PTR Glass Research Articles

Construction and investigation of a planar waveguide in photo-thermal-refractive glass by proton implantation

In this paper, the fabrication of optical planar waveguide in PTR glass by proton implantation is reported for the first time to our knowledge. The planar waveguide with a width of approximately 3.4 μm was generated under the condition of 400 keV H+ ion implantation with a dose of 8.0 × 1016 ions ‧ cm−2 at room temperature. The formation mechanism of PTR glass waveguide was illustrated with the aid of the SRIM program. The optical characteristics of the PTR waveguide, such as refractive index profile and near-field intensity distribution, were investigated in detail. The fabricated PTR waveguide presents a structure of both index-well and optical barrier according to the reconstructed refractive index profile. The finite-difference beam propagation method was carried out to simulate guiding modal distribution. Besides, in order to study the thermal stability of the H+-implanted PTR waveguide, annealing treatment was conducted at different temperatures.

Origin of refractive index change in photo-thermo-refractive glass

There are still some uncertainties in the origin of photo-thermo-induced refractive index change in PTR glass. On one side, according to Nikonorov N.V. it is assumed to be due to difference of refractive indexes of sodium fluoride nanocrystals (n ~ 1.32) and glass (n ~ 1.49). Later Glebov L.B. et al. suggested another mechanism. They assumed that the transformation of Na and F distributed in PTR glass matrix into crystalline NaF (chemical changes) are not the main causes of photo- thermo-induced refractive index change and accounted it for high residual stresses around the NaF crystals. So present experimental research is focused on revealing of the refractive index change origin in fluoride PTR glass. It is shown experimentally that residual stresses have no measurable effect on the refractive index change in PTR glass, that is in agreement with the estimations made using the Maxwell-Garnet theory.

Optical planar waveguides in photo-thermal-refractive glasses fabricated by single- or double-energy carbon ion implantation

Ion implantation has demonstrated to be an efficient and reliable technique for the fabrication of optical waveguides in a diversity of transparent materials. Photo-thermal-refractive glass (PTR) is considered to be durable and stable holographic recording medium. Optical planar waveguide structures in the PTR glasses were formed, for the first time to our knowledge, by the C3+-ion implantation with single-energy (6.0 MeV) and double-energy (5.5+6.0 MeV), respectively. The process of the carbon ion implantation was simulated by the stopping and range of ions in matter code. The morphologies of the waveguides were recorded by a microscope operating in transmission mode. The guided beam distributions of the waveguides were measured by the end-face coupling technique. Comparing with the single-energy implantation, the double-energy implantation improves the light confinement for the dark-mode spectrum. The guiding properties suggest that the carbon-implanted PTR glass waveguides have potential for the manufacture of photonic devices.

Double line and tubular depressed cladding waveguides written by femtosecond laser irradiation in PTR glass

Depressed cladding concepts such as double line depressed index and tubular low index cladding waveguide structures were fabricated in bulk photo-thermo-refractive glass using femtosecond laser pulses. Effects of the writing laser power, waveguide geometrical structures and heat treatment on the light guiding properties were investigated. The results show that depressed cladding tubular waveguide design exhibits better guiding performances than double line waveguides in PTR glass. For the tubular cladding waveguide, single and multimode fields can be obtained for a wide processing parameter window. The simulations agree well with the experiment results. Moreover, the normalized frequency can be tuned and the quality of the depressed cladding tubular waveguide can be improved by eliminating uneven stress distribution after heat treatment.

Photo-thermo-refractive glass with sensitivity to visible and near IR radiation

Photo-thermo-refractive glass (PTR glass) is a multicomponent silicate glass doped with Ce3+, which changes its refractive index after exposure to UV radiation followed by thermal development. It is extensively used for recording of trivial holograms (volume Bragg gratings) operating in the visible and near IR spectral regions. Ability to record complex holographic structures in PTR glass is of utmost interest as it would be advantageous for imaging and laser beam control applications. However, since photosensitivity of PTR glass is limited to the UV region, complex holograms for the visible and IR applications could not be recorded in PTR glass. To extend PTR glass sensitivity towards longer wavelengths the same glass matrix was doped with terbium, and then an excited state absorption mechanism was used for two-step excitation of the 5d14f7 band of Tb3+ ions by concurrent illumination by long wavelengths (449, 522, 808 or 975 nm) and UV (375 nm) photons. For the first time refractive index modulation exceeding 2 × 10−4 was observed after exposing the material to blue, green and near IR laser radiation. Complex holograms operating in the blue and green spectral regions were recorded in Tb-doped PTR-glass.

