Mode-division Multiplexing Link Research Articles

Mapping-varied modulation with labeling optimization for secure transmission in mode-division-multiplexed fiber-optic systems

To improve the physical-layer security of mode-division multiplexing (MDM) systems, a simple security scheme named mapping-varied modulation (MVM) is proposed in this paper by combining cryptographic and information-theoretic security. Specifically, on top of the information-theoretic security provided by the less-conditioned wiretap channel due to the larger mode-dependent loss induced by fiber-bend tapping, the proposed MVM security method varies the mapping rules of the adopted constellations for the subchannels (one subchannel corresponds to one mode) by using the inherently time-varying random channel state information (CSI) of the MDM fiber, under the assumption that an eavesdropper does not know the exact instantaneous CSI of the legitimate link. To maximize the difference among the binary labels of the constellation points in the same position for each subchannel, a labeling optimization method is proposed as well. Numerical results demonstrate the effectiveness of the proposed MVM method via bit-error ratio performance and secrecy rate, showing a potential way to improve the security of the MDM link for high-speed data transmission.

Chip-to-chip optical multimode communication with universal mode processors

The increasing amount of data exchange requires higher-capacity optical communication links. Mode division multiplexing (MDM) is considered as a promising technology to support the higher data throughput. In an MDM system, the mode generator and sorter are the backbone. However, most of the current schemes lack the programmability and universality, which makes the MDM link susceptible to the mode crosstalk and environmental disturbances. In this paper, we propose an intelligent multimode optical communication link using universal mode processing (generation and sorting) chips. The mode processor consists of a programmable 4 × 4 Mach Zehnder interferometer (MZI) network and can be intelligently configured to generate or sort both quasi linearly polarized (LP) modes and orbital angular momentum (OAM) modes in any desired routing state. We experimentally establish a chip-to-chip MDM communication system. The mode basis can be freely switched between four LP modes and four OAM modes. We also demonstrate the multimode optical communication capability at a data rate of 25 Gbit/s. The proposed scheme shows significant advantages in terms of universality, intelligence, programmability and resistance to mode crosstalk, environmental disturbances, and fabrication errors, demonstrating that the MZI-based reconfigurable mode processor chip has great potential in long-distance chip-to-chip multimode optical communication systems.

Ultra-Compact Ultra-Broadband Two-Mode Transverse-Electric Based SWG Multiplexer Demonstrated at 64 Gbps

We designed and experimentally demonstrated an ultra-compact and ultra-broadband two-mode de-multiplexer based on a subwavelength grating (SWG) asymmetrical coupler (ADC) on a silicon-on-insulator (SOI) platform. The device consists of a cascaded strip waveguide tapered to a multimode segment and a SWG-based single-mode waveguide. Tapering the strip waveguide in multi-segments guarantees a high fabrication tolerance with a wider wavelength range of operation. The parameters of the periodic structure are chosen to meet the Bragg condition and phase-match the TE1 mode of the strip waveguide. These features produce a faster mode evolution with a shorter coupling length of only 8.5 µm and an entire mode division multiplexing (MDM) link (Mux and Demux) of only 55 µm. The MDM link operates over a wide wavelength range, covering the entire S-, C-, and L-bands. The insertion loss (IL) and crosstalk (CT) achieved at 1550 nm are better than 1 dB and -18 dB, respectively. The device also has low fabrication sensitivity to deviations in the waveguide's parameters. Additionally, the transmission performance of the link is demonstrated at 1550 nm wavelength using non-return-to-zero (NRZ) on-off keying (OOK) modulation, where clear eye diagrams are obtained at 64 Gbps.

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.

Recent Progress on Novel DSP Techniques for Mode Division Multiplexing Systems: A Review

This paper provides an overview of latest progress on the novel advanced digital signal processing (DSP) techniques for long-haul mode division multiplexing (MDM) systems with high capacity. Space-division multiplexing (SDM) techniques have been developed for a period to increase the capacity of optical communication system by at least one order of magnitude through MDM techniques using few-mode fibers (FMFs) or multi-core multiplexing (MCM) using multi-core fibers (MCFs). The signals in MDM links are mainly impaired by the linear and nonlinear effects in FMFs, making DSP techniques become necessary to undo these impairments. In this paper, we not only review the advanced multiple-input multiple-output (MIMO) DSP techniques for compensating linear impairments in FMFs, but also enclose the state of the art of novel DSP techniques to deal with nonlinear effects. Firstly, we introduce the MIMO schemes for equalizing modal crosstalk and modal dispersion. Then, we focus on the fast tracking of time-varying (TV) channels in FMF links through frequency-domain (FD) recursive least square (RLS) algorithm. Besides, we also cover the mainstream DSP solutions for mode-dependent loss (MDL) and several possible methods to compensate nonlinearity in FMF. Moreover, artificial intelligence (AI) technologies are also discussed for its high nonlinearity tolerance and may bring a revolution in MDM systems on the process of channel equalization, link monitoring, etc. In the end, a brief conclusion and perspective will be provided.

