摘要： Spatial division multiplexing (SDM) in form of mode division multiplexing (MDM) is one solution to keep up with the increasing data traffic demand [1]. MDM is based on using individual modes of a few-mode or multi-mode fiber as individual transmission channels and requires the addressing of individual modes at the input and output of the multimode fiber link. Here, among others, mode-selective fused fiber couplers based on evanescent coupling can be applied for addressing individual modes of fiber optic transmission links. In this case, light coupling occurs when the mode propagation constants are equal and this can be achieved e.g. by applying grating structures and, thus, compensate for the mismatch of the propagation constants of the modes to be coupled [2]. In this work, we will present our latest efforts towards the development of grating assisted mode-selective fused fiber couplers. In order to realize these, the fabrication process is divided into two steps. First, the asymmetric fused fiber couplers are fabricated by using an etching, fusing and tapering process of a step index single-mode (sm) fiber from Fibercore SM1500 (4.2/125) and a four mode step index few-mode (fm) fiber from OFS. The cross-section of a fabricated asymmetric fused fiber coupler with a parabolic shape at the taper waist is shown in Fig. 1a. Following this step the grating structure is inscribed into the core of the sm fiber using the amplitude mask method. The grating structure is applied in order to obtain mode-selective light coupling between the sm and fm fibers. The amplitude mask method for inscribing the grating structure is realized by using a bronze foil mask (Optofab) and a KrF-excimer laser (Atlex FBG, ATL Lasertechnik). Moreover, the process parameters (repetition rate, pulse energy and exposure time) are taken from previous investigations of inscribing LPGs into the highly germanium doped sm fiber [3]. In order to determine the required grating period for mode selective light coupling a CAD model was designed in RSOFT. The model takes the fiber parameters as well as the heating zone, the fiber extension and the degree of fusion (DOF) of the coupler fabrication process into account. In Fig. 1b an illustration of the CAD model of the fabricated asymmetric fused fiber coupler is shown. To determine the required grating period for mode-selective light coupling and to simulate the performance of the grating assisted mode-selective fiber coupler the beam propagation method was applied (BeamPROP-toolbox in RSOFT). As illustrated in Fig. 1c, mode-selective light coupling can be obtained between the LP11 mode of the fm fiber and the LP01 mode of the sm fiber for a grating period of 500 μm.