Weakly-coupled Few-mode Fiber Research Articles

Mode-division-multiplexing self-homodyne coherent transmission over a weakly coupled few-mode fiber for optical interconnections.

Self-homodyne coherent transmission has recently received extensive investigation as a coherent lite candidate for high-speed short-reach optical networks. In this Letter, we propose a weakly coupled mode-division-multiplexing (MDM) self-homodyne coherent scheme using a multiple-ring-core few-mode fiber, in which one of the modes transmits a self-homodyne local oscillator (LO) and the rest are utilized for carrying signals. Multiple rings of index perturbations in the fiber core are applied to achieve low modal crosstalk, allowing the signals and the remote LO to be transmitted independently. We experimentally demonstrate a 7.2-Tb/s (5.64-Tb/s net rate) self-homodyne coherent transmission with an 800-Gb/s data rate for each of the nine information-bearing modes formatted in 80-GBaud probabilistic constellation-shaped 64-quadrature-amplitude modulation. To the best of our knowledge, this is the first experimental demonstration of an MDM self-homodyne coherent transmission with up to 10 spatial modes. The proposed scheme may pave the way for future high-capacity data center interconnections.

Modal dependence of Rayleigh scattering coefficients based on the bidirectional OTDR technique

In this work, a simple method based on optical time-domain reflectometry has been implemented to compare the Rayleigh scattering coefficient of different modes of few-mode fibers. The principle of the method is first validated in the case of single-mode fibers and then applied to two weakly-coupled few-mode fibers presenting different refractive index profiles. The results are compared with those obtained by more sophisticated experiments based on the analysis of the angular distribution of light scattering.

Optimizing graded-index few-mode fiber for space division multiplexing.

We investigate the graded-index few-mode fiber (GI-FMF) to realize a 4-LP-mode (i.e. LP01, LP11, LP21, and LP02) fiber for mode-division-multiplexed transmission. This study optimizes the GI-FMF for both, first, for large effective indices differences (Δneff), and second, for low differential mode delay (DMD) between any two LP modes, for different optimized parameters. Thus, it shows that GI-FMF is suitable for both weakly-coupled few-mode fiber (WC-FMF) as well as strongly-coupled few-mode fiber (SC-FMF) via adjusting the profile parameter (α), refractive index difference between core and cladding (nco - nclad), and core radius (a). We report the optimized parameters for WC-GI-FMF with large effective indices difference (Δneff) of 0.6 × 10-3 and low |DMD| of 5.4 ns/km while the minimum effective mode area (Min.|Aeff|) is 80 µm2 and bending loss (BL) of the highest order mode is 0.005 dB/turn (much lower than 10 dB/turn) at a 10 mm bend radius. Here, we could break down the degeneracy between LP21 and LP02 mode, which remains a challenging task in GI-FMF. To the best of our knowledge, this is the lowest DMD (5.4 ns/km) ever reported for such a weakly-coupled (Δneff = 0.6 × 10-3) 4-LP-mode FMF. Similarly, we optimized the parameters for SC-GI-FMF with Δneff of 0.1 × 10-3 and the lowest DMD of 0.9 ns/km while Min.|Aeff| is ≫ 100 µm2 and BL of higher order mode is 6 dB/turn (< 10 dB/turn) at 10 mm bend radius. Further, we investigate narrow air trench-assisted SC-GI-FMF to reduce the DMD and achieve the lowest DMD of 16 ps/km for a 4-LP-mode GI-FMF with a minimum Δneff of 0.7 × 10-5.

Five-LP-Mode IM/DD MDM Transmission Based on Degenerate-Mode-Selective Couplers With Side-Polishing Processing

Weakly-coupled mode division multiplexing (MDM) techniques supporting intensity modulation and direct detection (IM/DD) have attracted great attention for capacity enhancement of short-reach applications such as optical interconnections, for which mode multiplexers/demultiplexers (MMUX/MDEMUX) with low insertion loss (IL) and modal crosstalk are the key components. In this paper, we firstly propose the design method for degenerate–mode-selective couplers (DMSC) with side-polishing processing. Three kinds of DMSCs for different degenerate linearly-polarized (LP) modes (LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> , LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">21</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">31</sub> ) in a weakly-coupled few-mode fiber (FMF) are designed. Then a pair of MMUX and MDEMUX consisting of cascaded mode-selective couplers (MSC) and DMSCs for all the 5 LP modes of the FMF are fabricated with side-polishing processing. The measurement results show that the MMUX and MDEMUX achieve low back-to-back IL lower than 5.3 dB for all the 5 modes. Finally, stable digital-signal-processing (DSP)-free 5-mode IM/DD MDM transmission utilizing conventional optical modules over 10-km FMF are successfully demonstrated. The proposed low-IL MMUX/MDEMUX is scalable to support more modes and can promote the practical application of MDM techniques.

