For some applications (e.g. fiber-optic gyroscopes), some (often substantial) length of optical fiber needs to be wound up to form a coil. Although glass fibers are usually delivered in a similar form, wound up with many turns on some spool, that would often not be suitable for an application; special fiber coils are offered with well defined winding patterns and other specifications.
Applications of Fiber Coils
A typical application is in the fiber-optic gyroscopes, where the Sagnac effect is exploited in a Sagnac interferometer or in a fiber ring laser. Such coils are called fiber gyro coils. Here, various technical specifications are particularly important, because the addressed properties can directly influence the accuracy of the made gyroscopes, e.g. in terms of Shupe error.
There are also fiber-optic sensors using specialized fiber coils. For example, there are fiber-optic current sensors, which are suitable for measuring strong electrical DC currents based on the Faraday effect in the fiber: the magnetic field generated by the electric current causes a difference in propagation constants for light with left or right circular polarization..
In other cases, fiber coils are used as fiber-optic delay lines. An example is in devices for self-heterodyne linewidth measurements. Here, most requirements are substantially less critical than for application in interferometers or sensors.
Various kinds of specifications can be relevant for fiber coils:
- The geometrical dimensions are defined, in particular the inner and outer radius and the coil height.
- Usually, one has a well defined length of fiber (often several kilometers), and possibly also the precise number of turns per layer and the number of winding layers.
- Often, coils are made with a specified winding pattern, e.g. quadrupole, octupole, helical or flanged. (Customized winding patterns are also offered by some suppliers.) The quadrupole winding pattern, for example, or some slightly modified form, is common for application in fiber-optic gyroscopes, because it minimizes the effects of thermal gradients and vibrations. Sophisticated high precision coil winding machines are used for producing such coils; particularly for application in gyroscopes, the quality of the winding pattern and related properties such as regularity and winding tension can be of essential importance.
- A coil may be produced by winding the fiber on some frame made of aluminum or plastic, for example, or without such a part (frameless, freestanding) when the coil is mechanically stabilized with some adhesive.
Beyond that, various specifications for the used type of fiber can be relevant:
- Usually, it is a single-mode fiber which is suitable for a certain range of operation wavelengths. Particularly low propagation losses are achieved in the 1.5-μm wavelength region, but fiber coils are also available for other wavelengths such as 1.3 μm, 1–1.1 μm or 840 nm.
- One often uses polarization-maintaining fiber (PM fiber) and specifies a polarization extinction ratio. In addition, there can be specifications concerning polarization cross-talk, which can be important e.g. for application in interferometers. Both the average value and the maximum value of the polarization cross-talk can be relevant. Fibers with high quality (e.g. low fluctuations of core dimensions) exhibit low polarization cross-talk and a low temperature sensitivity of that quantity. Note that temperature changes may introduce mechanical stress through thermal expansion, and that may modify birefringence properties of the fiber.
- The effective mode area is also relevant, e.g. concerning coupling to other fibers or concerning nonlinear optical effects.
- Different fiber coatings can be used, which may have an influence on the possible number of turns, the mechanical stability of the bindings, etc.
- The bending of the fiber which is unavoidable for such a coil will introduce some bend losses for the transmitted light, which is usually kept at a rather low level, e.g. by using fiber with a sufficiently high numerical aperture. Most relevant is the resulting total insertion loss in decibels.
Fiber coils for critical applications such as gyroscopes have to undergo sophisticated testing procedures to ensure their performance and robustness.
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