Ultra-Broadband and Compact Two-Mode Multiplexer Based on Subwavelength-Grating-Slot-Assisted Adiabatic Coupler for the Silicon-on-Insulator Platform

We design and experimentally demonstrate an ultra-broadband and compact two-mode multiplexer based on subwavelength-grating-slot-assisted adiabatic coupler for the silicon-on-insulator platform. The introduction of subwavelength-grating slot effectively increases the refractive index of the gap region between two strip waveguides and enhances the coupling between them, leading to high modal transmission over a short mode-evolution region, particularly at the short-wavelength regime. As a result, our two-mode multiplexer has a compact design footprint, with a mode-evolution region length of only 25 $\mu$ m and an entire device length of only 55 $\mu$ m. Simulation shows that the mode-division multiplexing (MDM) link formed by the designed two-mode multiplexers has a record bandwidth of 740 nm spanning from 1260 nm to 2000 nm, where the crosstalk is lower than −18.5 dB and the insertion loss (IL) is lower than 0.32 dB. The MDM link has been fabricated using electron beam lithography and achieves measured crosstalk lower than −18.8 dB and IL lower than 2.6 dB, over a 100 nm bandwidth from 1260 nm to 1360 nm that covers the O-band and a 130 nm bandwidth from 1500 nm to 1630 nm that covers the C-band and the L-band.

Silicon mode (de)multiplexers with parameters optimized using shortcuts to adiabaticity.

We propose and experimentally demonstrate broadband silicon mode (de)multiplexers based on the optimization of system adiabaticity using shortcuts to adiabaticity (STA). The measured insertion losses and crosstalks were less than 1.1 dB and -24 dB, respectively, for the five two-mode mode-division-multiplexing (MDM) links over wavelengths ranging from 1500 nm to 1600 nm. The four-mode MDM link showed measured insertion losses and crosstalks less than 1.3 dB and -23 dB, respectively, within the same wavelength range. The method paves the way for future adaptation of the STA protocols to various components.

Broadband On-Chip Mode-Division Multiplexer Based on Adiabatic Couplers and Symmetric Y-Junction

The mode-division multiplexer/demultiplexer (MMUX/DeMMUX) is the crucial component for the implementation of mode-division multiplexing (MDM) transmission. We propose an on-chip two-mode MMUX/DeMMUX based on adiabatic couplers. Thanks to the principle of mode evolution, the proposed MMUX possesses advantages of broadband operation and high tolerance of fabrication error. The performance is experimentally evaluated by integrating the proposed MMUX and DeMMUX into an on-chip MDM link, with a low crosstalk of <–19 dB and insertion loss of <1.5 dB over a wavelength range of 90 nm. Furthermore, reasonable performance degradation can be observed for deviations of gap and etch depth from –50 to 50 nm.

Silicon mode multiplexer processing dual-path mode-division multiplexing signals.

Great strides have been made in the optical interconnect for a mode-division multiplexing (MDM) system. The mode multiplexer/demultiplexer (MUX/DEMUX) is the crucial component for the MDM system. We propose and demonstrate a scheme that is different from the reported multiplexing schemes, mode MUX that tackles dual-path MDM signals simultaneously and avoids mode conflict. The performance is experimentally demonstrated by integrating the proposed MUX and DEMUX into a MDM link, with ∼1 dB of insertion loss and ∼18 dB extinction ratio over the C-band. Single-wavelength, non-return-to-zero on-off keying signals at 10Gb/s carried on dual-path different modes are successfully processed with open and clear eye diagrams, and <1 dB power penalty are obtained.

Broadband and fabrication-tolerant on-chip scalable mode-division multiplexing based on mode-evolution counter-tapered couplers.

A broadband and fabrication-tolerant on-chip scalable mode-division multiplexing (MDM) scheme based on mode-evolution counter-tapered couplers is designed and experimentally demonstrated on a silicon-on-insulator (SOI) platform. Due to the broadband advantage offered by mode evolution, the two-mode MDM link exhibits a very large, -1 dB bandwidth of >180 nm, which is considerably larger than most of the previously reported MDM links whether they are based on mode-interference or evolution. In addition, the performance metrics remain stable for large-device width deviations from the designed valued by -60 nm to 40nm, and for temperature variations from -25°C to 75°C. This MDM scheme can be readily extended to higher-order mode multiplexing and a three-mode MDM link is measured with less than -10 dB crosstalk from 1.46 to 1.64μm wavelength range.