UV-written Bragg gratings

Abstract

It is no understatement to claim, that the Bragg grating [1,2] is the most significant fibre-optic invention since the erbium-doped fibre amplifier (EDFA). Unquestionably, it is the most versatile optical-fibre filter that exists and it has, hand-in-hand with the EDFA, facilitated dense wavelength division multiplexing (DWDM) in telecommunications, providing better processing of densely packed optical frequencies and easily out-performing any other available filter-technology. Further studies into the design, fabrication and applications of fibre Bragg gratings therefore are of major interest for future communication-systems, besides a diverse range of other applications including sensing, and short pulse fibre lasers. A Bragg grating is a periodic or almost-periodic structure, consisting of a variation of for example the refractive index with a typical period of ~0.5µm along the length of a waveguide and is typically formed within this waveguide using side-exposure to intense ultra-violet light [2]. By phase-matching the individual small reflections generated by the high-low, low-high index regions of difference ~10 along the waveguide, strong overall reflection of light incident on the grating is obtained within a well-defined spectral pass-band. Therefore, a Bragg grating can act as a band-rejection filter passing the wavelengths that are not in resonance with the grating and strongly reflecting wavelengths that satisfy the Bragg condition. The technology of Bragg gratings has matured significantly over the past years and with the recent advances in manufacturing capabilities of these obviously there will develop a demand for filters of even higher complexity to perform very specific filtering tasks. Currently it is often the imagination that sets the limits for what can be achieved experimentally. Of particular practical significance is the possibility to alter and control both the amplitude and phase of light reflected and/or transmitted through a grating. This possibility has been exploited in the past for a range of optical signal processing function including, for example, static chromatic dispersion compensation in optical fibre links [3], and the design and fabrication of Bragg grating filters for complex phase and amplitude coding applications similar to those used in microwave code-division multiple access (CDMA) communication systems [4]. These and many other recent new applications [5] of fibre Bragg gratings together with an overview of Bragg grating design- and fabrication-techniques will be taught in this lecture.

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