V. B. Neustruev

Germania-glass-core silica-glass-cladding modified chemical-vapor deposition optical fibers: optical losses, photorefractivity, and Raman amplification

Germania-glass-core silica-glass-cladding single-mode fibers (deltan as great as 0.143) with a minimum loss of 20 dB/km at 1.85 microm were fabricated by modified chemical-vapor deposition. The fibers exhibit strong photorefractivity, with type IIa index modulation of 2 x 10(-3). A Raman gain of 300 dB/(kmW) was determined at 1.12 microm. Only 3 m of such fibers is sufficient for constructing the 10-W Raman laser at 1.12 microm with a 13-W pump at 1.07 microm.

Optical absorption and luminescence of germanium oxygen-deficient centers in densified germanosilicate glass

Spectroscopic properties of germanium oxygen-deficient centers were studied in vapor axial deposition and modified chemical-vapor deposition germanosilicate glass after hydrostatic densification by as much as 19%. Gaussian decomposition showed a broadening and a significant shift of the initial absorption bands at 5.11 and 5.42 eV. A threefold decrease of the 3.15-eV luminescence band was revealed, whereas the 4.25-eV luminescence band was only slightly sensitive to densification. This shows that these two bands belong to different luminescence centers. A drastic growth of a new metastable absorption band near 7.3 eV was observed. The pressure-induced changes disappeared as a result of thermal annealing. These changes are discussed with respect to in-fiber grating phenomena.

Ultraviolet absorption and excess optical loss in preforms and fibers with high-germanium content

Nature of excess optical loss in fibers with high (up to 35 mol.%) GeO2 content silica core was studied experimentally. The absorption and scattering loss dependences on the drawing conditions were studied as well as the behavior of germanium-related oxygen- deficient centers (GODC) absorbing in UV spectral region. The peculiarity of such fibers was found to be in the drastic increase of the absorption and scattering loss when the core diameter diminish up to 3-3.5 micrometers . According to the excess loss analysis the absorption loss is caused by the enhanced GODC formation and the Ge-related drawing-induced defects. The excess scattering loss which dominates is assumed to be due to azimuthal and longitudinal core inhomogeneities caused by hydrodynamical instabilities of viscous glass during the fiber drawing. The germanium diffusion effect on optical loss is discussed.

Radiation resistance of optical fibres with fluorine-doped silica cladding

Spectra and kinetics of decay of (gamma) -irradiation induced loss in optical fibers with pure silica and Ge- and/or F-doped silica core and F-doped cladding made by MCVD and SPCVD methods were measured at room temperature. Induced loss in short-wavelength range is shown to decrease at Ge-content decrease and at F addition to the core glass. Minimum of induced loss is located in long-wavelength range, the best result (about 2 dB/km after the dose 10 kGy) being achieved on pure silica core MCVD fiber. Induced absorption band at 630 nm is observed in pure silica core fibers and it is suppressed by F-doping of the core glass.

Photoinduced refractive index subgratings in germanosilicate optical fibers

A new approach assuming nonadditive action of different mechanisms of the photorefractive effect is proposed to explain the features of fiber Bragg gratings (FBG) in germanosilicate fibers. By taking properly into account the properties of Ge-related defects, the causes of some discrepancies among the previous inferences about FBG are analyzed. The known mechanisms of the photorefractive effect are discussed from the point of view of superposition of subgratings, created by them. The published facts pointing to significant contribution of spatially periodic charge grating to (Delta) n are reviewed. To explain this contribution, an electrostriction mechanism is proposed. The results on FBG writing by laser radiation in the wavelength range 333 - 364 nm and new relevant data on the nature of germanium oxygen-deficient centers are discussed.

Raman gain properties of germania-based core silica fiber

Raman amplification in singlemode fiber with germania-based core and silica cladding was investigated for the first time. Very short efficient Raman lasers based on this fiber were developed in 1.1 and 1.6 mkm spectral bands.

Absorption and luminescence properties of Cr{sup 4+}-doped silica fibres

The absorption and luminescence spectra of silica fibres doped with chromium and various dopants are studied. Fibres doped with aluminium and gallium exhibit a broad (300 nm) luminescence band of Cr{sup 4+} ions at 1100 nm at room temperature. The quantum yield of luminescence is estimated to be 10{sup -4}-10{sup -5}. (optical fibres)

Mechanism of excitation and quenching of 3.15-eV luminescence in germanosilicate glass

The influence of hydrostatic densification of the parameters of the 3.15-eV luminescence and on the efficiency of photocoloration of germanosilicate glass is investigated. A four-fold decrease of the luminescence intensity under excitation at both 5 eV and 3.7 eV is revealed after 19% densification of the glass. A mechanism of relaxation of excitation of germanium oxygen-deficient centers is proposed based on the model of bistable oxygen vacancy. This mechanism explains the high sensitivity of the 3.15-eV luminescence to strain and temperature quenching.

Simple method for determining the parameters of singlemode fiber-optic waveguides

An experimental investigation was made of the possibility of using methods of measuring the near- and far-field distributions to determine the core diameter and the difference between the refractive indices of the core and cladding in single-mode fiber-optic waveguides for the near infrared region, by exciting them in a few-mode regime with ultraviolet (λ = 365 nm) radiation. It was found that even for a normalized frequency of V > 4 the output beam divergence corresponded to the numerical aperture of a single-mode optical waveguide, while the half-width of the radiation intensity distribution at the output end corresponded to the core diameter. The accuracy of the proposed method was not inferior to other methods specifically designed for single-mode optical waveguides, but the present method had the advantage of simplicity, especially in the case of a single-mode optical waveguide having a critical wavelength of > 1 μ.

Single-mode low-loss fiber waveguide

Chemical precipitation from a gaseous phase was used to prepare single-mode fiber waveguides with losses not exceeding 5 dB/km in the 0.87-1.02 μ spectral range. It was found that a high degree of polarization of light (up to 0.99) was retained over a length of 1000 m.