Mode Crosstalk Research Articles

Rayleigh-based OTDR with dynamic modal crosstalk suppression.

We propose and demonstrate a novel approach for measuring the modal attenuation of the splice loss of a purely LP11 mode group by using a Rayleigh-based OTDR with a dynamic modal crosstalk (XT) suppression technique for few-mode fibers (FMFs). With the proposed approach, the Brillouin loss interaction with a Brillouin Stokes beam co-propagating with the OTDR probe removes the modal XT caused at the modal conversion point and suppresses the accumulated modal XT that is detected. A preliminary experiment is demonstrated using spliced FMFs with a core-offset. Experiments revealed that the proposed technique can accurately measure the splice loss variations of a purely LP11 mode group.

Tunable mode-selective characteristics of a mode-filter petal-fiber with liquid rods

In this paper, a novel tunable mode-filter optical fiber consisting of a high-index core and petal-shaped cladding surrounded by a high-index outer ring is proposed. The cladding of the fiber is formed with periodically arranged liquid rods that support cladding modes with effective indexes. These cladding modes form a two-super-mode group. The mode-selection is realized by the coupling between the core mode and the super-mode group. With the petal-shaped cladding, cladding mode can be transmitted at high loss. With the liquid rods, the index-band of super-mode group can be adjusted by external temperature field, thereby achieving the purpose of tunable mode-selective. The super-mode group formed by the LP<sub>11</sub> mode of the liquid rods effectively increases its operating bandwidth and temperature tuning range. The numerical simulation results show that the mode-filter fiber with a length of only 71.4 mm can achieve a particular mode loss more than 20 dB, while other modes’ losses are below 1 dB. This special fiber can be used as a mode-filter in the few-mode fiber transmission system to reduce mode crosstalk of converters, multiplexer/demultiplexer, optical switch and optical routing.

Turbulence-induced optical loss and cross-talk in spatial-mode multiplexed or single-mode free-space communication channels

Single-mode or mode multiplexed free-space atmospheric optical channels draw increasingly more attention in the last decade. The scope of their possible applications spans from the compatibility with the telecom WDM technology, fiber amplifiers, and modal multiplexing for increasing the channel throughput to various quantum communication related primitives such as entanglement distribution, high-dimensional spatially encoded quantum key distribution, and relativistic quantum cryptography. Many research papers discuss application of specific mode sets, such as optical angular momentum modes, for communication in the presence of atmospheric turbulence. At the same time some basic properties and key relations for such channels exposed to the atmospheric turbulence have not been derived yet. In the current paper we present simple analytic expressions and a general framework for assessing probability density functions of channel transmittance as well as modal cross-talk coefficients. Under some basic assumptions the presented results can be directly used for estimation of the Fried parameter of the turbulent channel based on the measured statistics of the fundamental mode transmittance coefficient.

Reconfigurable Mode (De)Multiplexer Using Ge2Sb2Se4Te1 Based Triple-Silicon-Waveguide

A reconfigurable mode (de)multiplexer [(De)MUX] is proposed based on a triple-silicon-waveguide directional coupler, consisting of a central waveguide with a Ge2Sb2Se4Te1 (GSST) upper layer and two outer silicon waveguides. The output mode states can be switched by the self-holding phase transition between the amorphous and crystalline states of the GSST layer, and a low loss of the proposed device can be achieved benefiting from the low propagation losses of the GSST at both states. The proposed reconfigurable mode (De)MUX is optimized by using the full-vectorial finite-element method and the thee-dimensional full-vectorial finite-difference time-domain method. The numerical results reveal that the proposed reconfigurable mode (De)MUX is with a compact length of 17.65 $\mu \text{m}$ , insertion losses of 0.16 and 0.96 dB, and mode crosstalk of −15.91 and −26.97 dB at ON and OFF states, respectively. The proposed reconfigurable mode (De)MUX can operate over the S + C + L band with high performance. The fabrication tolerances are also studied in detail.

