Mode Division Multiplexing Applications Research Articles

Computation cost reduction in 3D shape optimization of nanophotonic components

Inverse design methodologies effectively optimize many design parameters of a photonic device with respect to a primary objective, uncovering locally optimal designs in a typically non-convex parameter space. Often, a variety of secondary objectives (performance metrics) also need to be considered before fabrication takes place. Hence, a large collection of optimized designs is useful, as their performance on secondary objectives often varies. For certain classes of components such as shape-optimized devices, the most efficient optimization approach is to begin with 2D optimization from random parameter initialization and then follow up with 3D re-optimization. Nevertheless, the latter stage is substantially time- and resource-intensive. Thus, obtaining a desired collection of optimized designs through repeated 3D optimizations is a computational challenge. To address this issue, a machine learning-based regression model is proposed to reduce the computation cost involved in the 3D optimization stage. The regression model correlates the 2D and 3D optimized structural parameters based on a small dataset. Using the predicted design parameters from this model as the initial condition for 3D optimization, the same optima are reached faster. The effectiveness of this approach is demonstrated in the shape optimization-based inverse design of TE0-TE1 mode converters, an important component in mode-division multiplexing applications. The final optimized designs are identical in both approaches, but leveraging a machine learning-based regression model offers a 35% reduction in computation load for the 3D optimization step. The approach provides a more effective means for sampling larger numbers of 3D optimized designs.

Switchable dual-wavelength mode-locked cylindrical vector beam fiber laser based on unpumped EDF saturable absorber and nonlinear polarization rotation

A switchable single- and dual-wavelength mode-locked pulse laser with cylindrical vector beam (CVB) based on an unpumped EDF saturable absorber and a nonlinear polarization rotation (NPR) hybrid mode-locked mechanism has been proposed and experimentally demonstrated. An unpumped EDF saturable absorber is utilized to generate a mode-locked operation that generates high-order laser pulses. Utilizing a hybrid mode-locked mechanism, the signal-to-noise ratio (SNR) and operating stability of the laser are improved compared to the mode-locked operation with a single unpumped EDF saturable absorber. Through adjusting the polarization controller, single- and dual-wavelength stable mode-locked pulses can be easily realized. The single-wavelength mode-locked pulse operates with central wavelengths of 1550.16 nm and 1549.80 nm, repetition frequencies of 6.33 MHz and 6.35 MHz, corresponding to pulse intervals of 158 ns and 157 ns. Further, the long-period grating (LPFG) is used as a mode converter to achieve pulse output with a purity exceeding 95 % in the TM01 and TE01 modes at different operating wavelengths. The proposed fiber laser with a simple structure has provided a flexible pulsed CVB laser source for laser processing, time division, and mode division multiplexing applications.

Investigation of hybrid spectrum slicing-wavelength division multiplexing (SS-WDM) in transparent medium for mode division multiplexing applications

Investigation of hybrid spectrum slicing-wavelength division multiplexing (SS-WDM) in transparent medium for mode division multiplexing applications

Efficient mode coupling between a few-mode fiber and multi-mode photonic chip with low crosstalk.

The mode-division multiplexing technology has been developing slowly in the field of fiber-chip-fiber optical interconnects, mainly limited by the inefficient mode coupling between few-mode fiber and multi-mode photonic chip. Here we propose and design a multi-mode coupler composed of tapered few-mode fiber and silicon integrated multistage waveguide tapers, which allows the direct coupling between high-order fiber modes and high-order waveguide modes with high coupling efficiency and low crosstalk. Based on the edge coupling scheme, the six linear polarization modes (LP01x/y, LP11a x/y, LP11bx/y) in few-mode fiber are firstly coupled into the integrated waveguide with low insertion loss, then the input modes are converted to the desired waveguide modes (TE0, TE1, TE2, TM0, TM1, TM2) with low mode crosstalk relying on the mode evolution principle. The obtained results show that the coupling efficiency is higher than 87%, while for the mode conversion process, the efficiency is higher than 99% and the mode crosstalk is lower than -25 dB. The favorable performance achieved may open new perspectives for diverse mode-division multiplexing applications, enabling efficient capacity scaling in fiber-chip-fiber optical interconnects and optical communication systems.

Radially anisotropic ring-core optical fiber: towards vector-vortex guided transmission using the full modal space

The radially anisotropic ring-core fiber with cylindrical birefringence is theoretically and numerically investigated as a novel platform for the transmission of vector-vortex beams with unique modal properties. First, we elucidate the parametric conditions where such fiber enables modal substitution in which either the donut-shaped azimuthal TE01 or radial TM01 mode replaces the normal Gaussian-like HE11 mode as the fundamental mode of the waveguide. We also demonstrate that it is possible to significantly engineer the waveguiding properties of the fiber via the addition of small radial birefringence (∼10−4) so as to make the (hitherto non-degenerate) TE0m and TM0m modes fully degenerate. The latter property is used to create stable vortex modes of high purity (>99%) with the newly degenerate modal pair – a feat not possible with standard few-mode fibers—all without affecting the co-propagating hybrid HE/EH modes that remain available as an independent basis set to produce vortex beams of similarly high purity. These new insights are relevant to the topical fields of mode-division multiplexing (MDM), structured light, fiber modelling and fabrication. With respect to MDM applications, the newly available vortex modes created with the degenerate TE/TM basis set can now be concurrently used with the more common vortex modes created via the HE/EH modal basis set.

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.

High-order mode conversion based on adiabatical mode evolution for mode division multiplexing applications.

