The Physics of Fiber optics

At this point, it is important to understand what fiber optics are and how they transmit light and images. Individual optical fibers are made of pure glass and can be as thin as a human hair. The diameter of the glass fibers is what gives them their ability to flex and bend. These individual optical fibers are bundled into what are called optic cables and can be of varied lengths. The bundles are used to transmit light signals over long distances. These fiber-optic bundles make up the bulk of the insertion tube of a VE.

Figure 4.9 depicts the structure of a single optical fiber. At the center of each fiber is the core, which is the thin pure glass through which the light travels. The next layer is the cladding, which is the outer reflective optical material surrounding the core. The reflective material reflects light back into the core. Finally, there is a buffer coating that serves to protect the cladding and the core from damage and moisture (Freudenrich 2001).

The bundles can be made up of hundreds or thousands of individual optical fibers. The bundles are protected by the insertion tubes' outer "jacket." In the case of medical endoscopes, these jackets are made from a tissue-neutral material, whereas industrial endoscope jackets are typically made from a fine stainless steel mesh, making them durable and rugged.

There are two types of optical fibers, including single-mode fibers and multimode fibers. Single-mode fibers have small-diameter cores of about 9 ^m and transmit infrared laser light (1300-1500 nm). In contrast, multimode fibers have larger core diameters of

Reflective Cladding (mirrored inner surface)

Reflective Cladding (mirrored inner surface)

A single optic fiber

A single optic fiber

Buffer coating

Figure 4.9 structure of a single optical fiber.

about 62.5 ^m. These fibers transmit infrared light (850-1300 nm) from light-emitting diodes (Freudenrich 2001).

Additionally, there are optical fibers that are made from plastic, typically having a large core diameter (1 mm), and transmit visible red light (650 nm).

In order to understand how an optical fiber works, consider the behavior of light. Light travels in a straight line. If you shine a flashlight straight into a cave, the light travels straight ahead. If the cave bends or twists, in order to illuminate that part of the cave with your flashlight, you may place a mirror at a specific angle, thus reflecting the light around that bend. If there are multiple bends, you could place many mirrors along the caves' contours and "bend" the light side to side along the cave walls. Optical fibers work in much the same way.

Essentially, the light in an optical fiber travels through the glass core. When the light meets a bend, it is reflected back into the core by constantly bouncing from the cladding or mirror-lined walls surrounding the core (Freudenrich 2001; see also Figure 4.10). This principle is called total internal reflection.

Different substances have different indices of refraction, that is, the manner in which the direction of light is altered. In the physics of refraction, the amount of light refracted is dependent upon the difference between the indices of refraction of the materials and what

Endoscopic light source

Cladding (Mirror-lined walls, moves light as inner surface bends)

Cladding (Mirror-lined walls, moves light as inner surface bends)

Emitted light from distal end of scope illuminating the field of view t

Emitted light from distal end of scope illuminating the field of view

Figure 4.10 Diagram showing how light traveling through the fiber core is reflected back into the core when a curve is encountered in the light path.

Critical Angle Internal Reflection

Cladding on the inner surface

Glass c

Critical Angle Internal Reflection

Glass c

Long axis

Critical angle Normal axis

Figure 4.11 Diagram of the critical angle associated with fiber optics.

Long axis

Critical angle Normal axis

Figure 4.11 Diagram of the critical angle associated with fiber optics.

is called the critical angle (Freudenrich 2001). In physics, the critical angle is that which is perpendicular to the surface. However, in fiber optics, the critical angle is described with respect to the parallel axis running down the middle of the core. Thus, the fiber-optic critical angle is 90° minus the physics critical angle (Figure 4.11).

In an optical fiber, the light travels through the core (high index of refraction) by constantly reflecting from the cladding (lower index of refraction) because the angle of the light is always greater than the critical angle. Light reflects from the cladding no matter what the angle of the fiber itself is, even if it runs in a full circle.

The cladding does not absorb any light from the core; therefore, the light wave can travel a considerable distance. Some of the light signal does degrade within the fiber. This is mostly due to impurities in the glass. The extent that the signal degrades depends on the purity of the glass and the wavelength of the transmitted light. For example, at 850 nm, there is a 60 to 75%/km degradation. At 1300 nm, there is a 50 to 60%/km and, at 1550 nm, there is a greater than 50%/km degradation. The higher the wavelength, the less the degradation of the light signal. Some premium optical fibers with an extremely pure glass core show much less signal degradation and can be less than 10%/km at 1550 nm (Freudenrich 2001).

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