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Encyclopedia of Physical Science and Technology EN011L-523 August 10, 2001 11:17

316 Optical Fiber Techniques for Medical Applications

Numerical aperture Light-gathering power of an optical phenomenon was probably known by ancient glass blow-
fiber. It is proportional to the sine of the acceptance ers, but the earliest demonstration of the effect was
angle. given by J. Tyndall in England in 1870. Thin rods
Optical fiber Thin thread of transparent material thro- that are used for light transmission are called optical
ugh which light can be transmitted by total internal fibers. Such fibers appear in nature. For example, tis-
reflection. sues of plant seedlings can guide light to coordinate their
Ordered bundle Assembly of optical fibers in which the physiology.
fibers are ordered in exactly the same way at both ends An ordered bundle of glass or plastic fibers may be used
of the bundle. for the transmission of images. Some of the earliest pa-
Power fiber Optical fiber that can transmit a laser beam pers and patents describing optical fibers, in the period
of high intensity. 1925–1930, involved the use of such bundles in medicine.
Rigid endoscope Sometime called “rigid telescope.” Only in the early 1950s, with the development of optical
Imaging in this device is carried out by a series of lenses fibers that consisted of inner core and outer cladding, did
that are placed inside a rigid tube. the flexible fiberoptic bundle become a powerful practical
Total internal reflection Reflection of light at the in- tool. At first the medical instruments, endoscopes, incor-
terface between media of different refractive indices, porating fiberoptic bundles, enabled physicians to obtain
when the angle of incidence is larger than a critical images from organs inside the body.
angle. During the 1970s and 1980s there has been rapid
Videoscope A special endoscope that contains a minia- progress in the development of glass fibers with extremely
ture electronic imaging device (e.g., charge-coupled low optical transmission losses. The main challenge in the
device) instead of an ordered imaging fiber bundle. use of these fibers is in communication systems. Light sig-
nals emitted from semiconductor lasers can be transmit-
ted through very long optical fibers and can be detected by
OPTICAL FIBERS are thin, flexible, and transparent semiconductor detectors. The laser light can be modulated
guides through which light can be transmitted from one in such a way that it may carry with it voice communica-
end to another. An ordered bundle of optical fibers forms tion, radio and television broadcasting, and computer data.
a fiberscope and can be used for transmitting images. A Thus, optical fibers will be the major building blocks in
medical endoscope incorporates such a fiberscope. Thin, future communication networks.
flexible endoscopes enable physicians to get images from There has been increasing recognition of the enor-
areas inside the body that were previously inaccessible, mous possibilities for utilizing the low-loss optical fibers
such as inside the bronchial tree or inside blood vessels in medical applications. Ultrathin endoscopes with im-
or even inside the fetus in the womb. Optical fibers can proved optical properties have been developed and uti-
be used for medical diagnostics by inserting them (e.g., lized for endoscopic imaging. Optical fibers have been
through catheters) inside the body and making physical used as diagnostic tools. Laser power has been transmit-
or chemical measurements through them. Measurements ted through optical fibers and used for ablation of tissue
such as blood flow, pressure, and gas content or sugar or for therapeutic applications inside the body. The diag-
content in blood can be performed quickly and reliably. nostic fibers and the “power”fibers can be incorporated
Optical power fibers may be used to transmit high laser in compound laser endoscopes. The unique capabilities of
power to areas inside the body. The laser radiation in this system may be enhanced by computer control. This
turn may be used to cut tissues, coagulate bleeding ves- will actually make it a robotic system that will interpret
sels, remove tumors, clean occluded arteries, and destroy the imaging and the diagnosis data, and will control the
cancer cells. Compound laser endoscopes may include therapeutic or the surgical laser operation inside the body.
several channels: fiberoptic ones for image transmission, We review here some of the physical principles involved
diagnostics, and laser power transmission, and ancillary and some of the revolutionary uses of optical fibers in
channels for injection of liquids or aspiration of debris. medicine.
Laser endoscopes are thin and flexible and used in myriad
applications.
II. OPTICAL FIBERS

I. INTRODUCTION In this section we describe the physical principles of light
guiding in optical fibers and in bundles of fibers. We dis-
Light can be transmitted through a cylinder of trans- cuss the materials and methods of fabrication of various
parent material by a series of internal reflections. This fibers and bundles.