Dense SDM Research Articles

Dense Space-Division Multiplexing Exploiting Multi-Ring Perfect Vortex

Vortex beams carrying orbital angular momentum (OAM) have gained much interest in optical communications because they can be used to expand the number of multiplexing channels and greatly improve the transmission capacity. However, the number of states used for OAM-based communication is generally limited by the imperfect OAM generation, transmission, and demultiplexing methods. In this work, we proposed a dense space-division multiplexing (DSDM) scheme to further increase the transmission capacity and transmission capacity density of free space optical communications with a small range of OAM modes exploiting a multi-ring perfect vortex (MRPV). The proposed MRPV is generated using a pixel checkerboard complex amplitude modulation method that simultaneously encodes amplitude and phase information in a phase-only hologram. The four rings of the MRPV are mutually independent channels that transmit OAM beams under the condition of occupying only one spatial position, and the OAM mode transmitted in these spatial channels can be efficiently demodulated using a multilayer annular aperture. The effect of atmospheric turbulence on the MRPV was also analyzed, and the results showed that the four channels of the MRPV can be effectively separated under weak turbulence conditions. Under the condition of limited available space and OAM states, the proposed DSDM strategy exploiting MRPV might inspire wide optical communication applications exploiting the space dimension of light beams.

Design, fabrication, and characterization of a low-index center and trench-assisted 7-ring-core 5-mode-group fiber for dense space-division multiplexing.

An optimized design of 7-ring-core 5-mode-group fiber for mode-group-based dense space-division multiplexing (DSDM) is proposed. It is found that decreasing the refractive index of the center and trench of each ring core can increase the available ring-core thickness and meanwhile suppress the radial higher-order modes. Based on the simulation, a fiber, with a thicker ring core and a relatively low refractive index contrast at the ring-core boundaries, is found of higher mode purity and lower macro-bending-caused mode coupling. In the experiment, the seven cores of the fabricated fiber have low transmission losses around -0.25 dB/km with only a few fluctuations. The three higher-order mode groups (MG2,1, MG3,1, and MG4,1) are verified to be in a weak-coupling state (crosstalk of which are less than -12 dB) over a transmission length of 23 km.

909.5 Tbit/s Dense SDM and WDM Transmission Based on a Single Source Optical Frequency Comb and Kramers-Kronig Detection

Space-division multiplexing (SDM) has become a promising technology for optical communications to seek a sustainable increment in data capacity to keep up with the ever-increasing bandwidth demand. SDM could provide massive parallelization in addition to existing WDM technology for a high capacity without compromising the signal-to-noise ratio requirement of the transmitter. However, the WDM-SDM combination requires arrays of laser sources with associated active wavelength controls. This is not favorable in terms of energy consumption and cost. Single source optical frequency comb (SS-OFC) is an appealing candidate to provide coherent WDM sources, replacing the laser arrays and surpassing the performance by providing the possibility of joint digital signal processing across multiple channels. In this paper, we have generated an SS-OFC based on highly nonlinear fibers (HNLF) for short-reach applications such as optical interconnects, taking advantage of the high pump-to-comb efficiency. The 50-GHz spaced SS-OFC across the whole C band provides 99 comb lines as WDM channel sources. Combining adaptive-rate modulation, a 7.9-km 37-core fiber, and a polarization diversity Kramers-Kronig receiver, we have successfully transmitted 3663 channels (37 SDM × 99 WDM) with reliably error-free performance, achieving 909.5 Tbit/s net rate with an aggregated spectral efficiency of 184.42 bit/s/Hz.

Orbital-angular-momentum-amplifying helical vector modes in Yb3+-doped three-core twisted microstructure fiber.

