All-fiber Mode Selective Coupler Research Articles

All-Fiber Low-Modal-Crosstalk Demultiplexers for DSP-Free IM/DD LP-Mode MDM Transmission

Weakly coupled mode-division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) transmission are promising methods of enhancing the capacity of short-reach scenarios in which low-modal-crosstalk-mode demultiplexers for degenerate linear polarized (LP) modes are highly desired. In this paper, we review two degenerate-mode reception schemes. Firstly, a low-modal-crosstalk orthogonal combined reception method for degenerate modes is proposed based on all-fiber mode-selective couplers, in which signals in both degenerate modes are demultiplexed into the LP01 mode of single-mode fibers and then are multiplexed into the mutually orthogonal LP01 and LP11 modes of a two-mode fiber (TMF) for simultaneous detection. Secondly, a novel degenerate-mode-selective coupler consisting of an input few-mode fiber and an output TMF is proposed, which could demultiplex degenerate LP modes without any digital signal processing (DSP). Both demultiplexers are achieved based on the taper and polish process. The fabricated devices are characterized and compared. The results show that the proposed schemes can pave the way to the practical implementation of DSP-free IM/DD LP-mode MDM transmission systems.

Heterogeneous optical network incorporating low-loss ring-core fiber and single-mode fibers seamlessly connected by all-fiber vortex (de)multiplexer

We propose and experimentally demonstrate heterogeneous optical network to make the space-division multiplexing (SDM) network with ring-core fiber (RCF) compatible with the legacy optical networks with conventional single-mode fiber (SMF). Two homemade all-fiber mode selective couplers are fabricated by fusing pre-tapered conventional SMF with multi-mode fiber (Type: Corning OM3), serving as orbital angular momentum (OAM) (de)multiplexer to enable seamless connection between heterogeneous optical network incorporating low-loss RCF (loss: 0.26 dB/km) based SDM network and ultra-low-loss SMFs (loss: 0.154 dB/km) based networks. We transmit 30-Gbit/s 8-ary quadrature amplitude modulation signals over two OAM modes (OAM1,1 and OAM2,1) in different mode groups without multiple-input multiple-output equalization techniques through 100-km heterogeneous RCF and SMF network (25-km ultra-low-loss SMF + 50-km low-loss RCF + 25-km ultra-low-loss SMF). The measured optical signal-to-noise ratio penalties for multiplexing transmission are less than 5.73 dB at a bit-error rate of 3.8 × 10−3, indicating that the demonstrated OAM-assisted heterogeneous optical network has a favorable performance.

All-Fiber Mode Selective Coupler Based on Vector Modes

All-Fiber Mode Selective Coupler Based on Vector Modes

Long-distance transmission of quantum key distribution coexisting with classical optical communication over a weakly-coupled few-mode fiber.

Quantum key distribution (QKD) is one of the most practical applications in quantum information processing, which can generate information-theoretical secure keys between remote parties. With the help of the wavelength-division multiplexing technique, QKD has been integrated with the classical optical communication networks. The wavelength-division multiplexing can be further improved by the mode-wavelength dual multiplexing technique with few-mode fiber (FMF), which has additional modal isolation and large effective core area of mode, and particularly is practical in fabrication and splicing technology compared with the multi-core fiber. Here, we present for the first time a QKD implementation coexisting with classical optical communication over weakly-coupled FMF using all-fiber mode-selective couplers. The co-propagation of QKD with one 100 Gbps classical data channel at -2.60 dBm launched power is achieved over 86 km FMF with 1.3 kbps real-time secure key generation. Compared with single-mode fiber using wavelength-division multiplexing, given the same fiber-input power, the Raman noise in FMF using the mode-wavelength dual multiplexing is reduced by 86% in average. Our work implements an important approach to the integration between QKD and classical optical communication and previews the compatibility of quantum communications with the next-generation mode division multiplexing networks.

A mode generator and multiplexer at visible wavelength based on all-fiber mode selective coupler

Abstract Using an all-fiber mode selective coupler (MSC) at the visible band, here we experimentally demonstrate a generating and wavelength multiplexing scheme for the cylindrical vector (CV) and vortex beams (VBs). The proposed MSCs act as efficient mode converters to produce spectrally insensitive high-order modes (HOMs) at the wavelength ranging from 450 to 980 nm, which have broad operation bandwidth (more than 7 nm), high mode conversion efficiency (94%), and purity (98%), and low insert loss (below 0.5 dB). By adjusting the polarization state and the phase shift of linear polarization (LP)11 mode respectively, the donut-shaped CVs and circular-polarization VBs are achieved. The focused intensity distribution of the donut beam on the cross- and axial-sections is monitored by using a confocal system. The all-fiber solution of producing and multiplexing HOMs opens a new route for stimulated emission depletion microscopy applications.

Ultra-Low-Loss Broadband All-Fiber Mode Selective Couplers for MIMO-Less MDM Transmission

Ultra-low-loss all-fiber mode selective couplers (MSCs) have been designed and fabricated using fused biconical taper technique with precise control on phase matching condition. An LP01 MSC with minimum insertion loss about 0.1 dB is fabricated. It is promising for single-mode excitation in large-mode-area few-mode fibers. The insertion loss of the demonstrated LP11 MSC is 0.3 dB at 1540 nm with mode extinction ratio higher than 28 dB. It has a broad operation wavelength from 1480 nm to 1640 nm with wavelength dependent loss about 0.3 dB. To the best of our knowledge, it shows the lowest loss in the reported broadband LP11 mode converters with high mode purity. The fabrication process is reliable and repeatable with smooth power curves, which is of great importance to the practical fabrication of these components. MIMO-less 2 × 25-Gb/s OOK signal and 2 × 20-Gb/s PAM-4 signal transmission over 15-km few-mode fiber have been demonstrated using the fabricated components. The low loss, large bandwidth and the high fabrication repeatability of the mode selective couplers make them promising for low-cost MIMO-less WDM-MDM transmission.

Generation of arbitrary polarized OAM mode based on a fiber mode selective coupler

In this paper, the generation mechanism of arbitrary polarized orbital angular momentum mode using an all-fiber mode selective coupler (MSC), composed of a single mode fiber and a two-mode fiber, are proposed and investigated. Theoretical investigation reveals that no matter which polarization state beam is launched into the MSC, first order arbitrary polarized orbital angular momentum (OAM) mode can be obtained by carefully adjusting a squeezer polarization controller. Additionally, the excitation of first order linearly-polarized and circularly-polarized OAM modes is experimentally demonstrated when the polarization state of input light is changed.

Fundamental-mode MMF transmission enabled by mode conversion

Modal dispersion in conventional multi-mode fiber (MMF) will cause serious signal degradation and an effective solution is to restrict the signal transmission in the fundamental mode of MMF. In this paper, unlike previous methods by filtering out higher-order modes, we propose to adopt low-modal-crosstalk mode converters to realize fundamental-mode MMF transmission. We design and fabricate all-fiber mode-selective couplers (MSC), which perform mode conversion between the fundamental mode in single-mode fiber (SMF) and fundamental mode in MMF. The proposed scheme is experimentally compared with center launching method under different MMF links and then its wavelength division multiplexing (WDM) transmission performance is investigated. Experimental results indicate that the proposed mode conversion scheme could achieve better transmission performance and works well for the whole C-band.