Mode division multiplexing chaotic encryption scheme based on key intertwining and accompanying transmission.

A mode division multiplexing (MDM) chaotic encryption scheme based on key intertwining and accompanying transmission is proposed in this paper. Based on the weakly coupled few-mode fiber (FMF), data and time-varying keys can be accompanied by transmission in two modes, LP01 and LP11, respectively. In order to generate a new key, the current key is XORed with all of the keys from all the preceding moments, one by one. To implement chaotic masking in the digital domain, the three chaotic sequences corresponding to the new key are adopted to encrypt the data at the constellation phase, data symbol block, and subcarrier levels. An 8.89 Gb/s encrypted 16QAM-OFDM signal transmission over 1 km weakly-coupled FMF is experimentally demonstrated. The receiver with the correct key can recover the data normally, while the BER of the illegal receiver remains around 0.5. In the case of the key transmission bit rate of 1 Gb/s, the cracking efficiency threshold of the time-varying key encryption scheme is 5.21 × 106 times that of the time-invariant key encryption scheme, which suggests that the proposed work is a promising candidate for future physical layer security.

Design of weakly-coupled ultra-flattened dispersion few-mode photonic crystal fiber

Design of weakly-coupled ultra-flattened dispersion few-mode photonic crystal fiber

Prototype of DSP-Free IM/DD MDM Transceiver for Datacenter Interconnection

New transmission techniques for short-reach optical transmission applications especially datacenter interconnections are highly expected to break current bandwidth bottleneck, among which mode division multiplexing (MDM) based on few-mode fiber (FMF) is promising by exploring different linearly-polarized (LP) modes or mode groups as spatial transmission channels. However, it has been a great challenge to deal with degenerate modes in FMFs. In this paper, a FMF design utilizing multiple ring perturbations in fiber core to suppress modal crosstalk during fiber transmission is firstly proposed and a 6-LP-mode double-ring-core (DRC) FMF is fabricated with a high minimum refractive index difference (min|neff|) of 1.4910-3 among all the modes. Secondly, a novel degenerate-mode-selective coupler (DMSC) converting each degenerate LPmn mode into the LP11 mode in a two-mode fiber is proposed. Then intensity-modulation/direct-detection (IM/DD) MDM transmission scheme without any digital signal processing (DSP) can be realized utilizing mode multiplexer/demultiplexer (MUX/DEMUX) consisting of cascaded mode-selective couplers (MSC) and DMSCs, in which each degenerate or non-degenerate LP mode is considered as an independent transmission channel. Then based on the proposed DRC-FMF and mode MUX/DEMUX, we demonstrate a 4-LP-mode prototype system using commercial single-mode 10 Gbps SFP + modules and 4K video transceivers without any hardware or software modifications. Finally, the intra-LP-mode dispersion (ILMD) effect in a weakly-coupled FMF is analyzed and a modified fixed-analyzer method is proposed for its measurement. Experimental results show that it may be one of the major impairments for IM/DD MDM transmission.

Optical Performance Monitoring for Intra-LP-Mode Dispersion in Weakly-Coupled Mode-Division Multiplexed Systems

Weakly-coupled mode-division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fiber (FMF) has been considered as a promising candidate for capacity enhancement of next-generation optical transmission systems. Similar to polarization mode dispersion in single-mode fibers (SMFs), signals in weakly-coupled FMFs suffer from the impacts of intra-LP-mode dispersion (ILMD) for both degenerate and non-degenerate LP modes.In this paper, we propose an effective method for ILMD monitoring in weakly-coupled MDM systems. The ILMD evaluation method for degenerate LP modes is designed by the analysis of channel transfer matrix in a digital coherent receiver. Then the ILMD monitoring scheme is experimentally validated for all the 6 LP mods in a weakly-coupled FMF. The influences of chromatic dispersion, optical signal to noise ratio and inter-LP-mode crosstalk to the method are also investigated.

