Mode-division Multiplexing Systems Research Articles

Nonparaxial Mode-Size Converter Using an Ultracompact Metamaterial Mikaelian Lens

Mode-division multiplexing (MDM) is a promising approach to increase data transmission capacity by using different modes as independent data channels. Intermodal crosstalk and coherent mode interference are detrimental to the performance of an MDM system and should be avoided in the mode converters and mode expanders for MDM. The Mikaelian lens is well known to be free from spherical aberrations and is therefore an excellent candidate for focusing high order waveguide modes which have main mode field distributions located off-axis. Here we proposed an ultracompact mode-independent mode-size converter using an integrated Mikaelian lens formed by refractive index engineering using a subwavelength grating. The Mikaelian lens is fabricated on the silicon-on-insulator (SOI) platform and can be used to convert the size of high order modes at the interface between the silicon waveguide and a multimode fiber in MDM systems. We present a proof-of-concept demonstration of a low loss spot size converter for three transverse electric (TE) modes between a 14 μm wide multimode waveguide grating coupler and a 1.56 μm wide multimode silicon waveguide. The measured insertion loss of the Mikaelian lens mode size converter is 0.21 dB for TE0 mode, 0.26 dB for TE1 mode and 0.28 dB for TE2 mode. 1-dB bandwidth of all the three modes are about 60 nm. The 15 μm long Mikaelian lens has comparable experimental performance as a 450 μm long adiabatic taper.

Second-harmonic generation of single-mode Laguerre-Gaussian beams with an improved quasi-phase-matching method.

In the second-harmonic generation processes involving Laguerre-Gaussian (LG) beams, the generated second-harmonic wave is generally composed of multiple modes with different radial quantum numbers. To generate single-mode second-harmonic LG beams, a type of improved quasi-phase-matching method is proposed. The Gouy phase shift has been considered in the optical superlattice designing and an adjustment phase item is introduced. By changing the structure parameters, each target mode can be phase-matched selectively, whose purity can reach up to 95%. The single LG mode generated from the optical superlattice can be modulated separately and used as the input signals in the mode division multiplexing system.

Modes power equalization based-singular value decomposition in mode division multiplexing systems for multi-hungry bandwidth applications

Optical fiber tendencies are pushing for changes towards upgrading scalability, agility, and unwavering quality for multi-hungry bandwidth applications. In the quest for future proof of those multi-hungry bandwidth applications, is vital to take advantage of new multiplexing technologies as the surge of network traffic that soon will overwhelm the capacity of multimode fiber (MMF). Due to the issue of MMF modal dispersion and mode coupling that caused Intersymbol Interference (ISI) which result in bandwidth degradation and limited range of length. Thus, Mode division-multiplexing (MDM) is a significant and elegant emerging technology, which harnesses the symmetry of modes by transmitting in parallel data through different modes. This paper models and analysis novel four-mode group demultiplexing MDM-based Singular Value Decomposition (SVD) to isolate the signals and fairly distribute the power to the system sub-channels, each to their respective groups. The novel MDM based-SVD system achieved an aggregated data rate of 100 Gbit/s on wavelengths 1550.12 nm over an existed graded-index MMF length of 3000 m. The performance of the proposed system has been evaluated through channel impulse response, channel spectrum, eye diagram, and Bit-Error-Rate (BER) matrices.

Broadband and compact silicon mode converter designed using a wavefront matching method.

A broadband and compact TE0-TE1 mode converter for a mode division multiplexing system designed using a wavefront matching method is realized. We present the first experimental demonstration of a silicon waveguide device designed by a wavefront matching method. In order to achieve broadband operation of the silicon mode converter, seven wavelengths are considered in its optimization process. The designed silicon mode converter is fabricated via a standard complementary metal-oxide-semiconductor technology, which enables low-cost mass production. Measurements performed using the fabricated mode converter correlate strongly with the calculated results.

Compact and broadband multimode waveguide bend by shape-optimizing with transformation optics

Multimode waveguide bend is one of the key components for realizing high-density mode-division multiplexing systems on chip. However, the reported multimode waveguide bends are either large, bandwidth-limited or fabrication-complicated, which hinders their applications in future high-density multimode photonic circuits. Here we propose a compact multimode waveguide bend supporting four TE modes simply by shape-optimizing with transformation optics. The shape of the waveguide is optimized in the virtual space with gradient distribution of the refractive index, so that the scattering loss and intermode cross talk are well suppressed. After conformal mapping back into the physical space, a compact (effective radius of 17 μm) multimode bending waveguide is obtained. Simulations show that the proposed multimode waveguide bend has little loss ( < 0.1 dB ) and low cross talk ( < − 20 dB ) throughout an ultrabroad wavelength range of 1.16–1.66 μm. We also fabricated the shape-optimized multimode bending waveguide on a silicon-on-insulator wafer. At 1550 nm wavelength, the measured excess losses for the four lowest-order TE modes are less than 0.6 dB, and the intermode cross talks are all below − 17 dB . Our study paves the way for realizing high-density and large-scale multimode integrated optical circuits for optical interconnect.

