| We have demonstrated electro-optic spectral tuning of the optical parametric gain spectrum for quasi-phase-matched (QPM) periodically poled LiNbO 3 (PPLN) crystals. Fabricating QPM gratings with an asymmetric-duty-cycle enabled electrooptic tuning, and numerical modeling accurately predicted the tuning rate. We demonstrated this tuning in a PPLN optical parametric oscillator (OPO) and in a PPLN optical parametric generator (OPG). With the PPLN OPO we achieved 8.91 cm-1, 2.28 nm of rapid spectral tuning with a linear tuning rate of 2.89 cm-1/(kV/mm), -0.746 nm/(kV/mm), by applying electric fields up to +/-1.5 kV/mm across the crystal. In a similar manner we demonstrated equivalent tuning in the PPLN OPG. In both cases we confirmed that the applied electric field shifted the entire gain bandwidth of the asymmetric-duty-cycle PPLN crystal linearly with respect to the applied electric field.; With the PPLN OPG, we not only confirmed the linearity of the tuning with respect to the applied electric field, but also with respect to the PPLN grating duty-cycle. For the latter demonstration, we characterized a unique multigrating PPLN crystal: each grating's duty-cycle was different, ranging from 50%/50% to 80%/20%. All gratings were first-order QPM, except for the three highest duty-cycle gratings, which were second-order QPM. The tuning rate for the first-order and second-order gratings with the same duty-cycles were similar, and the results agreed with our numerical predictions.; In addition, the asymmetric-duty-cycle PPLN crystals had an increased susceptibility to photorefractive behavior. A field-biased charge migration was evident since the application of a constant electric field caused the transmitted beams to focus or defocus depending upon the sign of the applied field. However, the dominant photorefractive behavior was attributed to the photogalvanic effect. With a simple numerical model we showed that the field-biased charge migration only diminished the observed tuning, whereas the photogalvanic charge migration, parallel to the spontaneous polarization of the LiNbO 3, caused a changing electro-optic spectral tuning rate for positive and negative applied fields. |
