There is a rapidly growing demand for high-quality optical Bragg gratings with arbitrary phase and index profiles, as these gratings are key elements in many components that are used in WDM networks. Over the past few years, several methods that improve the quality and the flexibility in the grating fabrication process have been developed. A straightforward approach is to scan a UV beam over a long phase mask in a fixed relative position to the fiber. Non-uniform profiles can in this case be fabricated either by post processing the illuminated region or by using a phase mask that contains the appropriate structure. Complex grating structures can also be synthesized by moving the fiber slightly relative to the phase mask during the scan.
In 1995, a novel versatile sequential technique for venting long and complex fiber gratings was demonstrated by R. Stubbe, B. Sahlgren, S. Sandgren and A. Asseh, in “Novel technique for writing long superstructured fiber Bragg gratings”, in Photosensitivity and Quadratic Nonlinearity in Glass Waveguides (Fundamentals and Applications), Portland, PD1 (1995) and by A. Asseh, H. Storøy, B. E. Sahlgren, S. Sandgren and R. A. H. Stubbe, in “A writing technique for long fiber Bragg gratings with complex reflectivity profiles”, J. Lightw. Techn. 15, 1419–1423 (1997). The idea was to expose a large number of small partially overlapping subgratings—each containing a few hundred periods or less—in sequence; where advanced properties such as chirp, phase shifts and apodization were introduced by adjusting the phase offset and pitch of the subgratings. In the setup that was used in the above-mentioned references, each subgrating was created by exposing the fiber with a short UV pulse while the fiber itself was translated at a constant speed. The UV pulses were triggered by the position of the fiber relative the UV beams, which was measured by a standard helium-neon laser interferometer.
Bragg gratings normally have their grating elements aligned normal to the waveguide axis. However, there is an increasing interest in producing gratings which have their elements at an angle to the waveguide axis, known as blazed Bragg gratings. Such blazed Bragg gratings are difficult to fabricate efficiently and with appropriate precision with previously know methods and apparatuses.
For example, U.S. Pat. No. 5,730,888 and U.S. Pat. No. 5,042,897 both relates to methods and apparatuses to photo-induce blazed gratings in optical fibers. The blazed gratings are formed by tilting the projection system relative to the fiber. However, there are several problems with these known methods. To be able to tilt the projection system relative to the fiber the equipment becomes complex and costly. Further, it is difficult to achieve an adequate focus on the fiber in the whole exposure area. Hereby, the known methods becomes slow and inefficient, with a low through-put. Still further, with the known methods it is only possible to produce blazed gratings with a limited blazing angle, whereas blazed gratings with larger angles of inclination is very complicated to produce, or even not possible to produce at all.