All-fiber fused directional coupler for highly efficient spatial mode conversion

Abstract

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 wavelength 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.

A 1- $\mu$ m Cylindrical Vector Beam Fiber Ring Laser Based on a Mode Selective Coupler

“…The principle of the coupler is to phase match the fundamental mode in the SMF with a high-order mode in the TMF, and achieve fundamental mode conversion to high-order modes [21]. Fig.…”

“…C 11 and C 22 are self-coupling coefficients, C 12 and C 21 are mutual-coupling coefficients. C 11 and C 22 are negligible with respect to C 12 and C 21 and approximately C 21 =C 12 =C. The power distributions of the two output ports of the MSC are:…”

“…In order to achieve high mode purity, we use weak fusion technique for maintaining the geometry of the SMF and the TMF [21], [25]. Furthermore, unlike other mode selective coupler fabrication techniques for which SMF (or FMF) must be pre-tapered, we use technique free of pre-tapering to fabricate the MSC.…”

“…Efforts are still needed to improve the laser slope efficiency as well as to extend the laser operating wavelength with a high CVB purity. Furthermore, the MSC's fabrication technology is relatively complex due to the pre-tapered process [19]- [21].…”

Multi-channel mode converters based on in-line fiber modal interferometer

“…The most well-known and widely used technique is based on spatial light modulators [6] or phase plates [7], but it is complicate and difficult for bulk devices to align the optical path, hence the loss of the mode coupling is large. Low loss and low-crosstalk mode converter based directional coupler have been demonstrated to realize two mode coupling [8], while coupling of the degenerated mode with the same effective indexes is not possible. Recently, photonic lantern was reported to convert multiple beams into the super-mode in FMF [9], while multiple-input and multiple-output processing is required to compensate the signal crosstalk.…”