Abstract

Picosecond pulses of visible light from a mode-locked dye laser are injected into a 600-m fiber designed to be monomode at 1300 nm. Up to seven modes are excited and can be time resolved at the fiber output by an ultrafast photodiode and a sampling oscilloscope. A computer-controlled XY micropositioner is used to make the photodiode scan the far field spatial power profile of each time-resolved mode. The photodiode has a sensitive area that is small compared to the mode size. By plotting the peak power of each time-resolved mode as a function of x and y, we obtain a picture of a mode in flight. This permits unambiguous identification of each mode. The time position of each mode is measured relative to the fundamental mode as the dye laser is tuned over the 560–605 nm spectral range. The group-velocity dispersion curves thus obtained are used to model the refractive- index profile of the optical fiber through its effect on waveguide dispersion. The method is sensitive enough to show the presence of a dip at core center for one of the fibers studied and to pin down the core diameter with submicrometer accuracy.

© 1990 Optical Society of America