Mode-division Multiplexing Systems Research Articles

On-chip mode-selective manipulation based on the modal-field redistribution assisted with subwavelength grating structures

Abstract Efficient mode-selective manipulation in multimode photonics has drawn much attention as a key technology for realizing scalable and flexible mode-division multiplexing (MDM) systems. A mode-selective manipulation scheme based on the modal-field redistribution assisted with subwavelength grating (SWG) structures is proposed and demonstrated for the first time. In particular, the proposed scheme focuses on manipulating the coupling coefficient κ as well as the ratio δ/κ for different mode channels. The SWG structures are used to engineer the refractive-index profile and redistribute the modal field distributions in the multimode bus waveguide, so that different modes are localized in different local regions. In this way, the undesired mode coupling can be suppressed significantly while the desired mode coupling can be enhanced. With such mode manipulation scheme, the fundamental and higher-order mode channels in the bus waveguide can be added/dropped independently and freely. As a proof of concept, a three-channel mode-selective add-drop coupler utilizing the proposed scheme is fabricated and demonstrated experimentally on silicon. The fabricated devices show low excess losses ranging from 0.1 to 1.9 dB over a wavelength range of 70 nm. The inter-mode crosstalks are lower than −19.4 dB in the wavelength range of 1525–1600 nm. The crosstalks for the drop and through ports (i.e., the residual power) are suppressed to be as low as −18 ∼ −30 dB in the wavelength range of ∼60 nm with the assistance of an additional coupler in cascade for performance improvement. The present concept of manipulating the evanescent coupling of the mode-channels paves the way for designing multimode silicon photonic devices with flexible mode-selective manipulation for MDM systems.

Design Consideration, Numerical and Experimental Analyses of Mode-Division-Multiplexed (MDM) Silicon Photonics Integrated Circuit with Sharp Bends.

Due to the popularity of different high bandwidth applications, it is becoming increasingly difficult to satisfy the huge data capacity requirements, since the traditional electrical interconnects suffer significantly from limited bandwidth and huge power consumption. Silicon photonics (SiPh) is one of the important technologies for increasing interconnect capacity and decreasing power consumption. Mode-division multiplexing (MDM) allows signals to be transmitted simultaneously, at different modes, in a single waveguide. Wavelength-division multiplexing (WDM), non-orthogonal multiple access (NOMA) and orthogonal-frequency-division multiplexing (OFDM) can also be utilized to further increase the optical interconnect capacity. In SiPh integrated circuits, waveguide bends are usually inevitable. However, for an MDM system with a multimode bus waveguide, the modal fields will become asymmetric when the waveguide bend is sharp. This will introduce inter-mode coupling and inter-mode crosstalk. One simple approach to achieve sharp bends in multimode bus waveguide is to use a Euler curve. Although it has been reported in the literature that sharp bends based on a Euler curve allow high performance and low inter-mode crosstalk multimode transmissions, we discover, by simulation and experiment, that the transmission performance between two Euler bends is length dependent, particularly when the bends are sharp. We investigate the length dependency of the straight multimode bus waveguide between two Euler bends. High transmission performance can be achieved by a proper design of the waveguide length, width, and bend radius. By using the optimized MDM bus waveguide length with sharp Euler bends, proof-of-concept NOMA-OFDM experimental transmissions, supporting two MDM modes and two NOMA users, are performed.

Mode converter using bent and twisted coupled-multicore fibers for group delay spread reduction

A mode converter using bent and twisted coupled-multicore fiber for the reduction of the group delay spread is proposed. The bent and twisted fiber, which is one of the periodic structures, can work as a long-period grating mode converter by properly setting the twist rate. To investigate the mode conversion characteristics, a simple and effective eigenmode analysis for three-dimensional periodic structures is proposed. We found that to convert the specified modes, the core layout is also important as well as the twist rate. For example, the mode conversion behaviors of 4CFs with square lattice and triangular lattice layouts are greatly different although each 4CF enables the specified mode conversion. It is revealed that, by switching the higher and lower order modes, the group delay can be equalized, resulting in the suppression of the group delay spread. It leads to reducing the complexity of the multiple-input multiple-output receiver in the mode division multiplexing system.

Mode add-drop technology based on few-mode fiber Bragg gratings.

