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Optical Fiber Techniques for Medical Applications 321

lasers for which the extinction length is small and if one IV. FIBEROPTIC ENDOSCOPES
uses short pulses, rather than a CW beam. The interaction
described above is basically photothermal. The interac- The name endoscope is based on two Greek words: endon
tion between excimer lasers (λ< 300 nm) and tissue may (within) and skopein (view). It is used to describe optical
be different. It involves the absorption of UV light, and instruments that facilitate visual inspection and photogra-
there are claims that ablation of tissue is possibly due to phy of internal organs. Endoscopes may be inserted into
photochemical processes that do not generate heat. The the body through natural openings (ear, throat, rectum,
excimer laser has important surgical applications. It has etc.) or through a small incision in the skin. The simplest
been used in cosmetic surgery for resurfacing of the skin, endoscopeisanopentube,andithasbeenusedinmedicine
such as for the removal of wrinkles, and in ophthalmology for thousands of years. The development of modern en-
for corneal reshaping (i.e., for the correction of vision). doscopes started about a hundred years ago with the addi-
There are several nonthermal effects which should also tion of artificial illumination (i.e., incandescent lamp) and
be mentioned. The interaction of tissues with low level lenses to the open tube (NITZE 1894). Rigid endoscopes
laser light (e.g., blue light or UV light) may give rise built in a similar manner are still in use. The nature of
to luminescence. This luminescence can be used for tis- endoscopy changed dramatically in the early 1950s when
sue diagnosis, for example, for distinguishing between optical-fiber bundles were introduced. These are used both
healthy and diseased tissues. Ar lasers or some new semi- for illumination and for imaging.
conductor lasers, which emit in the blue, can be used for
these applications. Another important application is pho- A. Light Guides for Illumination
tochemotherapy. Chemicals are introduced into the body
and are “triggered” by light of some specific wavelength 1. Light Sources
and then selectively interact with tissue. Such photochem-
In endoscopy light is transmitted through an optical fiber,
ical effects may be used for cancer treatment, to be dis-
in order to illuminate an internal organ. The light sources
cussed in Section VII, or for the treatment of age–related
that are suitable for this purpose should deliver enough
degenerate macula (ADM) and other diseases.
energy through the fiber to facilitate viewing or photog-
Medical laser systems have dramatically improved
raphy. In general, high-intensity light sources are needed,
during the last few years. Semiconductor lasers cover
such as tungsten lamps, mercury or xenon high-pressure
a broader range from the blue (e.g., GaN lasers) to the
arc lamps, or quartz iodine lamps. In most of these cases,
mid–IR (i.e., quantum cascade lasers). Solid-state lasers the light has to be focused on a fiber (or a fiber bundle),
pumped by semiconductor lasers are now more compact by means of a lens or a reflector. Special provisions are
and more powerful. There are gas lasers (e.g., CO 2 ) that often required to dissipate the excessive heat generated by
are very compact and easy to use. All these new lasers are the light sources. Some of the lasers that were mentioned
now more reliable and more efficient, most of them are earlier are suitable for special endoscopic illumination.
lightweight, easy to operate, and relatively inexpensive.
Many of the new lasers have already made their debut in
2. Light Guides (Nonordered Bundles)
the clinical setting.
The light from a regular high-intensity (noncoherent)
source cannot be focused to a spot whose size is equal
C. Lasers and Fibers
to the size of a thin optical fiber. Therefore, a bundle of
Laser beams in the visible and NIR (λ = 0.3–3.0 µm) are fibers is normally used for illumination. This assembly
transmitted by silica-glass fibers and have been used for consists of numerous clad fibers of a certain length, which
transmitting the radiation of Ar, dye, Nd:YAG, and GaAs are bundled together but not ordered. In order to increase
lasers. Excimer laser radiation at λ = 250–300 nm can the light collection efficiency, the fibers are designed to
be transmitted only through pure silica fibers, which are have a relatively high numerical aperture (NA 0.65–0.72).
not very useful at shorter wavelengths. Better fibers are The diameter of each individual fiber is 20–50 µm, so it is
needed for this spectral range. Er:YAG laser radiation can flexible. The ends of the fibers are cemented with epoxy
be transmitted through sapphire fibers. There has been a resin or fused. The remaining lengths are left free, so that
major effort to find suitable fibers for the transmission of the whole bundle can be bent. A thin plastic sleeve is nor-
the radiation of mid-infrared lasers, and especially that mally used to protect the fibers and often a metal tube
of a CO 2 laser (λ = 10.6 µm). At the moment, the best protects the ends of the bundle. It is also important that
fibers are hollow fibers or polycrystalline fibers made of the NA of the collecting fibers match the NA of the optical
halide crystals (e.g., AgClBr). The transmission of high element that focuses light from the light source onto the
laser power through fibers is discussed in Section VII. light guide.