Source: Thorlabs

Understanding Fiber Polarization Controllers

Fiber-optic devices, such as interferometers, often require an adjustable state of polarization of light within a fiber. This is where fiber polarization controllers come into play. These devices are crucial for managing the polarization state of light, which can be affected by various factors including fiber bending, temperature changes, and mechanical stress.

Types of Fiber Polarization Controllers

Bat Ear Controllers with Bent Fibers

One of the most widely used types of fiber polarization controllers is the “bat ear” controller. This type utilizes the birefringence induced by bending or coiling a fiber. Birefringence refers to the difference in the refractive indices in different directions within the fiber. The total retardation, or the amount of birefringence, is proportional to the length of the fiber and inversely proportional to its bending radius. The fiber type also plays a role in this effect.

Typically, a bat ear controller consists of three fiber coils arranged in sequence. The middle coil acts as a half waveplate, while the outer coils function as quarter waveplates. Each coil can be rotated around the axis in line with the input and output fibers. By adjusting the orientation of all three coils, the input state of polarization can be transformed into any desired output state. However, the effect on polarization can vary with wavelength, and high peak powers, such as those found in ultrashort pulses, may result in nonlinear polarization rotations.

The diameters of the fiber coils should be carefully selected to avoid excessive bend losses. A more compact variant of this controller uses strongly birefringent or polarization-maintaining fibers, which are less susceptible to nonlinear effects.

Polarization Controllers with Squeezed Fibers

Another type of polarization controller involves squeezing a section of the fiber with variable pressure. This method effectively creates a variable waveplate. By rotating the squeezed section around its axis, while keeping the fiber clamped at a distance, any output state of polarization can be achieved. This method is similar in effect to a Babinet–Soleil compensator, a bulk-optical device with birefringent wedges, although the operational principles differ.

Multiple squeezing sections can be used in series, with pressure variations rather than rotation angles. These devices can also include piezo-electric transducers, allowing them to function as polarization scramblers when driven with varying frequencies or random signals.

Conclusion

Fiber polarization controllers are essential tools in the field of fiber optics, enabling precise control over the state of polarization in various applications. Understanding the different types and their operational principles is crucial for optimizing the performance of fiber-optic systems. As technology advances, the development of more compact and efficient polarization controllers continues to evolve, promising even greater capabilities in the future.

Source: Thorlabs
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