Dual wavelength waveplates are THG-PR Polarisation rotators and used to manage the Polarisations of laser beams to obtain maximum conversion efficiency of third harmonic generations (THG), i.e., 1064 nm + 532 nm > 355 nm.
Generally, type I frequency-doubler (oo-> e) + type II frequency-tripler ( oe-> e) will be the best design for THG which needs no Polarisation rotator. However, when a type II KTP crystal is used as frequency-doubler and type II LBO as frequency-tripler (see the Figure), the laser Polarisations coming out from KTP are not optimized for THG. How to change it into its optimum? The THG-PR Polarisation rotator can do it!
Common 1/2 wave phase retardation plate could only rotate the Polarisation of 532 nm to a specific angle. However, the linear Polarisation of 1064 nm will be distorted to be elliptical. The THG-PR rotator is specially designed to maintain the linear Polarisation of 1064 nm (l plate) and change simultaneously 532 nm (l/2 plate) to the angle you need. The THG-PR can be applied to:
Type II (SHG) + Type II (THG)
Type I (SHG) + Type I (THG)
Type II (SHG) + Type I (THG)
(mm)