Mode conversion based on tilted-few-mode fiber assisted by dual arc waveguide for mode-division multiplexing system

In this paper, mode conversion based on tilted few-mode fiber assisted by the dual arc waveguide is reported. A multilayer core multimode fiber (MC-MMF) is used to model the device due to their low bending loss, high precession in mode confinement and ease of fabrication during manufacturing. In this report, an optimized 12°-tilted MC-MMF was used at the device input to provide perfect alignments with the arc waveguides and minimize radiation loss. A horizontal fiber of the same properties is connected at the output to collect the converted mode. This was achieved by making proper alignment between the core layers of the two fibers and the arc bends. Mode conversions of LP11-LP01, LP02-LP11, and LP21-LP01 were achieved with the conversion efficiency of 89.6 %, 91.2 %, and 94.34 %, respectively. The proposed conversion device indicates potential application in mode-division multiplexing communication.

A Flexible and Reconfigurable Optical Add-Drop Multiplexer for Mode Division Multiplexing Systems

Reconfigurable optical add-drop multiplexer (ROADM) is one of the key building blocks for on-chip optical networks, which can download the desired signals from the bus waveguide to the corresponding drop port and upload the local signals from the add port to the bus waveguide to achieve the signals dropping/adding between the local and the bus waveguide. In this letter, we propose a ROADM based on a Benes network for mode-division multiplexing systems. As a proof of the concept, a ROADM with three quasi-transverse electric modes (TE0-TE2) is fabricated and experimentally demonstrated, which can download signals from TEi (i = 0, 1, 2) mode channel in the bus waveguide to an arbitrary local drop port or upload local signals to TEi mode channel via an arbitrary add port. The excess loss of the fabricated device is less than 10.7 dB, and the crosstalk among different space or mode channels is lower than -13.7 dB. Finally, a 12 Gbps data transmission experiment is carried out to verify the performance of the device. The proposed device is reconfigurable and scalable, and thus, it is expected to be used for optical data processing in the mode-division multiplexing systems.

Evaluation of mode division multiplexed system by dynamic power transfer matrix characterization

We experimentally demonstrate a simple method to characterize the temporal dynamics of the power transfer matrix of a mode division multiplexed (MDM) system using the time series of the output power in each channel. We consider a 3 × 3 MDM system consisting of a pair of 3-channel photonic lanterns (PL) for mode (de)multiplexing and 1 km of few-mode fiber (FMF) to evaluate the time evolution of channel selectivity, insertion loss, channel-dependent loss, and accumulated cross-talk for each channel. We further compare the statistics of time evolution of the above parameters for MDM systems utilizing mode-selective and non-mode-selective photonic lanterns. Such results are used to evaluate the consequences of choice of photonic lanterns and their utility in long-haul and short-reach mode division multiplexed systems.

Design of a broadband single mode hybrid plasmonic waveguide incorporating silicon nanowire

In this work, a graphene hybrid plasmonic waveguide has been studied employing the finite element method. The graphene layers have been exploited here as optical absorber layers to extinguish the undesired plasmon modes in our system. Also, the silicon nanowires in our design, have been utilized to form a hybrid plasmon waveguide in order to achieve a high figure of merit of the desired plasmon mode. The multiwavelength operability and performance variation with respect to the alteration of waveguide cross-section are investigated here to demonstrate its high promise in broadband single mode operation, design of mode division multiplexing nanoplasmonic systems, and highly compact photonic integration. Furthermore, the effect of graphene’s optical anisotropy on waveguiding is explored here to predict the device performance more realistically.

Beyond 1.6 Tb/s Net Rate PAM Signal Transmission for Rack-Rack Optical Interconnects With Mode and Wavelength Division Multiplexing

