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

Optical Fiber Techniques for Medical Applications 327

Typically, an average laser power of 10–100 W is required. 3. Divergence. The light emitted from fibers is not
With the advent of low-loss fibers it is possible to trans- collimated, as is a regular laser beam. It is highly
mit such power levels through thin optical fibers. Power divergent, and the NA determines the divergence.
fibers may replace the cumbersome articulating arms for When transmitting the beam through the fiber, the
delivering the beam from the laser to the operating site. distal end of the fiber has to be kept clean, and it must
These power fibers may also be inserted inside the hu- not touch the tissue. If this end is held 2–3mmaway
man body through natural orifices, through catheters, or from the tissue, the power density at the tissue may be
through flexible or rigid endoscopes. Laser beams sent too low for incision.
through the fibers may then be used for a variety of med- 4. Dry field. Blood will coagulate under laser radiation.
ical applications: coagulation, therapeutic heating, tissue In order to prevent coagulation during fiberoptic laser
welding, ablation, incision, among others. surgery, one has to replace the blood near the distal
The three lasers that have been commonly used in the end of the fiber by saline solution or by transparent
past for laser surgery are the CO 2 , the Nd:YAG, and the blood substitute, or push the blood back using
Ar ion lasers. These are also the lasers that were tried first pressurized CO 2 gas.
with fiberoptic delivery systems. The Ar laser radiation
(λ = 514 nm) is in the visible spectral range, and its radia- The power handling capabilities of optical fibers has
tion is readily transmitted by pure silica glass (i.e., quartz) increased as a result of improvements in the properties of
fibers. The same is true for the Nd:YAG laser, whose ra- the fibers and better handling of the fiber end faces. Many
diation is in the NIR (λ = 1.06 µm). Fibers of diameters of the problems associated with coupling of high-power
0.1–0.2 mm and of lengths 1–2 m have often been used. laser beams into fibers have been solved. As a result, spe-
The power levels continuously transmitted through the cial fibers have been added to medical lasers as standard
fibers have been up to 10 W for the Ar ion laser, and up items for power transmission.
to 60 W for the Nd:YAG laser. The CO 2 1aser radiation is
in the mid-IR (λ = 10.6 µm), and it can be transmitted by
hollow fibers or by polycrystalline halide fibers (e.g., ones B. Laser Catheters
made of silver halide). In both cases, fibers of diameter of A catheter is a flexible plastic tube that can be inserted
about 1 mm and length of about1mhave been used. Power into various areas in the body. An optical fiber can be
levels of tens of watts have been continuously transmitted easily threaded into such a catheter, and laser energy de-
through these fibers. livered through the fiber can be used for diagnosis, therapy,
As for other lasers that are used now for medical or surgery. Many of the new devices are multichannel
applications, the GaAs type laser radiation or the Ho: catheters in which an optical “power”fiber is inserted in
YAG laser radiation can also be sent through silica one of the channels. Cooling liquid may also be injected
glass fibers. The transmission of the excimer laser radi- into this channel. Laser power sent through such a fiber
ation is more difficult, first because of the short wave- could be used either for tissue removal (i.e., surgery) or
length and second because of the need to transmit very for therapeutic heating, for instance, laser-induced laser
short pulses of high peak power. Specially manufactured therapy (LITT). Other channels are used for a diagnosis
quartz fibers are normally used. For the transmission fiber, for irrigation of tissue with saline solution, or for
of Er:YAG laser radiation, one can use fibers made of introducing drugs. A metal guide wire is often inserted
sapphire. in one of the channels to facilitate easy insertion of the
There are still many hurdles. Some of the problems catheter into the body. Recent progress in such catheters
involved in using optical fibers for power transmission are has led to their surgical and therapeutic use in cardiology,
as follows. gynecology, orthopedics, and other medical disciplines.
Several examples are discussed below.
1. Damage to the ends of the fibers. The power density
2
(W/cm ) at each end of the fiber is very high. It may C. Laser Endoscopes
4
2
easily reach values of 10 W/cm . Any defect on the
fiber end may increase the absorption of laser beam With the development of new flexible fiberscopes of very
on the end face. With these high power densities, the high optical quality and the concurrent development of
end face will be damaged. optical fibers for laser power transmission, the road was
2. Bending. With most fibers today, one cannot bend the clear for developing novel endoscopes. The endoscope
fibers beyond a certain value. Bending increases the would consist of several channels. One channel would in-
loss, and again, the fiber may be damaged at the clude a complete fiberscope, consisting of an imaging bun-
bend. dle and illumination light guides. Another channel would