O-Band 3D Optical Waveguide Fan-in/Fan-Out Devices for Few-Mode Multi-Core Fibers

The hybrid multiplexing of the wavelength and mode has been introduced to provide higher capacity density for data centers. One promising approach for further increasing the channel capacity is to combine them with a few-mode multi-core fiber (FM-MCF). Fan-in/fan-out (FI/FO) devices are important connectors between MCF and few-mode fiber (FMF). Here, an integrated three-dimensional optical waveguide broadband FI/FO device was demonstrated to convert light from a one-dimensional FMF array into a two-dimensional array matched with the FM-MCF. The device was fabricated by femtosecond laser direct writing and achieved three modes (LP01, LP11, LP21) transmission with high efficiency and low-wavelength dependence by optimizing the refractive index difference and waveguide length. The insertion losses (ILs) of LP01, LP11, and LP21 modes within the 7-cores at four wavelengths were less than 1.8-dB, 2.6-dB, and 6.5-dB, respectively. Based on the 7-core FI/FO device and FM-MCF, a multi-core four-wavelength (1271, 1291, 1311, 1331-nm) $\times $ two-mode (LP01, LP11) hybrid multiplexing communication system without any amplifier was demonstrated experimentally. 10-Gbit/s on-off keying signals communication below the forward error correction limit of 3.8E-3 per channel was achieved successfully using the direct detection technique. The proposed FI/FO device has great potential for hybrid coarse wavelength division multiplexing and space division multiplexing communication transmission in data centers.