Mode-division Multiplexing Networks Research Articles

Silicon‐photonic four‐mode triple‐band multiplexing device for hybrid wavelength/mode division multiplexing networks

SummaryWhile wavelength division multiplexing (WDM) technology combines several wavelengths onto a single waveguide, the technology of mode division multiplexing (MDM) allows many orthogonal modes of the same wavelength to operate simultaneously without interchannel crosstalk. Thus, the hybrid WDM and MDM network in which the two above‐mentioned techniques cooperate could give a several‐fold increase in the overall network capacity. Constructing this network requires hybrid wavelength‐and‐mode multiplexers, especially ones with high integration and complementary metal‐oxide‐semiconductor (CMOS) compatibility. In this paper, we propose a design of a four‐mode triple‐band multiplexer that is capable of multiplexing up to 12 separate optical signal flows by utilizing four eigenmodes (TE0, TE1, TE2, and TE3) and three‐wavelength windows, which center at 1310, 1490, and 1550 nm. The device is on silicon‐on‐insulator (SOI) platform, consisting of four butterfly‐shaped multimode interference (MMI) couplers, four directional couplers, and a cascaded asymmetric Y‐junction coupler. Via numerical simulations, the proposed design is verified to be able to operate effectively on the three aforementioned bandwidth slots with an optical conversion efficiency of over 93% in all functions. Moreover, it exhibits insertion loss less than 1.5 dB and crosstalk smaller than −16 dB within 25 nm bandwidth at each wavelength window. These results can affirm the success of wavelength–mode combination, which leads to a massive improve in the channel capacity on the same optical multiplexing system for optical telecommunications and photonics on‐chip interconnections.

Mode multicasting without parasitic wavelength conversion.

Optical multicasting, which involves delivering an input signal to multiple different channels simultaneously, is a key function to improve network performance. By exploiting individual spatial modes as independent channels, mode-division-multiplexing (MDM) can solve the capacity crunch of traditional standard single-mode fiber (SSMF) transmission system. In order to realize mode multicasting with high flexibility in future hybrid wavelength-division-multiplexing (WDM) and MDM networks, we propose a mode multicasting scheme without parasitic wavelength conversion, based on the inter-modal four-wave mixing (FWM) arising in the few-mode fiber (FMF). The operation mechanism including nonlinear phase shift for efficient mode multicasting is analytically identified. Then, based on the derived operation condition, we numerically investigate the impact of the dual-pump power and the FMF length on the performance of mode multicasting. By properly setting the pump wavelength and the dual-pump power, mode multicasting performance, in terms of mode multicasting efficiency, 3-dB bandwidth, and destination wavelength, can be tuned according to various application scenarios. After the performance optimization, mode multicasting of 25-Gbaud and 100-Gbaud 16-quadratic-amplitude modulation (16-QAM) signals is numerically demonstrated. The proposed reconfigurable mode multicasting is promising for future WDM-MDM networks.

Static analysis of gain control for a few-mode erbium-doped fiber amplifier employing pump power adjustment.

In mode-division multiplexing (MDM) optical transmission systems and MDM networks, the gain must be kept nearly constant even when the input signal power of the few-mode (FM), erbium-doped fiber amplifier (EDFA) changes. This paper investigates controllability of the gain in a 4-LP mode EDFA, whose EDF has signal modes of L P 01, L P 11, L P 21, and L P 02 or L P 01, L P 11, L P 21, and L P 31, by using analytical and full models. The results of calculation using the analytical model and a gain simulation using the full model show that the gain can be kept almost constant with an error less than 0.25dB simply by adjusting the L P 01 pump power of the EDFA with signal modes of L P 01, L P 11, L P 21, and L P 31. This result suggests that it is possible to control the gain simply by adjusting the pump power in FM-EDFAs where all signal modes have a radial mode number of one.

