Abstract
We model and demonstrate a simple mode selective all-fiber coupler capable of exciting specific higher order modes in two- and few-mode fibres with high efficiency and purity. The coupler is based on inter-modally phase-matching the propagation constants in each arm of the asymmetric fused coupler, formed by dissimilar fibres. At a specific coupler diameter, the launched fundamental LP(01) mode is coupled into the higher order mode (LP(11), LP(21), LP(02)) in the other arm, over a broadband wave-length range around 1550 nm. Unlike other techniques, the demonstrated coupler is composed of a multimode fiber that is weakly fused with a phase matched conventional single mode telecom fiber (SMF-28). The beating between the supermodes at the coupler waist produces a periodic power transfer between the two arms, and therefore, by monitoring the beating while tapering, it is possible to obtain optimum selection for the desired mode. High coupling efficiencies in excess of 90% for all the higher order modes were recorded over 100 nm spectral range, while insertion losses remain as low as 0.5 dB. Coupling efficiency can be further enhanced by performing slow tapering at high temperature, in order to precisely control the coupler cross-section geometry.
Highlights
Single mode fibers (SMF) propagating only a fundamental mode have been used as a longhaul transmission medium in optical fiber communications and developed for many photonics applications, including sensors, fiber lasers, spectroscopy and bio-medical uses
We further explore the possibility of obtaining an efficient mode conversion into high order modes with high modal purity, by using asymmetric fiber couplers formed by fusing pre-tapered SMF and either two or four mode fiber (FMF)
The mode purity was confirmed by the bending loss scheme with a modal extinction ratio of < 11dB,The pattern of the LP11 output shows an interference pattern, the reason for this interference could be the existence of the LP01 in the output of two mode fiber (TMF), due to mismatch in the coupler parameters resulted from some variation in the taper geometry, especially in the transition region of the coupler
Summary
Single mode fibers (SMF) propagating only a fundamental mode have been used as a longhaul transmission medium in optical fiber communications and developed for many photonics applications, including sensors, fiber lasers, spectroscopy and bio-medical uses. A more challenging task has been represented by the add-drop multiplexers for single channels For this task, all-fiber fused modal couplers promisies a cheap, compact and efficient mode conversion approach. One interesting approach [17, 18] was to use a tapered two mode fiber (TMF) to serve as a single mode fiber which was fused together with another TMF which had been pre-etched to remove the degeneracy between the high order modes [19]. We further explore the possibility of obtaining an efficient mode conversion into high order modes with high modal purity, by using asymmetric fiber couplers formed by fusing pre-tapered SMF and either two or four mode fiber (FMF)
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