Editing Polarization-maintaining optical fiber (section)

==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.