Fabrication of a Low-Loss Fused Fiber Spatial-Mode Coupler for Few-Mode Transmission

Spatial-mode couplers (SMCs) are critical devices for mode-division-multiplexed (MDM) systems. In this letter, we report a low-loss symmetric SMC fabricated with custom-designed two-mode fiber and commercially available standard fused biconical taper station. Sophisticated hydrofluoric etching or pre-pulling on the fiber sample is not needed for fabrication. We characterize the performance of the fabricated SMC, which shows a low loss of 1.1-2 dB and high modal extinction ratio of >= 20 dB for a broad wavelength range from 1539 to 1555 nm. We demonstrate transmission of 69.2-Gb/s LP01/LP11 dual-mode MDM coherent optical orthogonal-frequency division multiplexing signal over 4.5-km two-mode fiber using the fabricated SMC as mode multiplexer. The signal is successfully received at low (0.1/0.5 dB) optical signal-to-noise ratio penalty, and the estimated channel matrix confirms that there is very low modal crosstalk in both the SMC and the fiber.