Design for Ultra-Low Birefringence in Trench-Assisted Multicore Fibers via Stress Rod Implementation

Multi-core fibres (MCFs) have emerged as a highly promising alternative to traditional single-mode fibres (SMFs) for achieving higher transmission rates over long distances, with some early field trials already underway [1], [2]. One often overlooked aspect in the design process of MCFs is the birefringence characteristics of its cores. Unlike SMFs, which have only one core, the presence of other cores within the same cladding region will have an impact on the stress distribution in the MCF structure, ultimately affecting the birefringence. In the case of uncoupled heterogeneous (HT) MCFs, it has been reported that low birefringence can be beneficial for two key transmission parameters of MCFs: the inter-core crosstalk (IC-XT) and polarization-mode dispersion (PMD) [3]. This improvement arises from the reduction of random polarization-mode coupling effects enabled by lower birefringence [4]. As a result, designing MCFs with ultra-low birefringence characteristics is advantageous for fibre performance in long distance transmissions. In this work, we propose a method to achieve ultra-low birefringence in trench-assisted (TA) HT-MCFs by implementing a stress rod and optimizing its parameters.