Parasitic signals in hollow-core fibers used for SRS endoscopy

Stimulated Raman scattering (SRS) microscopy appears nowadays as a viable platform with strong potential in biology and medicine. The implementation of SRS is generally accompanied by parasitic signals stemming from other quasi-instantaneous nonlinear optical processes such as two-photon absorption, cross phase modulation (XPM), and thermal effects. Implementations which involve propagation of the excitation pulses in optical fiber, i.e. fiber delivery for SRS microscopes and SRS endoscopes are much more prone to generate parasitic signals. Hollow-core fibers, like photonic band-gap fibers and Kagomé-lattice hollow-core fibers (KL-HCF), are generally thought to minimize parasitic signals because light propagates in air rather than silica. However, in [2] we reported CARS and SRS using hollow core fiber; although Raman spectral features from the sample could be retrieved, these signals sat on a plateau whose origin was uncertain.