These nonlinear crystals are based upon quasi-phase-matching (QPM) in lithium niobate. Unlike birefringent phase matching, QPM enables manufacture of crystals appropriate for the phase matching of any optical interaction within the wavelength transparency range of lithium niobate (0.35-5.0µm).
Laser 2000 PPLNs utilise a multi-step lithographic arid electric field poling process to effect a permanent change in the nonlinear properties of lithium niobate materials. Substrates can be optically polished and AR coated to customer specification.
Several hundred milliwatts of Mid-IR light have been generated in PPLN OPO devices. By using a different angular component of the nonlinear optical tensor, PPLN exhibits a 4.5-fold increase in effective nonlinear coefficient relative to birefringently phase matched lithium niobate. Acceptance bandwidths may also be improved by the use of PPLN. The PPLN-based mid-infrared Nd:YAG pumped OPO, for example, exhibits ten- and fifty-fold advantages in temperature and angular acceptance respectively, over its birefringently phase matched counterpart.
Now available are crystals with poled gratings for harmonic and parametric generation over a broad range of wavelengths. Mid-infrared OPOs typically use a period in the 20-30µm range. Gratings and custom-patterned poling for specific SHG, DFG, or OPO applications can be designed.
PPLN is produced in a form appropriate for the generation of mid-infrared radiation. Samples are typically 5mm wide and 0.5mm thick. For YAG-pumped OPO applications, where the required period is near 30µm, samples have been produced and tested with a 2cm length. Also presently manufactured are samples as long as 4cm.