Tunable ultraviolet (UV) pulses shorter than 10 fs are generated by achromatic frequency doubling the output of a noncollinear optical parametric amplifier. We achieve first and second order achromatic phase matching with a suitable two-prism sequence and thereby increase the natural bandwidth of the nonlinear crystal by a factor of 80. Extremely broad UV spectra with a Fourier limit of 2.9 fs are generated in a 360 μm thick BBO crystal at a conversion efficiency of 20%. The angular dispersion and the first order chirp of the highly stable UV pulses is compensated with a second prism sequence. By controlling the higher order chirp with a deformable mirror we generate pulses as short as 7.1 fs.
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