The technology, properties and applications of novel germanium-free nitrogen-doped silica fibers are reviewed. The key features of the plasmachemical synthesis of low- hydrogen N-doped silica fiber preforms are discussed. Optoelectronic components fabricated from N-doped silica fibers are considered: thermostable Bragg and long-period gratings, mode field converters, and filters to smooth the erbium amplifier gain spectrum. Other important properties of the new fibers, such as high resistance to ionizing radiation and efficient third harmonic generation, are also discussed.

A technology has been developed for fabrication of single- mode fibers with a high level of phosphorous doping (10 - 17 mol% P₂O₅) in the core. Characteristics of such fibers intended for use in Raman lasers operating at 1.24 and 1.48 micrometers are investigated. A reduction of fiber drawing temperature and additional doping of the fiber core with fluorine allowed a reduction of optical losses below 1 dB/km in wavelength range 1.1 - 1.5 micrometers .

We have theoretically investigated for the first time the electrostrictive response in a single-mode ring-core fiber. It has been found that the electrostrictive response function strongly differs from that of standard fibers with a Gaussian light intensity profile.

IR absorption spectra of high-purity silica glass (SiO₂) containing from 0.1 to 10³ ppm of hydroxyl (OH) groups were studied and quantum-chemical calculation of OH groups embedded in different ways in the silica glass network was performed. On this basis a new approach to the problem of OH groups embedding in the glass network is proposed: hydroxyl occurs in high-purity SiO₂ mainly as single groups rather than paired groups and some of them are hydrogen- bonded to bridging oxygen atoms, and not only to each other, as it is considered now. New decomposition of the fundamental OH stretching vibrational band into components was proposed.

IR absorption spectra of phosphosilicate glasses containing 2.5 and 9 mol.% of P₂O₅ were measured and analyzed. Quantum-chemical calculation of interaction of hydroxyl groups with the phosphorus centers was performed. The explanation of experimentally observed changes of shape and position of OH vibrational absorption bands was proposed.

We have investigated absorption band corresponding to the principal stretching vibration of hydroxyl (OH) groups in (GeO₂)_x(SiO₂)_1-x glasses with x <EQ 25 mol.% made by VAD and MCVD methods and by melting of GeO₂ and SiO₂ powders. The literature data have been analyzed and quantum-chemical calculations of various possible ways of OH groups embedded in the germanosilicate glass atomic network have been performed. OH groups are shown to occur in the investigated specimens mainly as silanol groups, equalsV Si-OH, concentration of equalsV Ge-OH groups being less than 5%. The maximum at 3600 cm^-1 of OH absorption band in germanosilicate glass usually ascribed to equalsV Ge-OH groups is supposed to be caused by equalsV Si-OH groups hydrogen-bonded with equalsV Ge-O-GeequalsV linkages and, probably, with equalsVSi-O-GeequalsV linkages. The apparent discrepancy of data for various specimens of germanosilicate glasses are explained by assumption that different conditions of manufacturing yield different degree of homogeneity of glass specimens. The shape and position of the absorption bands of OH stretching vibration in GeO₂- SiO₂ glasses are proposed as an additional criterion in homogeneity of glass and for technological control.

The effect of using the multi-region crack growth model on the fiber lifetime prediction in an optical communication system is discussed. The weak flaw statistics model is compared with the safe stress model for mechanical reliability of optical fibers. Problems to be solved before putting the safe stress model into practice are analyzed.

Using the method of Fourier spectroscopy, the IR absorption and reflection spectra were obtained in the frequency region 240 divided by 5000 cm^-1 for different types of high- purity silica glasses (v-Sao₂), containing from 0.1 to 10³ ppm of the hydroxyl groups OH. The analysis of the positions of both the fundamental structural band (v approximately 1120 cm^-1) monitored in IR reflection mode and an overtone of this band (v approximately 2260 cm^-1) monitored in IR transmission mode was carried out for various as-received silica glasses and their fictive temperatures were determined. On the basis of the classical dispersion analysis the absorption band parameters and the spectra of optical constants ((epsilon) ', (epsilon) '', n and k) were calculated for vitreous Sao₂.

