Hollow Core Double-Clad Fiber Coupler for Nonlinear Micro-Endoscopy

We report the design, fabrication and characterization of the first hollow core double-clad fiber coupler for nonlinear micro-endoscopy. It is made of a pure silica negative curvature hollow core double-clad fiber and a highly multimode fiber fused along each other. The fabrication of such a coupler was made possible thanks to the use of a soft (borosilicate) glass multimode fiber which can be fused along the negative curvature fiber without altering its geometry, thanks to its much lower fusion temperature. A 60% power transfer from the double-clad fiber to the multimode one was measured at 500 nm. Demonstration of two-photon fluorescence and second harmonic generation imaging was performed on unlabeled biological tissues with an endoscopic head mounted on the hollow core double-clad fiber coupler to demonstrate its applicability. This device greatly simplifies the integration of nonlinear micro-endoscopes based on hollow core double clad fibers.