Applications of Fiber Bundles

This article introduces the application features and advantages of fiber optic bundles in endoscopy, image transmission, and light transmission.

Medical Endoscopes

The most important use of imaging fiber bundles is to allow physicians to look inside the body without surgery. This is done with special-purpose coherent fiber bundles called endoscopes, which are up to a couple of meters long. Versions called gastroscopes are threaded down the throat to examine the stomach. Colonoscopes are versions designed to examine the colon. Short rigid bundles are used for some medical examinations because of their high resolution, but flexible types are preferred for most purposes because they are easier to insert and manipulate through body orifices.
Traditional fiber-optic endoscopes use one set of fibers to transmit light inside the body and a separate set to collect and view the reflected light. Lenses on the end of the instrument focus light onto the fiber bundle, so it does not have to be pressed against tissue.
Endoscopes may include surgical tools to treat lesions in the stomach or colon. Some newer endoscopes use fibers to transmit light into the body, but collect light with a miniature CCD (charge-coupled device) imaging camera that is inserted into the body.
Some endoscopes include fibers capable of transmitting high laser powers as well as illuminating light, so physicians can perform laser surgery. For example, a surgeon performing microsurgery on the knee could make an incision to insert an endoscope. After viewing the area to be treated, the surgeon could look through the viewing fibers to align the instrument, look away, then fire laser pulses to treat the lesion. (Surgeons avoid watching during laser pulses to protect their eyes.) After each set of laser pulses, the surgeon looks back to check progress.

Fiber Optic Plate

Image transmission does not have to be over a long distance. Another common application of fiber-optic image transmission is the fiber-optic faceplate, a thin slice of a coherent bundle in which individual fibers are only a fraction of an inch long. Faceplates are cut from longer, fused coherent bundles like slices of salami, although generally one or both surfaces are not flat.
The job of a faceplate is to transmit an image between two stages of an imaging system that must amplify weak input light to generate a clearly visible image. It is typically used in military systems where faint light is amplified so soldiers can see an image of the scene.
Image amplifiers may go through multiple stages, each amplifying the input light by a certain factor. Infrared light is used to generate a visible image. The output stages often are strongly curved screens that can’t be focused onto flat input devices without distortion. A
faceplate can convert the curved output screen to a flat surface, as shown in Figure.
If the input of the next stage works best with a curved screen, the other side of the faceplate can be curved to match.

The big advantage of the faceplate is that it transfers light very efficiently between two surfaces that otherwise can’t be butted face to face. Suppose, for example, you’re trying to detect some very weak light from a scene illuminated only by starlight. A single-stage
image-intensifier camera makes the image brighter, but not bright enough to see clearly.
You want to add a second stage, but the output of the first stage is on a curved screen. Put a fiber faceplate between that output and the input of the second-stage tube and you lose very little light. An imaging lens would lose much more light. The first fiber-optic faceplates were developed for such military imaging tubes, and they remain in use for newer equipment.
Faceplates also can help flatten the curved image generated by some display screens, correct for distortion, and make the display appear brighter by concentrating light toward the viewer.

Image Manipulation, Splitting, and Combining

Coherent fiber bundles can do more than just transmit images; they can also manipulate them.
Twisting a coherent bundle by 180° inverts the image. You can do the same with lenses, but a fiber-optic image inverter does not require as long a working distance, which is of critical importance in some military systems. (Some image inverters are less than 1 in. long.)
Another type of image manipulation possible with fused fiber optics is the image combiner and splitter shown in Figure 30.8. This is made by laying down a series of fiber-optic ribbons, alternating them as if shuffling a deck of cards. One ribbon goes from the single input to output 1, the next from the input to output 2, the next to output 1, and so on.
Put a single image into the input, and you get two identical (but fainter) output images.
Put separate images into the two outputs, and you get one combined image.
Similar ideas could be used in other image manipulators or in devices to perform operations on optical signals. However, before you rush out for a patent application on your own bright idea, you must face the ugly reality of cost. Manufacture of the fiber-optic image combiner in Figure below requires time and exacting precision, making it too expensive for most uses. Image inverters are used in some systems, but only where less costly lens systems won’t do the job.

Light Piping and Illumination

llumination and light piping are the simplest applications of optical fibers. Light piping is Light piping simply the transfer of light from one place to another by guiding it through one or more optical fibers. It doesn’t matter how the fibers are arranged, as long as they deliver the light to the desire place. Thus fibers need not be arranged in the same way at both ends of an illuminating bundle. A single fiber may suffice for many applications.
Light piping for illumination is merely the delivery of light to a desired location. Why bother with optical fibers to do a lightbulb’s job? A flexible bundle of optical fibers can efficiently concentrate light in a small area, or deliver light around corners to places it otherwise could not reach, such as inside machinery. A fiber bundle also can deliver light without the heat generated by incandescent bulbs, and without bringing electric current near the illuminated spot. This can be important in locations where bulbs and current can’t be used because of explosive vapors or heat-sensitive materials. Fiber bundles also can be divided to deliver light from one bulb to many separate places. Light-piping fibers also can serve as indicators, to verify that an important bulb is operating.

Another important application of light piping is optical pow er delivery, transmitting laser beams for medical treatment or industrial material working. Conventional laser systems use lenses or mirrors to focus beams onto the desired spots. These systems use large optics, making them bulky, which is cumbersome for fine tasks such as delicate surgery. Optical fiber beam delivery systems are much easier to manipulate. Some are designed for surgeons to use with their hands; others are built for robotic control in factories.

Single large-core step-index fibers are best for many power delivery applications as long as the input light can be concentrated into a single core. Low-loss, large-core silica fibers can deliver fibers have surprisingly high power transmission capabilities, and can easily carry tens of watts over a pew meters Illuminating bundles transmitting lower powers can use smaller-core, step-index fibers of glass or plastic, as long as light intensities and heat levels are low.

If all fibers in an illuminating bundle go to the same place, they illuminate a single area.
If they are directed to different places, they can form a patterned image, such as the fiberoptic sign shown in Figure below. All the fibers collect input light from one bulb, then are splayed out to show the desired pattern. Diffusing lenses at the fiber ends can spread light to make large, easily visible spots. (The WALK sign makes a good example, but it’s not widely used.)