Editing Polarization-maintaining optical fiber

{{Short description|Single-mode optical fiber for linearly polarized light}}
{{Use American English|date=March 2021}}
{{Use mdy dates|date=March 2021}}

[[File:Panda fiber2.jpg|thumb|Image of the cross section of a polarization-maintaining optical fiber patch cord, taken with an illuminated microscopic viewer called a fiberscope. The two small, eye-like circles are the stress rods and the tiny circle between them is the core. The larger circle surrounding them is the cladding, usually 125 [[Micrometre|microns]] in diameter.]]

In [[fiber optics]], '''polarization-maintaining optical fiber''' ('''PMF''' or '''PM fiber''') is a single-mode [[optical fiber]] in which [[linear polarization|linearly polarized]] [[light]], if properly launched into the fiber, maintains a linear polarization during [[Wave propagation|propagation]], exiting the fiber in a specific linear polarization state; there is little or no cross-coupling of optical [[Power (physics)|power]] between the two polarization [[Normal mode|mode]]s. Such fiber is used in special applications where preserving polarization is essential.

==Polarization crosstalk==
In an ordinary (non-polarization-maintaining) fiber, two polarization modes (say vertical and horizontal polarization) have the same nominal [[phase velocity]] due to the fiber's circular symmetry. However tiny amounts of random [[birefringence]] in such a fiber, or bending in the fiber, will cause a tiny amount of crosstalk from the vertical to the horizontal polarization mode. And since even a short portion of fiber, over which a tiny coupling coefficient may apply, is many thousands of wavelengths long, even that small coupling between the two polarization modes, applied coherently, can lead to a large power transfer to the horizontal mode, completely changing the wave's net state of polarization. Since that coupling coefficient was unintended and a result of arbitrary stress or bending applied to fiber, the output state of polarization will itself be random, and will vary as those stresses or bends vary; it will also vary with wavelength. 

==Principle of operation==
Polarization-maintaining fibers work by ''intentionally'' introducing a systematic linear [[birefringence]] in the fiber, so that there are two well defined polarization modes which propagate along the fiber with very distinct phase velocities. The '''beat length''' L<sub>b</sub> of such a fiber (for a particular wavelength) is the distance (typically a few millimeters) over which the wave in one mode will experience an additional delay of one wavelength compared to the other polarization mode. Thus a length L<sub>b</sub> /2 of such fiber is equivalent to a [[Waveplate#Half-wave_plate|half-wave plate]]. Now consider that there might be a random coupling between the two polarization states over a significant length of such fiber. At point 0 along the fiber, the wave in polarization mode 1 induces an amplitude into mode 2 at some phase. However at point  1/2 L<sub>b</sub>  along the fiber, the same coupling coefficient between the polarization modes induces an amplitude into mode 2 which is now 180 degrees ''out of phase'' with the wave coupled at point zero, leading to [[Interference (wave propagation)|cancellation]]. At point L<sub>b</sub> along the fiber the coupling is again in the original phase, but at 3/2 L<sub>b</sub> it is again out of phase and so on. The possibility of coherent addition of wave amplitudes through crosstalk over distances much larger than L<sub>b</sub> is thus eliminated. Most of the wave's power remains in the original polarization mode, and exits the fiber in that mode's polarization as it is oriented at the fiber end. [[Optical fiber connector]]s used for PM fibers are specially keyed so that the two polarization modes are aligned and exit in a specific orientation.

Note that a polarization-maintaining fiber does not polarize light as a [[polarizer]] does. Rather, PM fiber maintains the linear polarization of linearly polarized light provided that it is launched into the fiber aligned with one of the fiber's polarization modes. Launching linearly polarized light into the fiber at a different angle will excite both polarization modes, conducting the same wave at slightly different phase velocities. At most points along the fiber the net polarization will be an [[elliptical polarization|elliptically polarized]] state, with a return to the original polarization state after an integer number of beat lengths. Consequently, if visible laser light is launched into the fiber exciting both polarization modes, scattering of propagating light viewed from the side, is observed with a light and dark pattern periodic over each beat length, since scattering is preferentially perpendicular to the polarization direction.

==Designs==
[[Image:PM optical fibres.svg|thumb|300px|right|Cross sections of three types of PM fiber.]]
Several different designs are used to create birefringence in a fiber.<ref>{{cite journal |title=PANDA-style fibers move beyond telecom |journal=Laser Focus World |date=August 2004 |first=Adrian |last=Carter |author2=Samson, Bryce}}</ref> The fiber may be geometrically asymmetric or have a refractive index profile which is asymmetric such as the design using an elliptical [[Cladding (fiber optics)|cladding]] as shown in the diagram. Alternatively, [[Stress (physics)|stress]] permanently induced in the fiber will produce [[Birefringence#Stress induced birefringence|stress birefringence]]; this may be accomplished using rods of another material included within the cladding. Several different shapes of rod are used, and the resulting fiber is sold under brand names such as "PANDA" and "Bow-tie". ("PANDA" refers to the resemblance of the fiber's cross-section to the face of a [[panda]], and is also an acronym for "Polarization-maintaining AND Absorption-reducing".) 

It is possible to create a [[Optical rotation#Theory|circularly birefringent]] optical fiber just using an ordinary (circularly symmetric) single-mode fiber and twisting it, thus creating internal torsional stress. That causes the phase velocity of right and left hand circular polarizations to significantly differ. Thus the two circular polarizations propagate with little crosstalk in between them

==Applications==
Polarization-maintaining optical fibers are used in special applications, such as in [[Fiber optic sensor|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 [[Absorption (optics)|attenuation]] than [[single-mode fiber]]. Another important application is [[fiber-optic gyroscope]]s, which are widely used in the aerospace industry. 

The output of a PM fiber is typically characterized by its [[polarization extinction ratio]] (PER)—the ratio of correctly to incorrectly polarized light, expressed in [[decibel]]s. The quality of PM [[patchcord]]s and [[Fiber pigtail|pigtail]]s can be characterized with a [[PER meter]]. Good PM fibers have extinction ratios in excess of 20 dB.

==References==
<references/>
*{{FS1037C}}
*MIL-STD-2196

==External links==
*[http://www.fujikura.co.jp/eng/resource/pdf/16pnb04.pdf Fujikura's PANDA Fiber] Specs for the most common type of PM fiber
*[https://lunainc.com/sites/default/files/assets/files/resource-library/Polarization-Crosstalk-in-PM-Fiber.pdf Polarization Crosstalk in PM Fiber]

[[Category:Optical fiber]]