Photo-thermo-refractive Glass Research Articles

Influence of UV-exposure on the crystallization and optical properties of photo-thermo-refractive glass

Photo-thermo-refractive (PTR) glass is a photosensitive multi-component silicate glass. Photoinduced crystalline phase precipitation results in refractive index variations in UV exposed areas of PTR glass. The precipitation of silver containing particles which occurs during photo-thermo-refractive process increases the optical absorption of the samples in the range 350nm to NIR wavelengths and the growth of sodium fluoride crystals and their aggregation increases light scattering in visible and NIR regions. We show that one effect of the UV-exposure is a decrease in the crystallization temperature by ∼50°C compared to the unexposed areas as measured by differential scanning calorimetry, which we attribute to an increase in nucleation rate. Using spectro-photometric measurements, a linear function is fitted to the changes in the amplitude of the absorption band of the silver containing particles versus the UV-dosage. The root mean square scatter of the data from the linear function is better than 0.99 and the slope of the function is 0.32±0.01cm/J. The IR absorption of PTR sample, measured by laser calorimetry shows that the increase of the absorption in infrared region at 1.1μm, is due to the tail of the absorption band of silver containing particles having maximum at 465nm. We finally show that after hyper-development, one effect of UV-exposure at 325nm on the crystallization kinetics of PTR glasses is a decrease in particle sizes from micron size to nanometers size. But additional investigations demonstrate that smaller dosage UV-exposures (a few tens of milliwatts) increase the optical scattering by one order of magnitude. Optical micrographs taken after UV-exposure and hyper-development reveal the use of smaller dosages enhances nucleation rate without preventing the growth of large crystals and therefore induces higher scattering.

Stable coherent coupling of laser diodes by a volume Bragg grating in photothermorefractive glass

Two separate laser diodes emitting near 980 nm were coupled by a thick volume Bragg grating recorded in a photothermorefractive glass. The coupled diodes emitted at the same wavelength with a spectral width narrowed to below the resolution of the spectrum analyzer (<30 pm). Coherent emission at a pumping level greater than five times above the threshold was observed for several months with near-unity visibility of the interference pattern.

Residual internal stress in partially crystallized photothermorefractive glass: Evaluation by nuclear magnetic resonance spectroscopy and first principles calculations

In some circumstances, the mechanical and optical properties of multiphase brittle materials strongly depend on the level of residual micromechanical stresses that arise upon cooling due to thermal and elastic mismatch between the constituent phases. Here we study the residual internal stress in a partially crystallized oxyfluoride glass, best known as photothermorefractive (PTR) glass. This material is composed of a glass matrix with embedded nanosize sodium fluoride (NaF) crystals. Using both the Selsing model and solid-state nuclear magnetic resonance in combination with first principles calculations we found that the crystals are under a tensile stress field of approximately 610–800MPa. For this stress level the estimated critical crystal diameter for spontaneous cracking is about 2300–1900nm, which greatly exceeds the observed diameters of 7–35nm. Hence no spontaneous cracking is expected for the PTR glasses. First principles calculations indicate that the stress induced change of the refractive index of the NaF crystals is about −0.08%, which agrees with the observed refractive index changes.

Nonlinear refractive index of photo-thermo-refractive glass

Nonlinear properties of a photo-thermo-refractive (PTR) glass are studied and compared with those in fused silica and a conventional optical glass. PTR glass is a new photosensitive medium for high-efficiency phase volume hologram recording which manifests a linear refractive index modulation after exposure to UV radiation followed by thermal treatment. Nonlinear optical properties of PTR glass exposed to femtosecond laser pulses are studied. Diffraction patterns in a propagated laser beam focused in the sample were detected by a CCD, while a nonlinear refractive index was measured by a collinear-orthogonal-polarization-pump-probe (COP3) method. It was found that nonlinear refractive index of PTRG is n 2 = 3.3 × 10 −20 m 2 /W (0.33 ppm cm 2 /GW) which is about the same as for the fused silica. It is important that n 2 in PTR glass does not vary after UV exposure and thermal development.

Coupled-wave analysis of apodized volume gratings

This work presents the use of longitudinal refractive index modulation (apodization) in photosensitive glass for improved sidelobe suppression in volume holographic optical elements. We develop a numerical model for both uniform and apodized volume holograms based on rigorous coupled-wave analysis. We validate the model by comparison with a transmissive 1.55- mum uniform volume grating in photothermorefractive glass. We then apply our numerical model to calculate the spectral response of apodized gratings. The numerical results demonstrate that apodization of the refractive index modulation envelope improves spectral selectivity and reduces first and second-order side-lobe peaks by up to 33 and 65 dB, respectively. We suggest a method for creating apodization in volume holograms with approximately Gaussian spatial refractive index profile.

Kinetics modeling in photosensitive glass

Abstract Kinetics of photoinduced process is studied in a new photosensitive material for volume hologram recording which is a photo-thermo-refractive glass (PTRG). A system of balance equations is derived which describes the processes of generation of electrons in a conduction band of these glass matrix by photoionization of Ce3+ and trapping of electrons by both silver ions Ag+ and hole centers (Ce3+)+. The main approach for the system solution is a calculation of low quasistationary concentration of free electrons in a conduction band, while concentrations of generated electron and hole centers are expected to be variable. A general solution of a system is found as a combination of exponential and hyperbolic functions. It is shown that an exponential solution is the result of the absence of a re-trapping of electrons, while a pure hyperbolic solution occurs in the case of equal probability of trapping and re-trapping. The approach developed for the first stage of photo-thermo-refractive process which is photoionization and trapping, is applied for the modeling of the final result of the whole process of photo-thermo-induced refractive index transformation in glass. It is found that the refractive index increment in PTRG is better described by hyperbolic function compare to the exponential one. Kinetic parameters of induced refraction in PTRG are determined.

