As described in previous chapters, the fabrication of gratings in optical fibers was significantly enhanced by the discovery made by Meltz and Morey, whereby FBGs are written sideways. By interfering two UV beams, it is possible to create RI modulations in the core of the optical fiber.
Chapter 4 presented the requirements and optical parts involved in a FBG laboratory. This chapter applies those assets to fabricate FBGs with the side-writing procedure. The first approach is based on inscribing Bragg gratings with only a single phase mask, which enables the fabrication of FBGs with a particular Bragg wavelength. In this case, the optical fiber is placed in contact or in close proximity to the phase mask, inside the near field where the diffracted beams interfere and the interference pattern is produced.
A phase mask is also used in a second approach, but it is positioned far from the fiber, and two mirrors redirect the +1 and –1 diffracted orders back to the fiber. That is, an interferometer scheme is employed, which is difficult to implement and align; nevertheless, with this arrangement it is possible to inscribe FBGs with different wavelengths by changing the angles of the mirrors.
The design and practical aspects will be discussed, including a systematic procedure for choosing the proper phase mask to obtain a certain Bragg wavelength, followed by an example. These writing schemes are fundamental techniques to fabricate Bragg gratings, but there are also some enhanced approaches to consider.
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