Design and analysis of an integrated optical coupler based on three-core fiber with long-period gratings

Oriented to the multicore fiber-based space-division multiplexing (SDM) networks, the inter-core coupling in multicore fiber is desired to transfer signals between cores. In this paper, we proposed and designed an integrated optical coupler based on three-core fiber (TCF) with long-period gratings (LPGs). The TCF with three cores distributed in the cladding connected with single-mode fiber at both ends through adjacent cores. The self-coupling characteristics of TCF are studied firstly, and the effects of core pitch and coupling distance on coupling efficiency are analyzed to rule out the direct coupling between adjacent cores, indicating that coupling distance increases with the core pitch increases. With LPGs induced in TCF, coupling between core mode and cladding mode in each individual core is realized, and then the power at resonance wavelength transfers among cores through the overlapped cladding modes. Moreover, with the LPGs, the efficient coupling among adjacent cores in TCF has been realized at the C-band. In addition, the influence of structural parameters such as core pitch, grating duty cycle and coupling co-efficiency do affect the coupling efficiency of TCF-LPGs. By adjusting the structural parameters properly, coupling efficiency of ~ 66% could be achieved to equalizing the output power in three cores. Therefore, the proposed TCF-LPGs could be used as an optical coupler, which would be helpful in multicore fiber-based SDM networks.