Design of a highly mode-selective photonic lantern for IM/DD MDM transmission

Recently mode multiplexers and demultiplexers (MMUX/MDEMUX) based on photonic lantern (PL) techniques for few-mode fiber transmission have attracted great interest. However, the design of PLs with high modal selectivity has become a great challenge for weakly-coupled mode-division-multiplexing (MDM) applications. In this paper, we propose a highly mode-selective PL-MMUX/MDEMUX design for intensity-modulation/direct-detection MDM transmission adopting each non-degenerate mode or each pair of degenerate modes as an independent channel. For the PL-MMUX, differential-phase injection (DPI) scheme is adopted for each pair of degenerate modes and a corresponding low-modal-crosstalk dissimilar core arrangement method is proposed considering the interleaving positive and negative phase characteristics of modal lobes of degenerate linearly-polarized (LP) modes, which achieve very high modal selectivity. For the PL-MDEMUX, simultaneous reception of degenerate modes is achieved by combining the optical power of both degenerated modes utilizing non-mode-selective PLs. Numerical simulations for a 6-LP-mode PL-MMUX/MDEMUX are performed. Results show that the PL-MMUX with DPI scheme and proposed cores distribution achieve much lower modal crosstalk than other cores distributions and the overall modal crosstalk of the PL-MMUX and MDEMUX in back-to-back case is lower than -20.63 dB over the C-band for all modes. The tolerance of our design to imperfect fabrication of differentiator or environmental impact are also discussed. The proposed DPI and dissimilar core arrangement method have high expansibility and provide a new way for the design of highly selective PLs and are beneficial to the practical applications of weakly-coupled MDM transmission systems and networks.

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.

Optical mode conversion based on silicon-on-insulator material [formula omitted]-junction coupler and multimode interferometer

In mode-division multiplexing (MDM) networks, converting modes back and forth between a higher-order mode and a lower-order mode plays a vital role in improving network capacity and flexibility. This paper presents a spatial optical mode conversion supporting three modes based on two Ψ-junction couplers and two multi-mode interference couplers (MMIs). The proposed device can arbitrarily convert a TEi mode to a TEj mode (where i, j = 0, 1, 2) by setting three phase-shifters at 0 or 180 degrees, which are introduced at the Ψ-junction and MMI couplers. Through numerical simulations using the spatial three-dimensions beam propagation method (3D-BPM), the mode converter shows a high conversion efficiency with the insertion loss smaller than 0.2 dB and the crosstalk below −20 dB at the center wavelength of 1.55 μm. The device works effectively in C band, partially in S and L bands within a wideband up to 80 nm. We believe that the proposed device would be a potential platform for applications in MDM networks and photonic integrated circuit systems.

Demonstration of various optical directed logic operations by using an integrated photonic circuit.

Optical directed logic is a novel logic operation scheme that employs electrical signals as operands to control the working states of optical switches to perform the logic operations. In this Letter, we propose and demonstrate an integrated photonic circuit which can implement five different optical logic operations by utilizing two optical modes. The proposed device is fabricated on a silicon-on-insulator substrate by using electron beam lithography and inductively coupled plasma etching processes. The static experimental results show that the fabricated device can implement five different operations correctly-XOR, XNOR, NOR, NOT, and AND-from which we can see that the signal-to-noise ratios are larger than 17.6 dB over the entire C band for all five logic functions. At last, all five logic operations with the speed of 10 Kbps are demonstrated. The proposed device with simple structure, large bandwidth, and versatility would be a promising candidate for information processing in optical mode division multiplexing networks.

Tunable broadband mode power dividers based on a wavelength-insensitive coupler using the thermo-optic effect for flexible modal power adjustment in a mode-division multiplexing network

We demonstrate tunable broadband two-mode power dividers based on a wavelength-insensitive coupler for a mode-division multiplexing system. Ultra-small T E 0 − T E 1 and T E 0 − T E 2 mode dividers based on Si-wire waveguides are designed based on the coupled-mode theory. Tunability is achieved by using the thermo-optic effect. In both dividers, arbitrary branching ratios can be realized over a wide wavelength range. We experimentally demonstrated for the first time, to the best of our knowledge, a tunable T E 0 − T E 1 mode divider based on Si-wire waveguides. From the experimental results, an arbitrary branching ratio can be achieved by adjusting the injection current to a TiN heater placed on top of one of the delay lines.

An investigation on data switching technology in mode division multiplexing networks

With the rapid growth of people’s demand for communication speed and capacity, optical communication technology has developed vigorously with its advantages of high transmission rate, large capacity and low power consumption. In order to increase transmission capacity and bandwidth, multiple multiplexing technologies such as wavelength division multiplexing (WDM), time division multiplexing (TDM), polarization division multiplexing (PDM) have been widely studied. However, there is a capacity crisis in standard single-mode optical fibers. Mode division Multiplexing (MDM) technology is considered to be an effective way to solve the capacity crisis because it can keep the bandwidth of a single transceiver constant and effectively increase the total transmission capacity. Data exchange is an important function in optical communication system, which can effectively increase the flexibility of the network. However, the data exchange function in MDM system is rarely reported. In this paper, data exchange technologies in MDM networks are studied in detail, including OAM mode exchange based on optical devices and spatial light modulator (SLM), and TE mode exchange based on asymmetric directional coupler (ADC), micro-ring resonator (MRR) and Y-waveguides. The principles, advantages and disadvantages of each mode switching technology are analyzed and discussed in detail.

