Broadband dual channel mode filter based on triple-core photonic crystal fiber

A novel mode filter based on a triple-core photonic crystal fiber (PCF) with a large bandwidth was proposed. The device uses silica as the substrate material, and its main core can support three modes: LP01, LP11, and LP21. By coating gold film on the inner walls of the air holes in the cladding of the side cores, the coupled LP01 and LP11 modes from the main core experience significant attenuation when propagating in the side cores. This enables the mode filter to output only the LP21 mode. The structure of the mode filter was optimized by using the eigenmode finite difference method and the eigenmode expansion method. The results indicate that when the input wavelength is 1.55 μm, the mode suppression ratios (MSRs) for the filtered LP01 and LP11 modes exceed 20 dB. Moreover, the mode filter operates in two working bands with bandwidths of 125 nm (1.5–1.625 μm) and 53 nm (1.766–1.819 μm), covering the S, C, and L bands. The total length of the device is only 46.5 mm. Especially, at 1550 nm, the insertion loss (IL) is as low as 0.024 dB, and the MSR is 19.84 dB. Owing to the advantages of wide bandwidth, low IL, high MSR, and compact structure, this mode filter will find great potential for applications in mode-division multiplexing fiber-optic communication/sensing, quantum information systems, and related fields.