Parallel six-mode-selective converter based on photonic crystal fiber without holes in the cladding.

Mode-selective converters (MSCs) play an indispensable role in mode division multiplexing (MDM) systems, and the commonly used cascaded waveguide-based MSCs not only have a relatively large size but also increase the insertion loss and mode crosstalk during the conversion process. In this paper, a parallel six-mode-selective converter (6-MSC) is proposed to enhance the integration of the device, which consists of a photonic crystal fiber (PCF) and six step-index fibers (SIFs). Here, a PCF without any holes in the cladding is proposed. Based on this structure, the distance between the PCF core and the SIF core can be arbitrarily adjusted, which greatly improves the design freedom of multi-core optical fiber devices. Simulation results show that at a wavelength of 1.55 µm, the proposed 6-MSC can achieve parallel conversion from the fundamental mode in each SIF to the corresponding modes ( 01, 01, 11 , 11 , 11 and 11 ) in the PCF at a device length of 800 µm. The maximum coupling efficiency is 96.22% and the crosstalk is below 3.45%. Moreover, the coupling efficiencies of all six mode pairs exceed 80% within the wavelength range of 1.538∼1.560 µm, and the 3-dB coupling efficiency bandwidth covers the entire C-band. To the best of our knowledge, this is the first report of parallel conversion between six orthogonal fiber modes and fundamental modes based on PCF. The proposed 6-MSC holds promising prospects for future integrated high-capacity MDM systems.