Search

The interplay of the Kerr-induced nonlinear refractive index changes and dispersion in nonlinear Fiber Bragg Gratings (FBGs) leads to a plethora of nonlinear phenomena, the most striking of which is perhaps the formation and propagation of gap solitons [1]. Whilst a considerable amount of theoretical work has been performed in this area [1,2,3,4] experimental observations of nonlinear grating behaviour are limited, principally by the difficulty in getting sufficiently high power densities within the core of a FBG in a suitable spectral and temporal range. In order to reduce the nonlinear threshold for gap soliton formation one can use the somewhat weaker dispersive properties of FBGs outside of the band gap and indeed recent experiments have yielded the first strong evidence of Bragg grating gap soliton formation by this means [5,6]. However, the strongest and most manifestly nonlinear effects are predicted to occur at wavelengths within the band gap, close to the Bragg wavelength of the grating structure and it is therefore essential to make measurements within this regime. In this paper we report what we believe to be the first clear experimental observation of nonlinearity within the band gap of an FBG, namely nonlinear self-switching and, at higher intensities, multiple gap soliton formation.

Recently much work on fibre lasers has concentrated on improving the maximum output energy and peak powers available from such systems. An important recent advance in this area has been the development of single transverse-mode, large mode area (LMA-) doped fibre1. The LMA gives increased energy storage reduces the impact of optical nonlinearities and improves the power handling capabilities of the fibre. We have developed LMA fibre designs allowing robust, single transverse-mode laser operation with fibre Mode Field Diameters (MFDs) in excess of 30 μm, i.e around 5 times larger than conventional doped fibre designs Q-switch fibre lasers based on this fibre have yielded pulse energies in excess of 0.5 mJ. Here we present the first demonstration of the use of such fibre in a passively mode-locked soliton laser. Subpicosecond, 1 nJ soliton pulses are obtained representing more than an order of magnitude increase in pulse energy relative to conventional soliton lasers.

We provide the first direct confirmation that light can be guided in holey fibers with non-periodically arranged air holes. The sensitivity of the holey fiber's optical properties to the air hole distribution is explored.

Recently Berger [1] introduced the idea of nonlinear photonic crystals (NPC) as the generalisation to higher dimensions of one dimensional quasi-phase matched structures. In a NPC the linear refractive index is constant while the second order susceptibility tensor varies periodically. Recently we fabricated what we believe is the first such two dimensional NPC in lithium niobate and our initial characterisation of the crystal has been reported elsewhere [2]. This crystal which was used here was poled in a hexagonal pattern, and was 7mm wide, 10mm long and 0.3mm thick. The poled fraction was ~30% and the period was 18.05µm.

The area of nonlinear pulse interactions in Bragg grating structures, although largely unexplored, contains many interesting and novel effects. Perhaps the simplest of these is the CW switching of a weak probe by a strong pump. Recall that a Bragg grating reflects strongly around the Bragg resonance frequency ω0 which is inversely proportion to the average refractive index. In a nonlinear medium the presence of a strong pump alters the refractive index, and thus frequencies which were reflected (or transmitted) by the grating can be transmitted (or reflected). This effect was first seen by LaRochelle et al. in 19901 and to date this is the only experimental work done in this area. However since then considerable theoretical work has been done onpulse interactions2 in fibre Bragg gratings (FBGs). Also experimental reports of nonlinear propagation in FBGs have started appearing in the literature3,4. A major factor in this upsurge of interest has been the development of techniques for writing long gratings at arbitrary wavelengths using the side illumination of fibres with UV light. This fact coupled with the latest generation of high power fibre sources allows the exploration of pulse interactions in FBGs in great detail.