Angular quasi-phase-matching : theory and first experiments

摘要

Advances in the periodic poling process have led to longer and larger PPLN crystals. Today it is also possible to prepare PPLN samples with a thickness of about 5 mm which allows the use of pump laser beams with a larger aperture, and so with a higher energy. Moreover thicker samples give the possibility to consider quasi-phase-matching (QPM) at any angle with respect to the grating periodicity. We called this scheme angular quasi-phase-matching (AQPM). In order to illustrate the potentiality of AQPM, we compared its tunability and spectral acceptance with that of BPM in the case of second harmonic generation (SHG) and difference frequency generation (DFG) in the periodically poled negative uniaxial 5%MgO:PPLN crystal, with a grating periodicity ∧ = 32.2 μm. We found that AQPM exhibits complementary spectral range and acceptances compared with BPM. We experimentally performed the first validation of the theory of AQPM by cutting 5%MgO:PPLN as a polished sphere with a diameter of 3.9 mm in order to propagate beams in any direction of the crystal by keeping normal incidence. It allowed us to measure any SHG and DFG AQPM direction, with the associated efficiencies, the spectral and angular acceptances. They are reported with calculations.