By increasing the number of modes of transmission in optical fiber, the mode-division multiplexing (MDM) technology can effectively improve the transmission capacity. The mode add-drop technology is an important part of the MDM system and a key element for realizing flexible networking. In this paper, a mode add-drop technology based on few-mode fiber Bragg grating (FM-FBG) is reported for the first time. The technology utilizes the reflection characteristics of Bragg grating to realize the add-drop function in the MDM system. The grating is written in parallel according to the optical field distribution characteristics of different modes. By changing the writing grating spacing Δa to match the optical field energy distribution of the few-mode fiber, the few-mode fiber grating with high self-coupling reflectivity for the high-order mode is fabricated, and the performance of the add-drop technology is improved. The mode add-drop technology is verified in a 3 x 3 MDM system, which uses quadrature phase shift keying (QPSK) modulation and coherence detection. The experimental results show that the excellent transmission, add, and drop of 3 x 8 Gbit/s QPSK signals in 8 km few-mode fibers are achieved. The realization of this mode add-drop technology only requires Bragg gratings, few-mode fiber circulators, and optical couplers. It has the advantages of high performance, simple structure, low cost, and easy implementation, and it can be widely used in the MDM system.

Effect of mode selection and core radius of graded-index multimode fiber on transmission performance of hybrid mode division and wavelength division multiplexing systems

The choice of linear polarization modes in optical fiber is a key factor affecting performance of mode division multiplexing system (MDM). Firstly, an intensity modulation and direct detection (IM-DD) optical fiber transmission system based on hybrid mode division and wavelength division multiplexing (HMWDM) is designed. Secondly, the transmission characteristics of LP01, LP11, LP02, LP12 and LP21 five modes in the HMWDM system are simulated by Optisystem15.0 software. It was shown that when LP12 mode was multiplexed with LP01, LP11 and LP02, the Q factor of the signal carried by LP12 was 5-higher than that of LP21 multiplexed with the three modes in the same system. Then, four modes of LP01, LP11, LP02 and LP12 are multiplexed, and the simulation results show that the optimum value of the Graded-index multimode fiber (GI-MMF) core radius was 21 μm ∼ 22 μm with the Q factor above 7 when the core radius changed from 19 μm to 25 μm. Furthermore, the optimal value does not change with the transmission distance and the bandwidth of de-multiplexer. Finally, in the HMWDM system with a 21 μm-optimal-value core radius, the IM-DD method was used to realize the short distance transmission at 2.0 km with bit error rate (BER) below 10−9 and a rate of 80Gbit/s. The extension of the HMWDM system also be proposed.

Broadband dual-mode 3 dB power splitter using bricked subwavelength gratings

The multimode power splitter plays an essential role in on-chip mode-division multiplexing systems. In this work, a broadband dual-mode optical power splitter based on the self-imaging effect in the multimode interference (MMI) engineered by bricked subwavelength gratings (BSWGs) is proposed. The dual-mode power splitter is optimized by combining the 3D finite difference time domain (FDTD) and the particle swarm optimization (PSO) method. For the input T E 0 mode, the insertion loss (IL) and the imbalance (IB) are lower than 0.55 dB and 0.65 dB, while the IL and IB for the input T E 1 mode are, respectively, lower than 0.7 dB and 0.37 dB over 150 nm bandwidth in the wavelength range of 1.5–1.65 µm. In addition, the cross talk (CT) is lower than − 25 d B for the two modes over this wavelength range. Furthermore, the minimum feature size of the gratings is as large as 135 nm, and only a single etch step is required, which is beneficial for the fabrication.

Broadband, Low‐Crosstalk, and Massive‐Channels OAM Modes De/Multiplexing Based on Optical Diffraction Neural Network

AbstractOptical communication technology based on the wavelength, time, polarization, and complex amplitude of light is approaching a bottleneck, while the spatial dimension is relatively unexplored. Orbital angular momentum (OAM) beams are an important group of spatial light beams that are promising for increasing the optical communication capacity based on their orthogonality. To effectively utilize this spatial dimension of light, broadband and low‐crosstalk OAM mode de/multiplexing devices are indispensable. In this work, by exploiting an optical diffraction neural network, a low‐crosstalk OAM de/multiplexer operating in the full C+L band is developed and demonstrated. The device can support 16 OAM modes (l = ±1 to ±8) with 5 phase plates (8‐level phase), and the designed insertion loss and average intermode crosstalk are better than −2.8 and −30.9 dB, respectively. In particular, multiplexing and demultiplexing are experimentally performed on the same device. The measured average insertion loss in mutltiplexing and demulteplexing is −6.1 to −5.4 and −6.8 to −5.8 dB, respectively, and the corresponding average intermode crosstalk is −27.2 to −22.7 and −26.5 to −22.4 dB. The results in this work have great application prospects for the design of mode de/multiplexers and mode division multiplexing communication systems.

