used to produce volume Bragg gratings for the visible and infrared regions, which have found applications in pulse stretching and compression [14], beam combining [15,16], and ultra-narrow spectral filtering [17,18]. In the near IR region PTR glass has an absorption coefficient of ~10 -4 cm -1 , which, coupled with its glass transition temperature of ~460 o C [12], makes a suitable substrate for high power and high temperature systems. In addition, forced air cooling can be applied to the sample without degrading the recorded profile or seriously Mask PTR Sample f z Fiber delivery from UV lamp Fig. 1: Recording geometry for producing a phase mask using the contact-copy technique.