We report on a novel scheme for generating a broad spectrum in the UV region. This scheme enables us to control the phase of the UV pulse through a frequency-mixing process in a nonlinear crystal. For group velocity matching, it is essential that a monochromatic beam should be sum-frequency mixed with an angularly dispersed beam having a broad spectrum in noncollinear geometry. We found analytically unique solutions for a noncollinear angle, for an angular dispersion of the broadband input beam, and for an angle of the beam from the optical axis in a nonlinear crystal, with the condition that there is no angular dispersion in the output beam. Based on the analysis of this scheme, we obtained UV pulses with a sufficiently broad spectrum for obtaining a sub-20-fs pulsewidths in the experiment. The improvement of conversion efficiency and compensation of chirp are also discussed.
© 2003 Optical Society of America
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