When light is incident from a material of higher index of refraction to a material of lower index of refraction, the light is refracted away from the normal to the interface between the two substances. At some angle of incidence, the refracted ray will be 90o to the normal. For all incident angles beyond this "critical" angle, the light is completely reflected at the interface. This is called total internal reflection (TIR). You can get a visual interactive experience with a Java Applet showing the reflection of light from a source below a pond.
The phenomena of total internal reflection has been known for centuries. Its use had been mostly limited to providing an efficient method of reflecting light in optical instuments such as binoculars. In more recent times, TIR in optical fibers has made enormous contributions to such fields as medicine, communications, star imaging and even entertainment. We'll concentrate on the first two of these fields.
Medicine
The phenomnea of TIR is exploited in the areas of fiber sensors, laser surgery, laproscopy, endoscopy, and arthroscopy. The fiber sensor uses light reflection from a single fiber to determine blood/tissue pH, oxygen content, and blood flow velocity from the Doppler effect. The kidney stone to the left is being blasted by a high powered laser pulse directed by an optical fiber. (Thanks to the University of Melobourne for the picture.)
The last three technologies (concentrating on different areas of the body) all use very thin fiber optic "hairs" bundled into a small cable that can be inserted through a small incision. The cable can be use to illuminate the area of interest and provide the surgeon with a high resolution image of the area. The surgeon can perform tasks such as removing cysts, trimming meniscal tears, and reconstructing torn ligaments without invasive surgery. Operations which, in the past, required weeks of recovery are now routinely "day operations". The Southern California Orthopedic Institute has a nice tutorial on arthoscopic surgery. It also has links to diagrams for various body parts, including some Quicktime movies of actual surgery.
Communications
Communication technology has pushed the science of fiber optics to the limit. Digital data transmitted over any medium is basically a series of 0's and 1's. When transmitted along a fiber optic cable, this takes the form of light pulses. Over simplifying: a light pulse is a "one', no light pulse is a "zero".
The factor that limits the rate at which data can be transmitted along a fiber is the pulse width and spacing. As the pulses travel down the fiber it widens. If the pulses widens too much, they will begin to overlap and the receiver will unable to interpret the original data. The picture to the right shows the shape of the pulses (perhaps representing 1101) initially and again after they have traveled far down the cable.
One of the factors which create this problem we have discussed in class...chromatic dispersion. If the pulse is composed of white light, then the pulse will widen because the propagation speed down the fiber is different for the different colors. The second cause of widening is called modal dispersion. To understand this phenomena, you will need to understand the effect of the cladding that surrounds the central optic fiber and explore the mechanics of TIR in optical fibers in more detail. A good start is ARC Electronics fiber optics tutorial . Dispersion can be defeated with repeaters and shapers and by improving the the actual construction of the fiber. Bell College has a
fibre optic theory site with great info on fiber construction.
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Questions:
What is the typical diameter of the surgical instruments used in arthroscopic surgery?
What are the incisions called through which the fiber optic cables and instruments are inserted?
What is the typical size of a single communications optic fiber?
How many data bits per second can be carried along an optic fiber?
When light is incident from a material of higher index of refraction to a material of lower index of refraction, the light is refracted away from the normal to the interface between the two substances. At some angle of incidence, the refracted ray will be 90o to the normal. For all incident angles beyond this "critical" angle, the light is completely reflected at the interface. This is called total internal reflection (TIR). You can get a visual interactive experience with a Java Applet showing the reflection of light from a source below a pond.
The phenomnea of TIR is exploited in the areas of fiber sensors, laser surgery, laproscopy, endoscopy, and arthroscopy. The fiber sensor uses light reflection from a single fiber to determine blood/tissue pH, oxygen content, and blood flow velocity from the Doppler effect. The kidney stone to the left is being blasted by a high powered laser pulse directed by an optical fiber. (Thanks to the University of Melobourne for the picture.) 
The factor that limits the rate at which data can be transmitted along a fiber is the pulse width and spacing. As the pulses travel down the fiber it widens. If the pulses widens too much, they will begin to overlap and the receiver will unable to interpret the original data. The picture to the right shows the shape of the pulses (perhaps representing 1101) initially and again after they have traveled far down the cable.