Low-crosstalk mode-group demultiplexers based on Fabry-Perot thin-film filters.

Mode-group multiplexing (MGM) can increase the capacity of short-reach few-mode optical fiber communication links while avoiding complex digital signal processing. In this paper, we present the design and experimental demonstration of a novel mode-group demultiplexer (MG DeMux) using Fabry-Perot (FP) thin-film filters (TFFs). The MG DeMux supports low-crosstalk mode-group demultiplexing, with degeneracies commensurate with those of graded-index (GRIN) multimode fibers. We experimentally demonstrate this functionality by using a commercial six-cavity TFF that was intended for 100 GHz channel spaced wavelength-division multiplexing (WDM) system.

Optimizing free space to few-modefiber coupling efficiency.

We evaluate the coupling efficiency from free space to few-mode fibers (FMF). A theoretical coupling model is developed based on the imaging properties of free-space modes and the inner products between the free-space modes and the FMF modes. Numerical results for six- and ten-mode free-space-to-FMF coupling are presented. It is found that in order to maximize the total coupling efficiency, the size of the highest-order free-space and fiber modes should be approximately matched, and there is an optimum value for the V number of the FMF. In addition, the axial alignment tolerance for free-space-to-FMF coupling is more stringent than for single-mode cases.