De Multiplexer Research Articles

Efficient mode coupling/(de)multiplexing between a few-mode fiber and a silicon photonic chip

Mode-division multiplexing (MDM) has attracted much attention due to its ability to further increase the transmission capacity of optical interconnects. While further developments of MDM optical interconnects are hindered by the coupling of few-mode fibers (FMFs) and silicon photonic chips, a high-efficiency, broadband, and scalable multimode FMF-chip interface is still eagerly desired. To address this challenge, a novel scheme for efficient multimode coupling is proposed by introducing a silica planar lightwave circuit as an intermediate. The core idea is to couple and demultiplex higher-order modes by leveraging the superiorities of silica optical waveguides for manipulating LP modes, facilitated through tailoring the mode conversion related to different mode symmetric properties. The demultiplexed modes are consequently butt-coupled to the silicon photonic chip in single-mode manner, thus being available for fulfilling further data transmitting/receiving/routing directly. As a proof of concept, a six-channel FMF-chip coupler working with the LP01-x/y, LP11a-x/y, and LP11b-x/y modes is designed with low coupling losses of 0.77–1.39 dB and low intermode crosstalk of <−27.2 dB in a broad bandwidth (>150 nm). Minimum coupling losses of 1.36–2.48 dB are experimentally demonstrated. It is the first demonstration for the integrated multimode FMF-chip coupler enabling the simultaneous coupling of six mode-channels, to the best of our knowledge. We believe that this work has the great potential for developing energy-efficient and low-cost chip-to-chip MDM interconnections in the future.

Gauge-flux-controlled orbital angular momentum mode conversion in silicon waveguides.

We propose a method to convert fundamental modes into orbital angular momentum (OAM) modes through chiral dynamics induced by gauge fluxes in silicon waveguides. By integrating a trench into a few-mode waveguide, we induce the rotation of TE10 and TE01 modes, naturally generating the gauge flux for the synthesized OAM modes. By precisely controlling the gauge flux, we achieve chiral dynamics that optimize the conversion efficiency of OAM modes at specific propagation distances, addressing challenges posed by mode degeneracy. Additionally, we demonstrate an on-chip OAM mode (de)multiplexer based on flux-controlled mode conversion. Our findings offer new, to the best of our knowledge, strategies for creating artificial gauge fluxes in straight waveguides and open up possibilities for manipulating OAM modes on photonic chips.

Experimental verification of phase behaviors of MMI couplers and their application to spectrally flat WDM (de)multiplexers

Phase and amplitude behaviors of several types of 2:2 multimode interference (MMI) couplers are theoretically analyzed and experimentally verified using a fabricated silicon-nanowire waveguide-based interferometer scheme. The phase response of various structural MMI schemes is critical for realizing highly efficient wavelength-division-multiplexing optical (de)multiplexers in terms of spectral flatness, low insertion loss, and minimal crosstalk. We experimentally demonstrate these aspects through the fabrication of a silicon-nanowire-based interference testing scheme. Finally, based on the experimental results, we propose a method for designing spectrally flat optical (de)multiplexers with good fabrication tolerance.

Ultra-compact thin-film-lithium-niobate photonic chip for dispersion compensation

Abstract Thin-film-lithium-niobate (TFLN) photonics has attracted intensive attention and become very popular in recent years. Here, an ultra-compact TFLN on-chip dispersion compensator is proposed and realized to provide a promising solution for dispersion control. The proposed dispersion compensator is composed of chirped multimode waveguide gratings (CMWGs) arranged in zigzag-cascade, enabling high footprint compactness and scalability. Particularly, these CMWGs are circulator-free and very convenient for cascading, owing to the TE0–TE1 mode conversion and the assistance of the TE0–TE1 mode (de)multiplexer. The present configuration with CMWGs in zigzag-cascade also overcomes the drawback of being unable to use waveguide spirals for large-range time delay and dispersion control due to the TFLN’s anisotropy. In addition, positive/negative dispersion control is realized by appropriately choosing the input port of the CMWGs. In the experiment, 2-mm-long CMWGs are used to provide a dispersion value of about +1.5 ps/nm and −1.2 ps/nm over a 21-nm-wide bandwidth, and there are up to 32 CMWGs in cascade demonstrated experimentally, showing a maximal dispersion of 49.2 ps/nm and −39.3 ps/nm. The corresponding average propagation loss is as low as 0.47 dB/cm, and the fabricated chip with 32 CMWGs in zigzag-cascade has a footprint as compact as 0.16 × 4.65 mm2. Finally, the present on-chip dispersion compensator is used successfully to compensate for the dispersion originating from a 5-km-long singlemode fiber (SMF) and high-quality eye-diagrams are achieved for the recovered 40 Gbps OOK signals, showing great potential for optical systems such as high-speed interconnects in datacenters.

