Mode Crosstalk Research Articles

Orbital angular momentum vector modes (de)multiplexer based on multimode micro-ring.

Orbital angular momentum (OAM) multiplexing has emerged as an important method to increase the communication capacities in future optical information technologies. In this work, we demonstrate a silicon integrated OAM (de)multiplexer with a very simple structure. By simply tapping the evanescent wave of two different whispering gallery modes rotating inside a multimodal micro-ring resonator, four in-plane waveguide modes are converted to four free-space vector OAM beams with high mode purity. We further demonstrate chip-to-chip OAM multiplexing transmission using a pair of silicon devices, which shows low-level mode crosstalk and favorable link performance.

Mode conversion based on dual-phase modulation utilizing interference of two-phase-modulated beams

To realize mode conversion with high accuracy and efficiency, we propose a new method based on dual-phase modulation (DPM) that utilizes the interference of two-phase-modulated beams. DPM realizes complex amplitude modulation with the interference of two-phase-modulated beams generated by phase-type spatial light modulators (PSLMs). DPM realizes highly efficient conversion using the interference of light rather than the diffraction that is conventionally used for complex amplitude modulation. Moreover, DPM enables the suppression of modal crosstalk (MXT) utilizing the destructive interference between two unwanted 0th beams that occur from PSLMs. In this paper, we experimentally confirmed that DPM can reduce the amount of the unwanted 0th beam. Then, we performed an experiment on mode conversion based on DPM and achieved a modal crosstalk of less than − 20 dB and an optical loss of less than 10 dB.

Free-Space Remote Sensing of Rotation at the Photon-Counting Level

In principle the rotational Doppler effect could be used to detect the turning of a distant object, but the approach has not progressed beyond the laboratory, due to low photon-collection efficiency and orbital angular momentum (OAM) mode crosstalk. The authors address these challenges and successfully observe the effect over a 120-m free-space link between two rooftops. By analyzing the photon counts for the relevant OAM components, they deduce the speed and symmetry of rotating bodies, even at the photon-counting level. This work is a significant step toward remote sensing in a realistic environment based on the rotational Doppler effect of ``twisted light''.

Reconfigurable three-dimensional mode (de)multiplexer/switch via triple-silicon-ITO-waveguide directional coupler.

A reconfigurable mode (de)multiplexer/switch (RMDS) is a pivotal component for the mode routing in mode-division multiplexing (MDM) networks. Here, we propose a three-dimensional (3D) RMDS via a triple-waveguide directional coupler, consisting of a lower doped silicon waveguide, a central plasmonic horizontal-slot waveguide with indium-tin-oxide (ITO) and an upper doped polycrystalline-silicon waveguide. The enhanced light-matter-interactions can be achieved via the central plasmonic metal-oxide-semiconductor (MOS) mode. The multiplexing states of the proposed 3D-RMDS can be switched by adjusting the applied voltage bias on the ITO layer. The simulation results reveal that a 3D quasi-TM0 and quasi-TM1 RMDS is with a compact length of 8.429 μm, the mode crosstalk of -20.3 dB (-9.2 dB) and the insertion loss of 0.06 dB (1.47dB) at the wavelength of 1550 nm for the "OFF" ("ON") state, respectively. The proposed 3D-RMDS can be applied in future 3D on-chip MDM networks to achieve a flexible mode-routing and further enhance the transmission capacity.

Compact and low-crosstalk mode (de)multiplexer using a triple plasmonic-dielectric waveguide-based directional coupler

A compact and low-crosstalk mode (de)multiplexer [(De)MUX] is proposed based on a triple-waveguide coupler, consisting of a hybrid plasmonic waveguide in between two outer silicon waveguides. The proposed quasi-TM0 and quasi-TM1 mode (De)MUX is optimized by using the full vectorial finite element method and 3D full vectorial finite difference time domain method sequentially. The numerical results show that the proposed mode (De)MUX has a compact coupling length of 7.5 μm, an ultralow mode crosstalk of −30 dB, an insertion loss of 0.32 dB at 1550 nm, and an extinction ratio of >15 dB over the full C-band. The fabrication tolerance of the proposed mode (De)MUX is also investigated in detail. The effects of thermal tuning and absorption heating are studied to correct the fabrication errors and to evaluate the thermal effects on the performance, respectively.