A helical Yb3+-doped three-core microstructure fiber (YTMF) amplifier is proposed in this paper, so as to solve the problem of generation and transmission of the orbital angular momentum (OAM) beams. The fiber is composed of three Yb3+-doped cores with a regular triangle shape and a longitudinal helical structure. The experimental results show that the 1064nm laser can be amplified due to the fluorescence amplification characteristics of the doped material Yb3+. Furthermore, theoretical analysis indicates the modes in YTMF at 1064nm, which is located in the amplified wavelength, can support nine modes carrying OAM. Therefore, the related experiments were performed and verified that the transmission modes can respectively carry 1, 2, and 3-order OAM at 1064nm in different coupling cases. These excellent properties indicate that the combination of doped materials and helical fiber provide favorable conditions for the generation and amplification of OAM, which provides a basis for the further development of OAM beams in the field of quantum communication and dense space division multiplexing.

Long-Haul Mode Multiplexing Transmission Enhanced by Interference Cancellation Techniques Based on Fast MIMO Affine Projection

In space division multiplexing (SDM) transmission utilizing mode division multiplexing (MDM) technologies, transmission performance of each spatial channel is highly dependent on a spatial mode that delivers information symbols over few-mode fibers (FMFs) or multi-mode fibers (MMFs). This still remains as a dominant barrier to realize a future deployable long-haul MDM transport systems. In this article, we review the transmission issues we face when transmitting MDM signals over long distances. We also present the review of key techniques that we have recently developed to overcome mode-relevant linear phenomena. By developing the projection technique for a signal detection application in multiple-input multiple-output (MIMO) systems, a novel linear filtering adaptation technique is proposed to enhance the performance of our interference canceling scheme with decreased complexity. Experimental evaluation using dense SDM transmission results over multicore FMFs verifies a fast convergence property brought by the proposed projection technique.

Mode- and wavelength-multiplexed transmission with crosstalk mitigation using a single amplified spontaneous emission source

We propose to use the low-coherence property of amplified spontaneous emission (ASE) noise to mitigate optical crosstalk, such as spatial, polarization, and modal crosstalk, which currently limits the density of photonic integration and fibers for dense space-division multiplexing. High optical crosstalk tolerance can be achieved by ASE-based low-coherence matched detection, which avoids dedicated optical lasers and uses spectrally filtered ASE noise as the signal carrier and as a matched local oscillator. We experimentally demonstrate spatial and modal crosstalk reduction in multimode fiber (MMF) and realize mode- and wavelength-multiplexed transmission over 1.5-km MMF supporting three spatial modes using a single ASE source. Performance degradation due to model dispersion over MMF is experimentally investigated.

Iterative Unreplicated Parallel Interference Canceler for MDL-Tolerant Dense SDM (12-Core × 3-Mode) Transmission Over 3000 km

We propose a novel multiple-input multiple-output (MIMO) detection scheme based on iterative unreplicated parallel interference cancelling (UPIC) technique for mode dependent loss (MDL)-impaired few-mode fibre (FMF) transmission. An optical MIMO channel over an FMF exhibits a mode-dependent property where each mode division multiplexed signal is passed through a different lossy channel, known as an MDL phenomena. One challenging issue for realising future deployable space division multiplexing (SDM) systems is establishing reliable and robust signal transmission techniques in the presence of MDL. The main contributions of this paper include first, a proposal of newly developed iterative interference cancellation technique with parallel processing for MDL-impact mitigation, second, a comparative study on performance of MIMO detection schemes, and third, a demonstration of the longest dense SDM transmission over multicore (MC)-FMF. The proposed UPIC with iterative processing is experimentally shown to improve Q-factors by more than 3 dB, and record-long dense SDM transmission reach exceeding 3000 km over 12-core × 3-mode MC-FMF was achieved.