Distributed Transverse Stress Sensor Based on Mode Coupling in Weakly-Coupled FMF

Distributed optical fiber sensors (DOFSs) have faced the challenge of measuring transverse stress along the fiber and the current main approach has been based on polarization coupling effect in polarization-maintaining fibers (PMFs), which has short sensing length and high dependence on direction of exerted stress. Instead, here we propose a novel distributed transverse stress sensor (DTSS) based on coupling effect between linearly-polarized (LP) modes in weakly-coupled few-mode-fibers (FMFs). In this scheme, multiple LP modes could be considered as independent spatial channels without stress perturbation because of ultralow inherent modal crosstalk, while quantifiable and spatially-resolvable mode coupling for a probe signal will occur under transverse stress satisfying phase-matching conditions. A proof-of-concept DTSS system is verified based on weakly-coupled two-mode fibers and mode-selective couplers for mode conversion. Moreover, we show that the scheme is little affected by mild common parameters including temperature, strain, twist, direction of stress, or state-of-polarization (SOP), which is crucial for accurate stress analysis under complex environmental conditions. The proposed DTSS scheme has simple structure, high flexibility for different sensing ranges and resolutions, and high collaborating capability with other sensing mechanisms.

Analysis and Measurement of Intra-LP-Mode Dispersion for Weakly-Coupled FMF

Weakly-coupled mode division multiplexing (MDM) technique based on linearly-polarized (LP) modes in ultralow-modal-crosstalk few-mode fibers (FMF) has been considered as a promising candidate for capacity enhancement of short-reach transmission applications. Similar to the polarization mode dispersion (PMD) in single-mode fibers (SMF), signals in weakly-coupled FMFs suffer from the influence of intra-LP-mode dispersion (ILMD). In this paper, the generation mechanism of ILMD is firstly analyzed and the mean ILMDs in a typical step-index circular-core FMF are calculated. Then numerical simulations are carried out to investigate its impact and the severe performance degradation indicates that it may be one of the major transmission impairments for intensity-modulation/direct-detection (IM/DD) MDM systems. Moreover, an Improved Fixed Analyzer Method (IFAM) is proposed for ILMD measurement, based on which a weakly-coupled FMF supporting 6 LP modes is measured. Experimental results show that the ILMD for each circularly-symmetric LP mode behaviors like PMD in a SMF, while it may be up to tens of times larger for the non-circularly-symmetric LP modes. This work evaluates for the first time the ILMD impact and is beneficial for the design of weakly-coupled FMFs and IM/DD MDM systems.

A switchable multi-wavelength EDFL using cascaded Sagnac fiber loops with mode differential delay line

We propose and experimentally demonstrate a switchable multi-wavelength Erbium-doped fiber laser that employs cascaded weakly-coupled few-mode fiber (FMF) Sagnac fiber loops with mode differential delay line. A comb filter of multiple specific wavelengths is performed by incorporating a weakly-coupled FMF in cascaded Sagnac fiber loops. The incorporation of the weakly-coupled FMF is realized with all-fiber mode multiplexer/demultiplexer (MUX/DEMUX), which can simultaneously generate transversal LP01 and LP11 modes. The comb filter characteristics of cascaded Sagnac fiber loops are simulated and analyzed by Jones matrix. By adjusting the polarization controllers (PCs), the fiber laser can operate in single-, dual-, and triple-wavelength laser states around C band. The side mode suppression ratio is larger than 30.0 dB. Moreover, the laser output stabilities for single-, dual-, and triple-wavelength lasing are investigated in a range of over 30 min at room temperature. Experimental results indicate that this work has potential applications in WDM systems, sensing, and instrument testing.

