Biomedical Engineering Reference
In-Depth Information
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FIGURE 17.21
Typical geometry of the main types of optical fibers including a (a) step index, (b) graded index,
and (c) single mode fiber.
discussed following, for endoscopic coherent fiber bundle fiber imaging, single fibers or
noncoherent bundles for sensing and diagnostics, and optical fibers made for high-powered
therapeutic applications.
In terms of the fiber material, three areas must be considered: the wavelength required,
the power, and the biocompatibility. For therapeutic applications the amount of power,
both for continuous lasers and pulsed lasers, needs to be considered. In particular, any
impurities in the fiber may absorb the light and either decrease the amount of light reaching
the probe for sensing or, for therapeutic applications, heat the fiber, which can then cause
fiber damage. The fiber itself may also cause a pulsed laser to be broadened in time due
to dispersion. In addition, besides impurities, the fiber material itself may work well for
one wavelength and not work at all for others. For instance, the standard silica or glass
fibers transmit well in the visible wavelength range, but beyond 2.0 micrometers these
fibers absorb the light and thus cannot be used for infrared light transmission. Other fibers
such as germanium, sapphire, barium fluoride, calcogenide, or hollow waveguide (i.e., gold
coated) can be used for these wavelengths, but these fibers, in addition to impurities, may
have problems for use in the body such as being too stiff, being hydroscopic (dissolves in
water), or being toxic. Thus, the biocompatibility is a function of the material in that it needs
to not be toxic to the patient and be able to function in the body without dissolving or
breaking off.
