Blues pulses 300 fs long have been generated from 150 fs infrared pulses by compressing second harmonic radiated in the Cerenkov geometry. Mode-locked Ti:sapphire pulses were prism coupled into a 1-cm long proton-exchanged MgO:LiNbO3 planar waveguide. The 2 ps Cerenkov output, chirped due to group velocity mismatch, was then compressed by a diffraction grating down to 300 fs. The use of mode-locked pulses results in a conversion efficiency of 1000%/W/cm, three orders of magnitude higher than by cw doubling in the same geometry. The Cerenkov geometry ensures that all wavelengths can be phase matched over long crystal length, so that even low power lasers can be efficiently doubled. The dispersion element ensures that the chirped output can be compressed into the femtosecond regime. This concept can be extended to femtosecond pulses in any two-photon process in any waveguide material and any mode-locked source.

We describe in this letter a novel approach to obtain a tunable midinfrared (mid-IR) laser output. By applying quasiphase-matched difference frequency generation in a periodically domain inverted LiNbO3 crystal placed inside a Cr3+:LiSAF laser cavity, more than 50 mW of mid-IR output was generated using only 1.8 watts of red diode or dye laser pumping. Wavelength tuning of the mid-IR signals from 3.3 to 4.2 μm was demonstrated by tuning only the wavelength of the Cr3+:LiSAF emission, without the need to reorient the LiNbO3 crystal for domain period adjustment.

Simultaneous phase- and group-velocity-matched generation of ultrashort optical second-harmonic pulses based on a quasi-phase-matched Cerenkov second-harmonic generation scheme is proposed. We compare our scheme with other schemes with respect to its ability to achieve high conversion efficiency, its enhancement with respect to cw second-harmonic generation, and its sensitivity to waveguide-period fluctuation; ion-exchanged KTP and LiNbO(3) waveguides are considered in the evaluation. We believe our proposed scheme is easy to implement, and there is no constraint on crystal length in achieving high efficiency as well as in preserving an ultrashort pulse width. 100% efficiency is potentially obtainable from sufficiently long crystals.

Planar LiNbO(3) waveguides with a 5-mm tapered region and a 10-mm homogeneous slab waveguide region were fabricated by a double proton-exchange process. The 4-microm waveguide entrance gives rise to a coupling efficiency as high as 93%; the precise control of the depth profile of the tapered region results in a low 1-dB power transfer loss from the tapered region to the 0.4-microm-depth working region. The techniques developed provide an alternative to prism coupling into planar waveguides in Cerenkov second-harmonic generation applications.

Ultrashort optical second-harmonic pulses based on a traditional phase-matched Cerenkov second-harmonic generation scheme, with an additional prism to equalize the arrival time, are proposed. Proton-exchanged KTP, LiNbO(3) waveguides, and several prism materials are considered in the evaluation. There is no constraint on crystal length, making high efficiency possible as well as preserving an ultrashort pulse width. The proposed approach may be a practical and efficient way to achieve convenient ultrashort harmonic pulses.