Multiple-Ring-Core FM-EDF for Weakly-Coupled MDM Amplification With Low Differential Modal Gain

Recently weakly-coupled mode-division multiplexing (MDM) transmission systems and networks based on ultralow-modal-crosstalk few-mode fiber (FMF) have attracted much interest, for which optical amplification technologies with low modal crosstalk and two-dimension mode-wavelength gain flattening are highly desired. In this paper, we propose for the first time a design method for few-mode Erbium-doped fiber (FM-EDF) with multi-ring-core (MRC) structures of both index and Erbium doping, which has high compatibility with the transmission MRC-FMF and low differential modal gain (DMG) with simple pump injection requirement. By sharing the boundaries of ring areas for both index and Erbium doping, the design method also helps to reduce the complexity of preform fabrication. We show by simulation that at 1550-nm signal wavelength, a 6-mode MRC-EDF can achieve a minimum DMG of 0.35-dB with only LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sub> pump light injection and a 10-mode MRC-EDF can achieve a minimum DMG of 0.60-dB with LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</sub> and LP <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">03</sub> pump lights injection. Besides, the tolerance of DMG to the variation of fiber parameters induced by fabrication is analyzed. The proposed FM-EDF is beneficial for the practical applications of weakly-coupled MDM transmission systems and networks.

Highly Nonlinear Organic-Silicon Slot Waveguide for Ultrafast Multimode All-Optical Logic Operations

Highly nonlinear multimode optical integrated devices hold great promise for the emerging large-capacity mode division multiplexing (MDM) technology, and they correspondingly enable multimode optical signal processing at nodes of the MDM network. Here, we propose a highly nonlinear multimode organic-silicon slot waveguide (HN-OSSW) whose nonlinearity is greatly improved by the intensive optical field confinement in the nano-gaps filled with highly nonlinear organic material. Specifically, the achieved nonlinear coefficients of the HN-OSSW under TE0 and TE1 modes are estimated higher than 7000 W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> and 5800 W <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> , respectively. Also, by phase mismatch optimization, this nanostructure exhibits superior intramode four-wave mixing conversion efficiency but ignorable intermode crosstalk. Coupling efficiency of the mode multiplexer and the optimal nonlinear interaction length are discussed in detail. Based on the HN-OSSW, we then numerically demonstrate a multimode all-optical hexadecimal addition and subtraction for sixteen phase-shift keying (16PSK) signals with the operation rate of 1.28Tb/s. Six different logic operations including A+B-C, A+C-B, B+C-A, A+B, A-B and B-A are simultaneously obtained with good performance evaluations, indicating the promising prospect of the HN-OSSW applied in ultrafast multimode signal processing.

Silicon Photonic Mode-Division Reconfigurable Optical Add/Drop Multiplexers with Mode-Selective Integrated MEMS Switches

Mode-division multiplexing (MDM) is an attractive solution for future on-chip networks to enhance the optical transmission capacity with a single laser source. A mode-division reconfigurable optical add/drop multiplexer (ROADM) is one of the key components to construct flexible and complex on-chip optical networks for MDM systems. In this paper, we report on a novel scheme of mode-division ROADM with mode-selective silicon photonic MEMS (micro-electromechanical system) switches. With this ROADM device, data carried by any mode-channels can be rerouted or switched at an MDM network node, i.e., any mode could be added/dropped to/from the multimode bus waveguide flexibly and selectively. Particularly, the design and simulation of adiabatic vertical couplers for three quasi-TE modes (TE0, TE1, and TE2 modes) based on effective index analysis and mode overlap calculation method are reported. The calculated insertion losses are less than 0.08 dB, 0.19 dB, and 0.03 dB for the TE0 mode, TE1 mode, and TE2 mode couplers, respectively, over a wavelength range of 75 nm (1515–1590 nm). The crosstalks are below −20 dB over the bandwidth. The proposed device is promising for future on-chip optical networks with flexible functionality and large-scale integration.

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.

On-chip enhanced electro-optic Kerr effect for manipulation of optical orbital angular momentum modes

The mode division multiplexing (MDM) technique using higher order orbital angular momentum (OAM) carrying modes through a channelized bandwidth provides enhanced capacity communication systems. Furthermore, encrypted channels via OAM based high-dimensional quantum key distribution (QKD) improve the transmission rate and security. In such applications, mode-selective manipulation of the spatially distributed (here, OAM) modes is a significant function to implement MDM and QKD networks. This paper proposes a novel versatile-designed chip which is configured in a Ycut periodically poled lithium niobate (PPLN) photonic wire. This integrated optical device acts as spatial mode converter for data modulated on higher order OAM = ±2ћ modes. The conversion is based on an enhanced electro-optic Kerr effect via phase-mismatched cascaded polarization coupling interactions of decomposed guided modes in two successive PPLN sections. The high-purity (91%) and low-voltage (<14.6 V) of the proposed device enable its operation in compatibility with applicable commercial modulators in OAM-based MDM and QKD systems.

