# Brief Review of Recent Developments in Fiber Lasers
## 1 Introduction  
The basic principle for light confinement and propagation in optical fibers is total internal reflection. Total internal reflection was first studied by [PERSON] in 1611 [1]. In 1621, it was mathematically described by [PERSON] ([PERSON]'s law), but remained unpublished during his lifetime. In 1690, [PERSON] in his Treatise on Light showed how [PERSON]'s law of sines could be explained by the wave nature of light [2]. Optical fibers are one of the major technological successes of the 20 th century. The first uncladded glass fibers were fabricated in the 1920s [3]. In the 1950s, a cladding layer was added [4; 5; 6]. The cladding layer considerably improved the fiber characteristics. The early fibers were very lossy (\(\sim\)1000 dB/km). The situation changed in the 1970s when silica fibers with a loss of \(\sim\)20 dB/km were developed [7]. In 1979, a loss level of \(\sim\)0.2 dB/km near the 1550 nm wavelength was achieved [8]. It was limited mainly by Rayleigh scattering. Such low-loss fibers have led to a revolution in optical fiber communications and to the advent of nonlinear fiber optics, fiber lasers and amplifiers, and fiber sensors. Stimulated Raman and Brillouin scattering in optical single-mode fibers were investigated both theoretically and experimentally in 1972 [9; 10; 11]. In 1973, it was suggested that optical fibers can support solitons [12]. Almost at the same time, optically induced birefringence, parametric four-wave mixing, and self-phase modulation were observed in optical fibers. The use of rare earth (RE)-doped optical fibers for gain in glass fiber lasers and amplifiers was first demonstrated by [PERSON] in 1961 [13]. He used a neodymium-doped silicate glass fiber emitting at 1060 nm. In the 1990s, erbium-doped fiber amplifiers and lasers were under intensified investigation [14]. These devises were commercialized in 1992 and employed in undersea fiber-optic communication systems by 1995. The state-of-the-art design of optical fibers is a careful trade-off between optical losses, optical nonlinearity, group-velocity dispersion, and polarization effects. In 1978, [PERSON] and colleagues proposed the idea of a photonic crystal fiber (PCF) [15]. They proposed to clad a fiber core with Bragg grating, which is similar to 1D photonic crystal. In 1992, [PERSON] invented a photonic crystal fiber made of 2D photonic crystal with an air core. In 1996, the first fabricated single-mode PCF was reported at the Optical Fiber Conference (OFC) [16]. Photonic crystals are periodic dielectric structures with the period being less than the wavelength of the light. They affect the motion of photons in the same way as an ionic lattice affects the electrons in semiconductors. PCFs are among the most specialized optical lightguides. For example, PCFs can range from fibers with low levels of nonlinearities supporting high-power pulses to highly nonlinear PCFs for supercontinuum generation. From a theoretical point of view, an optical fiber with a circular core has no birefringence, such that the polarization of a beam sent through the fiber may not change. In reality, however, a small amount of birefringence is always present in an optical fiber due to external perturbations or manufacturing imperfection. Such birefringence causes random power coupling between two polarization modes (polarization crosstalk) in an optical fiber, which is undesirable in a number of applications. A polarization-maintaining fiber (PMF) with intentionally induced birefringence along the entire fiber length maintains two polarization modes without polarization crosstalk. Larger birefringence better prohibits the polarization mode coupling. Birefringence can be induced by loss in rotational symmetry in the refractive index profile or by the stress distribution known as stress birefringence. The first commercially available PMF was introduced in 1983. Heat generation in fiber lasers is briefly discussed in Section 2.