We report on what is to our knowledge the first fabrication of fiber Bragg gratings by UV femtosecond radiation. The Bragg gratings, with photoinduced refractive-index modulation up to 1.92 x 10(-3) in H2-loaded SMF-28 and up to 1.05 x 10(-3) in Nufern GF1 fibers, were written by high-intensity (31-77-GW/cm2) femtosecond pulses at 264 nm. The dependence of the refractive-index modulation on intensity at equal fluences points to a two-photon absorption mechanism for grating inscription.

Strong high-quality fiber Bragg gratings with photoinduced refractive-index modulation of more than 10-(3) were written in a Corning SMF-28 fiber, a P(2)O5-doped-core fiber and a pure-silica-core fluorine-doped-cladding fiber by third-harmonic radiation (267 nm, 150 fs and 1.2-1.8x1011 W/cm(2)) of a femtosecond Ti:sapphire laser using a phase mask. We compare the 267-nm photosensitivity responses with the results of irradiation by 193-nm ArF and 157-nm F(2) excimer lasers. The dependence of the refractive-index change on the exposure dose and the annealing characteristics of the fabricated gratings are typical for Type-I UV-written fiber gratings.

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Fibre-Bragg-grating writing in single-mode optical fibres by the phase-mask method using 220-fs, 264-nm UV pulses of intensity 31-77 GW cm^ is reported for the first time. The achieved degree of modulation of the photoinduced refractive index was 1.9 x 10^ in an H2-loaded SMF-28 telecommunication fibre and 1.1 x 10 in a H2-free Nufern GFl fibre. The dependence of the induced refractive index on the intensity for the same irradiation fluences in the case of the H2-loaded SMF-28 fibre shows that the refractive index is induced due to nonlinear absorption.

Long-period fibre gratings (LPFGs) are versatile -spectrally-selective components which are widely used in integrated optical in-fibre devices for optical communications and optical sensing. Typical sources for LPFG fabrication are cw frequency-doubled Ar lasers (244 nm) or pulsed 248 nm KIF and 193 nm ArF excimer nanosecond lasers. These wavelengths correspond to strong linear absorption of the germanosilicate glass, common material of fibre core. In the case'of 248 nm (or 193 nm) irradiation the absorption of two photons (with sum energy of about I O or 12.8 eV) is one of the main primary photoprocesses. The mechanism of the refraction index change under UV excimer laser inadiation is not yet fully understood. In addition, it is not clear what kind of two-quantum absorption mechanism takes place: two-photon absorption through a virtual state or two-step absorption through an intermediate state. In both cases, the yield of the photochemical reactions involved is proportional to the squared intensity of the irradiation. Therefore, one can expect that the use of femtosecond UV pulses of much larger intensity would lead to an enhancement of the inscription process. In our previous shidy [ 11, we investigated the possibility of LPFG inscription using femtosecond laser light at 264 nm. In this case, there is no significant linear absorption at irradiation wavelength and the refractive index change is probably induced via the two-photon mechanism. In a separate investigation [2], a strong two-photon absorption at 264 nm in Ge-doped fused silica fibre preform was revealed. The details of our experimental setup were described in [I]. In this study the fibre was fmed on a 50 mm (1 pm resolution) computer controlled translation stage (Physik Instnunente PI 405.DG) and was exposed step by step with the 300 pm period. Three different single mode fibres: SMF-28 Coming, Fujikura telecommunication fibre and fiber made by Fiber Optics Research Center (FORC) were used for irradiation. Using this novel automated set-up, we have investigated the effects of light fluence and irradiance on the inscription process in H,-loaded and H2-outdiffused fibres under femtosecond 264 nm laser irradiation.A typical transmission spectrum for the fabricated LPFG in FORC fibre (70 grooves, incident fluence 80 J/cm2 per groove, incident laser intensity 160 GW/cm2) is presented below. The thermostability studies of the inscribed gratings were also performed.