Eight Modes Selective Elliptic-Core Photonic Lantern in MIMO-Free Mode Division Multiplexing Systems at S+C+L Bands

We propose an elliptical-core mode selective photonic lantern (EC-MSPL), which can be well matched to mode division multiplexing (MDM) systems without multiple-input multiple-output (MIMO). The taper region of the small package satisfying the adiabatic condition is obtained through two-stage taper related parameter optimization, which can reduce the length of the taper region without affecting the PL performance. Finally, we simulated the results for step-index EC-MSPL and graded-index EC-MSPL at taper lengths of 10 cm and 5 cm, respectively, using the beam propagation method. Both types of PL can achieve low-loss mode conversion and crosstalk below −15.6 dB at 1550 nm wavelength. At the same time, the coupling efficiencies of the two PL modes in the S+C+L bands are higher than 85% and 71%.

Amplification and Transmission System With Matching Multi-Layer Ion-Doped FM-EDFA

System stability and robustness are highly desired in a mode division multiplexing (MDM) transmission system. The matching of the active fiber and transmission FMF effectively reduces the transmission impairments and is conducive to transmission stability. In this paper, we proposed and fabricated a well-matched triple-layered-core few-mode erbium-doped fiber (FM-EDF) for low differential modal gain (DMG) with multilayer ions doping adjustment. Based on the fabricated FM-EDF, the amplifier is constructed experimentally. By a simple LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> pump mode, the modal gains are higher than 24 dB, and meanwhile, the DMG of 1.6 dB is obtained. The compatibility between the FM-EDF and the passive transmission FMF is verified in an MDM transmission system with a matching degree higher than 83%. Results show that after long-haul FMF transmission and single-stage amplification, the difference in the output signal power is lower than 1.8 dB at the receiver. The fabricated FM-EDF is beneficial for practical MDM systems with high mode stability, excellent fiber linkage integration, and signal channel equalization, which shows much potential for long-haul MDM transmission and multistage amplification.

Ultra-compact mode converters with nano strips for efficient phase shifting

Mode-division multiplexing (MDM) is an effective solution to improve the integration of photonic integrated circuits. MDM systems increase the number of channels for data transmission by introducing higher-order modes. The mode converter is a key component for MDM. In this work, we propose a novel ultra-compact mode converters in the silicon-on-insulator platform, combining beam splitters with nano strips. The nano strips are used to achieve the phase matching. The high-performance TE1-TE0, TE2-TE0, and TE3-TE0 mode converters exhibit high conversion efficiency and broad transmission bandwidth, in ultra-small footprints.

A novel multi-granularity flexible-grid switching optical-node architecture

A novel multi-granularity flexible-grid switching optical-node architecture is proposed in this paper. In our system, the photonic lanterns are used as mode division multiplexing/demultiplexing (MD-Mux/MD-Demux) for selecting mode. The wavelength division multiplexer/demultiplexer (WD-Mux/WD-Demux) and the fiber bragg gratings (FBGs) are used to select wavelength channels with the various grid. The experimental results show that the transmission bandwidth covers the C+L band, the average transmission loss is −13.4 dB, and the average crosstalk is −30.5 dB. The optical-node architecture is suit for mode division multiplexing (MDM) optical communication system.

