Fig. 1. 3D waveguide device and few-mode seven-core fiber. (a) One side of the 3D waveguide device is a waveguide array. (b) Top view of the 3D waveguide device. (c) Side view of the 3D waveguide device. (d) The other side of the 3D waveguide device is a seven-core arrangement.

Fig. 2. Numerically simulated effect of (a) different effective refractive index differences for a 3D waveguide length of 17 mm and (b) bending curvature on the waveguide transmission efficiency when the effective refractive index difference is 0.0026. Numerically simulated (c) LP01 mode and (d) LP11a mode distributions at different positions in the 3D waveguide device.

Fig. 3. Experimental intensity profiles of the (a) LP01 mode and (b) LP11a mode outputs from the 3D waveguide device. (c) Few-mode seven-core fiber. (d) Experimental setup of the few-mode fiber array coupled to an FM-MCF. Experimental intensity profiles of the (e) LP01 mode and (f) LP11a mode outputs from the few-mode seven-core fiber. (g) LP01 and LP11a modes insertion losses from the few-mode fiber array to the few-mode seven-core fiber.

Fig. 4. Measured cross talk between different waveguides when the (a) LP01 mode, (b) LP11a mode, and (c) LP11b mode propagate in the waveguide. Measured cross talk between different fiber cores when the (d) LP01 mode, (e) LP11a mode, and (f) LP11b mode propagate in the 1-km few-mode seven-core fiber.

Fig. 5. (a) The setup of SDM communication system based on the few-mode seven-core fiber; TX, transmitter; SMF, single-mode fiber; PC, polarization controller; VOA, variable optical attenuator; EDFA, erbium-doped fiber amplifier; RX, receiver, PED, programmable error detector. Bit error rate (BER) curves of (b) cores 3 and 4, (c) cores 3 and 6, (d) cores 1 and 5.

Fig. 6. (a) The setup of SDM QPSK communication system based on the few-mode seven-core fiber. Constellations of the QPSK signals: (b) with fiber cores 3 of LP11a mode; (c) with fiber cores 3 of LP01 mode; (d) with fiber cores 4 of LP11a mode; (e) with fiber cores 4 of LP01 mode.

Fig. 7. Fabrication setup for 3D waveguide devices based on femtosecond laser direct writing.

Fig. 8. (a) The effective refractive index difference versus radius of a fiber core in FM-MCF. Numerical simulation calculations show that the FM-MCF supports (b) LP01, (c) LP11a or LP11b, (d) LP21a or LP21b, (e) LP02, (f) LP31a or LP31b, (g) LP12a or LP12b modes transmission.