Principle of Waveplate
Waveplates (retardation plates or phase shifters) are made from materials which exhibit birefringence. The velocities of the extraordinary and ordinary rays through the birefringent materials vary inversely with their refractive indices. The difference in velocities gives rise to a phase difference when the two beams recombine. In the case of an incident linearly polarized beam this is given by a=2pd(ne-no)/l(a-phase difference; d-thickness of waveplate; ne,no-refractive indices of extraordinary and ordinary rays respectively; l-wavelength). At any specific wavelength the phase difference is governed by the thickness of the retarder.Half Waveplate
The thickness of a half waveplate is such that the phase difference is 1/2-wavelength (ture-zero order) or some multiple of 1/2-wavelength (multiple order).
A
linearly polarized beam incident on a half waveplate emerges as a linearly polarized beam
but rotated such that its angle to the optical axis is twice that of the incident beam.
Therefore, half-waveplates can be used as continuously adjustable polarization rotators.
Half-waveplates are used in rotating the plane of polarization, electro-optic modulation
and as a variable ratio beamsplitter when used in conjunction with a polarization cube.
Quarter Waveplate
The thickness of the quarter waveplate is such that the phase difference is 1/4 wavelength (ture-zero order) or some multiple of 1/4 wavelength (multiple order).
If
the angle q (between the electric field vector of the incident
linearly polarized beam and the retarder principal plane) of the quarter-waveplate is 45,
the emergent beam is circularly polarized. When a quarter waveplate is double passed, i.e.
by mirror reflection, it acts as a half waveplate and rotates the plane of polarization to
a certain angle. Quarter waveplates are used in creating circular polarization from linear
or linear polarization from circular, ellipsometry, optical pumping, suppressing unwanted
reflection and optical isolation.
Dual-wavelength Waveplate
The THG-PR polarization rotator is an example of dual-wavelength waveplates and used to manage the polarizations 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 polarization 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 polarizations coming out from KTP are not optimized for THG. How to change it into its optimum? CASIX's THG-PR polarization rotator can do it!
Common 1/2 wave phase retardation plate could only rotate the
polarization of 532 nm to a specific angle. However, the linear polarization of 1064 nm
will be distorted to be elliptical. The THG-PR rotator is specially designed to maintain
the linear polarization 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:
1. Type II (SHG) + Type II (THG)
2. Type I (SHG) + Type I (THG)
3. Type II (SHG) + Type I (THG)