Mode conversion based on adiabatical mode evolution in a two-core configuration is investigated. The configuration can convert all the launching modes to higher-order modes from one port and convert all the launching modes to lower-order modes from another port. Mode conversion between the two degenerated high-order modes is also demonstrated numerically. The mode conversion feature is only dependent on the relationship between the effective mode indices of the two cores in the configuration, which shows the characteristics of high flexibility and large fabrication tolerance.

Engineering of closely packed silicon-on-isolator waveguide arrays for mode division multiplexing applications

On-chip optical communications require high-performance, energy-efficient, and scalable multiplexing techniques for enabling high parallelism. Mode division multiplexing (MDM) is gaining momentum, but its scalability is limited by the crosstalk and the (de)multiplexing complexity. To overcome such limitations, this paper proposes the exploitation of super-modes in closely spaced arrays of waveguides. A theoretical approach demonstrates the generation of super-modes achieved by perturbing the uniformity of waveguide arrays. Spectral characterization shows a crosstalk of about −20 dB on the C band, in a nonuniform array of five silicon-on-isolator waveguides.

Mode-selective coupling between few-mode fibers and buried channel waveguides.

With intensive computation, we analyze in detail butt-coupling between few-mode fibers and buried channel waveguides and discuss, in particular, the conditions to achieve mode-selective coupling for the first 6 spatial modes. In the case of coupling a buried rectangular-core waveguide to a step-index or a parabolic-index circular-core fiber, a modal crosstalk smaller than -20 dB for all the 6 spatial modes can be achieved, when the shape of the waveguide core is sufficiently close to a square (within ± 0.7% variation in its aspect ratio) and the waveguide and the fiber have close mode volumes. In the case of coupling a buried rectangular-core waveguide to a step-index elliptical-core fiber, it is possible to achieve a modal crosstalk smaller than -20 dB with many combinations of waveguide and fiber parameters. The aspect ratios of the elliptical core and the best matched rectangular core can be very different (e.g., 0.85 for the elliptical core and 1.05 for the rectangular core) and an elliptical core that has a moderate ellipticity and an area not close to the upper limit allowed for supporting 6 spatial modes is preferred. The use of a parabolic-index profile in the elliptical core can further improve the mode selectivity with greatly relaxed tolerances on both the aspect ratio and the area of the rectangular core required. In general, when the fiber and the waveguide core are matched for low-crosstalk performance, the butt-coupling losses for all the 6 spatial modes as well as the mode-dependent loss are also small (typically well below 1 dB). Our results are useful for the design of mode-selective waveguide devices, such as mode (de)multiplexers, mode converters, and mode switches, for fiber-based mode-division multiplexing applications.

Compact Si photonic multimode interference-based optical circuit for mode division multiplexing applications

A design for a compact Si photonic two mode demultiplexer for mode division multiplexing (MDM) applications is presented. The design uses the self-imaging in multimode interference structures to achieve MDM with an insertion loss less than 0.5 dB and a cross talk better than 20 dB over the C band. The imaging is achieved within a length that is half the length reported in the literature, and its overall dimensions are 42 μ m × 3 μ m . The minimum cross talk is affected by the structure geometry. The tolerance of the design to variations in the dimensions is also studied.

Silicon three-mode (de)multiplexer based on cascaded asymmetric Y junctions.

A three-mode (de)multiplexer based on two cascaded asymmetric Y junctions is proposed and experimentally demonstrated on a silicon-on-insulator platform for mode-division multiplexing applications. Within a bandwidth from 1537 to 1566 nm, the best demultiplexing crosstalk of the fabricated device, composed of a three-mode multiplexer, a multimode straight waveguide, and a three-mode demultiplexer, is up to -31.5 dB, while in the worst case it is -9.7 dB. The measured maximum insertion loss is about 5.7 dB at a wavelength of 1550 nm. The mode crosstalk and insertion loss can be further improved by high-quality fabrication processes.

Supermodes in Coupled Multi-Core Waveguide Structures

Optical supermode dynamics are nowadays receiving an ever increasing attention in mode-division multiplexing applications. In this paper, we systematically analyze and characterize these modes in various structures based on coupled-mode theory. In particular, we investigate the structure of supermodes in honeycomb multi-ring arrangements of any size. In addition, we show that higher order supermodes in coupled few-mode multi-core waveguide arrays can be strongly affected by angle-dependent couplings, leading to different modal field profiles. Analytical solutions are provided for linear, rectangular, and ring arrays. Higher order supermodes are observed for the first time in a coupled few-mode three-core fiber system using the S2 imaging method.

Proposal of an InP-based few-mode transmitter based on multimode interference couplers for wavelength division multiplexing and mode division multiplexing applications

An InP-based monolithically integrated few-mode transmitter aiming at the combination of wavelength division multiplexing (WDM) and mode division multiplexing (MDM) technologies is proposed. The core elements of the proposed transmitter are mode converters and a wavelength-mode division multiplexer that are all based on multimode interference (MMI) couplers. Simulations show that the wavelength-mode division multiplexer has a large fabrication tolerance of 30 and 0.5 μm for the length and the width of the device, respectively. A low loss below 0.26 dB for the passive parts of the transmitter is obtained in the whole C-band wavelength range.

Supersymmetric mode converters.

Originally developed in the context of quantum field theory, the concept of supersymmetry can be used to systematically design a new class of optical structures. In this work, we demonstrate how key features arising from optical supersymmetry can be exploited to control the flow of light for mode-division multiplexing applications. Superpartner configurations are experimentally realized in coupled optical networks, and the corresponding light dynamics in such systems are directly observed. We show that supersymmetry can be judiciously used to remove the fundamental mode of a multimode optical structure while establishing global phase-matching conditions for the remaining set of modes. Along these lines, supersymmetry may serve as a promising platform for versatile optical components with desirable properties and functionalities.