Optical and Quantum Electronics 28 (1996) 93-135 TUTORIAL REVIEW UV-written in-fibre Bragg gratings I. BENNION, J. A. R. WILLIAMS, L. ZHANG, K. SUGDEN, N. J. DORAN Department of Electronic Engineering and Applied Physics, Aston University, Aston Triangle, Birmingham B4 7ET, UK Received 31 May; accepted 8 August 1995 The techniques for fabrication and the properties of periodic and aperiodic fibre Bragg gratings produced by UV exposure in photosensitive optical fibres are reviewed with an emphasis on applications. 1. Introduction In 1978, Hill and coworkers reported experiments in which they had succeeded in producing gratings directly written into germanosilicate optical fibres as periodic perturbations of the refractive index along the fibre length [1, 2]. In those experiments, the grating was formed by the standing-wave interference pattern set up by counterpropagating beams of light at 488nm or 514.5nm from an argon-ion laser: gratings of this type have since come to be known as 'Hill gratings'. Permanent, very narrowband Bragg reflection filters of order 1 m in length were formed with reflectivities approaching 100%, with exposures of several minutes' duration. Although the potential of this technique excited significant interest at the time, with the exception of a small number of further studies the subject was not extensively pursued until it was reported by Meltz et al. in 1989 that gratings could be written by two-beam holographic exposure through the side of the fibre using UV radiation [3]. This work overcame two dis- advantages of the original Hill gratings by dramatically improving the writing efficiency and by demonstrating the possibility of producing gratings with an arbitrarily selected Bragg wave- length simply by adjusting the angle between the exposing beams. Since then, interest and activity in the field has increased rapidly to the point where fabrication methods have been significantly refined; many different fibre types have been used for grating fabrication, includ- ing even several nongermanosilicate compositions; many applications have been identified and demonstrated; and ranges of gratings are now available commercially from several suppliers. Grating-based structures in guided-wave optics have long been recognized as potentially very important owing to the large number of device functions they can facilitate. From the ear- liest days of planar integrated optics, surface-relief gratings were investigated in many Wave- guide materials for a wide variety of filtering and coupling functions [4]. The same concepts were subsequently explored in fibre form and many devices were demonstrated [5-14], all of which have since been pursued using gratings written directly into photosensitive fibre, as described in this paper. In order to produce a surface-relief fibre grating, it is necessary to side-polish the fibre to reveal the core, which is then patterned by holographic exposure and 0306-8919 9 1996 Chapman & Hall 93