attenuation is measured in units of decibels per kilometer (dB/km) or decibels per
meter (dB/m). Recall that Sect.1.4gives a discussion of decibels. A variety of
materials that exhibit different light-attenuation characteristics in various spectral
bands are used to make the diverse types of opticalfibers employed in biophotonics
applications. The basic reason for such a selection is that each material type exhibits
different attenuation characteristics as a function of wavelength. For example, the
silica (SiO 2 ) glass material used in conventional opticalfibers for telecom appli-
cations has low losses in the 800-to1600-nm telecom spectral range, but the loss is
much greater at shorter and longer wavelengths. Thus,fibers materials with other
attenuation characteristics are needed for biophotonics applications at wavelengths
outside of the telecom spectral band.
3.3.2 Bend-Loss Insensitivity.
Specially designedfibers with a moderately higher numerical aperture (NA) than in
a conventional single-mode fiber are less sensitive to bending loss [ 5 ].
Bend-insensitive or bend-tolerantfibers are available commercially to provide
optimum low bending loss performance at specific operating wavelengths, such as
820 or 1550 nm. Thesefibers typically have an 80-μm cladding diameter. In
addition to low bending losses, this smaller outer diameter yields a much smaller
coil volume compared with a standard 125-μm cladding diameter when a length of
this low-bend-lossfiber is coiled up within a miniature optoelectronic device
package or in a compact biophotonics instrument.
3.3.3 Mechanical Properties.
A number of unique mechanical properties make opticalfibers attractive for
biomedical applications [ 5 ]. One important mechanical factor is that opticalfibers
consist of a thin highlyflexible medium, which allows various minimally invasive
medical treatment or diagnostic procedures to take place in a living body. As
described in subsequent chapters, these applications can include various endoscopic
procedures, cardiovascular surgery, and microsurgery.
A second important mechanical characteristic is that by monitoring or sensing
some intrinsic physical variation of an opticalfiber, such as elongation or refractive
index changes, one can createfiber sensors to measure many types of external
physical parameter changes [ 5 ]. For example, if a varying external parameter
elongates thefiber or induces refractive index changes at the cladding boundary,
this effect can modulate the intensity, phase, polarization, wavelength, or transit
time of light in thefiber. The degree of light modulation then is a direct measure of
changes in the external physical parameter. For biophotonics applications the
external physical parameters of interest include pressure, temperature, stress, strain,
3.3 Performance Characteristics of Generic Optical Fibers 67