Frequency Conversion based on Three-Wave Parametric Solitons

Parametric three-wave mixing in quadratic nonlinear crystals provides a versatile means of achieving widely tunable frequency conversion of laser light, thus permitting a substantial extension of the wavelength coverage of laser sources across broad wavelength regions. For mode-locked lasers, the effective parametric interaction length among short pulses at different wavelengths is limited by crystal dispersion, which leads to group velocity walk-off. Nevertheless, as well known since the 1970s, the frequency conversion of short pulses may be significantly enhanced by means of the optical soliton concept (Zakharov & Manakov, 1973). Indeed, the collision of two soliton input pulses at different frequencies, with proper durations and peak powers, leads to a time compressed pulse at the sum-frequency (Ibragimov & Struthers, 1996). However such pulse is unstable, since it rapidly decays into two, time-shifted replicas of the input pulses. On the other hand, two resonant bright pulses and a kink (or phase jump across a CW background) pulse may propagate locked together as an optical simulton, in spite of their different linear group velocities, owing to their nonlinear mutual trapping (Nozaki & Taniuti, 1973; Degasperis et al., 2006).