Reduction of self-phase modulation in double-clad photonic crystal fiber for nonlinear optical endoscopy.

Double-clad photonic crystal fiber and double-clad fiber have been widely used in multiphoton-excited fluorescence or second-harmonic generation (SHG) endoscopy. We provide a useful comparison of two fibers used in nonlinear optical microendoscopy. While a double-clad fiber is found to have a higher percentage of the output power from its core, which results in the efficient utilization of the power of the excitation laser, a double-clad photonic crystal fiber has a higher threshold of the nonlinearity, which effectively reduces the self-modulation effect and thus leads to a higher degree of polarization of the excitation beam. Consequently, the use of the double-clad photonic crystal fiber facilitates bright two-photon fluorescence imaging as well as polarized SHG imaging.