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.

On-chip switchable and reconfigurable optical mode exchange device using cascaded three-waveguide-coupling switches.

Data exchange between different data channels can offer more flexible and advanced functions for many optical networks. In this paper, we propose a switchable and reconfigurable data exchange device for arbitrary two optical mode channels based on three-waveguide-coupling (TWC) switches in mode-division multiplexing (MDM) networks. The working principle of the TWC switches is numerically analyzed using the coupled supermode theory. As a proof of concept, switchable data exchange between arbitrary two mode channels among the first three-order quasi-transverse electric modes is experimentally demonstrated successfully. The insertion losses of the device are less than 5.6 dB, including the coupling loss of the multiplexer and demultiplexer, while the mode crosstalk is less than -13.0 dB for all functions. The proposed device is expected to offer more flexibility to on-chip MDM networks due to its low loss, low crosstalk and good scalability.

Ultra-Broadband Mode Splitter Based on Phase Controlling of Bridged Subwavelength Grating

An ultra-broadband mode splitter is proposed based on the phase controlling of the bridged subwavelength grating (BSWG) waveguides. An asymmetric directional coupler composed of two BSWG waveguides with an identical waveguide-width but two different bridge-widths is introduced to split the TM0 and TM1 modes based on the mode-sensitive phase-matching condition. The proposed mode splitter is optimized by utilizing the 3D full-vectorial finite difference time domain method. The results indicate that the proposed mode splitter can achieve a compact coupling length of 5.75 μm, the insertion losses of 0.41 dB and 0.53 dB, and the mode crosstalks of −19.6 dB and −27.8 dB for the TM0 and TM1 modes, respectively. The broad bandwidths of 430 nm and 372 nm can be achieved with the mode crosstalk of ≤−15 dB for the TM0 and TM1 modes, respectively. The fabrication tolerances are also studied in detail. The proposed mode splitter could be employed for signal selection and routing in the mode division multiplexing networks.

Compact and Nonvolatile Mode-Selective Switch With Nano-Heater

A compact and nonvolatile mode-selective switch (MSS) is proposed based on a 3D triple-waveguide directional coupler integrated with a nano-heater and Ge2Sb2Se4Te1 (GSST) phase-change material. The proposed MSS is optimally designed for the quasi-TM0 and quasi-TM1 modes, with a compact length of μ m and mode extinction ratios (ERs) of 21.22 dB and 19.03 dB at “OFF” and “ON” states, respectively. The mode ERs are higher than 17.92 dB and 15.81 dB over a 100 nm bandwidth. A nano-heater is implemented to adjust GSST phase states, which can achieve the tuning times of 2.88 μs and 6.57 μs with the heating power of 18 mW and 62.2 mW for “ON” and “OFF” states, respectively. The absorption heating is also investigated to evaluate the thermal effect on the performance of the optimized device. The proposed MSS can be applied in photonic integration based on-chip reconfigurable mode division multiplexing networks.

Proposal and performance analysis of an optical mode-division multiplexing random-access protocol for data centers

A random-access protocol for optical direct-detection mode-division multiplexing networks is proposed. The protocol is suitable for both data centers and on-chip communications systems. A mathematical description of the protocol is developed using both a Markov chain model analysis and an equilibrium point analysis. Several performance measures—specifically average network throughput, average packet delay, and average blocking probability—are derived. In our analysis, the effects of both multiplexing crosstalk and receiver noise are taken into account. The effect of several design parameters on the obtained performance measures are examined with the aid of a set of numerical examples.

Design of 20-polarization-maintaining-mode “pseudo-rectangle” elliptical-core fiber for MIMO-less MDM networks

Abstract We propose a polarization-maintaining few-mode fiber, which is characterized by the combination of an elliptical core structure and two stress-applying areas. Numerical simulations indicate that such fiber could support 20 distinct polarization-maintaining modes. The influences of elliptical-core size and stress-applying-area position are investigated for the 20 spatial modes. Moreover, the stress distribution, the modal birefringence distribution and the bending loss for the 20 polarization-maintaining modes of the designed fiber are also calculated. With appropriate geometrical parameters, the minimum effective refractive index difference (minΔneff) between adjacent modes is 2.01 × 10−4 at C band. The proposed fiber is a promising candidate to simplify or eliminate multiple-input-multiple-output digital-signal-processing in mode-division multiplexing networks.