Specialty optical fibres and devices for the internet of things

Abstract

Multicore fibers, coherent communications, polarization and modal multiplexing have been recently proposed to address the perceived lack of data transmission capacity in optical fiber communication systems [1]. While fibers, and amplifiers have been promptly developed in mode division multiplexing (MDM), add/drop multiplexing still represents a challenge. Here a weakly-fused microfiber coupler manufactured from a single mode fiber (SMF) and a few-mode fiber (FMF) allows for the efficient multiplexing of a high order mode in the FMF from a fundamental mode at the SMF input. Unlike conventional couplers, which are manufactured at high temperature and thus exhibit a circular cross section, intermodal couplers are manufactured at low temperature allowing for the tackling of the two microfibers without any circularization. Because of the circular geometry of the microfibers composing the coupler, the modes propagating in the two microfibers have a profile extremely similar to those of single microfibers in isolation and coupling between the SMF fundamental mode and the FMF high order mode is simply achieved by matching the propagation constants of the modes propagating in the two fibers [2]. When microfibers have a size smaller than 10µm, modes are guided by the silica/air interface and the modal effective index is determined by the microfiber diameter. Because of the larger overlap with the surrounding lower refractive index cladding, higher order modes have a lower effective index, thus matching the propagation constants of different modes in different microfibers can be achieved by using different microfiber diameters, i.e. with asymmetric coupler cross sections. In the coupler uniform waist region, coupling mostly occurs between the mode propagating in the SMF microfiber and the mode with closest propagation constant in the FMF microfiber, as the coupling strength between different modes is inversely proportional to the mismatch between the mode propagation constants.
Intermodal optical fibre couplers were fabricated using the modified flame brushing technique [3]. The SMF was a commercially available telecom fiber with cut-off at lambda=1.25µm, while the FMF consisted of a four-mode fiber with outer diameter of OD=125µm. The ratio between the microfibers was obtained from the eigenvalues of the propagation equation for a silica strand in air, assuming that the modes had the same modal effective index in the coupling region. The device length was optimized to have complete power transfer. The average insertion loss recorded in the couplers was of the order of 0.1dB. The spectral purity has been evaluated using CCD cameras by comparing intensity levels between the maximum and the center of the mode, confirming that for higher order modes coupling in excess of 96% have been achieved.

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Identifiers

ORCID for Gilberto Brambilla: orcid.org/0000-0002-5730-0499

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