Permanent Refractive Index Change Research Articles

Photorefractivity in nanostructured tin-silicate glass ceramics: a radiation-induced nanocluster size effect.

The possibility of obtaining permanent photoinduced refractive index changes, up to -10(-3), in nanostructured silica-based composites has been demonstrated in SiO(2):SnO(2) optical-grade glass ceramics exposed to ultraviolet radiation. Optical and electron paramagnetic resonance data suggest that the negative refractive index change involves a modification at the surface of the SnO2 nanoclusters, leading to a reduction of their crystalline size.

Anisotropic refractive-index change in silica glass induced by self-trapped filament of linearly polarized femtosecond laser pulses

Permanent refractive-index change can be induced by a self-trapped filament of intense ultrashort laser pulses in silica glass. We investigated the dependence of refractive-index change on polarization of incident laser pulses. As the region of refractive-index change can be selectively etched by hydrofluoric acid (HF), we first polished the sample and etched the cross section of the regions. The HF-etched cross section of refractive-index change that was induced by linearly polarized pulses was seen to be elliptical when observed with a scanning-electron microscope. By polarimetric analysis of diffraction from gratings fabricated by polarized ultrashort laser pulses, the index ellipsoid of refractive-index change was confirmed to be uniaxial and negative.

Refractive-index changes of standard telecommunication fiber through exposure to femtosecond laser pulses at 810 cm

We report on permanent refractive-index changes as great as 6 x 10(-3) in standard fibers on irradiation with tightly focused femtosecond (120-fs) near-IR (lambda = 800-nm) pulses. The refractive-index increase was measured through changes in the transmission spectrum of an UV photowritten Bragg-grating-based Fabry-Perot interferometer. We depict the dependence of the index variation on exposure time. The changes in refractive index can be optimized after a few seconds of irradiation. The measured induced excess loss increases during near-IR exposure.

Permanent refractive-index changes in GeO 2 glass slabs induced by irradiation with sub band gap light

Permanent, positive refractive-index changes in pure GeO 2 glass slabs are induced by illumination with light having energy far below that of the fundamental absorption edge. The refractive-index change is tentatively explained by the Kramers–Krönig relationship for an additional absorption band induced in the glass via two competitive photochemical reactions.

Photoviscous flow and glass macroproperties

The interest in glass photosensitivity has been renewed due to the importance of photosensitivity phenomena in potical fibers devices which promise a host of novel applications. Among these developments, one is the ability to imprint periodic and permanent refractive-index changes (Bragg gratings) on monomode germanosilicate optical fibers using excimer lasers. Defect formation in the structure of the glass core mainly germanium GODC (germanium-related oxygen-deficient centers) defects are supposed to be responsible for the grating formation. Here the phenomena are analyzed under another perspective that is related with high temperature gradient inside monomode optical fibers and its consequences. Because temperatures much higher than the germanium silicate glass softening point could be achieved during exposure to excimer lasers, the formation of Bragg gratings is a two step process to modify vitreous media. First, UV energy should be absorbed at the fiber core - by defects, dopants and glass components - and a second step where this energy is thermalised. There is no ablation in the glass core and energy transfer is governed by viscous flow or photoviscous flow and internal structural rearrangements. Alternatives to enhance optical fiber glass photosensitivity via glass composition and viscous behaviour are discussed.

Characterisation of GaLaS chalcogenide glass thin-film optical waveguides, fabricated by pulsed laser deposition

The fabrication of stoichiometric thin-film optical waveguides of GaLaS via a pulsed laser deposition technique is reported. Stoichiometric films are grown by ablating GaLaS bulk glass with a KrF excimer laser (λ = 248 nm) at an incident laser flux ⩾ 3.5 J/cm 2. The composition of the films is determined by energy-dispersive X-ray analysis and the refractive index is measured by a dark-mode prism coupling technique. Photoinduced structural rearrangement of the as-deposited films leads to a blueshift in the visible absorption edge and a permanent refractive index change, Δ n, of −1%. On the basis of these results, grating structures have been written with both blue light, and e-beam addressing, and their suitability for integrated optical structures assessed.

Ultraviolet-induced permanent Bragg gratings in Ce-doped fluorozirconate glasses or optical fibres

A holographic method was used to write Bragg gratings in fluorozirconate glass plates or fibres doped by cerium. UV pumping of a cerium transition results in the creation of a permanent change in refractive index of ≈ 2 × 10−5.

Thermal lensing and permanent refractive index changes in rare-earth-doped glasses

The characteristics of thermal lensing were measured for Eu3+- and Nd3+-doped phosphate glasses under resonant and nonresonant excitation conditions. The relative strengths and response times of the laser-induced lenses were determined and intepreted in terms of the thermal properties of the glass hosts for nonresonant excitation and the coupling of the rare-earth ions to the local vibrations for resonant excitation. Significant enhancement of the strength of thermal lensing was observed for resonant excitation, and this effect was much greater for the Eu-doped glass than for the Nd-doped glass. The response times for thermal lensing were also different for resonant and nonresonant excitation. In addition, a permanent change in the refractive index of Δn = 1.23 × 10−5 was observed for the Eu-doped glass and not for the Nd-doped glass. These results are discussed in terms of the different distributions of vibrational modes generated by the different types of laser excitation of these two rare-earth ions.