Wavelength-Insensitive Weakly Coupled FMFs and Components for the MDM-GPON

Mode division multiplexing (MDM) has been widely investigated to enhance the capacities of passive optical networks (PONs), and weakly coupled transmission schemes are highly preferred to reduce the cost and complexity. However, wavelength insensitivities are scarcely studied for such applications. In this letter, an evolution scheme of the cascaded mode division multiplexing PON (MDM-PON) and conventional Gigabit PON (GPON), which supports bidirectional 1310/1490-nm transmission, is proposed. This scheme is enabled by wavelength-insensitive weakly coupled (WIWC) few-mode fiber (FMF) and optical components. Then, the realization of both WIWC few-mode wavelength division multiplexers (FM-WMUXs) and mode-selective couplers (MSCs) is analyzed. Based on the analysis, WIWC FM-WMUXs and MSCs that support two linear polarization modes and 1310/1490-nm transmission with low modal crosstalk are designed and fabricated. Finally, the bidirectional MDM-GPON transmission scheme with 2.5 Gb/s on-off-keying modulation and simple direct detection over 10-km FMF and 10-km single-mode fiber are experimentally demonstrated. The proposed MDM-GPON evolution scheme can achieve maximum compatibility by maintaining both the optical distribution network and the optical network units.

Ultra-broadband and compact graphene-on-silicon integrated waveguide mode filters

Increasing bandwidth demands in optical communications necessitates the introduction of mode-division multiplexing (MDM) on top of the existing wavelength-division multiplexing (WDM) systems. Simultaneous management of both multiplexing systems will be a complex task, and there is the possibility of signal degradation through modal crosstalk. Here, we propose graphene-on-silicon (GOS) integrated waveguide mode filters to suppress the propagation of spurious waveguide modes at the telecommunications wavelength. Graphene’s high fabrication tolerance potentially enables surgical tailoring and deployment at targeted segments on the waveguide to absorb the undesired TE0 or TE1 modes. The proposed GOS waveguide mode filters can potentially improve the performance and reduce the device footprint of MDM systems.

Propagation properties of partially coherent Lommel beams in non-Kolmogorov turbulence

Based on generalized Huygens–Fresnel principle, the analytical expressions of the partially coherent Lommel beams propagating in turbulent atmosphere are derived. The propagation properties of vortex mode intensity (including mode probability density and crosstalk probability density) of partially coherent Lommel beam in non-Kolmogorov turbulence are investigated in this paper. The research results show that coherence width of partially coherent Lommel beam plays a key role to determine the mode intensity in the process of propagation. The poor coherence of source beams leads to serious dispersion of spiral spectrum distribution. It is worth noting that crosstalk probability density increases to its peak and gradually decreases due to the enhancement of other-order crosstalk. Furthermore, long wavelength and small asymmetry parameter are helpful to mitigate the influence of atmospheric turbulence and restrain the mode crosstalk on the orbital angular momentum mode transmission.

2.6 Tbit/s On-Chip Optical Interconnect Supporting Mode-Division-Multiplexing and PAM-4 Signal

In this letter, we combine the use of dense-wavelength-division-multiplexing and mode-division-multiplexing (MDM) with advanced-modulation formats for on-chip optical interconnects. A three-mode MDM multiplexer (MUX) and de-multiplexer (DEMUX) was designed and used. Each wavelength carrier had an aggregate data rate of 192-Gbit/s using 4-level pulse-amplitude-modulation. The mode crosstalk of the MDM MUX/DEMUX was measured to be below −20 dB in the 40-nm wavelength band from 1520 to 1560 nm. Fourteen wavelength channels are multiplexed and successfully transmitted with an aggregate transmission capacity of 2.6 Tbit/s (14 wavelengths $\times$ 3 modes $\times$ 64 Gbit/s) with all channels satisfying the hard-decision forward-error-correction threshold. Numerical analysis of the mode crosstalk is also performed.

Si-based Mach-Zehnder wavelength/mode multi/demultiplexer for a WDM/MDM transmission system.

We propose and experimentally demonstrate a low-loss and low-crosstalk Mach-Zehnder mode/wavelength multi/demultiplexer for WDM/MDM transmission based on a Si-photonics platform. A broadband 3-dB mode divider, which is also newly devised here, makes it possible to compose a Mach-Zehnder filter for "mode" and "wavelength" simultaneously. Transmission characteristics of fabricated 3-dB mode dividers are in excellent agreement with theoretical results. Mach-Zehnder filters using the 3-dB mode divider with a free spectral range (FSR) of 20 and 1 nm are also fabricated and the modal crosstalk is less than -24 dB in the 40-nm wavelength range for the MZ filter with an FSR of 20 nm. The tuning of the peak wavelength position by the TiN heater is also demonstrated.