It is shown that under certain conditions, the chief mechanism limiting the maximal light intensity transmittable through silica fiber is stimulated Raman scattering. The threshold intensities leading to irreversible fiber damage via overheating have been estimated on the basis of an approximate theoretical model of thermal effect of stimulated Raman scattering. Dependencies of the threshold intensity on the physico-chemical properties of the fiber materials and on the sizes of the fiber cross-section have been determined.

A brief history, state of the art and main tendencies in development and application of Raman fiber amplifiers are discussed.

The results of experimental research and numerical modeling of the 1.3 micrometers Raman fiber amplifier based on the high Gao₂ doped fiber are presented. The Raman amplifier was pumped by the P₂O₅-doped fiber Raman laser. The measurements of gain and noise figure in broad range of experimental conditions are fulfilled. The amplifier gain coefficient was measured to be 42 dB/W.

Extremely simple and efficient 1.24 micrometers phosphosilicate fiber-based Raman laser was developed. The cavity of the Raman laser was formed by the Bragg gratings written directly in the phosphosilicate fiber. The investigation of the laser parameters, mathematical simulations and optimization of the Raman laser were carried out. As a result of optimization the 1.24 micrometers output power of 2.4 W was reached at the neodymium fiber laser pump power of 3.6 W, that corresponds to the Raman laser quantum efficiency of 77%.

We present results of the research and development of double-clad Yb³⁺-doped fibers for high-power fiber lasers. The quality of the fabricated fibers and optimization of reflecting Bragg gratings allowed us to demonstrate fiber lasers with a quantum efficiency close to 90%. These fiber lasers were used in Raman converters emitting at various wavelengths.

A 1.43 micrometers fiber laser with an output power of 1.4 W is reported. The laser is based on Raman conversion of the Yb- fiber laser emission in a phosphorous doped fiber. The light conversion is based on both phosphorous and silica Raman shifts in the same P-doped fiber. This device can find medical applications, since its emission wavelength coincides with one of the water absorption bands.

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.

Single-mode germanosilicate-core optical fibers were fabricated by the MCVD process modified by sintering of the deposited soot in a reduced atmosphere (helium- or nitrogen- containing one). The fibers showed a higher photoinduced refractive index change and a higher efficiency of (chi) ⁽²) formation as compared to an ordinary germanosilicate fiber. The sintering process in nitrogen and helium atmospheres was shown to increase the concentration of germanium oxygen-deficient centers. Besides, nitrogen appears to enter into the glass network in a sufficient concentration to modify the glass network and to additionally increase photosensitivity of the fibers.

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.

The main goal of this paper is not to explain all experimental features observed in connection with UV-written refractive index gratings in silica glass-based fibers, but it is an attempt to present some ideas and possible way of thinking to help researchers in their work. We would like also to show usefulness and facilities of quantum chemical calculations in understanding of microstructure of silica glass, its point defects and processes initiated by UV laser irradiation. We show that the oxygen vacancy relaxation to the puckered state can explain several features of the UV- induced refractive index change in doped silica glass.

Interaction of hydrogen atoms and molecules with equalsVSi-O- SiequalsV, equalsVSi-O-GeequalsV and equalsVGe-O-GeequalsV atomic linkages and equalsVSi-SiequalsV, equalsVSi-GeequalsV and equalsVGe- GeequalsV oxygen vacancies (silicon and germanium oxygen- deficient centers) in silicon dioxide is studied with the help of MNDO and PM3 quantum-chemical methods in isolated molecular cluster model. Main experimental features of interaction of hydrogen with oxygen-deficient centers and its role in photostructural processes in silica and germanosilicate glass are explained.

