Second harmonic generation in a LiNbO3 crystal fiber

We have observed the second harmonic generation from a single crystal LiNbO3 fiber with a diameter of 63 mu m. Femtosecond laser pulses Corm a modelocked oscillator was focused perpendicularly onto the fiber. Scattered second harmonic was analyzed in polarization and the conversion efficiency was estimated. Comparing to a bulk nonlinear crystal, the striking feature of the SHG from the microcylinder is that the nonlinear conversion takes place out off phase-matching condition. For the fundamental at 850 nm the phase-mismatching is negative, even so, a relative important conversion efficiency of 3.7 x 10(-4) was obtained for incident 1.1-nJ and 150-fs pulses (peak intensity of 1.0 x 10(9) W/cm(2)). This conversion efficiency, independent to the phase matching, should not exhibit the sharp variation as for a bulk crystal when the incident fundamental is tuned across the phase-matched wavelength. The fiber provides then a wide bandwidth doubler. We applied this microdoubler in a second order autocorrelator.