Origin and suppression of parasitic signals in Kagomé; lattice hollow core fibers used for SRS microscopy and endoscopy
Résumé
Hollow core fibers are considered as promising candidates to deliver intense temporally overlapping pi-cosecond pulses in applications such as stimulated Raman scattering (SRS) microscopy and endoscopy because of their inherent low nonlinearity compared to solid-core silica fibers. Here we demonstrate that-contrary to prior assumptions-parasitic signals are generated in Kagomé-lattice hollow core fibers. We identify the origin of the parasitic signals as an interplay between the Kerr nonlinearity of air and frequency dependent fiber losses. Importantly, we identify the special cases of experimental parameters that are free from parasitic signals making hollow core fibers ideal candidates for noise free SRS microscopy and endoscopy. Stimulated Raman scattering (SRS) [1] has received much attention since its implementation in microscopy [2, 3] as it provides a label free contrast mechanism with chemical sensitivity that is identical to spontaneous Raman scattering. SRS imaging has been implemented at video rate [4] and successfully applied to a variety of topics in cell biology and biomedical sciences [5] to appear nowadays as a viable platform with strong potential in biology and medicine. Although SRS per se does not suffer from intrinsic non-resonant background such as the one found in coherent anti-Stokes Raman scattering (CARS) [6], the implementation of SRS is generally accompanied by parasitic signals stemming from other quasi-instantaneous nonlinear optical processes such as two-photon absorption [7] and cross phase modulation (XPM) [8] but also thermal effects [9]. When implemented with excitation beams propagating in free-space, it has been previously demonstrated that SRS microscopy can be made free of these spurious signals using stimulated Ra-man gain and opposite loss detection [10]. 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. Numerous attempts to perform CARS fiber beam delivery [11-13] or probe/endoscopic imaging [14, 15] have been reported. SRS fiber delivery [16] or scanning probe [17] have been less studied. But in general CARS and SRS using optical fibers have been hampered by the strong background signals arising from the wave mixing interactions in the fiber silica core. We concentrate here on Kagomé lattice hollow core fibers [18] (KL-HCFs) that are more suitable than band gap hollow core fiber. Indeed, the broad spectral transmission band of Kagomé fibers has been exploited recently for development of Kagomé-based Raman probes [19], demonstrating that hollow core fibers lend themselves well to spectroscopic applications utilizing a single exci-tation beam, like spontaneous Raman. CARS and SRS are a different matter; in [16] we reported CARS and SRS using hollow core fibers that are known to exhibit extremely weak nonlinear effects because light propagates in the air core [20]; although Ra-man spectral features from the sample could be retrieved, these signals sat on a plateau whose origin was uncertain. In the present paper our aim is to understand the origin of the residual parasitic signals in hollow core fibers, a topic that has important ramifications for the implementation of SRS beam delivery and endoscopy. Most importantly, we identify the experimental parameters leading to the complete cancellation of the residual parametric signals. Fig. 1(a) shows a SEM micrograph of the silica KL-HCF