Biophotonics_Concepts_to_Applications

index differenceor simply theindex difference. Typical values ofΔrange from 1 to
3 % for MMFs and from 0.2 to 1.0 % for SMFs. Because the core refractive index
is larger than the cladding index, electromagnetic energy at optical frequencies is
made to propagate along thefiber waveguide through total internal reflections at the
core-cladding interface.
The remainder of Sect.3.1describes operational characteristics of step-index
fibers and Sect.3.2describes graded-indexfiber structures.

3.1.1 Ray Optics Concepts

When the opticalfiber core diameter is much larger than the wavelength of the
light, a simple geometrical optics approach based on using light rays (see Sect.2.1)
can explain how light travels along afiber. For simplicity, the analysis considers
only those rays (calledmeridional rays) that are confined to the meridian planes of
thefiber, which are the planes that contain the axis of thefiber (the core axis).
Figure3.3shows a light ray entering thefiber core from a medium of refractive
index n at an angleθ 0 with respect to thefiber axis and striking the core-cladding
interface inside thefiber at an angleφrelative to the normal of the interface. If it
strikes this interface at such an angle that it is totally internally reflected, then the
ray follows a zigzag path along thefiber core, passing through the axis of the guide
after each reflection.

Index profiles Nominal dimensions

Step-index

single-mode

fiber

Step-index

multimode

fiber

Graded-index

multimode

fiber

Fig. 3.2 Comparison of
conventional single-mode and
multimode step-index and
graded-index opticalfibers

3.1 Light Guiding Principles in Conventional Optical Fibers 57