In 1995 we demonstrated the possibility to synthesize silicon oxynitride in the form of fiber preform core with an ultra low hydrogen content. Note that silicon oxynitride decays at temperatures above 1800 C regardless of the glass deposition regimes. Therefore, fabrication of silicon oxynitride fibers might appear to be highly conjectural. However, such fibers were successfully produced despite the high-temperature procedures of tube collapsing and fiber drawing.

Further improvement of the technology allowed a reduction of optical losses in N-doped silica fibers to the level close to that in standard telecommunication fibers. It was a demonstration of an alternative low-loss fiber, free of expensive germanium and consisting of the three most widespread elements on Earth - Si, O, and N. This achievement generated intense interest in the new fiber. Apart from the economical advantages, silicon oxynitride determines a number of other interesting and important features of optical fibers:

• N-doped silica fibers turned out to be incomparably more resistant to ionizing radiation than the standard fibers.
• The peculiarities of the near-IR loss spectrum in N-doped silica optical fibers can be used to smooth the gain spectrum of erbium-doped fiber amplifiers.
• Interesting peculiarities of hydrogen-free silicon oxynitride were revealed in the UV spectral region. A comparatively low UV absorption along with a high numerical aperture allow application of N-doped silica fibers for nonlinear transformation of near-IR laser radiation into its third harmonic.
• The presence of a considerable amount of oxygen-deficient centers in the glass network of N-doped silica fibers made it possible to write photoinduced in-fiber Bragg gratings. In contrast to similar gratings written in traditional germanosilicate fibers, gratings in N-doped silica fibers remain functional at very high temperatures (~ 600 C). This fact, as well as high radiation resistance of N-doped silica fibers, are of much importance for designing various fiber sensors intended for use in harsh environments.
• Long-period gratings written in N-doped silica fibers by the thermodiffusion technique permitted extension of the temperature range of fiber sensor operation up to 1200 C.
• The thermodiffusion technique was also applied to fabrication of mode-field converters (tapers) which are another important component of modern fiber optics.
• Co-doping of germanosilicate fibers with nitrogen allowed drastic improvement of their photosensitivity.
 
 
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