High Speed MIMO-free Transmission Experiment of Mode Division Multiplexing System with Photonic Lanterns

High Speed MIMO-free Transmission Experiment of Mode Division Multiplexing System with Photonic Lanterns

Low Power Consuming Mode Switch Based on Hybrid-Core Vertical Directional Couplers with Graphene Electrode-Embedded Polymer Waveguides

We propose a mode switch based on hybrid-core vertical directional couplers with an embedded graphene electrode to realize the switching function with low power consumption. We designed the device with Norland Optical Adhesive (NOA) material as the guide wave cores and epoxy polymer material as cladding to achieve a thermo-optic switching for the E11, E21 and E12 modes, where monolayer graphene served as electrode heaters. The device, with a length of 21 mm, had extinction ratios (ERs) of 20.5 dB, 10.4 dB and 15.7 dB for the E21, E12 and E11 modes, respectively, over the C-band. The power consumptions of three electric heaters were reduced to only 3.19 mW, 3.09 mW and 2.97 mW, respectively, and the response times were less than 495 µs, 486 µs and 498 µs. Additionally, we applied such a device into a mode division multiplexing (MDM) transmission system to achieve an application of gain equalization of few-mode amplification among guided modes. The differential modal gain (DMG) could be optimized from 5.39 dB to 0.92 dB over the C-band, together with the characteristic of polarization insensitivity. The proposed mode switch can be further developed to switch or manipulate the attenuation of the arbitrary guided mode arising in the few-mode waveguide.

In-line mode-dependent loss equalizer with femtosecond laser induced refractive index modification.

We demonstrate an in-line all-fiber mode-dependent loss (MDL) equalizer with femtosecond laser induced refractive index (RI) modification. By inscribing an RI-modified structure into the core of a few-mode fiber (FMF), a differential mode attenuation (DMA) can be achieved for LP01 and LP11 modes. The DMA can serve as an in-line MDL equalizer for the long-haul mode-division multiplexing transmission system. Through numerical simulations, we identify that the LP01 mode has a larger attenuation than that of higher-order modes, where the sign of DMA is contrary to that of the conventional FMF links and devices. Finally, a proof-of-concept experiment is implemented by inscribing an RI modified region with a width of 4 µm, a height of 13 µm, and a length of 200 µm into the FMF core. An average additional attenuation of 8.4 dB and 3 dB can be applied to the LP01 and LP11 modes over the C-band, respectively, leading to an MDL equalization range of 5.4 dB. Meanwhile, the average polarization dependent loss (PDL) of the LP01 and LP11 modes induced by the in-line MDL equalizer is approximately 0.3 dB over the C-band. Power matrix measurement indicates that the in-line MDL equalizer has a negligible mode coupling. The proposed in-line MDL equalizer with a wider range and low insertion loss is feasible by precise manipulation of femtosecond laser inscription.

Over 100 nm Bandwidth Orbital Angular Momentum Modes Amplification For MDM and WDM Transmission With a Ring-Core Bi/Er Co-Doped Fiber

We designed and fabricated a ring-core Bi/Er co-doped fiber (RC-BEDF), and thus investigated the broad-spectrum amplification of orbital angular momentum (OAM) modes. The deviation from the average gain of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> modes enabled gains of <1.8 dB and <1.9 dB respectively, and the differential mode gain (DMG) between the two modes was <1.0 dB over 100 nm bandwidth (1525–1626 nm). In particular, the 3 dB bandwidth of the gain spectra exhibited 74 nm (1552–1626 nm) and 69 nm (1557–1626 nm), respectively. Furthermore, four beams of different wavelengths, i.e., 1525, 1560, 1590, and 1626 nm, were separately extracted from the broad spectra to detect the amplified modes, by which the interferograms of first- and second-order OAM modes were obtained. The amplified OAM mode purities of the wavelengths were all above 95%. Moreover, dual-channel combination of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> mode at 1550 nm and OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mode at 1600 nm was amplified, which indicated realization of different-OAM-modes amplification with wavelength-division multiplexing (WDM). Herein, for the first time, we extended the C+L band OAM modes amplification to 1626 nm and achieved OAM multiplexed amplification of WDM signals. This ultra-broad-spectrum and ultra-low-DMG OAM amplification has been widely considered as a promising technology for long-haul mode division multiplexing and WDM fiber-optics communication systems and networks.

