Chapter 5 Fiber Lasers

A fiber amplifier can be converted into a laser by placing it inside a cavity designed to provide optical feedback. Such lasers are called fiber lasers, and this chapter is devoted to them. Section 5.1 covers general concepts such as pumping, cavity design, and laser threshold. The characteristics of continuous-wave (CW) fiber lasers are covered in Sec-tion 5.2. It includes a discussion of high power Yb-doped fiber lasers and the nonlinear phenomena that limit the power level. Sections 5.3 and 5.4 focus on the Q-switching and mode-locking techniques used to generate short optical pulses from fiber lasers. Section 5.3 covers Q-switching as well as active mode-locking schemes, whereas Sec-tion 5.4 is devoted to passive mode-locking techniques for which the nonlinear effects play a dominant role. The effects of fiber dispersion and fiber nonlinearities on the mode-locking process are considered in more detail in Section 5.5. The Ginzburg– Landau equation that was the focus of Chapter 4 is used for this purpose and applied to the cases of both the anomalous and normal types of dispersion. 5.1 Basic Concepts Many different rare-earth ions, such as erbium, neodymium, and ytterbium, can be used to make fiber lasers capable of operating over a wide wavelength range extending from 0.4 to 4 μm. The first fiber laser, demonstrated in 1961, used a Nd-doped fiber with the 300-μm core diameter [1]. Low-loss silica fibers were used to make diode-laser-pumped fiber lasers in 1973 soon after such fibers became available [2]. Although there was some research activity in between [3], it was not until the late 1980s that fiber lasers were fully developed. The initial emphasis was on Nd- and Er-doped fibers [4]–[14], but other dopants such as holmium, samarium, thulium, and ytterbium were also used [15]–[18]. Starting in 1989, erbium-doped fiber lasers (EDFLs) became the object of intense focus [19]–[23]. Such lasers are capable of producing short optical pulses in the 1.55-μm spectral region and are useful for a variety of applications [24]– [26]. Ytterbium-doped fiber lasers attracted renewed attention after 2000 because of their potential for producing high power levels [27]