Alexander L. Tomashuk

Low-hydrogen silicon oxynitride optical fibers prepared by SPCVD

The performance of the first samples of nitrogen-doped silica optical fibers, a novel type of optical fiber, is investigated. The fiber preforms containing up to /spl sim/3 at% of nitrogen in the core have been synthesized by reduced-pressure plasmachemical deposition (SPCVD) and drawn into fibers. By eliminating hydrogen-containing components from the gas mixture, low-hydrogen silicon oxynitride has been obtained. Optical loss in fibers in the range 1.3-1.6 /spl mu/m, beyond OH- and NH-group absorption peaks, is several dB/km and, apparently, can be further reduced by optimizing the preparation processes. >

Performance of Bragg and long-period gratings written in N- and Ge-doped silica fibers under /spl gamma/-radiation

In-fiber Bragg and long-period gratings as well as Mach-Zehnder interferometers based on germanium- and nitrogen-doped silica fibers have been investigated under /spl gamma/-rays. The majority of the experimental results suggest that both types of gratings in both types of fibers are stable with respect to /spl gamma/-ray doses of up to 1.47 MGy.

Round-robin irradiation test of radiation resistant optical fibers for ITER diagnostic application

Fused silica core optical fibers are expected to play crucial roles especially in the size-reduced International Thermonuclear Experimental Reactor (ITER-FEAT). Several radiation resistant optical fibers have been developed in Japan and the Russian Federation. The task force on radiation effects in diagnostic components in the ITER-EDA (engineering and design activity) promoted international round-robin irradiation experiments on the developed optical fibers. Ten different optical fibers were tested in a cobalt-60 gamma-ray irradiation facility and in the Japan Materials Testing Reactor. The paper reports results obtained on five different optical fibers, which include purified, hydrogen loaded, and fluorine doped ones. Results show that the developed optical fibers could be deployed in remote handling and out-of-vessel applications. But, for the in-vessel diagnostics in the visible range optical spectroscopy, further improvement of the radiation resistance of optical fibers will be needed.

Super-high-strength metal-coated low-hydroxyl low-chlorine all-silica optical fibers

High-purity KS-4V synthetic silica developed in the Silicate Chemistry Institute of the Russian Academy of Sciences is tested as the core material for radiation hardened optical fibers. Pure-silica-core fluorine-doped-silica-cladding optical fibers with polymer (acrylate) or metal (aluminum) coating are produced as the experimental samples. The light-reflecting fluorine-doped silica cladding is synthesized by the plasma outside deposition process. The aluminum coating technology used provides a very high strength of the fibers, unattainable for polymer coatings, and expands the fiber operating range up to 400/spl deg/C. It is established that the metal coating application can result in the annealing of the drawing-induced color centers with an absorption peak at 630 nm. Post-/spl gamma/-irradiation loss spectra in KS-4V-based fibers measured in 1-2 hours after 2 MGy irradiation at a dose rate of 8.3 Gy/s in the spectral range 350-700 nm are discussed. The 630 nm absorption peak is practically absent from the post-irradiation loss spectra of aluminum-coated KS-4V-based fibers.

/spl gamma/-radiation-induced absorption in pure-silica-core fibers in the visible spectral region: the effect of H/sub 2/-loading

/spl gamma/-radiation-induced absorption in the visible spectral region is studied in H/sub 2/-loaded and untreated optical fibers with high-OH low-Cl KU core and low-OH low-Cl KS-4V core. H/sub 2/-loading is found to suppress the low-dosage transient absorption band centered at 670 nm in KS-4V fibers. After irradiation to 1.7 MGy(Si), H/sub 2/-loaded fibers show losses many times lower than the untreated fibers. H/sub 2/-loaded and irradiated fibers are found to retain very high radiation resistance under subsequent irradiations even in the absence of molecular hydrogen. It is concluded that H/sub 2/-loading followed by pre-irradiation can be used as an efficient radiation hardening technique for pure-silica optical fibers.

Radiation Resistant Er-Doped Fibers: Optimization of Pump Wavelength

H-free and H-loaded pieces of a carbon-coated erbium-doped fiber (EDF) are gamma-irradiated to doses in the range 0.1-10 kGy. In three months after the irradiation, optical loss spectra and lasing efficiency of the fibers are studied. It is found that the slope efficiency of a laser based on an irradiated EDF quickly grows under the action of pumping at the wavelength of 980 nm, owing to photobleaching of radiation-induced color centers. Photobleaching is found to be much more efficient in H2-loaded EDFs. No photobleaching occurs with pumping at 1480 nm. Pumping at 980 nm is argued to ensure a sufficiently long service life of H2-loaded EDFs in space, much longer than in the case of pumping at 1480 nm.

Radiation-Resistant Erbium-Doped Fiber for Spacecraft Applications

Radiation-induced absorption and lasing efficiency of a hermetically coated erbium-doped fiber saturated with molecular hydrogen are studied. It is shown that H2-loading of hermetically coated erbium-doped fibers prolongs their service time in space more than in 5 times, making such fibers promising for space applications.

Enhanced Radiation Resistance of Silica Optical Fibers Fabricated in High O

Seven F-doped- and undoped-silica-core optical fibers were produced by the MCVD-method, the ratio of the O2 and SiCl4 molar flow rates in the vapor-gas mixture during the fabrication of the perform core being varied among the performs. The fibers were γ-irradiated to 8.1 kGy (0.75 Gy/s), radiation-induced absorption (RIA) being measured during and after the irradiation in the spectral range 1.1-1.7 μm. The fiber optical loss spectra were also measured after γ-irradiation to 1.31 MGy. Three RIA mechanisms affecting the near-IR region have been revealed, namely, short-wavelength RIA tails due to the Cl0-center and to the self-trapped holes of the second type (STH2) and a long-wavelength RIA tail due to the self-trapped holes of the first type (STH1). STH2 are argued to result from the strain frozen-in in the fiber glass in the fiber drawing process. The RIA due to STH 1 is shown to anticorrelate with the RIA due to STH2. All the RIA mechanisms revealed are shown to be strongly suppressed by providing a high O2 excess in the vapor-gas mixture during the perform core synthesis. The O2-excess technique is therefore proposed as a very promising one for the development of radiation-resistant fibers.

_{\bf 2}

We have investigated radiation induced absorption in Al-doped, P-doped and Ge-doped optical fibres under 60Co source gamma-radiation up to a total dose of 71 kGy at two temperatures 30 and 80 °C. The Al-doped and P-doped fibres demonstrated high radiation sensitivity required for the optical fibre dosimetry. The RIA response to temperature increase from 30 to 80 °C depended on the dopant. In Al-doped fibres the absorption level decreased by 25% whereas in P-doped fibres it increased by at least 10%. For comparison we also tested standard telecom-grade Ge-doped fibres. Such fibres demonstrated a monotonous rise of the RIA during the whole irradiation with a small decrease of sensitivity at the higher temperature.

Excess Conditions

The design of a fiber-optic dosimeter, which determines the radiation dose from the difference of radiation-induced attenuation (RIA) Δα measured in a P-doped silica fiber at λ = 413 and 470 nm, is presented along with its first test results under gamma-radiation (dose rates 0.00064 and 0.0066 Gy/s, maximal dose ~2Gy). The dose-dependence of Δα as well as of RIA at individual wavelengths is found to be well described by a power law, the exponent lying in the range 0.90-0.94. In contrast to RIA at individual wavelengths, Δα is found not to depend on dose rate and to decay only slightly on termination of irradiation. Therefore, using Δα for dosimetry is argued to be more promising.