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

The detailed structure of the field distribution in an arbitrary femtosecond pulse passing through straight and tapered optical waveguides is found using the scalar diffraction theory and the method of images. It is shown that the wave propagates down the guide as the superposition of many pulses that diffract in the off-axis direction and interfere with each other. For the straight multimode guide the usual result is recovered; the spatial profile of the wave is undistorted along its path of propagation provided the input pulse is mode matched. In the case of the tapered guide excited by the 10-fs pulse, which propagates in the off-axis direction and has the parabolic field distribution and spherical wavefront, the new phenomenon is manifested by the appearance of the sub-fs pulse inside the guide.