are of interest to biophotonics, as Fig.1.6illustrates. Therefore, it is essential that
biophotonics researchers and implementers know whichfiber is best suited for a
certain application. This chapter provides the background that is necessary to
understand how opticalfibers function and describes various categories offibers
that are commercially available for use in biophotonics. Subsequent chapters will
illustrate how the diverse types of opticalfibers are used for specific biophotonics
analysis, imaging, therapy, and measurement applications.
First, Sects.3.1and3.2discuss the principles of how light propagates in two
categories of conventional solid-corefibers. Here the term“conventional”refers to
the configuration of opticalfibers that are used worldwide in telecom networks.
These discussions will be the basis for explaining how light is guided in other types
of opticalfibers.
Next, Sect.3.3describes opticalfiber performance characteristics. These include
optical signal attenuation as a function of wavelength, sensitivity of thefiber to an
increase in power loss as thefiber is progressively bent into small loops, mechanical
properties, and optical power-handling capabilities.
Using this background information, Sect.3.4through3.13then present a variety
of other opticalfiber structures and materials that are appropriate for use in the
wavelength spectrum that is relevant to biomedical research and clinical practice.
Table3.1summarizes the characteristics of these opticalfibers, which can be
constructed from materials such as standard silica, UV-resistant silica, halide
glasses, chalcogenides, and polymers [ 5 ]. The biophotonics applications in this
table have been designated by the following three general categories with some
basic examples:
- Light care: healthcare monitoring; laser surfacing or photorejuvenation
- Light diagnosis: biosensing, endoscopy, imaging, microscopy, spectroscopy
- Light therapy: ablation, photobiomodulation, dentistry, laser surgery, oncology
3.1 Light Guiding Principles in Conventional Optical Fibers.
3.2 Graded-Index Optical Fibers
This section describes the principles of how light propagates in a conventional
solid-corefiber [ 1 – 6 ]. Such afiber normally is a cylindrical dielectric waveguide
that operates at optical frequencies. Electromagnetic energy in the form of light is
confined within thefiber and is guided in a direction parallel to thefiber axis. The
light propagation along such a waveguide can be expressed in terms of a set of
guided electromagnetic waves called themodesof the waveguide. Each guided
mode consists of a pattern of electric and magnetic field distributions that is
repeated periodically along thefiber. Only a specific number of modes that satisfy
54 3 Optical Fibers for Biophotonics Applications