Larissa Glebova

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.

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.

Volume-chirped Bragg gratings: monolithic components for stretching and compression of ultrashort laser pulses

Abstract. An innovative type of optical component—a volume Bragg grating—has recently become available commercially and has found wide applications in optics and photonics due to its unusually fine spectral and angular filtering capability. Reflecting volume Bragg gratings, with the grating period gradually changing along the beam propagation direction (chirped Bragg gratings—CBGs) provide stretching and recompression of ultrashort laser pulses. CBGs, being monolithic, are robust devices that have a footprint three orders of magnitude smaller than that of a conventional Treacy compressor. CBGs recorded in photo-thermo-refractive glass can be used in the spectral range from 0.8 to 2.5 μm with the diffraction efficiency exceeding 90%, and provide stretching up to 1 ns and compression down to 200 fs for pulses with energies and average powers exceeding 1 mJ and 250 W, respectively, while keeping the recompressed beam quality M2<1.4, and possibly as low as 1.1. This paper discusses fundamentals of stretching and compression by CBGs, the main parameters of the gratings including the CBG effects on the laser beam quality, and currently achievable CBG specifications.

Incoherent combining of 100-W Yb-fiber laser beams by PTR Bragg grating

Volume diffractive gratings (Bragg gratings) in photo-thermo-refractive (PTR) inorganic glass are proposed for incoherent laser beam combining because they have narrow spectral selectivity and diffraction efficiency greater than 95% from visible to near IR regions. They showed no laser-induced damage, no thermal lens, and no Bragg angle shift under CW Yb-fiber laser (1096 nm) irradiation at 100 kW/cm2. It opens the way to rugged, low-cost, efficient optics for high-power laser systems. Based on theoretical modeling of PTR Bragg gratings, we have designed a high-efficient technology for incoherent combining of two or several laser beams with certain wavelength shift. Two 100 W beams of Yb-fiber lasers in the range of 1080-1100 nm with the wavelength separation of 11 nm were combined with efficiency exceeding 75% while material losses did not exceed 2-4%. No fading or parameter change of PTR Bragg grating working in two 100 W beams were found. It was found that the process limiting efficiency of incoherent beam combining is the spectral widening of radiation of Yb-doped fiber lasers. At high power, their spectral width exceeds spectral selectivity of Bragg grating and causes a decrease of diffraction efficiency.

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 inde...

Strong Bragg Gratings in Highly Photosensitive Photo-Thermo-Refractive-Glass Optical Fiber

A new type of photosensitive fiber is demonstrated. Long lengths (>;100 m) of coreless optical fiber are fabricated from highly photosensitive photo-thermo-refractive glass. A minimum loss of <;2 dB/m is measured. A holographic technique using low power near-UV two-beam interference patterns is applied to record strong and robust Bragg gratings inside the fiber. The gratings show no degradation when heated up to 425 °C for several hours.

Continuous-wave diode-pumped laser action of Nd 3+ -doped photo-thermo-refractive glass

Laser action of the photo-thermo-refractive (PTR) glass, which is the photosensitive material for holographic recording of volume Bragg gratings (VBGs), was demonstrated for the first time by introducing Nd³⁺. Nd:PTR glass has a bandwidth of 27.8 nm and 16.0 nm for luminescence and absorption, respectively. An uncoated 2 mm thick Nd:PTR element generated cw laser output of 124 mW, with a slope efficiency of 25%, by laser diode pumping. This Nd:PTR glass also performed wide bandwidth laser action at 1053.9-1063.3 nm, where the decrease of the pump-absorption efficiency was held off below 30%, even under a 3.5 nm shift of pump wavelength from its absorption center.

Near-IR absorption in high-purity photothermorefractive glass and holographic optical elements: measurement and application for high-energy lasers

Volume Bragg gratings (VBGs) in photothermorefractive (PTR) glass are widely used for laser beam control including high-power laser systems. Among them, spectral beam combining based on VBGs is one of the most promising. Achieving 100+ kW of combined laser beams requires the development of PTR glass and VBGs with an extremely low absorption coefficient and therefore methods of its measurement. This paper describes the calorimetric method that was developed for measuring a low absorption coefficient in PTR glass and VBGs. It is based on transmission monitoring of the intrinsic Fabry-Perot interferometer produced by the plane-parallel surfaces of the measured optical elements when heated by high-power laser radiation. An absorption coefficient at 1085 nm as low as 5×10(-5) cm(-1) is demonstrated in pristine PTR glass while an absorption coefficient as low as 1×10(-4) cm(-1) is measured in high-efficiency reflecting Bragg gratings with highest purity. The actual level of absorption in PTR glass allows laser beam control at the 10 kW level, while the 100 kW level would require active cooling and/or decreasing the absorption in PTR Bragg gratings to a value similar to that in virgin PTR glass.

Evolution of Absorption Spectra in the Process of Nucleation in Photo-Thermo-Refractive Glass

Photo-thermo-refractive (PTR) glass belongs to a class of photosensitive multicomponent silicate glasses which demonstrate photo-thermo-induced precipitation of a crystalline phase. Photo-induced crystallization of sodium fluoride results in refractive index variations in exposed areas of PTR glass which was successfully applied for phase hologram recording. However, even if this glass is widely used for advanced optical applications such as the stabilization of laser diodes or spectral beam combining, the detailed mechanism of its photosensitivity is not well known. We present in this paper an analysis of the evolution of the absorption spectra of UVexposed PTR glass at the early stages of the thermal treatment (nucleation). Absorption spectra were measured on samples nucleated at constant temperature for different periods. Analysis of the structure of these spectra shows that the optimum of the nucleation process is associated with the appearance of a new absorption band in visible range that disappears when the crystal growth process starts. Analogous study was performed during the nucleation process. High temperature spectroscopic measurements were performed during the thermal treatment. It was found that absorption spectra evolve with a change of temperature, i.e. the band structure of these spectra is different whether or not the temperature is decreased below a temperature of ~400°C. We associate this process with the change of phase of the silver bromide nuclei (liquid/solid) that appears during the cooling or heating processes.

Magneto-induced microwave conductivity in Mn2+-doped silicate glass

Abstract A non-resonant microwave absorption induced by low magnetic field (magneto-induced microwave conductivity) was observed in Na 2 O–CaO–MgO–SiO 2 glasses doped with Mn in addition to the usual EPR signal of Mn 2+ . The non-resonant response increases with the increase of Mn 2+ concentration in glasses. In contrast to the EPR signal, the magnitude of non-resonant absorption does not show saturation at high microwave power and grows much faster as the temperature decreases. The magneto-induced microwave conductivity in the glass dielectric is explained by the magnetic field-dependent electron tunneling between non-bridging oxygen ions adjacent to paramagnetic Mn 2+ ions. The high probability of tunneling is provided by aligning the electron spins in Mn 2+ and adjacent non-bridging oxygen ions in the external magnetic field.