I. Introduction
In 1986, Mitschke and Mollenauer discovered the nonlinear optical effect of soliton self-frequency shift (SSFS) in optical fibers [1]: as ultrashort optical pulse propagates in anomalous dispersion fibers, a soliton may form and later shift to the longer wavelength. This effect can be explained by intra-pulse stimulated Raman scattering: the shorter wavelength content of the soliton continuously transfers its energy to the longer wavelength content, leading to continuous red shift. So far, SSFS has been experimentally demonstrated in all kinds of optical fibers with anomalous dispersion [1]–[8], including standard single mode fibers, index-guided photonic-crystal fibers (PCFs), larger-mode-area (LMA) fibers, higher-order-mode (HOM) fibers, photonic-crystal (PC) rods, and hollow-core fibers. The broadband wavelength tunability, Gaussian-like spatial mode, and ultrashort pulse width, make soliton pulses generated through SSFS an exceptionally useful technique for the field of nonlinear optical microscopy. For example, used as excitation source, the soliton pulses enable the largest nonlinear optical imaging depth in animal models in vivo [9].