All optical fiber mode selective coupler based on non-weak guided fiber

An all fiber mode selective coupler (MSCs) is proposed, which is based on the non-weak guided fiber using weak fusion technology. According to the mode coupling theory and supermode theory, the structure size of the coupler is calculated. The coupler is composed of a single-mode fiber (SMF) and a graded index fiber designed by ourselves. We have designed this kind of few mode fiber which can breaks modal degeneracy. This kind of coupler is different from other types in that it does not require pre-tapering in the manufacturing process, only needs to control the distance of the core. When the circular polarization HE11 mode is launched into the SMF, and passed through the coupling region, the higher-order mode can be excited in a section of the multimode graded fiber. The circular polarization HE21 mode in the graded-index multimode fiber would be OAM modes. When the HE11 mode is launched into the multimode fiber there is no energy coupling to the single mode fiber at the output end of the SMF, So the donut and Gaussian beams are delivered from the same fiber. Over the past decade, this mode selective coupler (MSCs) has been introduced as an essential component for many multimode fiber devices, such as sensors, mode multiplexing (WDM). If the coupler is used in the visible band, it also can be a new way to realize stimulated emission depletion microscopy applications.