Nonlinear symmetrization of picosecond pulses in optical fibers

Preliminary results of experiments to determine the controllable nonlinear change in the envelope structure of picosecond laser pulses in an optical waveguide are reported. The source of the ultrashort pulses was an yttrium aluminum garnet laser with passive mode locking. The pulses were observed with an image converter camera with a phototransistor trigger. Changes in the length and the shape of the pulses were detected as they entered the optical waveguide. Analysis of the symmetrization of the pulses for various pulse lengths showed that the process occurs more rapidly for long pulses than for short pulses, and may involve two types of pulse spectrum deformation: (1) a broadening of the spectrum during time-varying absorption in the film shutter; and (2) a self-modulation of the pulse in the fiber. The combined effects of spectral distortions on the broadening of long pulses are discussed.