Enhanced Atmospheric Turbulence Resiliency With Successive Interference Cancellation DSP in Mode Division Multiplexing Free-Space Optical Links

We experimentally demonstrate the enhanced atmospheric turbulence resiliency in a 137.8 Gbit/s/mode mode-division multiplexing free-space optical communication link through the application of a successive interference cancellation digital signal processing algorithm. The turbulence resiliency is further enhanced through redundant receive channels in the mode-division multiplexing link. The proof of concept demonstration is performed using commercially available mode-selective photonic lanterns, a commercial transponder, and a spatial light modulator based turbulence emulator. In this link, 5 spatial modes with each mode carrying 34.46 GBaud dual-polarization quadrature phase shift keying signals are successfully transmitted with an average bit error rate lower than the hard-decision forward error correction limit. As a result, we achieved a record-high mode- and polarization-division multiplexing channel number of 10, a record-high line rate of 689.23 Gbit/s, and a record-high net spectral efficiency of 13.9 b/s/Hz in emulated turbulent links in a mode-division multiplexing free-space optical system.

Analysis of Localized Modes in Disordered Optical Waveguides Mediated by Transverse Anderson Localization

Disordered waveguides mediated by transverse Anderson localization (TAL) can be used as mode division multiplexing system. The whole cross-section of the disordered waveguides is covered by all localized modes, which function as independent space channels. The localized modes are independent of the boundary and external excitation. The numerical results reveal that the average width tends to be steady in one particular realization of the disordered waveguide with a sufficient number of localized modes. In this paper, we proposed that localization length can be measured by the average width of localized modes. Besides, we explore the impact of the disordered waveguide design parameters, such as refractive index contrast, fill-fraction, and feature size, on the average width of localized modes.

Carrier Depletion High-Speed Tunable Dual-Mode Converter for Mode-Division Multiplexing Networks

Mode-division multiplexing (MDM) is a promising technique to further enhance the channel capacity in modern optical communications. Tunability of the mode converter in MDM systems is very important for flexible network deployment. In this letter, we demonstrate a high-speed tunable dual-mode converter on silicon-on-insulator (SOI) platform for MDM networks. The proposed device is based on carrier depletion mechanism, which enables ultrafast conversion between TE0 and TE1 mode on the order of sub-nanoseconds. The experimental results show that the insertion losses are less than 4.6 dB and the crosstalk values are lower than −19 dB in the wavelength range from 1525 nm to 1565 nm. To verify the function of high-speed mode transition, 10 Gb/s on-off key optical payload signal is performed to control the dual-mode converter. The performance indicates this device has great potential to be a pivotal component in high-speed MDM networks.

Highly scalable and flexible on-chip all-silicon mode filter using backward mode conversion gratings.

Mode filters are fundamental elements in a mode-division multiplexing (MDM) system for reducing modal cross-talk or realizing modal routing. However, the previously reported silicon mode filters can only filter one specific mode at a time and multiple modes filtering usually needs a cascade of several filters, which is adverse to highly integrated MDM systems. Here, we propose a unique concept to realize compact, scalable and flexible mode filters based on backward mode conversion gratings elaborately embedded in a multimode waveguide. Our proposed method is highly scalable for realizing a higher-order-mode-pass or band-mode-pass filter of any order and capable of flexibly filtering one or multiple modes simultaneously. We have demonstrated the concept through the design of four filters for different order of mode(s) and one mode demultiplexer based on such a filter, and the measurement of two fabricated 11μm length filters (TE1-pass/TE2-pass) show that an excellent performance of insertion loss <1.0dB/1.5dB and extinction ratio >29dB/28.5dB is achieved over a bandwidth of 51.2nm/48.3nm, which are competitive with the state-of-the-art.

High capacity terahertz communication systems based on multiple orbital-angular-momentum beams

Structured electromagnetic waves carrying orbital angular momentum (OAM) have been explored in various frequency regimes to enhance the data capacity of communication systems by multiplexing multiple co-propagating orthogonal OAM beams (i.e. mode-division multiplexing (MDM)). Terahertz (THz) communications in free space have gained interest as THz waves tend to have: (a) larger bandwidth and lower beam divergence than millimeter-waves, and (b) lower interaction with matter conditions than optical waves. In this paper, we review recent experimental demonstrations of OAM-based THz MDM communication systems, including (a) THz MDM system with two multiplexed OAM beams; (b) THz OAM multiplexing together with frequency-division-multiplexing and polarization-division-multiplexing; (c) multiplexing a full set of two-dimensional Laguerre–Gaussian () beams; and (d) THz integrated OAM emitter for OAM mode generation and multiplexing. System performance of THz OAM links with the effect of turbulence, divergence, and multipath is also simulated and analyzed.