In this article, a beyond 1.6 Tb/s net rate pulse amplitude modulation (PAM) communication system is experimentally demonstrated for 20 m rack-rack optical interconnects with low cost intensity modulation direct detection (IM-DD) architecture, which is enabled by mode division multiplexing (MDM) and wavelength division multiplexing (WDM). The MDM with three linearly polarized modes is realized by a multiplane light conversion (MPLC) mode multiplexer with the maximum modal crosstalk less than - 20 dB. The experimental results show that 2.01 Tb/s (1.84 Tb/s net rate) PAM-6 signal carried on three modes and four wavelengths is successfully transmitted over 20 m standard OM2 multimode fiber (MMF) with the BERs of received signals from all channels below hard decision forward error correction (HD-FEC) threshold of 3.8 × 10-3. Look-up table (LUT) pre-distortion and Volterra nonlinear equalizer (VNLE) are employed for nonlinear impairment mitigation to improve the system performance. Compared to linear feedforward equalizer (FFE), 0.3 dB and 1 dB receiver sensitivity improvements are obtained by LUT and VNLE, respectively. To the best of our knowledge, our proposed scheme achieves the highest bit rate of 167.5 Gb/s per wavelength in MDM system with IM-DD structure. The experimental results show that our proposed PAM MDM communication scheme is a promising candidate for future 1.6 Tb/s short reach rack-rack optical interconnects.

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.

Silicon Integrated Nanophotonic Devices for On-Chip Multi-Mode Interconnects

Mode-division multiplexing (MDM) technology has drawn tremendous attention for its ability to expand the link capacity within a single-wavelength carrier, paving the way for large-scale on-chip data communications. In the MDM system, the signals are carried by a series of higher-order modes in a multi-mode bus waveguide. Hence, it is essential to develop on-chip mode-handling devices. Silicon-on-insulator (SOI) has been considered as a promising platform to realize MDM since it provides an ultra-high-index contrast and mature fabrication processes. In this paper, we review the recent progresses on silicon integrated nanophotonic devices for MDM applications. We firstly discuss the working principles and device configurations of mode (de)multiplexers. In the second section, we summarize the multi-mode routing devices, including multi-mode bends, multi-mode crossings and multi-mode splitters. The inverse-designed multi-mode devices are then discussed in the third section. We also provide a discussion about the emerging reconfigurable MDM devices in the fourth section. Finally, we offer our outlook of the development prospects for on-chip multi-mode photonics.

Inverse design of a single-step-etched ultracompact silicon polarization rotator.

We propose and experimentally demonstrate a novel ultracompact silicon polarization rotator based on equivalent asymmetric waveguide cross section in only single-step etching procedure for densely integrated on-chip mode-division multiplexing system. In the conventional mode hybridization scheme, the asymmetric waveguide cross section is employed to excite the hybridized modes to realize high performance polarization rotator with compact footprint and high polarization extinction ratio. However, the fabrication complexity severely restricts the potential application of asymmetric waveguide cross section. We use inverse-designed photonic-crystal-like subwavelength structure to realize an equivalent asymmetric waveguide cross section, which can be fabricated in only single-step etching process. Besides, a theory-assisted inverse design method based on a manually-set initial pattern is employed to optimize the device to improve design efficiency and device perform. The fabricated device exhibited high performance with a compact footprint of only 1.2 × 7.2 µm2, high extinction ratio (> 19 dB) and low insertion loss (< 0.7 dB) from 1530 to 1590 nm.

Machine learning aided inverse design for few-mode fiber weak-coupling optimization.

Few-mode fiber (FMF) supporting many modes with weak-coupling is highly desired in mode division multiplexing (MDM) systems. The multi-parameter design of FMF becomes comparably difficult, inaccurate and time-consuming when it comes for complex fiber structures and many high order modes. In this work, we demonstrate a machine learning method using neural network to inversely design the desired FMF based on multiple-ring structure. By using the minimum index difference between adjacent modes as the weak-coupling optimization aim, we realize the inverse design of 4-ring step-index FMFs for supporting 4, 6 and 10 -mode operation, and 6-ring step-index FMF for supporting 20-mode operation. This method provides high-accuracy, high-efficiency and low-complexity for fast and reusable design of optical fibers, including particularly weak-coupling FMF in this work. It can be widely extended to a lot of fibers and has great potential for instantaneous applications in the optical fiber industry.

Efficient 3D silicon mode (de)multiplexer based on a sidewall-aligned vertical coupler.

An efficient three-dimensional (3D) quasi-TE0 and quasi-TE1 mode (de)multiplexer [(De)MUX] is proposed and optimized based on a dual-waveguide 3D-coupler, in which the sidewalls of a bottom silicon waveguide and an upper poly-silicon waveguide are aligned vertically. The full-vectorial finite element method and 3D full-vectorial finite difference time domain method are utilized to study the performances of the proposed 3D mode (De)MUX. The results indicate that the proposed mode (De)MUX can achieve a compact coupling length of 6.88 µm, a mode crosstalk of -30.04dB, an insertion loss of 0.02 dB, and an ultra-broad 1 dB bandwidth of 300 nm. The proposed 3D mode (De)MUX based on a sidewall-aligned vertical coupler has the potential to extend the functionality and to increase the integration of the mode division multiplexing (MDM) systems.

