Abstract

The nonlinear optical material potassium niobate (KNbO₃, KNB) has established itself as an excellent mid-infrared frequency converter. To date, all work using this biaxial material has been carried out with beams propagating in the principal planes of the crystal. This necessarily dictates the use of Type I phase matching since the effective nonlinear coefficient d_eff is zero in the principal planes for Type II phase-matched processes. Here, the possibility of using Type II phase matching by propagating beams outside the principal planes is reported. Type II phasematching has the advantages of broader angular acceptance [for second harmonic generation (SHG) and frequency up-conversion] and narrower emitted linewidth [for optical parametic oscillators (OPOs) and frequency down-conversion]. Calculated and experimental SHG and OPO tuning curves will be presented, in addition to data that determines the relative sign of d₃₁ and d₃₂ in KNB. This relative sign has a critical impact on the magnitude of d_eff for the Type II conversion process.

© 1992 Optical Society of America