Frequency doubling in LiNbO3 using temperature dependent QPM

We report the application of temperature-dependent quasi-phase matching (QPM) for second harmonic generation of green light using periodically field poled LiNbO3. In contrast to the usual QPM devices, here the fundamental and second harmonic waves are polarized orthogonally so that the second harmonic signal corresponds to the extraordinary wave. This requires the utilization of the d31 component of the nonlinear tensor (i.e. the same component as used for ordinary birefringent phase matching). d31 is smaller than the d33 component usually used in QPM devices and therefore yields a lower efficiency. However, the use of QPM in our geometry with orthogonally polarized waves results in a greatly enhanced temperature tunability, which increases the versatility of the devices. Moreover, the domain inversion grating period required in this geometry for first-order QPM at the Nd laser wavelength 1064 nm and room temperature is relatively large (350 ...m), and therefore easier to fabricate. Compared with birefringent phase matching, the QPM technique allows for phase matching at any wavelength and eliminates the walk-off effect since the fields propagates along one of the dielectric principal axes. The interaction path can therefore, in principle, be extended over arbitrarily long distances.