Sergey L. Semjonov

Tomographic stress profiling of arc-induced long-period fiber gratings

Long-period fiber gratings (LPGs) have been inscribed in nitrogen-doped fibers by electrical arc discharge. The influence of drawing tension as well as external load applied during arc discharge on coupling strength has been investigated. The influence of drawing tension on the gratings coupling strength is found to be negligible, whereas the coupling strength increases considerably with external load. Tomographic stress profiles of the fiber have been recorded before and after electric arc discharge. The axial stress modulation in the core region of the grating was found to be smaller than 10 MPa and is thus too small to be the dominating mechanism for grating formation.

Fabrication and investigation of single-mode highly phosphorus-doped fibers for Raman lasers

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 .

CW highly efficient 1.24 /spl mu/m Raman laser based on low-loss phosphosilicate fiber

We report the first demonstration of an extremely simple and efficient 1.24 /spl mu/m phosphosilicate fiber-based Raman laser with Bragg gratings written directly in the fiber used. The laser pumped by Nd fiber laser exhibits a slope efficiency of 80%.

All-solid photonic bandgap fiber with large mode area and high order modes suppression

We present all-solid bandgap fiber design with the small ratio of cladding elements diameter to pitch (0.24), with MFD of 36 m at 1 m wavelength and higher order mode suppression caused by the propagation loss difference.

Phosphosilicate-core single-mode fibers intended for use as active medium of Raman lasers and amplifiers

Highly phosphorus doped (7 - 17 mol%) single-mode fibers for the application in Raman laser have been manufactured. It has been established that with increasing the P2O5 concentration level, both optical losses and the fiber Raman gain coefficient increase. Using the fiber technology developed, the maximum efficiency of a single-cascaded Raman laser is achieved at a phosphorous pentoxide doping level of 12 - 14 mol% P2O5.

Fiber performance in hydrogen atmosphere at high temperature

Optical losses induced in fibers at 300oC and in hydrogen atmosphere were studied. A non-linear dependence of hydrogen penetration through the carbon coating on hydrogen pressure was observed. It was demonstrated that carbon coating could not defend the fiber from hydrogen penetration for a long time period. At some time, the hydrogen presence in the fiber core resulted in high optical losses in all spectral range in the case of Ge-doped fibers. It was found that the short-wavelength loss edge (SWE) in a Ge-doped fiber co-doped with a small amount of phosphorus was significantly smaller than that in Ge-doped fibers without co-doping. Nevertheless, P-codoping effect did not decrease optical losses related with SWE completely.

Active material for fiber core made by powder-in-tube method: subsequent homogenization by means of stack-and-draw technique

A procedure for the preparation of optically homogeneous glass for fiber preforms through sintering of coarse oxide particles and further processing of the resultant glass, including several drawing and stacking steps, is described. Reducing the pressure to 10-2 Torr during sintering considerably reduced the amount of gas bubbles in Yb/Al-doped silica glass and decreased the background loss to 100 dB/km after the third drawing-stacking-consolidation cycle. For comparison, a fiber singly doped with alumina was fabricated by the same procedure as above. The level of wavelength- independent losses in that fiber was 65 dB/km.

Reliability of aluminum-coated fibers at high temperature

At temperatures higher than 400 degree(s)C the decrease of aluminum coated fiber strength due to interaction between glass and aluminum was observed. The rate of this decrease appears to depend on the interface between glass and metal. The influence of atmosphere above the metallizer at the application of a metal coating by the freezing method was investigated. Some decrease of the initial bending strength, yet longer lifetime at high temperatures, have been observed for fibers sealed in atmosphere containing oxygen, which can create an alumina layer between silica glass and aluminum. In order to exclude the chemical reaction between silica and aluminum, an intermediate layer of carbon was applied during the fiber drawing. Results of the carbon coating influence on strength and lifetime of aluminum coated fibers at high temperatures are presented.

1480 nm two-cascaded highly efficient Raman fiber laser

Summary form only given. Lasers based on high P/sub 2/O/sub 5/-doped fibers can potentially have a higher efficiency than Raman fiber lasers (RFL) based on standard Ge-doped fibers. An increase of phosphorus content in the fiber core gives rise to an increase of the Raman gain coefficient. Despite a rise of optical losses, heavily P/sub 2/O/sub 5/-doped fibers enable one to develop extremely efficient Raman lasers. A two-cascaded Raman laser based on a P/sub 2/O/sub 5/-doped fiber with the frequency shift of 1330 cm/sup -1/ per cascade and with Bragg gratings written directly in the P/sub 2/O/sub 5/-doped fiber demonstrated the highest ever achieved efficiency in RFL.

Mechanical behavior of low- and high-strength carbon-coated fibers

The factors limiting the maximum strength of carbon coated fibers are considered. In spite of the variations in the inert strength, at room temperature the strength of fibers under investigation depends not on the drawing conditions, but on the properties of the carbon coating. The strength of weak carbon coated fibers with melted-in zirconia particles is also investigated. It grows with increasing carbon thickness (i.e. decreasing of fiber electrical resistance). When the carbon coating is thick enough (electrical resistance is less than 10 kOhm/cm), the fiber strength practically does not depend on the coating thickness and environment humidity and is more than two times higher than that of polymer coated fibers.