Fiber Polarization Controllers
Definition: devices which allow one to control the state of polarization of light within fibers
More general term: polarization controllers
German: Faser-Polarisationseinsteller, Polarisationsregler
Categories: fiber optics and waveguides, photonic devices
Author: Dr. Rüdiger Paschotta
Cite the article using its DOI: https://doi.org/10.61835/4xc
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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.
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 maybe not fully sufficient if the retardations are not accurate.
Note, however, that the effect on the polarization has some wavelength dependence. For high peak powers, as often occur with ultrashort pulses, there can also be nonlinear polarization rotations.
The diameters of the fiber coils should not be too small, as otherwise the bending would cause excessive bend losses.
A more compact variant, which is also less sensitive to nonlinear effects, uses strongly birefringent (polarization-maintaining) fibers instead of fiber coils.
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 in 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 piezo-electric transducers. That kind of device can also serve as a polarization scrambler, when the piezos are driven with different frequencies or random signals.
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See also: fibers, polarization-maintaining fibers, polarization of light, birefringence, waveplates, nonlinear polarization rotation
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2023-01-26
Could you provide some insight on why many “bat ear” polarization controllers contain two quarter wave plates and one half wave plate, instead of just one QWP and one HWP? If I am not mistaken, the latter should be enough to reach every possible point on the Poincaré sphere.
The author's answer:
I think you are right with that statement; you should need only two handles to control a point on a 2D surface. For example, two QWP should in principle also do.
On 2023-07-13, another user kindly contributed the following explanation:
Fiber polarization controllers usually have three “paddles” which are QWP, HWP, QWP because they are not exactly quarter- and half-wave plates. The extra paddle provides enough freedom of adjustment to reach any point on the Poincare sphere, whereas two not-quite-right wave plates would not. Usually, one paddle has one turn of fiber and acts like an almost-quarter-wave plate, the next paddle has two turns and acts like an almost-half-wave plate, and the third paddle has three turns and acts like an almost-three-quarters-wave plate.