Dopant diffusion during optical fibre drawing

Analysis of material concentration in step-index fibers with alumina cores produced by means of the powder-in-tube technique

Step-index fibers (SIFs) with alumina cores were fabricated employing the powder-in-tube technique. The fabricated SIFs have alumina concentrations of up to 32 mol%, which is the highest value reported so far for fibers with core diameters smaller than 25 μm. The mixing mechanisms between alumina and silica during fiber drawing were revealed by energy dispersive X-ray analysis of the neck-down area of the preform. The results of the measurements and simulations indicate that besides diffusion, fluid dynamics between softened silica and alumina powder also play an important role in the resulting alumina and silica concentrations in the fiber. The influence of different drawing parameters on the alumina and silica concentrations of the fibers is also presented.

Enhancement of spectral response of Bragg gratings written in nanostructured and multi-stepped optical fibers with radially shaped GeO2 concentration

We present experimental results on fiber Bragg gratings inscription in nanostructured graded-index (nGRIN) and multi-step index (MSIN) optical fibers, both having non-uniform radial distribution of GeO₂ dopant in the fiber cores. In particular, the positive role of radial shaping the GeO₂ distribution in the fiber core on grating reflection efficiency is reported. We postulate that an appropriate spatial distribution of the germanium concentration that matches the fundamental mode profile improves grating spectral response due to more efficient grating-mode interaction, as compared with uniformly doped step-index optical fibers with the same overall doping level. Moreover, we show that radially shaped fibers exhibit moderately higher temperature responses than their step-index counterparts.

Preparation and characterizations of Nd:YAG ceramic derived silica fibers drawn by post-feeding molten core approach

This paper presents a novel molten core approach - post-feeding molten core approach - to draw Nd:YAG ceramic derived silica fibers. This technique can effectively mitigate the diffusion of silica from cladding. The diffused silica concentrations decrease from 73.76 wt.% to 45.08 wt.% at the center of cores, by using the post-feeding method. Micro-Raman spectra indicate that the core materials of those fibers are amorphous and maintain an environment similar to YAG glass. The output laser power and slope efficiency are greatly improved. The enhanced performance of this approach shows that it has considerable potential in fabricating hybrid fibers.

Identification of Nanocrystalline Inclusions in Bismuth-Doped Silica Fibers and Preforms

The nature of nanocrystalline inclusions and dopant distribution in bismuth-doped silicate fibers and preforms are studied by scanning and transmission electron microscopy, and energy and wavelength-dispersive X-ray microanalysis. The core compositions are Bi:SiO2, Bi:Al2O3-SiO2, Bi:GeO2-SiO2, Bi:Al2O3-GeO2-SiO2, and Bi:P2O5-Al2O3-GeO2-SiO2. Nanocrystals of metallic Bi, Bi2O3, SiO2, GeO2, and Bi4(GeO4)3 are observed in these glasses. These inclusions can be the reason for the background optical loss in bismuth-doped optical fibers. The bismuth concentration of 0.0048±0.0006 at% is directly measured in aluminosilicate optical fibers with effective laser generation (slope efficiency of 27% at room temperature).

Diffusion and Interface Effects during Preparation of All-Solid Microstructured Fibers

All-solid microstructured optical fibers (MOF) allow the realization of very flexible optical waveguide designs. They are prepared by stacking of doped silica rods or canes in complex arrangements. Typical dopants in silica matrices are germanium and phosphorus to increase the refractive index (RI), or boron and fluorine to decrease the RI. However, the direct interface contact of stacking elements often causes interrelated chemical reactions or evaporation during thermal processing. The obtained fiber structures after the final drawing step thus tend to deviate from the targeted structure risking degrading their favored optical functionality. Dopant profiles and design parameters (e.g., the RI homogeneity of the cladding) are controlled by the combination of diffusion and equilibrium conditions of evaporation reactions. We show simulation results of diffusion and thermal dissociation in germanium and fluorine doped silica rod arrangements according to the monitored geometrical disturbances in stretched canes or drawn fibers. The paper indicates geometrical limits of dopant structures in sub-µm-level depending on the dopant concentration and the thermal conditions during the drawing process. The presented results thus enable an optimized planning of the preform parameters avoiding unwanted alterations in dopant concentration profiles or in design parameters encountered during the drawing process.

Arc fusion splicing effects in large-mode-area single-mode ytterbium-doped fibers

For the first time the effects of arc fusion splicing on the residual stress and refractive index of large-mode-area single-mode ytterbium-doped fibers (YDFs) are investigated using a state-of-the-art three-dimensional concurrent stress-index measurement method. The results, based on a commercially available fiber, describe a host of perturbations that decrease the core/cladding refractive index difference by as much as 1.74 × 10(-3) over an axial length of many hundreds of wavelengths. Simulations indicate that these perturbations result in an expansion of the mode-field-diameter by 39.6% and, based on the measured sample, result in an extra splice loss of 20.8%. The results of this investigation will be useful in the design and optimization of high-power all-fiber YDF lasers and amplifiers.

Three mode Er3+ ring-doped fiber amplifier for mode-division multiplexed transmission

We successfully fabricate three-mode erbium doped fiber with a confined Er(3+) doped ring structure and experimentally characterize the amplifier performance with a view to mode-division multiplexed (MDM) transmission. The differential modal gain was effectively mitigated by controlling the relative thickness of the ring-doped layer in the active fiber and pump launch conditions. A detailed study of the modal gain properties, amplifier performance in a MDM transmission system and inter-modal cross-gain modulation and associated transient effects is presented.

Large-mode-area fused-fiber combiners, with nearly lowest-mode brightness conservation

We show the feasibility of producing a low-mode all-fiber combiner fabricated from a large core and extremely small NA fibers. Although these fibers support multiple modes, the combiner that we produce can be operated nearly at the single mode regime while preserving the brightness of the combined beam almost perfectly with respect to the inputs. The M-square parameter of the combined beam was 2.3 and the power transfer efficiency was close to 100%. Such an all-fiber beam combining device is a rugged solution for high-brightness, high-efficiency beam delivery.

Analysis of changes in optical fibers during arc-fusion splicing by use of quantitative phase imaging

A non-interferometric imaging technique in conjunction with Abel inversion is used to directly and quantitatively examine the changes in optical fibers due to the heating produced during arc-fusion splicing as a function of fusion arc parameters. Phase images in the vicinity of a fusion splice are obtained using Quantitative Phase Microscopy, allowing the refractive-index change to be reconstructed with high spatial resolution. This simple, nondestructive method confirms that, for a fixed arc current, while the fusion time increases, the refractive-index of both fiber cores within the fusion region decreases in magnitude, the core region broadens, and the axial gradient decreases.

Effect of ionizing radiation on the properties of arc-induced long-period fiber gratings

We experimentally study the effect of ionizing radiation on the properties of long-period gratings fabricated in two pure-silica-core fibers with the arc-discharge technique. It is observed that the spectra of the gratings remain almost unchanged after being subjected to doses in excess of 0.5 MGy. The results also show that the gratings' temperature and strain sensitivities are not affected by gamma radiation.