Low-Loss and Low-Mode-Dependent-Loss Fan-In/Fan-Out Device for 6-Mode 19-Core Fiber

This paper describes a low-loss fiber-bundle-type fan-in/fan-out (FI/FO) device for 6-mode 19-core fiber, which exhibits a low mode-dependent loss (MDL) with a physical-contact connection for all the cores. We reveal that reducing the offset at the connection of few-mode fibers (FMFs) at butt coupling does not necessarily result in a minimum MDL, and the FMFs should be aligned so as to minimize the MDL, by experimentally investigating the MDL as a function of the lateral offset at butt coupling. We develop a precise rotational alignment mechanism to accurately adjust the core position without any complex alignment equipment, and we introduce a hexagonal microhole into a ferrule to accurately arrange the fibers. With these technologies, we successfully suppress the mode-dependent loss of the FI/FO device. The device provides a low MDL variation of less than 2 dB for all the cores with good optical properties such as a low excess loss, low core-to-core crosstalk, and a high return loss. The fabricated FI/FO device also achieved the highest reported spatial multiplicity connection of more than 100 with a small footprint as well as a low connection loss. Other optical properties that we obtained such as the modal crosstalk are also described.