Mode-multiplexed Transmission Research Articles

OAM-SDM Solution Toward a Submarine Cable

Determining how to add more signal channels without enlarging the cable volume is continually an issue in managing submarine cables. To improve the cost per bit and the power efficiency, space division multiplexing (SDM) is regarded as a promising solution that could be used to address optical density and cable capacity challenges regarding submarine cables. In this study, we focus on angular momentum (OAM) modes supporting few-mode ring-core fiber (RCF) solutions for submarine cables. Considering the inhibition of the hydrogen loss for the submarine cable, we propose a center-region-lifted 3-OAM-mode-group supporting RCF (abbreviated to 3-OAM-MG RCF) with a core-cladding index contrast of 0.6 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> , and meanwhile we also design and fabricate a matched 3-OAM-MG erbium-doped fiber (EDF). To confirm the short-haul transmission ability, we conduct a wavelength/mode/polarization division multiplexing (WDM-MDM-PDM) transmission test through a 25.3-km 3-OAM-MG RCF, where the coupled modes inside MG are demultiplexed by using 2 × 2 multiple-input multiple-out (MIMO) equalization. The transmission capacity is approximately 35 Tbit/s and the bit error rates (BERs) of all the channels are less than 2.4 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$\times 10^{-2}$</tex-math></inline-formula> . For the application scenario of long-haul submarine communication, a cascade WDM-MDM transmission system consisting of 3-OAM-MG RCF and 3-OAM-MG EDF is set up, which makes the BERs of all the mode and wavelength channels below the 20 <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">${\%}$</tex-math></inline-formula> soft-decision forward error correction (SD-FEC) threshold. It indicates that data-carrying OAM mode multiplexing transmission has potential application in a long-haul submarine cable system.

Raman-amplification-assisted twisted light multiplexing transmission over large-core fiber

Twisted light-carrying orbital angular momentum (OAM) with a helical phase front and doughnut intensity profile has been widely investigated in fiber-optic communications. To facilitate long-distance OAM mode multiplexing transmission, an efficient OAM (de)multiplexer and a suitable inline OAM amplifier are highly desired. Here, we demonstrate three OAM modes and 22 wavelength channels multiplexing transmission with an OAM distributed Raman amplifier (DRA) over 110-km large-core fiber assisted by a homemade all-fiber OAM (de)multiplexer. To characterize the performance of the OAM DRA, two pump configurations including the first-order bidirectional pumping and second-order backward pumping are proposed and demonstrated. The obtained results indicate the successful implementation of the long-distance OAM mode multiplexing transmission assisted by the DRA and an all-fiber OAM (de)multiplexer with favorable performance. These demonstrations may open up new perspectives for ultra-long-haul capacity scaling fiber-optic communications employing OAM modes.

Weakly guiding graded-index ring-core fiber supporting 16-channel long distance mode division multiplexing systems based on OAM modes with low MIMO-DSP complexity.

We propose a weakly guiding graded-index ring-core fiber (RCF) with trench-assisted structure. The simulation analysis indicates that such a special fiber design is able to support 8 orbital angular momentum (OAM) mode groups (MGs) with low inter-group crosstalk (< -25 dB/km) and low intra-group differential mode delay (DMD) (< 125 ps/km) for higher order OAM MGs with topological charge |l| = 4, 5, 6, 7. The designed RCF also shows favorable tolerance characteristics to ellipticity and bending. Moreover, stable and distinguished broadband performance of proposed RCF is verified over the whole C band ranging from 1530 nm to 1565 nm. This kind of fiber design could be employed in small-scale multiple-input multiple-output digital signal processing (MIMO-DSP) intra-group modes multiplexing transmission combined with MIMO-free inter-group mode multiplexing transmission. The simulated results of the designed RCF show its great potential of the 16-channel long-distance mode division multiplexing (MDM) transmission with low MIMO-DSP complexity.

Divergence-degenerate spatial multiplexing towards future ultrahigh capacity, low error-rate optical communications

Spatial mode (de)multiplexing of orbital angular momentum (OAM) beams is a promising solution to address future bandwidth issues, but the rapidly increasing divergence with the mode order severely limits the practically addressable number of OAM modes. Here we present a set of multi-vortex geometric beams (MVGBs) as high-dimensional information carriers for free-space optical communication, by virtue of three independent degrees of freedom (DoFs) including central OAM, sub-beam OAM, and coherent-state phase. The novel modal basis set has high divergence degeneracy, and highly consistent propagation behaviors among all spatial modes, capable of increasing the addressable spatial channels by two orders of magnitude than OAM basis as predicted. We experimentally realize the tri-DoF MVGB mode (de)multiplexing and data transmission by the conjugated modulation method, demonstrating lower error rates caused by center offset and coherent background noise, compared with OAM basis. Our work provides a potentially useful basis for the next generation of large-scale dense data communication.

Design of steering wheel-type ring depressed-core 10-mode fiber with fully improved mode spacing.

