An optically nonlinear crystal is arranged for frequency-doubling an input pulse. The crystal has parallel facets each coated with a reflective coating. The crystal is arranged with respect to the input pulse such that the input pulse makes a plurality of forward and reverse passes between the coatings. A frequency-doubled pulse is generated on the forward passes. The input pulse and the frequency-doubled pulse propagate with different group velocities in the crystal such that temporal separation the pulses occurs. The crystal and reflective coatings are configured such that the temporal separation does not exceed a predetermined value.
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