The mechanisms of radiation-induced absorption in silica optical fibers in the visible spectral region and in the telecom spectral windows as well as the technological means to lower the induced absorption are analyzed. Hydrogen loading of large-core silica optical fibers is shown to drastically reduce the induced absorption at megagray doses. It is shown that low-dose transient absorption can degrade the performance of pure-silica-core fibers at (lambda) approximately 1.55 micrometers . N-doped silica fibers are argued to be the best candidates for low-dose applications (e.g. in space). At megagray doses, the long-wavelength induced absorption is found to be the main induced absorption mechanism. Its origin is not known with certainty, whereas its value may be different in pure-silica-core fibers obtained under different preform fabrication conditions. Different types of radiation-sensitive fibers are investigated with the aim to develop fiber-optic dosimeters. An optimum wavelength region for the operation of P-doped silica fiber dosimeters has been determined. Novel types of dosimeters and neutron detectors are proposed based on the effect of irreversible radiation-induced increase of OH- group absorption.

For the first time, two-channel concentric-core optical fibers consisting of a broadband central single-mode lightguide for the wavelength of 1.3 micrometers and a ring multimode lightguide, 10 micrometers in thickness, have been developed. The outer fiber diameter is 125 micrometers . The fibers are meant for two-channel communication systems. Optical losses of less than 1 dB/km for the both cores have been achieved, the channel-to-channel crosstalk being less than -40 dB. A dependence of optical losses in the ring lightguide on the radius of curvature and strain of a coiled fiber has been revealed. The optimal refractive index profile of the ring core has been found to minimize optical losses in coiled and straightened fibers. A fiber-optic communication link, 12 km in length, has been constructed, and the loss distribution along the link has been investigated using the OTDR-technique. It is shown that there is no distributed coupling between the two cores and the channels remain independent even at large distances.

The article overviews the main properties, fabrication techniques and ares of application of long-period fiber gratings. The basic theoretical equations describing spectral properties of the gratings are given. Experimental investigation of the cladding modes excited by long-period gratings, as well as sensitivity of the long-period grating spectrum to external perturbations are discussed. The most common fabrication techniques of long-period gratings are examined with reference to their advantages and disadvantages. The most important applications of long- period gratings are discussed. Particular attention is paid to the recent results obtained with participation of the authors.

Inexpensive temperature sensor, using an in-fiber Bragg grating as a sensitive element, has been developed. A long- period grating is used to transform the shift of the reflected light wavelength into the variation of amplitude of the detected signal.

This paper reports recent achievements in the field of infrared optical fibers based on vitreous arsenic chalcogenides (As₂S₃, As₂Se₃, As₂Se_1.5Te_1.5). The minimum optical losses of the fibers from arsenic sulfide, arsenic selenide and arsenic telluride are equal to 20 - 30 dB/km at 2.2 and 3.3 micrometers , 80 - 100 dB/km at 4.3 micrometers , and 100 - 300 dB/km and 6.7 micrometers , respectively. Among them the double polymer coated two-layer arsenic-sulfide optical fibers have the lowest optical losses (20 - 25 dB/km) and the highest mechanical bending strength (1 - 1.5 GPa).

For the first time crystalline silver halide optical fibers with optical losses lower than 50 dB/km in a broad IR region from 9 to 14 micrometers were fabricated by an extrusion process. The optical loss mechanism essential to the lowest absorption in silver halide materials and fibers, namely intraband absorption by free holes in the valence band is proposed. The crystalline fibers with Rayleigh-type (lambda) (superscript -4) optical scattering were obtained. An IR region near 13 micrometers in AgBrI fibers with optical losses of less than 10 dB/km was discovered. Non-aged, stable IR polycrystalline silver halide optical fiber cables with losses lower than 1 dB/m in the region from 3 to 20 micrometers are demonstrated. Various applications of the developed low- loss silver halide fibers in remote spectroscopic chemical sensing, non-contact temperature monitoring and IR laser power delivery are discussed.