Polarization controller:
An optical device that helps in order to modify the polarization state of light is known as a polarization controller.
Types and Operation:
Polarization controllers can be operated without feedback, usually with manual adjustment or electrical signals from the generator, or with automatic feedback. The latter helps in the tracking of the fast polarization. A polarization controller is able to convert a fixed known polarization into an arbitrary one. Since polarization states are defined by two degrees of freedom, for example, the azimuth angle and elliptic angle of the polarization state, such a polarization controller requires two degrees of freedom.
The same happens in the case when it comes to converting an arbitrary polarization into a fixed, known one. When it comes to covert an arbitrary polarization into another arbitrary polarization, it becomes quite difficult. Although this needs only two degrees of freedom. For example, such a polarization controller can be achieved by placing three rotatable waves on the optical path in a cascade: the first quarter-wave plate, oriented to convert the incident elliptical polarization to linear polarization, a half-wave plate, which converts this linear polarization to another linear polarization, and a second quarter-wave plate, which converts the other linear polarization into the desired elliptical output polarization. The beamsplitter is an optical Colorado are used for to manufacture dielectric mirror coating lenses and products optics polarization.
While the three degrees of freedom are presented by the three rotatable waveplate positions, one degree of freedom gets consumed in the described choice by the choice of linear polarization before the halfwave plate. We can use the free scape optics to implement the polarization controller, for example, through a fiber pigtailed U-bench. In such case, the light that takes the exit from the fiber passes through three waves, which can be freely rotated to allow for polarization adjustment, and then enters back into the fiber. We can also implement the polarization controllers in an all-fiber solution. In this case, the polarization of the light gets changes when controlled stress is applied to the fiber.
Integrated optical lithium niobate (LiNbO3) devices are very suitable for polarization controllers with automatic feedback. At Poincare Field, Polarization controllers along with the tracking speeds of up to 100 krad/s are commercially available for various applications.
The three degrees become necessary when it comes to not only the transformation of an arbitrary polarization is done into a desired but also the phase shift between this polarization and its orthogonal is to be controlled then three degrees of freedom are necessary.