Enabling Shape Sensing in MIS Using a Two-Photon Polymerized Waveguide

Shape sensing optical fibers have attracted increasing attention in the last decade as a promising tool for miniature shape detection in confined spaces [1]. Due to their small footprint, they are considered a prime candidate for installation in endoscopic and laparoscopic surgery tools. They can be used not only as a shape sensor to detect organ/tissue curvatures while a tool is navigated, but also as part of an all-optical palpation sensor [2] to measure interaction forces. A shape-sensing optical fiber protruding from a laparoscopic instrument can provide spatial coordinate information of its bent tip which can be used to map the underlying surface and in conjunction with a second pressure sensitive sensor, such as a membrane, provide stiffness information. Shape sensing optical fibers are typically realized in single or multi-core optical fibers (MCFs) by the inscription of Fiber Bragg Gratings (FBGs) in each individual core of the optical fiber. Shape sensing by means of such optical fibers has been reported as early as 2000 [3]. It was later demonstrated that selective inscription of Fiber Bragg grating in the individual cores can be realized, providing freedom in the central Bragg wavelength that can be back-reflected from each core [4]. However, a common denominator in all multi-core optical fiber sensing approaches is the need to individually interrogate all cores, or to obtain the sensing signal by fusing all cores into a single optical fiber. Typically, this is realized either by utilizing expensive optical fiber assemblies, such as fan-outs, or by tapering the multi-core optical fiber and fusing it to a single core fiber. In the present work we report on the use of the two- photon polymerization (TPP) technique for the interrogation of four FBGs with different central wavelengths inscribed in a four-core optical fiber using a TPP produced waveguide. The TPP technique has been used extensively in recent years for several applications, including waveguide inscription for a wide variety of applications [5]. The design capabilities of the method allow the creation of a miniaturized waveguide structure that can fuse the optical signals from each core of a multi- core optical fiber into the core of a single core optical fiber. This technique is a low-cost alternative method to solve the interconnection problem of such fibers that are used in MIS sensors.