Density-matrix formalism for modal coupling and dispersion in mode-division multiplexing communications systems.

Borrowing methodology from quantum mechanics, we propose and develop a density-matrix formalism for modal coupling and dispersion in mode-division multiplexing communications systems. The central concept in our formalism is the density matrix, from which all observable information of an optical field can be handily accessed. In the formalism, we derive fundamental evolution equations and concatenation rules for the key elements that characterize essential modal properties, and construct a statistical model ready for the numerical analysis of stochastic light propagation in randomly perturbed fibers. Unlike the Stokes-vector formalism that requires J2 - 1 auxiliary Gell-Mann matrices, the density-matrix formalism can be directly formulated for arbitrary modal-space dimension J. Based on the density-matrix formalism, the statistical modal properties of a 4-mode fiber under random perturbation are numerically investigated, which raises an interesting possibility of optimizing the modal dispersion by manipulation of the random perturbation.

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.

Analysis and simulation of reducing nonlinear interaction by shaping envelopes of transmitted signals in mode-division multiplexing systems

Nonlinear interaction induced by Kerr effects is a significant problem in the efforts made to ensure the successful development of transmission systems, such as mode-division multiplexing (MDM)-based optical multimode communication systems. A technique for reducing the interaction of the modes in MDM systems based on shaping envelopes of propagated signals is proposed. The envelopes of modes that carry m-array quadrature amplitude modulation (mQAM) are optically formed with a sinusoidal envelope to lower fiber nonlinearities by reducing the effective intensity and interference time between modes. The transmission performance of the proposed system is analytically characterized by developing an analytical model to estimate induced nonlinear phase noise and numerically investigated by examining the signal-to-noise ratio versus mode power. The effect of sinusoidal enveloped (SE)-4QAM format on the transmission distance is also explored for different mode combinations. Single-, two-, and five-mode transmissions are carried out to investigate the proposed method’s efficiency on the reduction of nonlinear interaction. In our system, the modes carry SE-4QAM format at rate 20 Gsymbol / s. The results show a significant enhancement in the performance of the MDM system when modes are modulated by SE-4QAM format. For example, the transmission distances of LP01 and LP11 are lengthened by 56.5% and 150% at the symbol error rate of 10 − 5, respectively.

Ultra-broadband on-chip multimode power splitter with an arbitrary splitting ratio

The multimode power splitter is a basic component in mode-division multiplexing systems. In this paper, we propose an ultra-broadband silicon multimode power splitter enabling arbitrary power splitting ratios. The proposed multimode splitter is designed based on a waveguide crossing with an obliquely embedded subwavelength grating (SWG) transflector. The incident multiple guided-modes can be split into two beams with low excess losses and low crosstalk by the SWG transflector where the thin-film interference effect happens. As an example, a silicon multimode power splitter is designed to work with the three lowest-order modes of TE polarization. Any desired splitting ratio ranging from 0% to 100% can be achieved by engineering the structural parameters of the SWG. Moreover, the desired splitting ratio can be very uniform over an extremely broad bandwidth of ≥ 415 nm, covering O-, E-, S-, C-, L- and U-bands. The intermodal crosstalk is &lt; −20 dB for all the input modes in theory. To the best of our knowledge, the proposed structure is the first multimode power splitter enabling any desired power splitting ratios in all the optical communication bands.

Fiber impairments mitigation in OFDM based cognitive optical networks

An adaptive impairments assessment is necessary to evaluate the impact of various linear and nonlinear effects in future generation cognitive optical transport links. In this paper, an Adaptive Fiber Impairment Mitigation (AFIM) algorithm is proposed to identify a suitable mitigation scheme for the cognitive environment. The AFIM algorithm will assess fiber impairments and adaptively select a suitable mitigation scheme with minimum complexity based on the present network conditions and user performance target. The performance of AFIM algorithm is compared with Fixed Fiber Impairment Mitigation approach in terms of outage probability and outage capacity analysis. An Orthogonal Frequency Division Multiplexing based Mode Division Multiplexing system with Few Mode Fiber (FMF) is suggested as a solution to increase the nonlinearity threshold limit of the system. The $$\hbox {L}_2$$ -by-3 nonlinear transform based Peak to Average Power Ratio reduction technique is implemented to mitigate fiber nonlinear effects in FMF based backbone and backhaul links. The performance analysis of the FMF system has been evaluated and compared with that of Single Mode Fiber system. The proposed analytical model and mitigation schemes are integrated with the AFIM algorithm to realize the cognitive optical network. Further, the result shows that AFIM algorithm enhances the system capacity by more than 6-folds at an outage probability of 0.5 and reduces the outage probability to 0.6 at the capacity range of 20 Gbps.