Weakly-Coupled MDM-WDM Amplification and Transmission Based on Compact FM-EDFA

Recently optical amplification technologies for mode-division multiplexing (MDM) transmission systems and networks have attracted great interest. In this article, we first propose a compact structure of few-mode Erbium-doped fiber amplifier (FM-EDFA) for weakly-coupled MDM amplification, which is characterized by weakly-coupled few-mode Erbium-doped fiber (EDF), cascaded mode-selective couplers (MSCs) for spatial mode control of pump lights, and few-mode wavelength multiplexer (FM-WMUX) for pump/signal coupling. Compared to previous studies, the proposed FM-EDFA has a similar structure with commercial single-mode EDFA (SM-EDFA) and can support weakly-coupled MDM transmission without inducing large modal crosstalk. Then related components are designed and fabricated for a 2-mode proposed FM-EDFA. The mode-field mismatching between transmission FMF and EDF under central alignment is investigated by simulation. Thanks to the effective suppression of modal crosstalk for the optical components, the transmission FMF, the EDF and their coupling, we experimentally demonstrate the first weakly-coupled MDM-WDM amplification and transmission over 15-km 2-mode FMF with 10-Gb/s optical on-off keying (OOK) modulation and direct detection. The proposed FM-EDFA can be extended to MDM transmission systems and networks supporting more 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.

Switchable multi-wavelength linearly-polarized lasing oscillations in a figure eight EDF laser based on spatial-mode beating by means of weakly-coupled FMF

A switchable multi-wavelength figure eight Erbium-doped fiber (EDF) laser with linearly-polarized (LP) beam generation in the C-band region is proposed and experimentally demonstrated. Spatial-mode beating occurs owing to effect of multi-mode interference in the weakly-coupled few-mode fiber (FMF). A bandpass filter of multiple specific wavelengths is created by incorporating a weakly-coupled FMF with a Sagnac fiber loop. The LP modes are generated by cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). Combination of the cascaded MSCs and a simple N × 1 optical switch is employed to switch the LP modes. In experiments, the proposed fiber laser delivers versatile lasing outputs in states of single-, dual-, triple-wavelength oscillations for LP01, LP11 and LP21 modes. We investigate the stabilities for LP01, LP11, LP21 modes in 24-min. Moreover, the mode intensity profiles, optical signal-noise ratios (OSNRs) of lasing output and slope efficiencies are also measured. The proposed fiber laser has promising applications in WDM-MDM optical fiber communication systems, optical fiber sensor systems, optical instruments and spectroscopy.

Statistics of mode couplings induced by concatenated multiple connections in weakly-coupled few-mode fibers

We numerically investigate the statistical characteristics of mode couplings induced by concatenated multiple few-mode fiber connections with lateral misalignments in weakly-coupled mode division multiplexed (MDM) systems. For the currently-feasible connectors, the probability of the crosstalk ratio being smaller than -20 dB is remarkably degraded as increasing the number of the connections. The currently-feasible connectors are not applicable to the weakly-coupled MDM systems. In order to suppress the mode crosstalk by smaller than -20 dB, the misalignment deviation is required to be smaller than 0.04, which corresponds to be 0.3% of the mode field diameter of the fundamental mode. In addition, we propose the simple model giving us intuitional understanding for mode couplings induced by few-mode fiber connections.

Applications of weakly-coupled few-mode fibers [Invited

Space-division multiplexing (SDM) has attracted significant attention in recent years because larger transmission capacity is enabled by more degrees of freedom (DOFs) in few-mode fibers (FMFs) compared with single-mode fibers (SMFs). To transmit independent information on spatial modes without or with minor digital signal processing (DSP), weakly-coupled FMFs are preferred in various applications. Several cases with different use of spatial DOFs in weakly-coupled FMFs are demonstrated in this work, including single-mode or mode-group-multiplexed transmission, and spatial DOFs combined with time or frequency DOF to improve the system performance.

Design and Characterization of Nanopore- Assisted Weakly-Coupled Few-Mode Fiber for Simpler MIMO Space Division Multiplexing

We report on a novel weakly-coupled 6-LP-mode fiber that features a nanopore-assisted step-index core with double-cladding profile. The purpose of this fiber design is to alleviate modes coupling and bend sensitivity, thus simplifies the multiple-input multiple-output (MIMO) process and the environmental requirements in which the fiber is used. Through numerical simulations, we show that the minimum effective index difference (Δfl eff ) of larger than 1.8*10 -3 among all LP-mode over C and L band can be achieved by a simple nanopore with 170 nm radius in the center of the core. Furthermore, the dependences of both bending loss and chromatic dispersion on fiber structures are investigated by the finite element method (FEM) to show the good broadband performance. The proposed fiber targets applications in simpler MIMO space division multiplexing (SDM) systems such as data center and short-reach large-capacity transmission.

