fiber polarization controllers (original) (raw)

Definition: devices which allow one to control the state of polarization of light within fibers

Categories: article belongs to category fiber optics and waveguides fiber optics and waveguides, article belongs to category photonic devices photonic devices

Related: fiberspolarization-maintaining fiberspolarization of lightbirefringencewaveplatesnonlinear polarization rotation

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DOI: 10.61835/4xc Cite the article: BibTex BibLaTex plain textHTML Link to this page! LinkedIn

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Contents

What is a Fiber Polarization Controller?

Various fiber-optic devices such as interferometers require an adjustable state of polarization of light in a fiber. For that purpose, different types of fiber polarization controllers have been developed.

Bat Ear Controllers with Bent Fibers

A popular type of fiber polarization controller exploits the birefringence which is induced by bending (coiling) a fiber. The total retardation (amount of birefringence) is proportional to the length of fiber and inversely proportional to its bending radius. It also depends on the fiber type. Some simple holders allow to arrange for some number of fiber turns with a given bending diameter, so that approximately a ($\lambda / 2$) or ($\lambda / 4$) retardation is achieved, for example.

fiber polarization controller

Figure 1: A “bat ear” polarization controller, containing three fiber coils which can be rotated around the input fiber's axis.

Frequently, one uses three such fiber coils (“bat ears”) in a sequence, where the middle coil approximately acts as a half waveplate and the outer ones as quarter waveplates. Each coil can be rotated around an axis in line with the input and output fibers. By adjusting the orientation of all three coils, one can transform a given input state polarization at a fixed wavelength into any output state of polarization. In some cases, one uses only two coils, which is in principle sufficient, but may not be fully sufficient if the retardations are not accurate.

Note, however, that the effect on the polarization has some wavelength and temperature dependence. For high peak powers, as often occur with ultrashort pulses, there can also be a nonlinear polarization rotation.

The diameters of the fiber coils should not be too small, as otherwise the bending would cause excessive bend losses.

Polarization Controllers with Squeezed Fibers

One can realize an effective variable waveplate with a device which allows one to squeeze some length of fiber with a variable amount of pressure. By rotating the squeezed bit of fiber around its axis while keeping the fiber clamped at some distance from the squeezed part, one can obtain an arbitrary output state of polarization. In effect, one achieves the same as with a Babinet–Soleil compensator (a bulk-optical device containing two birefringent wedges), although the operation principle is different.

It is also possible to use several squeezing sections in series, where only the pressure but no rotation angle is varied. This variant may also be equipped with piezoelectric transducers. That kind of device can also serve as a polarization scrambler, when the piezos are driven with different frequencies or random signals.

Frequently Asked Questions

This FAQ section was generated with AI based on the article content and has been reviewed by the article’s author (RP).

What is a fiber polarization controller?

A fiber polarization controller is a fiber-optic device used to adjust the state of polarization of light within an optical fiber, as required for components like interferometers.

How does a 'bat ear' fiber polarization controller work?

It uses the birefringence induced by bending a fiber. Typically, three rotatable fiber coils act as quarter- and half-waveplates. Adjusting their orientation transforms a given input polarization into any desired output polarization.

What is a squeezed-fiber polarization controller?

This controller applies variable pressure to a section of fiber to induce adjustable birefringence. By controlling the pressure and the orientation of the squeezed section, it functions as a variable waveplate to achieve arbitrary polarization control.

What are the limitations of fiber coil polarization controllers?

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