High-speed two-dimensional laser scanner based on Bragg gratings stored in photothermorefractive glass

A high-speed free-space wavelength-multiplexed optical scanner with high-speed wavelength selection coupled with narrowband volume Bragg gratings stored in photothermorefractive (PTR) glass is reported. The proposed scanner with no moving parts has a modular design with a wide angular scan range, accurate beam pointing, low scanner insertion loss, and two-dimensional beam scan capabilities. We present a complete analysis and design procedure for storing multiple tilted Bragg-grating structures in a single PTR glass volume (for normal incidence) in an optimal fashion. Because the scanner design is modular, many PTR glass volumes (each having multiple tilted Bragg-grating structures) can be stacked together, providing an efficient throughput with operations in both the visible and the infrared (IR) regions. A proof-of-concept experimental study is conducted with four Bragg gratings in independent PTR glass plates, and both visible and IR region scanner operations are demonstrated.

Comparative study of photo-induced variations of X-ray diffraction and refractive index in photo-thermo-refractive glass

Spontaneous and photo-induced crystallization have been investigated in fluorinated silicate glass by means of X-ray diffraction and optical interferometry. This glass is a photo-sensitive material for high-efficiency phase volume hologram recording. Variations of a refractive index in this glass are controlled by UV irradiation followed by a thermal development which is photo-thermo-refractive (PTR) process. A method of discrimination of weak narrow crystalline lines from a broad diffractive pattern of a vitreous material was developed, and quantitative measurements of small concentrations of crystalline phase in glass matrix were performed. The sensitivity of the method was about 0.01 wt% of crystalline phase of NaF in a silicate glass. This crystalline phase with concentration below 0.1 wt% was detected even in a highly transparent PTR glass with a modified refractive index produced by PTR processing. A correlation between the intensity of X-ray diffraction peaks of NaF and the induced refractive index was found in equally developed PTR glass samples exposed to different dosages of UV radiation.

Measurement of the induced refractive index in a photothermorefractive glass by a liquid-cell shearing interferometer

A liquid-cell shearing interferometer was developed to measure refractive-index variations (delta n) in transparent materials. The cell was filled with a liquid having a matched refractive index. The achieved resolution was better than 1/1000 of a fringe shift and resulted in a delta n measurement sensitivity down to 10(-7) for 1-mm-thick samples. A refractive-index increment in photothermorefractive glass of up to 5 x 10(-6) was observed after UV exposure at 325 nm. A refractive-index decrement of up to 1 x 10(-3) was observed after thermal development of the exposed sample. It was proved that photothermorefractive glass obeys the reciprocity law; i.e., delta n depends on the UV dosage but does not depend on the irradiance.

Influence of Glass Composition on the Refractive Index Change upon Photothermoinduced Crystallization

The influence of the main components (SiO2, Al2O3, ZnO, and Na2O) of a photothermorefractive glass matrix and Br–ions on the change in the refractive index is studied in the course of photothermoinduced crystallization in glasses. The optimum concentrations of these components at which the change in the refractive index is maximum are determined. The regularities revealed are explained within the modern concepts of the structure of polychromatic glasses and the nature of the photothermorefractive effect.

High-frequency Bragg gratings in a photothermorefractive glass

Holographic UV mirrors were recorded in a volume of photothermorefractive glass. This photosensitive silicate glass doped with silver, cerium, and fluorine is transparent in the near-UV, visible, and near-IR spectral regions, and its induced refractive-index change reaches 10(-3). UV radiation of a He-Cd laser at 325 nm was used for Bragg grating recording with a spatial frequency of approximately 9200 mm(-1). The absolute diffraction efficiency of the recorded mirror reached 12.6% at 325 nm and did not deteriorate under the long-time effects of optical irradiation and heating up 400 degrees C.

High-efficiency Bragg gratings in photothermorefractive glass

Photosensitive silicate glasses doped with silver, cerium, fluorine, and bromine were fabricated at the Center for Research and Education in Optics and Lasers. Bragg diffractive gratings were recorded in the volume of these glasses with a photothermorefractive process (exposure to UV radiation of a He-Cd laser at 325 nm is followed by thermal development at 520 degrees C). Absolute diffraction efficiency of as much as 93% was observed for 1-mm-thick gratings with spatial frequencies up to 2500 mm(-1). No decreasing of diffraction efficiency was detected at low spatial frequencies. Original glasses were transparent (absorption coefficient less than 1 cm(-1)) from 350 to 4100 nm. Induced losses in exposed and developed glass decreased from 0.3 to 0.03 cm(-1) between 400 and 700 nm, respectively, and did not exceed 0.01-0.02 cm(-1) in the IR region from 700 to 2500 nm. Additional losses caused by parasitic structures recorded in the photosensitive medium were studied.