ISSN 1063-7842, Technical Physics, 2006, Vol. 51, No. 8, pp. 1035–1045. © Pleiades Publishing, Inc., 2006. Original Russian Text © S.A. Kukushkin, A.V. Osipov, M.G. Shlyagin, 2006, published in Zhurnal Tekhnicheskoœ Fiziki, 2006, Vol. 76, No. 8, pp. 73–84. 1035 INTRODUCTION The photosensitivity of germanium-doped quartz glass fibers and the feasibility of recording holographic Bragg gratings in them were first reported more than 20 years ago [1]. It was found that the intense radiation of a 488-nm Ar laser may induce a space-periodic refractive index grating in the core, this grating persist- ing after the writing light has been switched off. Ten years later, a flexible high-sensitivity technique of grat- ing formation was developed in which the fiber is illu- minated from one side by the interference pattern pro- duced with a powerful source of UV radiation [2]. Since then, this line of inquiry has rapidly progressed, especially when modified fibers began to be used as various sensors in fiber-optic communication. Today, such gratings are being widely employed in fibers and planar lightguides for wavelength division multiplexing and filtering of optical signals. Specifically, they are applied as mirror resonators in fibers and semiconduc- tor lasers, smoothing filters in optical amplifiers, dis- persion compensators in communication channels, etc. [3]. Fiber Bragg gratings are also used as sensors. The reflection spectrum of the simple Bragg grating has a single narrow resonance line at a wavelength depending on external actions. From the shift of the resonance reflection line, one can judge, for example, temperature or mechanical stress variations [3]. In the years, the optical grating technology has dra- matically evolved. UV lasers tailored for recording Bragg gratings have been developed and put in produc- tion. The most widely used are pulsed 248- and 193-nm excimer lasers and cw frequency-doubling Ar lasers at a length of 244 nm. For recording strongly reflective gratings, a high level of accumulated energy (energy exposure) is necessary (on the order of 10 2 –10 4 kJ/m 2 and so powerful pulsed lasers are preferable for this purpose. Special fibers with an increased germanium content in the core have been developed aimed at sim- plifying the recording process and improving the sensi- tivity to the writing beam [4], as well as fibers addition- ally doped by boron, which offer the highest sensitivity today. It has also been found [3] that keeping a standard fiber in hydrogen under a pressure of several tens of atmospheres for several days raises its sensitivity by several orders of magnitude. In spite of the above breakthroughs in the Bragg grating technology, mechanisms responsible for grating photosensitivity are not yet fully understood. The grat- ing formation dynamics is essentially nonlinear. More- over, the reflection amplitude grows nonmonotonically in a number of fibers, tending to a maximum (type-I gratings, which feature consolidation of the material). In others, the reflection amplitude, having reached a maximum, falls nearly to zero and then grows again, tending to saturation (type-IIA gratings). If the light pulse energy is high, one pulse may suffice to record a high-reflectivity grating. Such gratings are the most thermally stable: they withstand long-term heating at temperatures near 1000°C. Different dynamics of Bragg grating formation and much differing thermal stabilities of Bragg gratings suggest several mecha- Formation of Pores in the Optical Fiber Exposed to Intense Pulsed UV Radiation S. A. Kukushkin a , A. V. Osipov a , and M. G. Shlyagin b a Institute of Problems in Machine Science, Russian Academy of Sciences, Vasil’evskiœ Ostrov, Bol’shoœ pr. 61, St. Petersburg, 199178 Russia b Centro de Investigación Cientifica y de Educación Superior de Ensenada (CICESE), Ensenada, Baja California, 22860 Mexico Received December 27, 2005 Abstract—A new method of forming refractive index gratings (type-IIA Bragg gratings) in the optical fiber by exposing it to intense UF laser radiation is suggested. Central in this method is the generation and propagation of micropores in the parts of the fiber where mechanical stresses are localized. It is shown that such parts are the center of the core and the core–cladding interface. The temperature of the radiation-heated germanium- doped core is found experimentally and theoretically. Thermal stresses arising in the heated fiber are calculated. A theory of cracking and pore formation in the fiber exposed to intense laser pulses is worked out. The pore size distribution, pore formation and growth rate, and pore density versus time of laser action are estimated. PACS numbers: 42.81.-i DOI: 10.1134/S1063784206080135 OPTICS, QUANTUM ELECTRONICS