X-ray diffraction study of NaF nano-crystals in photo-thermo-refractive glass

PTR glass is a photosensitive silicate glass which imparts refractive index change after UV-exposure and thermal development. The origin of photosensitivity is explained by the precipitation of NaF crystals inside the glass matrix. While nucleation of NaF in PTR glass has been thoroughly studied, the mechanisms of growth of NaF crystals have drawn a limited interest. To study the NaF growth, several techniques were combined: optical spectroscopy, interferometry, DSC and XRD. It was found that the size of NaF crystals in UV-exposed PTR glass, as determined by the broadening of the diffraction lines does not exceed ~20nm. The NaF crystals' growth is limited by the exhaustion of Na+F− within the glass matrix as well as the increase of the viscosity of the glass surrounding the crystals that act as a kinetics barrier for further growth. X-ray diffraction analysis of NaF powder allowed calculating the crystals' volume fraction and their average size. A combination of these parameters with Rayleigh scattering model allowed the prediction of scattering at any wavelength and any thermal treatment, proving that scattering is determined by single crystals.

Scaling the spectral beam combining channels in a multiplexed volume Bragg grating

In order to generate high power laser radiation it is often necessary to combine multiple lasers into a single beam. The recent advances in high power spectral beam combining using multiplexed volume Bragg gratings recorded in photo-thermo-refractive glass are presented. The focus is on using multiple gratings recorded within the same volume to lower the complexity of the combining system. Combining of 420 W with 96% efficiency using a monolithic, multiplexed double grating recorded in PTR glass is demonstrated. A multiplexed quadruple grating that maintains high efficiency and good beam quality is demonstrated to pave a way for further scaling of combining channels.

Crystallization and absorption properties of novel photo-thermal refractive glasses with the addition of B2O3

A new boron-containing photo-thermal-refractive (boron-PTR) glass was prepared by two-step melt-quenching techniques at relative lower glass melting temperature. The influence of Ag+ doping concentration, UV exposure dosage and thermal development parameters on the boron-PTR glass' spectroscopic absorption and crystallization property were investigated and compared with that of the known boron-free PTR glass explored by L. B. Glebov et al. The introduction of B2O3, to some extent, can increase the strength of the silicate bonding thus preventing the liquid–liquid phase separation in this new PTR system when compared to the boron-free PTR system. The developed boron-PTR glass with the processing of 15s-UV exposure and appropriate thermal development (480°C/3h, 500°C/2h) exhibits good photo-induced crystallization performance, high VIS-near IR transmittance (670 to 2600nm) and moderate photo-induced refractive index change (1.1×10−3), which at present status enables this new PTR glass basic ability to record holographic elements.

Non-collinear generation of third harmonic of IR ultrashort laser pulses by PTR glass volume Bragg gratings

Three conditions for non-collinear third harmonic generation by a PTR glass volume Bragg grating are demonstrated using infrared ultrashort pulse illumination. Each condition corresponds to a different angle of grating orientation and a separate generation mechanism. We identify the mechanisms as corresponding to sum-frequency generation, Bragg diffraction of 3 omega, and a non-resonant Bragg condition involving three omega photons interacting with a nonlinear grating vector. Theoretical modeling is performed using wave vector additions and the results are compared to experimental measurements.

Modeling of phase volume diffractive gratings, part 1: transmitting sinusoidal uniform gratings

A detailed model of the diffraction of plane and Gaussian beams on plane uniform phase Bragg gratings based on Kogelnik's theory of coupled waves is presented. The model describes transmitting gratings with arbitrary orientation in a plane-parallel plate taking into ac- count spectral width and angular divergence of laser beams along with material dispersion of a photosensitive medium. The model results are compared with experimental data for high-efficiency Bragg gratings in a photothermorefractive PTR glass. © 2006 Society of Photo-Optical Instrumenta- why our modeling is illustrated for those variations of grat- ing parameters that are typical for PTR VBGs: the refrac- tive index is in the range of 1.5 at wavelengths ranging from 0.4 to 2.7 m, refractive index modulation is up to 1000 ppm 10 3 , and grating thickness is from 0.2 to 20 mm. The goal of this work is to reduce Kogelnik's theory to practical formulas that enable practical modeling and de- sign of diffractive optical elements based on VBGs. This part of modeling considers diffraction of plane monochro- matic, divergent, and polychromatic laser beams on uni- form sinusoidal lossless transmitting volume gratings and compares the model with experimental results in PTR Bragg gratings. Further parts will describe modeling of re- flecting volume gratings holographic mirrors as well as the application of both transmitting and reflecting grating for spectral beam combining for different types of lasers.