Abstract

Holey fibers (HF) are very attractive for the realization of all-optical fiber devices based on nonlinear phenomena.¹⁻³ We have investigated the nonlinear behaviour of a HF characterized by a very small effective area (A_cff < 3 μm²) that enhances its nonlinear properties and by a strong asymmetry in the core region that induces a high birefringence, leading to a polarization maintaining behaviour. Our experimental results are obtained by coupling a 110 fs pulse train at 1.55 μm (80 MHz repetition rate) generated by an optical parametric oscillator (OPO) in a 55 cm long HF span. The maximum power measured at the fiber output was about 25 mW. By increasing the input power we observed the onset of stimulated Raman radiation followed by a progressive broadening of the spectrum due to the four wave mixing effect. In addition to the broad infrared continuum the optical spectrum (see Fig. 1) shows a strong and sharp peak at λ = 430 nm that corresponds to the third harmonic of the 1.3 μm radiation produced by super-continuum generation. We also observe a weaker peak at λ = 517 nm that can be ascribed to the third harmonic of the 1.55 μm pump. As shown also in the phase matching condition for the nonlinear interaction is guaranteed by the modal dispersion. The blue light has a spatial pattern corresponding to a high-order mode and is characterised by eight radial lobes. It is interesting to note that, by rotating the polarization of the input beam, it is possible to vary the relative efficiency of green and blue radiation generation. This fact can be explained by considering that, due to the fiber birefringence, the modal matching is strongly dependent on the fields polarization.

© 2002 Optical Society of America