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 HG<sub>1</sub> conversion. The second had a binary four-sector phase profile, and was capable to perform fundamental mode conversion to a TEM<sub>01</sub>, TEM<sub>10</sub>, or TEM<sub>11</sub>. Mode conversion efficiency and thermal stress on each type of phase element were investigated using a 150 W of continuous wave power with a TEM<sub>00</sub> profile Yb:fiber laser.
Vortex beams with different helical modes are used for optical computing, free space optical communications, laser machining, and micro manipulating. We demonstrate holographic vortex phase masks produced in photo-thermo-refractive (PTR) glass. PTR glass is a photosensitive silicate glass that enables permanent refractive index change after UV exposure and thermal development. It is extensively used for recording of volume Bragg gratings (VBGs) and phase masks. A master phase mask is recorded by exposure of a PTR glass plate to UV radiation with a spatial intensity profile produced by a digital micromirror device. It provides a proper phase profile in a transmitted UV beam. This master phase mask is placed in one of the legs of an interferometer used for recording of a transmitting volume Bragg grating (VBG). Therefore, an additional phase profile is holographically encoded into the VBG resulting in the same phase profile in a diffracted beam. Such a device is a holographic phase mask (HPM) that enabled two exceptional features. First, it is tunable and could be used for different wavelengths. Second, holograms in PTR glass could be multiplexed and several HPMs could be fabricated in the single volume of glass. Owing to exceptionally low absorption of PTR glass and high thermal stability of holograms, holographic phase masks recorded in PTR glass can be used for mode conversion of high power laser beams. Such multiplexed HPM can split an incident Gaussian beam into several diffracted beams with different modes encoded.
Phase masks for mode conversion and other laser beam transformation are usually produced by surface profiling using spatially selective etching or deposition. Such fabrication techniques for making complex phase masks take significant time, effort, and expense. Surface damages and contaminations restrain the wide application of such phase elements as well. We propose an alternative, where volume phase masks were produced by spatially selective refractive index change in the bulk of plane-parallel plate of photo-thermo-refractive (PTR) glass. Those phase masks show high tolerance to harsh environments and high-power laser radiation. The approach uses a light amplitude modulating element called digital micromirror device (DMD) which has millions of micro-mirrors that tilt to ‘on’ or ‘off’ position based on the voltage applied to them. Selecting different time intervals for the mirrors to be ‘on’ or ‘off’, allows for grey level images to be generated and projected by the DMD device. Using a broadband UV light source, the desired amplitude image was projected onto a PTR glass plate for a designated period of time. The variable light amplitude exposure of the glass is converted to a refractive index change after thermal treatment step. The phase masks are recorded in the volume and are practically unsusceptible to surface damage, changes in the environment and other factors from which surface created phase masks suffer. The simplicity in fabricating grey level phase masks, the flexibility to computer design, and the robustness are the main advantages of the approach when compared to the standard phase mask fabricating techniques.
Photo-thermo-refractive (PTR) glass) is a photosensitive silicate glass doped with Ce<sup>3+</sup> where a permanent refractive index decrement is produced by UV exposure followed by thermal development. This material provides high efficiency and low losses combined with high thermal, ionizing and laser tolerance of holographic optical elements (HOEs). This is why PTR glass is widely used for holographic recording of volume Bragg gratings (trivial holograms produced by interference of two collimated beams) and phase plates operating in near UV, visible, and near IR spectral regions. It would be very beneficial though to record also complex HOEs (lenses and curved mirrors) for those spectral regions. However, PTR is not sensitive to visible or IR radiation and therefore does not allow the recording of nonplanar holograms for these regions. The present paper describes a technique for recording complex HOEs using visible radiation in Ce<sup>3+</sup> doped PTR glass. This two-step technique includes a blank exposure to UV radiation followed by structured exposure to a visible beam. It was found that the second exposure decreases the refractive index decrement induced in the UV exposed glass after thermal development. This means that areas, which underwent double exposure, have refractive index lower than in unexposed areas but higher than in just UV exposed ones. Thus, this technique provides refractive index increment after visible irradiation of UV exposed PTR glass. Using this approach, complex holograms (curved mirrors and lenses) operating in the visible region, were recorded in PTR glass.
Planar holographic optical elements (volume Bragg gratings, VBGs) recorded in photo-thermo-refractive (PTR) glass are widely used for fine spectral filtering and laser beam control. PTR glass provides photosensitivity in near UV region. Therefore, while planar holographic elements operate in the whole window of transparency - near UV, visible and near IR spectral regions, application of complex (nonplanar) elements is restricted to near UV. A method has been proposed to create high-efficiency diffractive optical elements in PTR glass using visible light. The method employs excited state absorption in PTR glass doped with Tb<sup>3+</sup>. UV radiation was used for excitation to a metastable level of Tb<sup>3+</sup> and pulsed radiation at 532 nm was used for hologram recording. Both planar VBGs and holographic lenses operating at 532 nm were demonstrated. Complex holographic optical elements in PTR glass can provide attractive solutions for lasers and spectroscopy replacing conventional optical components.
Photo-thermo-refractive (PTR) glass is a multicomponent silicate glass doped with Ce<sup>3+</sup> and Ag<sup>+</sup> which is extensively used for holographic recording of volume Bragg gratings (VBGs). Possibility of recording of advanced, complex holograms in the PTR glass is of current interest as it offers great opportunities in imaging and laser systems control. However, the glass does not have capabilities for recording of complex holograms with using light from the visible / IR spectral region due to its UV photosensitivity. Extension of the PTR-glass sensitivity range into the visible / IR spectral region was carried out by doping the original glass with trivalent terbium ions. Photosensitivity mechanism was implemented by means of excited state absorption using a UV photon and a visible photon for excitation of the Tb<sup>3+</sup> 5d<sup>1</sup>4f<sup>7</sup> band. For the first time refractive index modulation on the order of 2x10<sup>-4</sup> was obtained in PTR glass after exposure to the visible / IR light. Resulting magnitude of induced refractive index allows for high efficiency complex hologram fabrication in Tb<sup>3+</sup> doped PTR glass for use which in the visible / IR region. Holographic capabilities of Tb<sup>3+</sup> doped PTR glass were demonstrated by recording a complex hologram in the glass using green and blue light.