and the molecular composition of a liquid or gas surrounding thefiber. Operational
details and examples of these biosensor applications are presented in Chap. 7.
3.3.4 Optical Power-Handling Capability.
In some biomedical photonics applications, such as imaging andfluorescence
spectroscopy, the opticalfibers carry power levels of less than 1μW[ 5 ]. In other
situations thefibers must be able to transmit optical power levels of 10 W and
higher. A principal high-power application is laser surgery, which includes bone
ablation, cardiovascular surgery, cosmetic surgery, dentistry, dermatology, eye
surgery, and oncology surgery.
Hard-clad silica opticalfibers with fused silica cores that have very low con-
taminants are described in Sect.3.4. Thesefibers are capable of conducting high
optical power from either a continuous wave (CW) or pulsed laser [ 5 ]. Otherfibers
that are capable of transmitting high optical power levels include conventional
hollow-corefibers (see Sect.3.5), photonic crystalfibers (see Sect.3.6), and ger-
manate glassfibers (see Sect.3.9). The challenge is the launching of high optical
power levels into afiber. Artifacts such as dust or scratches on the end face of the
fiber can form absorption sites that generate elevated temperature levels at thefiber
tip. In standard connectors where thefibers are glued into the connector housing,
these temperature levels can cause the surrounding epoxy to break down and give
off gases. The gases ignite and burn onto the tip of thefiber, thereby causing
catastrophic damage to thefiber and the connector. To handle high power levels,
various manufacturers have developed specialfiber optic patch cords that have
carefully preparedfiber end faces and specially designedfiber optic connectors that
greatly reduce the susceptibility to thermal damage.
3.4 Conventional Solid-Core Fibers
As a result of extensive development work for telecom networks, conventional
solid-core silica-based opticalfibers are highly reliable and are widely available in a
variety of core sizes [ 5 ]. Thesefibers are used throughout the world in telecom
networks and in many biophotonics applications. Figure3.6 shows the optical
signal attenuation per kilometer as a function of wavelength. The shape of the
attenuation curve is due to three factors. First, intrinsic absorption due to electronic
absorption bands causes high attenuations in the ultraviolet region for wavelengths
less than about 500 nm. Then the Rayleigh scattering effect starts to dominate the
attenuation for wavelengths above 500 nm, but diminishes rapidly with increasing
wavelength because of its 1/λ^4 behavior. Thirdly, intrinsic absorption associated
with atomic vibration bands in the basicfiber material increases with wavelength
and is the dominant attenuation mechanism in the infrared region above about
68 3 Optical Fibers for Biophotonics Applications