What are polarization maintaining fibers?
In the most common an optical fiber in which the polarization of linearly polarized light waves launched into the fiber is maintained during propagation, with little or no cross-coupling of optical power between the polarization modes. Such fiber is used in special applications where preserving polarization is essential.
What is polarization maintaining(PM) fibers ?
Polarization Maintaining (PM) optical fiber is a key component of Fiber Optic Gyroscopes, devices that measure rotation in missiles, aircraft, ships and satellites. They are a type of interferometric sensor in which the phase difference between two light paths is measured.The polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature. Specialised fibers are required to achieve optical performances, which are affected by the polarization of the light travelling through the fiber.optical fibers always exhibit some degree of birefringence, even if they have a circularly symmetric design, because in practice there is always some amount of mechanical stress or other effect which breaks the symmetry. As a consequence, the polarization of light propagating in the fiber gradually changes in an uncontrolled (and wavelength-dependent) way, which also depends on any bending of the fiber and on its temperature.
Principle of polarization maintaining(PM) fibers
The mentioned problem can be fixed by using a polarization-maintaining fiber, which is not a fiber without birefringence, but on the contrary a specialty fiber with a strong built-in birefringence (high-birefringence fiber or HIBI fiber, PM fiber). In general, optical fiber telecommunications applications, PM fiber is used to guide light in a linearly polarised state from one place to another. To achieve this result, several conditions must be met. Input light must be highly polarised to avoid launching both slow and fast axis modes, a condition in which the output polarization state is unpredictable.
Provided that the polarization of light launched into the fiber is aligned with one of the birefringent axes, this polarization state will be preserved even if the fiber is bent. The physical principle behind this can be understood in terms of coherent mode coupling. The propagation constants of the two polarization modes are different due to the strong birefringence, so that the relative phase of such copropagating modes rapidly drifts away. Therefore, any disturbance along the fiber can effectively couple both modes only if it has a significant spatial Fourier component with a wavenumber which matches the difference of the propagation constants of the two polarization modes. If this difference is large enough, the usual disturbances in the fiber are too slowly varying to do effective mode coupling.In addition, connectors must have been installed on the PM fibers in such a way that internal stresses do not cause the electric field to be projected onto the unintended axis of the fiber.
PM optical fibers are used in special applications, such as in fiber optic sensing, interferometry and quantum key distribution. They are also commonly used in telecommunications for the connection between a source laser and a modulator, since the modulator requires polarized light as input. They are rarely used for long-distance transmission, because PM fiber is expensive and has higher attenuation than singlemode fiber.Optical fibers may be applied in measurements of electrical current, particularly as so-called optical current transformers. Electric current sensors, in which optical fibers are used are small, light, cheap and safe. Their sensitivity is, however, due to the restricted magnetootpic properties of optical fibers, rather small. Moreover, these sensors are susceptible to deformations of the optical fiber. An increase of their sensitivity consists in lengthening the distance of optical fiber on which the magnetic field acts.PM fibers are applied in devices where the polarization state cannot be allowed to drift, e.g. as a result of temperature changes. Examples are fiber interferometers and certain fiber lasers. A disadvantage of using such fibers is that usually an exact alignment of the polarization direction is required, which makes production more cumbersome. Also, propagation losses are higher than for standard fiber, and not all kinds of fibers are easily obtained in polarization-preserving form.
Fiber-MART offer polarization components can be utilized in high power optical amplifiers and optical transmission system, test and measurement. the launch conditions at the optical fiber end face must be consistent with the direction of the transverse major axis of the fiber cross section. Fiber-Mart Polarizing Beam Combiner/Splitter (PBC/PBS) is a compact high performance light wave component that combines two orthogonal polarization signals into one output fiber, and also can split the incoming light into two orthogonal states. We also supply the Isolator type (IPBC/IPBS) which provides both polarization beam combining and optical isolation in one integrated component.for more information,you can visit www.fiber-mart.com.pls feel free to contact with us for any question . E-mail: [email protected]
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