This invention relates to the creation of Bragg gratings in photosensitive optical waveguides. It is known that such a grating can be created by illuminating the photosensitive waveguide from the side, writing the lines simultaneously with an interferometrically generated grating fringe pattern of light. Such a fringe pattern can be created using two-beam interferometry, or as a fringe pattern generated in the vicinity of a diffraction grating, typically a phase grating, through which light is caused to pass.
Two broad regimes for producing gratings in photosensitive optical waveguides are known, respectively characterised as type I and type II. A type I grating is created by relatively low fluence exposure to a fringe pattern, and the refractive index change that is produced grows towards a saturation level with increasing fluence, which is normally, but not necessarily, provided on a cw basis. The manufacture of such type I gratings is for instance described in U.S. Pat. No. 4,474,427 (K O Hill et al) and U.S. Pat. No. 4,725,110 (W H Glenn et al). A type II grating is created by significantly higher fluence exposure to a fringe pattern, and the effect has been attributed to some `damage` effect upon the core of the fibre. The manufacture of such a type II grating is for instance described by J L Archambault in a paper entitled `High Reflectivity and Narrow Bandwidth Fibre Gratings Written by Single Excimer Laser Pulse`, Electronics Letters (7 Jan. 1993), Vol. 29 No. 1, pp 28-29. Published papers on type II grating creation have recited the use of ultra-violet radiation at 248 nm from a KrF excimer laser, the fluence being confined to a single pulse typically of about 20 ns duration. The suggestion that the observed effect is a `damage` effect is based upon observation of the effects of such fluences incident upon fibre preforms similar to those from which the optical fibres themselves were drawn. Damage is inferred when ablation from the surface of preforms is detected using a sensor that directs an interrogation beam of laser light laterally through a zone just above the point where the 248 nm ultra-violet light is incident upon the preform surface. When using a KrF excimer laser in this way, it is found that there is a relatively narrow safety margin between the fluence necessary to reach the lower threshold of the onset of refractive index modifying `damage`, and the fluence necessary to reach the upper threshold at which catastrophic disruption of the fibre is liable to occur.
For the purpose of this invention the term `damage`, as used in the context of Bragg grating creation, is defined to cover a process of creating a Bragg grating in a photosensitive waveguide using a fluence sufficient to produce ablation effects when applied to an optical fibre preform having the same optical core composition as that of the waveguide.