Michael Fokine

Thermal stability of chemical composition gratings in fluorine-germanium-doped silica fibers.

A model based on diffusion of dopants in a periodic structure has been applied to describe thermal decay of chemical composition gratings in fluorine-germanium-doped silica fibers. The good agreement between previously reported values and the diffusion coefficients derived here from experiments and models in the 1000-1200 degrees C temperature range indicate that fluorine diffusion is the main mechanism of grating decay. Experimental results also indicate that the presence of phosphorous significantly increases the decay rate of chemical composition gratings.

Grating formation in pure silica-core fibers

Strong grating formation in pure silica-core fibers by use of 193-nm ArF-laser radiation is reported. Unsaturated refractive-index changes of Dn~0.3x10(-3) were observed in nontreated fiber, and changes of Dn~0.5x10(-3) were observed in fibers with a high hydroxyl concentration. Possible mechanisms of photosensitivity in pure silica-core fibers are discussed.

Large increase in photosensitivity through massive hydroxyl formation

We report a large increase in photosensitivity of germanium-doped silicate fibers by rapid heat treatment of hydrogen-loaded fibers at 1000 degrees C before exposure of the fibers to 242-nm radiation. The increase in photosensitivity is compared with thermally induced absorption caused by introduction of massive amounts of hydroxyl species. The absorption loss was measured to be 0.02 dB/cm mol.% OH at 1.55 mum. Strong gratings (Dn > 1 x 10(-4)) in germanium-free phosphorous-doped fibers in the presence of 242-nm radiation have also been manufactured by this technique.

All-fiber devices based on photonic crystal fibers with integrated electrodes

A special kind of microstructured optical fiber is proposed and fabricated in which, in addition to the holey region (solid core and silica-air cladding), two large holes exist for electrode insertion. Either Bi-Sn or Au-Sn alloys were selectively inserted into the large holes forming two parallel, continuous and homogeneous internal electrodes. We demonstrate the production of a monolithic device and its use to externally control some of the guidance properties (e. g. polarization) of the fiber.

Thermal stability of oxygen-modulated chemical-composition gratings in standard telecommunication fiber

Experimental results and a discussion on the formation and decay of oxygen-modulated chemical-composition gratings in a standard telecommunication fiber are presented. Comparison between the decay experiment and the model provides a diffusion coefficient with an activation energy of 490 kJ/mol, which is in close agreement with reported values of oxygen self-diffusion in silica. The gratings have a diffusion-controlled decay behavior, with more than 50% of the reflectivity remaining after 7.5 h at a temperature of 1230 degrees C. The gratings show higher thermal stability when heated in air than in an inert argon atmosphere.

Strong upconversion from Er3Al5O12 ceramic powders prepared by low temperature direct combustion synthesis

Crystalline ceramic powders of Er3Al5O12 were obtained by low temperature direct combustion synthesis. Irradiating the sample with a low-power continuous-wave infrared (1.48 mu m) diode laser led t ...

Growth dynamics of chemical composition gratings in fluorine-doped silica optical fibers

The refractive-index modulation of chemical composition gratings in fluorine-germanium-doped silica fibers as a function of thermal treatment during manufacturing has been studied. The final grating strength was found to depend strongly on an intermediate annealing step, with an optimum temperature near 600-700 degrees C, before development at a fixed temperature of 1000 degrees C. Low-temperature treatment, aimed at removing any remaining hydrogen from the fiber, performed at 100 degrees C for 20 h before the annealing step, also significantly increased the final refractive-index modulation.

Fabrication and optical characterization of silica optical fibers containing gold nanoparticles.

Gold nanoparticles have been used since antiquity for the production of red-colored glasses. More recently, it was determined that this color is caused by plasmon resonance, which additionally increases the materials nonlinear optical response, allowing for the improvement of numerous optical devices. Interest in silica fibers containing gold nanoparticles has increased recently, aiming at the integration of nonlinear devices with conventional optical fibers. However, fabrication is challenging due to the high temperatures required for silica processing and fibers with gold nanoparticles were solely demonstrated using sol-gel techniques. We show a new fabrication technique based on standard preform/fiber fabrication methods, where nanoparticles are nucleated by heat in a furnace or by laser exposure with unprecedented control over particle size, concentration, and distribution. Plasmon absorption peaks exceeding 800 dB m(-1) at 514-536 nm wavelengths were observed, indicating higher achievable nanoparticle concentrations than previously reported. The measured resonant nonlinear refractive index, (6.75 ± 0.55) × 10(-15) m(2) W(-1), represents an improvement of >50×.

Novel Tellurite Glasses and Nanocluster Doped Silica for Photonics

The topic of this review is a summary of the photonics research currently performed at the Material Science and Chemical Engineering Department at Politecnico di Torino, with an emphasis on novel tellurite glasses and nanocluster doped silica for photonics. The fabrication and characterization of novel tellurite glasses and their optical fibers, with the aim of obtaining laser sources operating in the near infrared, will be reviewed. Doping with Tm, Yb and Ho have been investigated and different pumping schemes are evaluated in order to optimize laser performance. Nanocluster doped glasses, which are of great interest for nonlinear applications in telecommunications as well as for different types of plasmonic sensors, will also be discussed. Specifically, the fabrication and characterization of nano-metal glass composites manufacturing by RF sputtering.

Large Increase in Photosensitivity through Massive Hydroxyl Formation

For efficient fabrication of fiber Bragg gratings several methods of enhancing the photosensitivity involving hydrogen can be used.