A variable waveplate is tunable retarder with a variable
(controlled) birefringence or thickness. These waveplates can
tune the retardance to quarter-wave or half-wave for a specific
wavelength of light, shift the retardation value for a single
wavelength of light, or as a pure phase modulator for a linear
polarized light beam. There are different kinds of variable
waveplates.
Berek Tunable Waveplate
The Berek tunable waveplate consists of a plate of uniaxial crystalline
material with surfaces perpendicular to the optic axis. At normal incidence,
the Berek variable waveplate is isotropic and introduces zero phase retardance.
The birefringence is varied by tilting the plate. Berek tunable Waveplate is
variable true zero order waveplate.
Soleil-Babinet Compensator
The Soleil-Babinet compensator is a continuously variable, zero-order retarder
(waveplate) that operates over a broad wavelength range. The classical design
of the Soleil-Babinet compensator consists of a long birefringent wedge and a
fixed wedge mounted to a compensator plate. The retardance is adjusted by moving
the position of the long wedge relative to the short wedge. This allows the
retardation to vary continuously while maintaining a uniform retardance across
the aperture at any given setting.
Electrically Controllable Waveplate
The electrically controllable waveplate (also called active retarder or voltage-controlled waveplate) is a variable waveplate that its phase retardation can be actively controlled by varying the applied voltage. Electro-optic crystal or liquid crystal are used for making such variable waveplate. Pockel Cell and Photoelastic modulator(PEM) are Electrically Controllable Waveplates.
Variable waveplate can be used to compensate precisely for small phase shifts produced by other optics. Vary the retardation and orientation of the waveplate to produce very small retardation until system is optimized. Combined with polarizer or polarizing beamsplitter, variable waveplate can be used for making devices such as Variable Attenuator.