We present a steering wheel-type ring depressed-core few-mode fiber (SWTR-DC-FMF) that features a central depressed step-index core and a novel SWTR structure consisted of two symmetrical high-index parts and low-index parts, respectively. The DC and SWTR make great contribution to separate the non-degenerated LP modes and spatial modes in the circular symmetry core, resulting in fully improved mode spacing. The designed fiber is able to support 10 spatial modes with the minimum effective index difference (Min Δneff) between adjacent spatial modes larger than 1.93 × 10-4 and the Min Δneff between adjacent LP modes above 1.51 × 10-3 at the same time, facilitating potential fiber spatial mode multiplexing transmission with less multiple-input multiple-output (MIMO-less) digital signal processing technique. The broadband performance including neff, Δneff, effective mode area (Aeff) and differential mode delay (DMD) is comprehensively investigated over the whole C and L band. Moreover, the birefringence and fabrication tolerance are discussed. The designed fiber targets emerging applications in short-reach weakly coupled space-division multiplexing (SDM) optical networking to increase transmission capacity and spectral efficiency and further reduce the system complexity effectively.

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.

Demonstration of 6 &lt;inline-formula&gt; &lt;tex-math notation="LaTeX"&gt;$\times$ &lt;/tex-math&gt; &lt;/inline-formula&gt; 10-Gb/s MIMO-Free Polarization- and Mode-Multiplexed Transmission

$6\times 10$ Gb/s polarization- and mode-multiplexed transmission, without MIMO equalization, is experimentally demonstrated over a 200-m elliptical core few-mode fiber with two PANDA stress rods. This fiber simultaneously breaks the degeneracies in space and polarization in a mode group. A polarization extinction ratio of <−16 dB and loss <0.63 dB/km were achieved.

Nonlinear impairment scaling with the number of mode groups in mode-multiplexed transmission over a 50 µm multimode fiber.

We numerically investigate the impact of nonlinear effects in mode-multiplexed transmission over a 50 µm graded-index multimode fiber. Such a fiber supports 36 spatial modes, well suited for a mode-multiplexed transmission. The number of mode groups used for transmission is subsequently increased to identify the nonlinear penalty occurring due to the Kerr-effect-based nonlinear interaction between the spatial modes. It is shown that the nonlinear penalty scales less than proportional with the number of modes and hence, is no obstacle for using such a fiber for a mode-multiplexed transmission. Consequently, we clarify the potential to upgrade the transmission capacity over time with such a fiber.

Orbital angular momentum mode multiplexed transmission in heterogeneous few-mode and multi-mode fiber network.

Mode-division multiplexing (MDM), which employs the spatial modes of light as information carriers, has been widely investigated to increase the transmission capacity. Few-mode fibers (FMFs) and multi-mode fibers (MMFs) have been used for MDM fiber transmission. One of the MDM techniques known as twisted light multiplexing using orbital angular momentum (OAM) modes has recently attracted increasing interest. In this Letter, by splicing two FMFs together with a conventional OM3 MMF, we propose and demonstrate OAM-based MDM in a heterogeneous fiber-optic network, i.e., two OAM mode (OAM01 and OAM-11) multiplexing transmission in the heterogeneous FMFs and MMF network. We transmit 20-Gbit/s quadrature phase shift keying signals over two OAM modes in different mode groups without multiple-input multiple-output equalization techniques and achieve less than 2.8dB optical signal-to-noise ratio penalties at a bit-error rate of 2×10-3. The experimental results show favorable transmission performance of OAM-based MDM in heterogeneous FMFs and MMF network compared to the one in FMF.

Mode-multiplexed transmission over conventional graded-index multimode fibers.

We present experimental results for combined mode-multiplexed and wavelength multiplexed transmission over conventional graded-index multimode fibers. We use mode-selective photonic lanterns as mode couplers to precisely excite a subset of the modes of the multimode fiber and additionally to compensate for the differential group delay between the excited modes. Spatial mode filters are added to suppress undesired higher order modes. We transmit 30-Gbaud QPSK signals over 60 WDM channels, 3 spatial modes, and 2 polarizations, reaching a distance of 310 km based on a 44.3 km long span. We also report about transmission experiments over 6 spatial modes for a 17-km single-span experiment. The results indicate that multimode fibers support scalable mode-division multiplexing approaches, where modes can be added over time if desired. Also the results indicate that mode-multiplexed transmission distance over 300 km are possible in conventional multimode fibers.

Influence of Discrete Mode Coupling on the Nonlinear Interaction in Mode-Multiplexed Systems

In this letter, we analyze the impact of discrete mode coupling on mode-multiplexed transmission links that employ few-mode fibers. We show that discrete coupling between spatial modes, as introduced, e.g., by splices, highly degrades the performance of mode-division multiplexed transmission links due to an additional nonlinear penalty. This stands in contrast to the performance improvement that is introduced by frequently appearing random mode coupling.

Modal birefringence in weakly guiding fibers

We examine wave propagation in few-mode and multimode fibers with a small index difference between core and cladding, where linearly polarized (LP) “modes” serve as a highly useful simplified solution. However, for a nonzero index difference, each LP “mode” decomposes into two true fiber waveguide modes, typically an HE and an EH mode. These two constituent modes have different group delays, which results in an effect termed modal birefringence. This effect needs to be understood in the design of mode-multiplexed transmission systems. We report an analysis of modal birefringence including scaling rules for fiber design, and provide numerical results for about 50 of the lower order modes.