Fiber Lasers and Optical Solitons

In the Sect. 11.1.7 we discussed the self-phase modulation of an optical pulse in a fiber because of the intensity-dependent refractive index n = no- -n2l t). While the resultant spectral broadening of the pulse leads in a medium with normal negative dispersion dno/dX 0) to a spatial broadening of the pulse, an anomalous linear dispersion dn /dk 0) would result in a pulse compression. Such anomalous dispersion can be found in fused quartz fibers for k 13 p.m [11.66,11.67]. For a suitable choice of the pulse intensity the dispersion effects caused by no(k) and by n2l(t) may cancel, which means that the pulse propagates through the medium without changing its time profile. Such a pulse is named a fundamental soliton [11.68,11.69]. 

Introducing the refractive index n = no + n2/ into the wave equation (11.19) yields stable solutions that are called solitons of order N. While the fundamental soliton N — 1) has a constant time profile /(r), the higher-order solitons show an oscillatory change of their time profile I t) the pulsewidth decreases at first and then increases again. After a path length which depends on the refractive index of the fiber and on the pulse intensity, the soliton recovers its initial form I(t) [11.70,11.71]. 

Optical solitons in fused quartz fibers can be utilized to achieve stable femtosecond pulses in broadband infrared lasers, such as the color-center laser or the Tiisapphire laser. Such a system is called a soliton laser [11.72-11.79]. Its experimental realization is shown in Fig. 11.31. 

A KC1 TL color-center laser is synchronously pumped by a mode-locked Nd YAG laser. The output pulses of the color-center laser at A. = 1.5nm pass the beam splitter S. A fraction of the intensity is reflected by S and is focused into an optical fiber where the pulses propagate as solitons, because the dispersion of the fiber at 1.5 p.m is dn/dk 0. The pulses are compressed, are reflected by M5, pass the fiber again, and are coupled back into the laser resonator. If the length of the fiber is adjusted properly, the transit time along the path M -S-M5-S-Mo just equals the round-trip time T = Idjc through 

With such a KC1 T1° color-center soliton laser, stable operation with pulse widths of 19 fs was demonstrated [11.78]. This corresponds at A, = 1.5 pm to only four oscillation periods of the infrared wave. More about soliton lasers can be found in [11.72-11.80]. 

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