Novel double clad fiber coupler for endoscopy Wendy-Julie Madore 1 , Etienne De Montigny 1 , Olivier Ouellette 1 , Étienne Duchesne de Lamotte 1 , Mathias Strupler 2 , Nadir Goulamhoussen 1 , Nicolas Godbout 1 , Caroline Boudoux 1,2,* 1 Engineering Physics Department, Ecole Polytechnique Montréal, 2900, Boulevard Edouard-Montpetit, Montreal (QC) H3C3A7, Canada 2 Research Center, Sainte-Justine Hospital, 3175 Côte Sainte-Catherine Road, Montreal (QC) H3T1C5, Canada *caroline.boudoux@polytml.ca Abstract: A novel design for asymmetric fiber couplers for use in endoscopy applications using double clad fibers is presented. Improvements in coupling ratio over symmetric couplers are presented, along with their advantages for endoscopic imaging. OCIS codes: (060.2340) Fiber optics components, (170.2150) Endoscopic imaging, (170.4580) Optical diagnostics for medicine 1. Introduction Double clad fibers (DCF) have recently been used in endoscopy for several biomedical applications requiring coherent illumination through their single mode (SM) core and incoherent collection through their multimode (MM) inner cladding. Such applications include speckle removal for wide field endoscopy [1], multimodal fluorescence- OCT [2- 4] and nonlinear microscopy [5]. One key factor in implementing DCF in clinical settings is low insertion loss in the SM core and maximum extraction of the inner cladding modes. Several DCF couplers have been proposed [6, 7] having SM transmission efficiencies of approximately 90% and MM collection efficiencies ranging from 10-45%. Higher collection efficiencies have allowed for collection of weak signals in confocal endomicroscopy [8] and refractive index sensing [9]; they have also been used in combined fluorescence-OCT needle probe systems [10]. We present here a novel double clad fiber coupler (DCFC) scheme which provides an improvement of collection efficiency by a factor of more than two using a combination of DCF and MM fibers in a fusion-tapering fiber coupling setup. 2. Materials and Methods Figure 1(a) shows a schematic of the double clad fiber coupler. The DCFC consists of two fibers: a double clad fiber (Nufern 9:105:125, 0.12, 0.2) and a multimode fiber (Nufern 200:220, 0.12) assembled as a DCFC on a custom fusion tapering fiber coupler setup [11]. Chemical etching is performed on both the fibers’ outer cladding prior to processing; the fibers are then fused until the desired transmission specifications are reached. The central portion of the coupler is then packaged in stainless steel tubing and sealed at both ends. Figure 1 a) Schematic of the DCFC. b) Single- and multi-modal transmission curves. Characterization of the coupler was performed by injecting broadband illumination into the single mode core and measuring the transmission with an optical spectrum analyzer. Multimode transfer efficiency is measured by introducing diffuse broadband illumination into the DCF and inspecting the portion that is coupled through to the multimode fiber. 3. Results A comparison of the DCFC with a previously published version is presented in Fig. 1(b) [7]. Transmission through the core is flat across the spectrum (1250-1350 nm) and is now quasi-lossless (98% transmission). The asymmetry