Reconfigurable Mode (De) Multiplexer via 3-D Triple-Waveguide Directional Coupler With Optical Phase Change Material

A three-dimensional reconfigurable mode (de) multiplexer (3DRM-(De)MUX) is proposed based on a triple-waveguide directional coupler with optical phase change material, which consists of an input waveguide on the upper layer, a bus waveguide on the lower layer, and an intermediate waveguide on the upper layer with a Ge2Sb2Se4Te1 (GSST) cap layer. The self-holding “ off ” and “ on ” states of the proposed 3DRM-(De)MUX can be switched by triggering the amorphous and crystalline states of GSST, respectively. The proposed 3DRM-(De)MUX is optimized by using the full-vectorial finite element method and 3-D full-vectorial finite difference time domain method. The simulated results show that the proposed 3DRM-(De)MUX for switching between the quasi-TE0 and quasi-TE1 modes is with a compact coupling length of only 12.0 μm, a broad bandwidth of 100 nm, the mode extinction ratios of 18.25 dB and 14.0 dB, and the insertion losses (ILs) of –0.38 and –1.14 dB for the “ off ” and “ on ” states, respectively. A more compact 3DRM-(De)MUX can also be achieved for the quasi-TM1 mode with the coupling length of 8.28 μm, IL of –0.12 dB (–1.98 dB), and mode crosstalk of –16.37 dB (–13.23 dB) for the “ off ” (“ on ”) state, respectively. The proposed 3DRM-(De)MUX can be applied in 3-D on-chip mode division multiplexing networks for flexible mode routing and switching.

Crosstalk-aware routing, spectrum, mode and time assignment using FMF with partial MIMO equalization in flexible grid datacenter networks

Abstract Recently, lots of datacenters (DCs) have been built to enhance reliability and expand processing ability for computing service in cloud computing. Furthermore, spatial division multiplexing (SDM) technology and elastic optical network (EON) are also considered as the candidate solutions to solve the requirements of huge bandwidth and flexible connections. However, it is much more difficult to use few mode fiber (FMF) in DCs-based EON due to the existing modal crosstalk among different modes. Faced with the challenges, we propose a routing, spectrum, mode and time assignment algorithm to decrease the blocking probability and increase the spectrum utilization. To deal with the problem more efficiently, we propose a method called flexible sliding three-dimensional block (3D block) which represents the dimensions of spectrum, space and time in our algorithm. 3D block slides in the resource space to find out proper resources. The simulations are conducted on datacenter networks (DCNs) and the results show that lower blocking probability and higher spectrum utilization are achieved by efficiently utilizing the spectrum, space and time domains.

Dual communication windows polarization filter based on photonic crystal fiber with nano-scale gold film

Abstract A photonic crystal fiber (PCF) polarization filter with dual communication bands is proposed in this paper. The polarization characteristics of the double-hole gold-coated PCF are numerically analyzed by utilizing the finite element method (FEM). As we know, the filter can achieve polarization filtering effect on input signals at 1310 nm (y-polarized mode) and 1550 nm (x-polarized mode) bands simultaneously in two orthogonal polarization states. In the above two communication bands, the confinement losses of the unwanted polarization mode can reach 192.7 dB/cm (y-polarized mode) and 427.5 dB/cm (x-polarized mode), whereas other polarization mode losses are very low. Furthermore, when the transmission length of the PCF is 800 μm, x and y polarization mode crosstalk can reach −275.4 and 126.9 dB in the 1550 and 1310 nm bands, respectively. Meanwhile, the crosstalk is less than −20 dB or better than 20 dB, the optical bandwidth can reach 220 and 150 nm, respectively.

Three-dimensional compact mode (de)multiplexer based on triple-waveguide directional coupler

A three-dimensional (3D) compact mode (de)multiplexer ((De)MUX) is proposed based on a triple-waveguide directional coupler to (de)multiplex both the horizontal and vertical quasi-TM1 modes, in which a horizontal-slot waveguide (WG) is inserted in between a silicon WG at the bottom layer and a poly-silicon WG at the upper layer. The characteristics of the proposed 3D mode (De)MUX are studied by utilizing the full-vectorial finite element method and 3D full-vectorial finite difference time domain method. The numerical results reveal that the proposed 3D mode (De)MUX is with a compact coupling length of 5.536 μm, a low mode crosstalk of −30.7 dB, and a low-insertion loss of 0.1 dB for (de)multiplexing the horizontal quasi-TM1 mode at the wavelength of 1550 nm. For (de)multiplexing the vertical quasi-TM1 mode, the optimized 3D mode (De)MUX is with the coupling length of 5 μm, the insertion loss of 0.74 dB, and the mode crosstalk of −19.5 dB. The fabrication tolerances of the proposed 3D mode (De)MUX are also investigated for achieving a robust fabrication process.

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.