What is a fiber optic polarization controller?

What is a fiber optic polarization controller?
Definition: A device that can control the polarization state of light in optical fibers. Many fiber optic devices, such as interferometers, require the ability to control the polarization state of light in the fiber. Therefore, different types of fiber polarization controllers have been developed.
Bat ear controller in bent optical fiber
A common polarization controller is achieved by bending (or winding) optical fibers to obtain birefringence. The total delay (birefringence size) is proportional to the length of the fiber and inversely proportional to the bending radius. It is also related to the type of optical fiber. In some cases, the optical fiber can be wound several times with a specific bending radius to obtain a delay of λ/2 or λ/4.

Figure 1: Bat ear polarization controller, consisting of three fiber optic coils that can rotate along the axis of the incident fiber.
Usually, three coils are used to form a column, with the middle coil as a half wave plate and the two sides as quarter wave plates. Each coil can rotate along the axis of the incident and outgoing optical fibers. By adjusting the orientation of the three coils, the polarization state of a specific wavelength of incidence can be converted into any output polarization state. However, the impact on polarization is also related to wavelength. At high peak power (usually occurring in ultra short pulses), nonlinear polarization rotation occurs. The diameter of the fiber optic coil cannot be too small, otherwise bending will introduce additional bending losses. Another more compact type, and less sensitive to nonlinearity, utilizes the strong birefringence (polarization preservation) of optical fibers rather than fiber coils.
Compressed Fiber Polarization Controller
There is a device that can obtain variable waveplates, which can compress the length of optical fibers to a certain extent under varying pressures. By gradually rotating and compressing the optical fiber around its axis, and clamping it at a certain distance from the compression section, any output polarization state can be obtained. In fact, the same performance as the Babinet Soleil compensator (a type of bulk optical device containing two birefringent wedges) can be obtained, although their working principles are different. Multiple compression positions can also be used, where only the pressure, not the rotation angle, changes. Pressure changes are usually achieved using piezoelectric transducers. This device can also serve as a polarizer, where the piezoelectric is driven by different frequencies or random signals.