The popularization and application of multi-core fiber cannot be realized without integrated devices with various functions. Therefore, how to build micro-optical devices with higher integration, better performance, and richer functions in a single multi-core fiber has become a research hotspot in recent years. This paper proposes a diagonal core reflection coupler based on multi-core fiber, which can connect the optical path of the diagonal core with an arcuate shape frustum on the multi-core fiber tip.

The proposed diagonal core reflection coupler is suitable for multi-core fibers with centrosymmetric core distribution (Fig. 1). With seven-core fiber as an example, after the end of the fiber is fused with a short coreless fiber, a 45° frustum is prepared by precision grinding, as shown in Fig. 2 (a). The frustum can realize the connection of three pairs of diagonal core optical paths. For example, the light in the fiber core a can be reflected twice through the surface of the frustum with a base angle of 45° and then transmitted in reverse to the fiber core b, and vice versa. When the beam leaves the core a and enters the coreless fiber, diffraction diffusion will occur. Then a large part of the beam cannot be coupled into core b after it is reflected twice by the 45° frustum. To illustrate the diffraction divergence of the beam during reflection, this paper builds a three-dimensional optical model as shown in Fig. 2 (c) through the finite-difference time-domain method for core spacing d=38 μm, and five key monitors are constructed along the beam propagation path. The monitor can provide feedback related to the optical field distribution of the section at this position, and the results are respectively displayed in Fig. 2 (d)-(h). The monitor M3 shows the light field distribution of two symmetrical side core profiles in Fig. 2 (b). Serious diffracted beam does occur during transmission and reflection. The light field is matched as much as possible with the fundamental mode field of core b, the coupling efficiency when the beam is transmitted to core b is improved, and the 45° frustum at the fiber end is optimized by an arc melt-shaping method to improve the divergence effect of the beam in the xoz plane. As shown in Fig. 3 (a) and (b), if the reflecting interface of the frustum is arcuate, the beam emitted from core a will have a focusing effect after being reflected at point A. Since cores a and b are symmetrical, when the focusing and divergence effects offset each other, the beam can be efficiently coupled into core b after the second reflection. Paraboloid is selected as the optimized shape, and the general shape function Eq. (1) is obtained.

The 45° frustum is prepared by precision grinding of the fiber end, and the arcuate shape frustum is prepared by the arc melting shaping method. Then the shape outline of the arcuate shape frustum is extracted and compared with the theoretical design. The tests indicate that the reflection coupling efficiency of the arcuate shape frustum is improved from 61.1% of the 45° platform before optimization to 72.6%, showing an obvious optimization effect. Finally, this paper discusses two important factors that affect the coupling efficiency in the preparation of a diagonal core reflection coupler, including the core shift of the multicore optical fiber and the base angle shift of the frustum. The results reveal that the arcuate shape frustum can provide greater tolerance in the preparation.

In this paper, a diagonal core reflection coupler on multi-core fiber tip is presented and optimized to realize the low loss connection of the optical path of multi-core fiber symmetric core. The device is designed to fabricate a reflection frustum at the end of a multi-core fiber by the precision grinding method. The surface shape of the 45° frustum is optimized by an arcuate shape frustum to improve the coupling efficiency of the diagonal core and reduce the insertion loss. Through the finite-difference time-domain method, the three-dimensional models of the 45° frustum and the arcuate shape frustum are built respectively. The results show that the optimized arcuate shape frustum has better preparation tolerance and performance than that before optimization. By precision grinding and arc melting optimization, the 45° frustum and the arcuate shape frustum are fabricated successfully, and the insertion loss test results are basically consistent with the simulation results. This kind of device realizes low loss connection of multi-core fiber diagonal fiber core, and the whole device is prepared on the fiber end of the fiber with a compact structure, which can be employed in Raman distributed temperature measurement, especially in the case of narrow space such as oil field and oil wells.