Ultra-broadband multimode 3dB optical power splitter using an adiabatic coupler and a Y-branch.

As an essential component of mode division multiplexing (MDM) system, a multimode 3dB power splitter with low loss, high power balance, and low mode crosstalk is highly desired. In this paper, we propose an ultra-broadband on-chip multimode 3dB optical power splitter using an adiabatic coupler and an S-bend based Y-branch. As an example, a splitter for the four-lowest modes of a rib waveguide on silicon on insulator (SOI) platform is designed. Simulation results show that the device exhibits < 0.12dB insertion losses, within ± 0.38dB power imbalances, and < -18.5dB mode crosstalks for the four-lowest modes within a large operating wavelength range of 165 nm (from 1400 nm to 1565 nm). The fabrication tolerance of gap size at the output end of the adiabatic coupler is also analyzed.

Ultra-compact and fabrication-tolerant mode multiplexer and demultiplexer based on angled silicon waveguides

An ultra-compact and fabrication-tolerant mode multiplexer (MUX) and demultiplexer (DeMUX) is proposed based on the angled silicon waveguides. The proposed mode (De)MUX is optimized by using the full-vectorial finite element method (FV-FEM) and three-dimensional full-vectorial finite difference time domain method (3D-FV-FDTD). The simulated results show that the optimal mode (De)MUX is with an ultra-compact length of 7.0μm, a high mode conversion efficiency of 98.5%, an ultra-low excess loss of 0.05 dB and a low mode crosstalk of −18.2 dB at the operating wavelength of 1550 nm. The fabrication tolerances are also studied for the proposed mode (De)MUX.

Controlling abruptly autofocusing vortex beams to mitigate crosstalk and vortex splitting in free-space optical communication.

Orbital angular momentum (OAM) mode crosstalk induced by atmospheric turbulence is a challenging phenomenon commonly occurring in OAM-based free-space optical (FSO) communication. Recent advances have facilitated new practicable methods using abruptly autofocusing light beams for weakening the turbulence effect on the FSO link. In this work, we show that a circular phase-locked Airy vortex beam array (AVBA) with sufficient elements has the inherent ability to form an abruptly autofocusing light beam carrying OAM, and its focusing properties can be controlled on demand by adjusting the topological charge values and locations of these vortices embedded in the array elements. The performance of a tailored Airy vortex beam array (TAVBA) through atmospheric turbulence is numerically studied. In a comparison with the ring Airy vortex beam (RAVB), the results indicate that TAVBA can be a superior light source for effectively reducing the intermodal crosstalk and vortex splitting, thus leading to improvement in the FSO system performance.

Effective area measurement of few-mode fiber using far field scan technique with Hankel transform generalized for circularly-asymmetric mode.

We report on a measurement method for the effective area of the few-mode fiber. We derived a transform equation between a near-field pattern and a far-field pattern generalized for circularly-asymmetric higher-order modes of a cylindrical core, and enabled effective area measurement of the higher-order modes using high-dynamic-range far-field scan technique and low-crosstalk mode multiplexer. The measured effective area values agreed well with the values that were numerically predicted using a finite-element method from the refractive index profile, when the modal crosstalk was suppressed.

Reconfigurable all-optical on-chip MIMO three-mode demultiplexing based on multi-plane light conversion.

We present, to the best of our knowledge, the first experimental demonstration of reconfigurable all-optical on-chip multi-input-multi-output three-mode demultiplexing based on multi-plane light conversion. The demultiplexer consists of cascaded phase shifter arrays and multimode interference couplers integrated on a compact silicon chip. By optimizing the phase shifters, reconfigurable three-mode demultiplexing is experimentally realized with wavelength-dependent loss of less than 3dB and modal crosstalk of less than -10 dB over a 23nm optical bandwidth. Error-free mode demultiplexing of 40Gbps non-return-to-zero signal is also demonstrated.

Integrated Circularly Polarized OAM Generator and Multiplexer for Fiber Transmission

Unlike linearly polarized modes in fiber, modes exploiting orbital angular momentum (OAM) are circularly polarized when propagating in fiber. The use of OAM modes for spatial multiplexing requires efficient, low cost mode generators and multiplexers. We propose such a device based on the standard 220-nm silicon-on-insulator platform, taking multiple single-mode data-modulated signals, and imprinting these signals on right- and left-circularly polarized OAM channels on a single, multiplexed output. The device is designed to easily couple to an OAM fiber with a ring-shaped core. This approach treating circular polarization within the multiplexer allows us to avoid the losses associated with filtering out unwanted polarization to create a single polarization. Designing the device to have an output matched to the OAM fiber mode profile also avoids mode size conversion. We describe our design methodology and optimization techniques using a transfer-matrix model and the finite-difference time-domain method. A candidate design is simulated and modal crosstalk is examined, showing that low-crosstalk OAM multiplexing can be achieved through direct fiber-to-chip coupling.

