Frequency doubling in LiNbO3 using temperature-dependent QPM

We report the application of temperature-dependent quasi-phase matching (QP M) for second harmonic. generation of green light using periodically field poled LiNbO3. In contrast to the usual QPM devices, here the fundamental an d second harmonic waves are polarized orthogonally so that the second harmo nic signal corresponds to the extraordinary wave. This requires the utilization of the d(31) component of the nonlinear tenso r (i.e. the same component as used for ordinary birefringent phase: matchin g). d(31) is smaller than the d(33) component usually used in QPM devices a nd therefore yields a lower efficiency. However, the use of QPM in our geom etry with orthogonally polarized waves results in a greatly enhanced temper ature tunability, which increases the versatility of the devices. Moreover, the domain inversion grating period required in this geometry for first-or der QPM at the Nd laser wavelength 1064 nm and room temperature is relative ly large (350 mu m), and therefore easier to fabricate. Compared with biref ringent phase matching, the QPM technique allows for phase matching at any wavelength and eliminates the walk-off effect since the fields propagates a long one of the dielectric principal axes. The interaction path can therefo re, in principle, be extended over arbitrarily long distances.