Detailed structure of fs pulses passing through straight and tapered optical waveguides

The detailed structure of the field distribution in an arbitrary femtosecon d pulse passing through straight and tapered optical waveguides is found us ing the scalar diffraction theory and the method of images. It is shown tha t the wave propagates down the guide as the superposition of many pulses th at diffract in the off-axis direction and interfere with each other. For th e straight multimode guide the usual result is recovered; the spatial profi le of the wave is undistorted along its path of propagation provided the in put pulse is mode matched. In the case of the tapered guide excited by the IO-fs pulse, which propagates in the off-axis direction and has the parabol ic field distribution and spherical wavefront, the new phenomenon is manife sted by the appearance of the sub-fs pulse inside the guide. (C) 2001 Elsev ier Science B.V. All rights reserved.