Chapter 7: Glass Fibers for High Power Lasers

7.1 Introduction of Optical Fibers

The phenomenon of total internal reflection, responding for guiding of light in optical fibers, has been well known for two hundreds years. ^[1] Although glass fibers were made in the 1920s ^[2], their use became practical only in the 1950s, when the use of a cladding layer led to considerable improvement in their guiding characteristics. Before 1970, optical fibers were used mainly for medical imaging over short distance. Their use for communication purposes was considered impractical at that time because of high loss (-1000 dB/km). However, the situation changed drastically in 1970's, following an earlier suggestion ^[3] the loss of optical fibers was reduced to about 20 dB/km. Nowadays the loss of optical fibers is only 0.1 dB/km at the 1.55 ?m optical communication window. The availability of low-loss fibers led to a revolution in the field of optical fiber communications.

Fiber laser has a history almost as long as that of the laser itself. Since its invention in 1963 by Elias Snitzer, almost two decades of development on fiber lasers were spent before the first commercial device appeared on the market in the late 1980's. These lasers used single-mode diode pumping, emitted a few tens of milliwatts, and attracted users because of their large gains and the feasibility of single-mode continuous-wave (CW) lasing for many transitions of rare-earth ions not achievable in the more-usual crystal-laser version. The most well-known application of fiber-laser technology is 1550-nm erbium-doped fiber amplifiers ^[4]

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