Off-axis dispersion-managed metasurface for routing orbital angular momentum mode and wavelength multiplexing channels

Offering the strengths of mode orthogonality and physical independence from wavelength dimension, optical orbital angular momentum (OAM) modes hold immense potential for enhancing communication capacity through multi-dimensional channel multiplexing. Although methods using angle-separated gratings have seen advances in multi-dimensional (de)multiplexing, routing these multiplexed channels is impeded by the resultant mode-wavelength dispersion resulting from Bragg diffraction of grating. This induces not only multi-level mode conversion but also confined wavelength separation scope, posing obstacles in spatial reallocation of both OAM modes and wavelengths for routing channels. To tackle these issues, we propose a wavelength-dependent multi-foci transformation solution for OAM mode-wavelength routing that utilizes an off-axis dispersion-managed metasurface. Incorporated with composite lens phases and helical modulations, the metasurface enables the precise manipulation of OAM mode-wavelength dispersion upon multi-foci Fourier transform without multi-level diffraction, facilitating both efficient mode conversion and versatile wavelength separation. Harnessing the multi-dimensional independence, this approach establishes a high-dimensional linear mapping relationship among OAM modes, wavelengths, and spatial positions, thereby achieving the routing of mode-wavelength hybrid channels. In a proof-of-concept simulation, we demonstrated the successful routing of 20 channels with five OAM modes and four wavelengths across four depth planes, with the average channel crosstalk below -15.2 dB. Additionally, 16-ary quadrature amplitude modulation signals carried by these 20 multiplexed channels were successfully routed, and the bit-error-rates approach 1 × 10-5. Our method shows flexibility in spatial reallocation of both OAM modes and wavelengths, as further explored by altering the number of routing channels and their spatial positions, which may provide new insights for advanced OAM-based optical communications.

Time Evolution of Orbital Angular Momentum Modes for Deep-Routing Multiplexing Channels

Optical orbital angular momentum (OAM) mode multiplexing has emerged as a promising technique for boosting communication capacity. However, most existing studies have concentrated on channel (de)multiplexing, overlooking the critical aspect of channel routing. This challenge involves the reallocation of multiplexed OAM modes across both spatial and temporal domains—a vital step for developing versatile communication networks. To address this gap, we introduce a novel approach based on the time evolution of OAM modes, utilizing the orthogonal conversion and diffractive modulation capabilities of unitary transformations. This approach facilitates high-dimensional orthogonal transformations of OAM mode vectors, altering both the propagation direction and the spatial location. Using Fresnel diffraction matrices as unitary operators, it manipulates the spatial locations of light beams during transmission, breaking the propagation invariance and enabling temporal evolution. As a demonstration, we have experimentally implemented the deep routing of four OAM modes within two distinct time sequences. Achieving an average diffraction efficiency above 78.31%, we have successfully deep-routed 4.69 Tbit·s−1 quadrature phase-shift keying (QPSK) signals carried by four multiplexed OAM channels, with a bit error rate below 10–6. These results underscore the efficacy of our routing strategy and its promising prospects for practical applications.

Electrically switchable 2N-channel wave-front control for certain functionalities with N cascaded polarization-dependent metasurfaces.

Metasurfaces with tunable functionalities are greatly desired for modern optical system and various applications. To increase the operating channels of polarization-multiplexed metasurfaces, we proposed a structure of N cascaded dual-channel metasurfaces to achieve 2N electrically switchable channels without intrinsic loss or cross-talk for certain functionalities, including beam steering, vortex beam generation, lens, etc. As proof of principles, we have implemented a 3-layer setup to achieve 8 channels. In success, we have demonstrated two typical functionalities of vortex beam generation with switchable topological charge of l = -3 ~ +4 or l = -1 ~ -8, and beam steering with the deflection direction switchable in an 8×1 line or a 4×2 grid. We believe that our proposal would provide a practical way to significantly increase the scalability and extend the functionality of polarization-multiplexed metasurfaces. Although this method is not universal, it is potential for the applications of LiDAR, glasses-free 3D display, OAM (de)multiplexing, and varifocal meta-lens.

Multi‐Object Silicon Photonic Spectrometer

AbstractA multi‐object silicon photonic spectrometer with N input ports is proposed and realized by integrating a multi‐channel passband optical filter (POF), a tunable narrow‐band optical filter as well as a calibration‐free N × 1 Mach–Zehnder switch (MZS) array. Here, the multi‐channel POF consisting of a multimode waveguide grating (MWG) and a mode (de)multiplexer is used to achieve a broadened working window and an enhanced dynamic range for the present spectrometer, while the narrow‐band optical filter is realized with a thermally‐tunable Euler micro‐ring resonator (EMR) for achieving a very high spectral resolution. The introduction of the N × 1 MZS enables the time‐division‐multiplexed (TDM) spectrum analysis for multiple objects. In this paper, a multi‐object silicon photonic spectrometer with 16 input ports is demonstrated with an on‐chip loss of less than 3 dB and inter‐channel crosstalk as low as −25 dB. This multi‐object spectrometer can be used to analyze the spectra of 16 objects one by one by setting the 16 × 1 MZS, the resolution is as high as 50 pm, and the working window is ≈84 nm.

