Optical fiber is a thready material that makes use of optical total internal reflection (TIR) to guide light waves. The fiber loss was predicted low enough to transmit optical signals for long distances in the 1960s, and the low loss silica fiber was fabricated in the 1970s. Since then, optical fibers have been used in telecommunication systems in tremendous amounts and with great success. Their applications in sensor and other science and technology fields are also developed quickly, playing increasingly important roles in various fields.
1.1 HISTORICAL REVIEW AND PERSPECTIVE
The optical fiber was proposed and fabricated earlier in the 1920s [1,2], demonstrating light propagation in a glass waveguide based on the principle of TIR. The invention of optical fiber broke the limitation of the straight propagation of light. Fibers with cladding were invented later to reduce propagation loss, caused by the outer medium of air, for the earlier fibers without cladding. This improvement resulted in practical applications using optical fibers, such as image transmissions in bundles . It was predicted theoretically in 1966 by K.C. Kao in his initiative paper that optical fiber with extremely low loss could be realized, and its application to telecommunications was proposed . Soon after, a fiber with loss down to a few tens dB/km was fabricated . Combined with lasers, especially the semiconductor laser, which came into being in the same decade [6–8], the fiber ...