CARS endomicroscopic imaging probe enabled by a double-core double-clad fiber and related focus-combining micro-optical concept

摘要

Non-linear endoscopic imaging probes allow for in-vivo, label free tissue histology and thus bring tumour treatment to a new level providing accurate, real time diagnostics. Current R&D efforts focus on developing robust instruments for daily surgery routine. Here, we present an endomicroscopic probe for coherent anti-Stokes Raman scattering (CARS) imaging based on a fiber piezo scanner on the distal side of the probe. One of the main hurdles in the implementation of CARS imaging in all-fiber solutions is the generation of a background four-wave-mixing (FWM) signal within the delivery fiber by the Stokes- and pump lasers involved in the process of generation of the nonlinear image [1]. Until now, different complex special fibers, such as hollow-core or large-mode-area fibers were proposed to inhibit FWM [2, 3]. We developed a novel solution based on a solid silica double-core double-clad (DCDC) fiber, which allows a separate guiding of the exciting Stokes- and pump laser radiation in two separate cores of the delivery fiber, avoiding the generating of undesired FWM in the fiber. An outer cladding allows to collect the signal generated in the tissue and guides it to the proximal detection system (see Fig. 1). Using a linear diffraction grating, the optical design of the miniature distal probe head (see Fig. 2) provides a perfect overlap and focusing of the Stokes and pump lasers at each position of the field of view of 184 urn with high NA of 0.55 on the sample. Images are acquired by resonantly scanning the tip of the DCDC, mounted in a 4-quadrant-piezo tube, in the intermediate image plane of the miniature objective (see Fig. 3). The exemplary multimodal CARS/TPF and SHG image of dura tissue in figure 4 was recorded with the probe of only 2.4 mm diameter using an all-fiber laser at 795/1030 nm with nominal average powers of 20/110 mW, pulse duration of 20/60 ps and a repetition rate of 1 MHz [4]. It proves the concept to enable high contrast und sub-micron resolution multimodal imaging of unstained tissue with a less complex all fiber based setup compared to what is known from the state of the art.