Monolithic mode-selective few-mode multicore fiber multiplexers

With the capacity limits of standard single-mode optical fiber fast approaching, new technologies such as space-division multiplexing are required to avoid an Internet capacity crunch. Few-mode multicore fiber (FM-MCF) could allow for a two orders of magnitude increase in capacity by using the individual spatial modes in the different cores as unique data channels. We report the realization of a monolithic mode-selective few-mode multicore fiber multiplexer capable of addressing the individual modes of such a fiber. These compact multiplexers operate across the S + C + L telecommunications bands and were inscribed into a photonic chip using ultrafast laser inscription. They allow for the simultaneous multiplexing of the LP01, LP11a and LP11b modes of all cores in a 3-mode, 4-core fiber with excellent mode extinction ratios and low insertion losses. The devices are scalable to more modes and cores and therefore could represent an enabling technology for practical ultra-high capacity dense space-division multiplexing.

High-capacity dense space division multiplexing transmission

Abstract In this paper, we review space division multiplexing (SDM) transmission experimental demonstrations and associated technologies. In past years, SDM achieved high capacity transmission through increased spatial multiplicity, and long-haul transmission through improved transmission performance. More recently, dense SDM (DSDM) with a large spatial multiplicity exceeding 30 was demonstrated with multicore technology. Various types of multicore and multimode SDM fibers, amplification, and spatial multi/demultiplexers have helped achieve high-capacity DSDM transmission.

Few-mode multicore fibers for long-haul transmission line

Abstract Few-mode multicore fibers (FM-MCFs) that enable dense space-division multiplexing (DSDM) have the potential to drastically improve the fiber capacity. In designing the FM-MCFs, several issues that originate from multicore fibers and few-mode fibers must be considered. In this paper, these design issues such as inter-core crosstalk (IC-XT) and dispersion mode delay (DMD) are discussed. A three-mode 12-core fiber with low-DMD low-IC-XT achieves long-haul DSDM transmission over 500 km. The design concept, fiber design, and characteristics of the fabricated three-mode 12-core fiber are also described.

Dense Space-Division Multiplexed Transmission Systems Using Multi-Core and Multi-Mode Fiber

In this paper, we describe recent progress in space-division multiplexed (SDM) transmission, and our proposal and demonstration of dense space-division multiplexing (DSDM), which offers the possibility of ultra-high capacity SDM transmission systems with high spatial density and spatial channel count of over 30 per fiber. We introduce the SDM transmission matrix, which cross indexes the various types of multi-core multi-mode transmissions according to the type of light propagation in optical fibers and how the spatial channels are handled in the network. For each category in the matrix, we present the latest advances in transmission studies, and evaluate their transmission performance by spectral and spatial efficiencies. We also expound on technologies for multi-core and/or multi-mode transmission including optical fiber, signal processing, spatial multi/demultiplexer, and amplifier, which will play key roles in configuring DSDM transmission systems, and review the first DSDM transmission experiment over a 12 core × 3 mode fiber.

Dense SDM (12-Core x 3-Mode) Transmission Over 527 km With 33.2-ns Mode-Dispersion Employing Low-Complexity Parallel MIMO Frequency-Domain Equalization

We propose long-haul space-division-multiplexing (SDM) transmission systems employing parallel multiple-input multiple-output (MIMO) frequency-domain equalization (FDE) and transmission fiber with low differential mode delay (DMD). We first discuss the advantages of parallel MIMO FDE technique in long-haul SDM transmission systems in terms of the computational complexity, and then, compare the complexity required for parallel MIMO FDE as well as the conventional time-domain equalization techniques. Proposed parallel MIMO FDE that employs low baud rate multicarrier signal transmission with a receiver-side FDE enables us to compensate for 33.2-ns DMD with considerably low-computational complexity. Next, we describe in detail the newly developed fiber and devices we used in the conducted experiments. A graded-index (GI) multicore few-mode fiber (MC-FMF) suppressed the accumulation of DMD as well as intercore crosstalk. Mode dependent loss/gain effect was also mitigated by employing both a ring-core FM erbium-doped fiber amplifier and a free-space optics type gain equalizer. By combining these advanced techniques together, we finally demonstrate 12-core $\times$ 3-mode dense SDM transmission over 527-km GI MC-FMF without optical DMD management.