Fiber Coils

Introduction to Fiber Coils

Fiber coils are specialized forms of optical fiber that are wound up to form a coil. While glass fibers are typically delivered on spools with many turns, special fiber coils are required for some applications where a well-defined winding pattern and other specifications are necessary. Fiber coils are used in various applications, including fiber-optic gyroscopes, fiber-optic sensors, fiber-optic delay lines, and fiber polarization controllers. Let's look into wht the key applications of Fiber Coils are.

What are the key uses of Fiber Coils?

Fiber-optic gyroscopes are one of the main applications of fiber coils. In this application, the Sagnac effect is exploited in a Sagnac interferometer or a fiber ring laser, and the fiber coil is used as a fiber gyro coil. Technical specifications are critical in this application, as properties such as Shupe error can directly impact the accuracy of the made gyroscopes.

Fiber-optic sensors: There are also fiber-optic sensors using specialized fiber coils. For example, fiber-optic current sensors 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.

Fiber-Optic Delay Lines: In other cases, fiber coils are used as fiber-optic delay lines. For example, devices for self-heterodyne linewidth measurements use fiber coils. While most requirements for this application are less critical than for use in interferometers or sensors, the fiber coils still require defined specifications.

Fiber Polarization Control: Fiber coils with only a few windings are used in fiber polarization controllers. In this application, one exploits the birefringence which is introduced by the bending.

Fiber Disk Lasers: Another type of fiber coil, made of rare-earth doped fiber, is used for a not very common type of fiber lasers called side-pumped fiber disk lasers.

What to look for when selecting a Fiber Coil

Various specifications can be relevant for fiber coils. The geometrical dimensions of the coil are defined, in particular the inner and outer radius and the coil height. Usually, there is 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. Coils are made with a specified winding pattern, such as quadrupole, octupole, helical, or flanged. Sophisticated high precision coil winding machines are used for producing such coils.

Fiber coils may be produced by winding the fiber on some frame made of aluminum or plastic or without such a part (frameless, freestanding) when the coil is mechanically stabilized with some adhesive. Different types of fiber coatings can be used, which may have an influence on the possible number of turns and the mechanical stability of the bindings.

Polarization-related properties are particularly critical for application in interferometers. The sensitivity to thermal influences can also be important. 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 for application in interferometers. Both the average value and the maximum value of the polarization cross-talk can be relevant. Fibers with high quality exhibit low polarization cross-talk and a low temperature sensitivity of that quantity.

Temperature changes may introduce mechanical stress through thermal expansion, which may modify birefringence properties of the fiber. The effective mode area is also relevant, concerning coupling to other fibers or nonlinear optical effects. 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 by using fiber with a sufficiently high numerical aperture. Most relevant is the resulting total insertion loss in decibels.