Full C-Band 3060-km DMD-Unmanaged 3-Mode Transmission With 40.2-Tb/s Capacity Using Cyclic Mode Permutation

We have achieved the longest three mode-multiplexed full C-band transmission yet attained over 3060 km. In wideband mode-multiplexed transmission over weakly-coupled fewmode fibers (FMFs), the width of signal impulse responses is dependent on wavelength, and also exhibits a linear growth with increased transmission distance because of the presence of differential mode delay (DMD). Those properties are technically challenging issues for future deployable space division multiplexing (SDM) transport systems since they make a system design complicated, especially for terrestrial FMF transmission links. In this article, we describe how we applied a cyclic mode permutation (CMP) technique to achieve wideband long-haul FMF transmission where spatial channels are cyclically interchanged in each span to suppress DMD-induced pulse broadening. The CMP technique enabled DMD-unmanaged long-haul transmission across 4.4-THz optical bandwidth over two-mode FMF whose DMD varied in the range from 33.7-44.3 ps/km, resulting in 3060-km FMF transmission with a net capacity of 40.2 Tb/s.

A Degenerate-Mode-Selective Coupler for Stable DSP-free MDM Transmission

Weakly-coupled mode division multiplexing (MDM) has been intensively investigated in recent years. In order to be compatible with conventional intensity modulation and direct detection systems, effective solutions to handle degenerate non-circularly-symmetric linearly-polarized (LP) modes ( $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes, l ≥ 1) of weakly-coupled few-mode fibers (FMFs) are highly desired. In this paper, a novel degenerate-mode-selective coupler (DMSC) consisting of an input FMF and an output two-mode fiber (TMF) is proposed based on coupling mechanisms of a FMF directional coupler. Optical powers in both $\text{LP}_{lm{\rm{a}}}$ and $\text{LP}_{lm{\rm{b}}}$ modes of the FMF of $\text{LP}_{lm}$ DMSC can be selectively and simultaneously transferred to the LP11a and LP11b modes of the TMF. Therefore, the DMSC can demultiplex degenerate non-circularly-symmetric LP modes without any digital signal processing (DSP). Necessary conditions for the DMSC are theoretically investigated. LP11 and LP21 DMSCs are numerically simulated to verify the principles and fabricated in the form of fused-type fiber couplers. Compared with regular mode-selective couplers (MSCs), the fabricated DMSCs significantly improve the power stability for demultiplexing randomly-rotated non-circularly-symmetric LP modes. Based on a mode demultiplexer composed of cascaded LP02 MSC, LP21 DMSC, LP11 DMSC and LP01 MSC, 4-LP-mode MDM transmission over 10-km of weakly-coupled FMF is demonstrated with 10-Gb/s on-off-keying and DSP-free direct detection.

A 6-LP-mode ultralow-modal-crosstalk double-ring-core FMF for weakly-coupled MDM transmission

Weakly-coupled mode division multiplexing (MDM) technique based on few mode fiber (FMF) with low linear-polarized (LP) modal crosstalk is promising for capacity enhancement of short-reach transmission. In this paper, we propose a double-ring-core FMF for simultaneous 6-LP-mode weakly-coupled MDM transmission. FMF with ring-core structure is theoretically analyzed by deriving from a step-index circular-core FMF with perturbed ring areas. Then simplified method is proposed to define boundaries of ring areas in FMF and increase the minimum effective refractive index difference (min|Δneff|) among all the modes. Fabricated by commercial fabrication technique, the FMF achieves a min|Δneff| up to 1.49×10−3 while its maximum Δn between core and cladding is only 0.828%, which greatly reduces the difficulty of fiber fabrication. Thanks to the ultralow modal crosstalk of the FMF, we successfully demonstrate simultaneous 6-LP-mode (LP01, LP11, LP21, LP02, LP31 and LP12) 4-wavelength 71-km weakly-coupled MDM transmission using 10 Gb/s optical on-off-keying (OOK) intensity modulation and direct detection (IM/DD) without any digital signal processing (DSP). The design method can be extended to weakly-coupled FMFs with more ring areas.