Fiber Optics: Beyond Simple Light Pipes

November 3, 2010
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Optical fibers are amazingly versatile optical components, used for everything from simple, compact go/no-go sensing to imaging to digital communication to precision gaging. They continue to be used to solve difficult problems from industrial automation to information technology to molecular diagnostics.

Basically, optical fibers efficiently transmit light. They are usually dielectric-coated, cylindrical transparent tubes. Near 100% transmission efficiency is obtained by the use of total internal reflection, but this efficiency can be significantly reduced by impurities within the fibers.

Fiber optic structures range from simple use of individual fibers (or fiber bundles) to more complex geometries with variations both perpendicular to and parallel to the fiber axis.

Different applications have been developed by selecting appropriate physical distribution of fibers in fiber bundles, modifying the fiber and coating materials, using internal and external structures including suitable fixtures at both ends. Fiber units can be rigid or flexible, with small or larger diameters, sometimes tapered. In addition, cabled fiber bundles can contain other functional components such as small knives for surgery on internal organs.

More specialized versions keep emerging: as spectral probes for relatively inaccessible sensing (as for molecular processes in the brain), for detecting very small changes along the fiber bundles to determine the 3-D position of surgical knives for remote, battle-field robotic surgery as well as to detect changes in the structural integrity of structures such as buildings, dams and airplanes.

A large application area has been long electro-magnetic-interference insensitive communication lines. And even here, a relatively small but highly significant change-using multiple wavelengths in wavelength division multiplexing (WDM)-changed the economics of fiber optical telecommunications by increasing the band-width of existing fiber networks many times over.



Illumination and Sensing

A simple and adaptable arrangement for illumination and sensing is the use of two separate fibers or bundles, one for each function. This can provide great flexibility, both literally and figuratively. For example, the fibers in the distal (or distant) end of a fiber optic bundle can be shaped into a line or hollow circle or other geometry suitable to the task.

To place a fiber bundle into a recessed hole, the fibers used for illumination and sensing near the light source and sensors can be put into one steel or plastic sheath to form a Y.

Thus, the long leg of the Y is a single fiber bundle containing a mixture of illuminating and sensing fibers.



Imaging

Imaging fiber optics are used in medicine, manufacturing , neuroscience research and elsewhere to view internal and/or hard-to-access features.

The imaging is simple: use a fiber optic bundle with the fibers in a fixed geometry at the operator’s end. Then, maintain this fixed geometry at the distal end, the working end. Whatever image is formed at the distal end will be transmitted through inches (or feet) of fiber bundle to the operator.

Fiber optic imaging provides both structural simplicity and flexibility.



Telecommunications and Digital Signals

Fiber optics are increasingly used to transmit digital optical pulses (or signals) for telecommunications. Two major advances in use in these areas have been (1) the significant reduction of absorption along the fiber that permits signals to be transmitted distance over 100 miles between repeaters (2) use of individual wavelengths to carry digital signals or WDM.

[Note: Charles Kao won the Nobel Prize in Physics (2009) for discovering that impurities in fiber material were the primary cause of limited transmission distances.]



Fiber Optic Sensors

Fiber optic sensors can detect and measure a large variety of physical properties: different interactions with the fibers provide different measurement capabilities.

Sensing can be done by the changing a light characteristic inside the fiber. In one type of internal fiber optic sensor, embedded diffraction (Bragg) components are established inside the fibers, perpendicular to the fiber axis. They provide very sensitive changes in grating-spacing as fibers are bent, altering wavelengths transmitted. This can be used to measure strain and distortion.

On the other hand, external sensing is done by causing a direct change to a non-fiber optical sensor or an electronic sensor. The sensor then transmits the change to an optical transmitter in the fiber-optic circuit.

There are very many different types of internal and external fiber optic sensors.



Summary

Fiber optics has been successfully applied to solve problems and simplify hardware in a great many different applications.

One of the very many different types of fiber optics can be applied, or modified, to solve or simplify sensing, illumination, data communication, IT, imaging, gaging and other problems.



Key Words

Bifurcated fiber optics, borescopes, endoscopy, fiber optics, fiber optic imaging, fiber optic sensing, fiber optic sensors, fiber optic sensors chemical, fiber optic telecommunications, Intuitive Surgical, Lenox Boroscope, Luna Innovations, medical fiber optics, optogenetics, robotic assisted surgery, robotic catheter system, structural fiber optic sensors, total internal reflection.

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