Characterization of modes excited in nonlinear photonic crystal fibre using low coherence interferometry

We have excited both LP₀₁ and LP₁₁ modes using a high magnification objective lens (60×) in a nonlinear photonic crystal fibre (PCF) of core diameter 2.2μm and simultaneously detected the modes using low coherence interferometry. We placed the nonlinear PCF of length ∼11cm in one arm of an interferometer, and then interfered the output with light in the reference arm onto a photodetector via a single mode collection fibre positioned at a point in a near-field image of the fibre endface. More than one fringe packet was observed in the interferogram, indicating the presence of two modes propagating in the fibre core. To uniquely identify the dispersion curves we need to know which mode corresponds to each fringe packet in the interferogram. In the same experimental setup we replaced the photodetector with a digital CCD camera to record the 2-D interference pattern across the image as function of group delay. A Fourier analysis technique was used to compute the intensity and phase of the mode field patterns corresponding to the various interferograms. Using this technique we can simultaneously measure the group velocity dispersion and the mode profile with phase information of the modes excited in a multimode PCF.