Abstract

The introduction of short-pulse generation in optical fibres has generated a need for optical fibres that can carry large modeintensities without distorting the pulses. The usual way of doing this has been by extending the dimensions of a waveguide, hereby enlarging the effective areas of the modes propagating. However this offers very little spatial control of the pulses, and introduces loss in the fibre, due to coupling to less stable higher order modes. Recently, a novel type of optical fibres has been developed, that allows stable propagation of higher order modes with ultralarge modal areas. It has been shown that they can carry a higher order mode several meters without significant distortion of the mode. The stability of the mode is unaltered through bend radii >7 cm [1]. This fibre is of great interest, since it is able to carry a larger amount of pulsed energy, before reaching the regime of non-linear pulse propagation, in a flexible manner. However control of energy coupling between the fundamental and higher-order modes is required, if one wants to use the fibre as a high-energy transport fibre. Coupling between modes under controlled conditions may be achieved with a long-period fibre grating (LPFG) [2]. Structuring the axis of propagation with a periodically varying index of refraction, coupling between specific modes in an optical fibre is achieved. The refractive index may be changed exposing the fibre to UV or IR light. The exposure is usually controlled by an amplitude mask or point by point writing. Using an amplitude mask reduces the sensitivity to fibre-core alignment, and is an easy way to write an LPFG. However this approach lacks the opportunity to expose the fibre core locally. Point by point writing provides much more flexibility, although it is not as time efficient as writing with an amplitude mask.

© 2007 IEEE