A hybrid multiplexer/de-multiplexer for wavelength-mode-division based on photonic crystals

A hybrid multiplexer/de-multiplexer (HMUX/HDeMUX) for wavelength-mode-division based on photonic crystals (PCs) is presented. The proposed device consists of a point-defect cavity, a wavelength-selective cavity and asymmetrical parallel waveguides. Coupled-mode theory (CMT) is applied to the analysis, and the finite-difference time-domain (FDTD) method is used for the simulations. The simulation results show that the device can multiplex the fundamental and first-order modes of 1550 and 1310 nm. It exhibits not only a low insertion loss (<0.37 dB) but also low mode crosstalk (<−13 dB). Thus, the PC-based HMUX/HDeMUX has considerable potential for application in large-capacity optical communication systems.

Weakly-coupled 4-mode step-index FMF and demonstration of IM/DD MDM transmission.

Weakly coupled-mode division multiplexing (MDM) over few-mode fibers (FMF) for short-reach transmission has attracted great interest, which can avoid multiple-input-multiple-output digital signal processing (MIMO-DSP) by greatly suppressing modal crosstalk. In this paper, step-index FMF supporting 4 linearity polarization (LP) modes for MIMO-free transmission is designed and fabricated for the first time, to our knowledge. Modal crosstalk of the fiber is suppressed by increasing the mode effective refractive index differences. The same fabrication method as standard single-mode fiber is adopted so that it is practical and cost-effective. The mode multiplexer/demultiplexer (MUX/DEMUX) consists of cascaded mode-selective couplers (MSCs), which are designed and fabricated by tapering the proposed FMF with single-mode fiber (SMF). The mode MUX and DEMUX achieve very low modal crosstalk not only for the multiplexing/demultiplexing but also for the coupling to/from the FMF. Based on the fabricated FMF and mode MUX/DEMUX, we successfully demonstrate the first simultaneous 4-modes (LP01, LP11, LP21 & LP31) 10-km FMF transmission with 10-Gb/s intensity modulation and MIMO-free direct detection (IM/DD). The modal crosstalk of the whole transmission link is successfully suppressed to less than -16.5 dB. The experimental results indicate that FMF with simple step-index structure supporting 4 weakly-coupled modes is feasible.

Reducing group delay spread using uniform long-period gratings

Despite the promise of an orders-of-magnitude increase in transmission capacity, practical implementation of mode-division multiplexing faces a number of challenges. The most important among them is the complexity of digital signal processing (DSP) for compensating mode crosstalk and modal dispersion. The most promising method proposed so far for reducing this DSP complexity is strong mode coupling. We propose and demonstrate, for the first time, a method of inducing strong mode coupling and reducing group delay spread using uniform long-period gratings (LPGs). Even though the LPGs have a fixed grating period, mode coupling is effective among all mode groups and over a broad wavelength range. Both insertion loss and mode-dependent loss can be significantly reduced by optimizing the index profile of and the number of modes supported by the fiber in which the LPG is applied.

Orbital-angular-momentum mode-group multiplexed transmission over a graded-index ring-core fiber based on receive diversity and maximal ratio combining.

An orbital-angular-momentum (OAM) mode-group multiplexing (MGM) scheme using high-order mode groups (MGs) in a graded-index ring-core fiber (GIRCF) is proposed, in which a receive-diversity architecture is designed for each MG to suppress the mode partition noise resulting from random intra-group mode crosstalk. The signal-to-noise ratio (SNR) of the received signals is further improved by a simple maximal ratio combining (MRC) technique on the receiver side to efficiently take advantage of the diversity gain of the receiver. Intensity-modulated direct-detection (IM-DD) systems transmitting three OAM mode groups with total 100-Gb/s discrete multi-tone (DMT) signals over a 1-km GIRCF and two OAM mode groups with total 40-Gb/s DMT signals over an 18.4-km GIRCF are experimentally demonstrated, respectively, to confirm the feasibility of our proposed OAM-MGM scheme.