Hybrid WDM/MDM (De) multiplexer based on Fabry–Perot resonators with Bragg grating reflectors

The traveling-wave-like Fabry–Perot (TW-like F-P) resonators, utilizing transverse-mode conversion, have been thoroughly investigated as on-chip filters. However, the asymmetric directional coupling (ADC) between the phase shifter and the output waveguide in this structure is not fully utilized, resulting in a rare implementation of hybrid wavelength division multiplexing (WDM) and mode division multiplexing (MDM). In this paper, using the transfer matrix method (TMM), we investigate methods to effectively enhance the quality factor (Q-factor) of TW-like F-P resonators. This is achieved by increasing the phase shifter length and reducing the coupling coefficient between these waveguides, without significantly impacting the channel drop efficiency. MDM can be achieved by adjusting the width of the output waveguides, utilizing the ADC between the phase shifter and the output waveguide. We design nine-channel hybrid WDM-MDM multiplexers based on TW-like F-P resonators. The variational-finite-difference time-domain (varFDTD) method is utilized to analyze the device’s performance, and its single channel extinction ratio (ER) values can reach −20dB. This work paves the way for TW-like F-P-resonator-based large capacity optical communications and interconnections.

Photonic crystal-connected bidirectional micro-ring resonator array for duplex mode and wavelength channel (de)multiplexing

The progress of on-chip optical communication relies on integrated multi-dimensional mode (de)multiplexers to enhance communication capacity and establish comprehensive networks. However, existing multi-dimensional (de)multiplexers, involving modes and wavelengths, face limitations due to their reliance on single-directional total internal reflection and multi-level mode conversion based on directional coupling principles. These constraints restrict their potential for full-duplex functionality and highly integrated communication. We solve these problems by introducing a photonic-like crystal-connected bidirectional micro-ring resonator array (PBMRA) and apply it to duplex mode-wavelength multiplexing communication. The directional independence of total internal reflection and the cumulative effect of the subwavelength-scale pillar within the single-level photonic crystal enable bidirectional mode and wavelength multiplexed signals to transmit among multi-pair nodes without interference, improving on-chip integration in single-level mode conversion. As a proof of concept, we fabricated a nine-channel bidirectional multi-dimensional (de)multiplexer, featuring three wavelengths and three TE modes, compactly housed within a footprint of 80 μm×80 μm, which efficiently transmits QPSK-OFDM signals at a rate of 216 Gbit/s, achieving a bit error rate lower than 10−4. Leveraging the co-ring transmission characteristic and the orthogonality of the mode-wavelength channel, this (de)multiplexer also enables a doubling of communication capacity using two physical transmission channels.

A demultiplexer-based dual-path switching true random number generator

This paper presents a demultiplexer (DEMUX) based dual-path switching true random number generator (TRNG). Unlike the classical single chain TRNG, the proposed TRNG utilizes DEMUXs to separate the cumulative jitter and occurrence of metastability in two paths. It also uses Ring Oscillator (RO) to increase the uncertainty of path switching time. Therefore, two sources of entropy are skilfully combined and no post-processing circuitry is required. The proposed structure is implemented on Xilinx Virtex-7 FPGA development board. The experimental results show that the proposed structure is able to achieve a high throughput of 500 Mbps with 33 Look-up Tables (LUTs) and 4 Flip-Flops (FFs). This effectively improves the throughput with high quality entropy and low hardware overhead.

Inverse Design of a Wavelength (De)Multiplexer for 1.55- and 2-μm Wavebands by Using a Hybrid Analog-Digital Method

Inverse Design of a Wavelength (De)Multiplexer for 1.55- and 2-μm Wavebands by Using a Hybrid Analog-Digital Method

Experimental demonstration of a silicon four-mode (de)multiplexer based on cascaded triple-waveguide couplers

Silicon photonics based on-chip mode-division multiplexing (MDM) is an appealing technology for enhancing the interconnect capacity in both short- and long-haul optical communication. Here, a silicon four-mode (de)multiplexer [(De)MUX] is proposed, optimized, fabricated, and characterized for on-chip MDM systems, based on three cascaded triple-waveguide couplers (TWCs), with coupling lengths below 24.25 µm. A four-mode MDM-link consisting of two back-to-back mode-(De)MUXs was fabricated and measured, yielding a crosstalk of less than −15.0dB over a bandwidth wider than 72.6 nm for four modes. In addition, the TWC based mode (De)MUX demonstrates a 1-dB bandwidth greater than 40 nm, and can be considered as a robust component for multimode on-chip networks.

Inverse-Designed Low-Crosstalk CWDM (De)Multiplexer Assisted by Photonic Crystals

Inverse-Designed Low-Crosstalk CWDM (De)Multiplexer Assisted by Photonic Crystals

Demultiplexing-free ultra-compact WDM-compatible multimode optical switch assisted by mode exchanger.

Silicon-based optical switches are integral to on-chip optical interconnects, and mode-division multiplexing (MDM) technology has enabled modes to function as carriers in routing, further boosting optical switches' link capacity. However, traditional multimode optical switches, which typically use Mach-Zehnder interferometer (MZI) structures and mode (de)multiplexers, are complex and occupy significant physical space. In this paper, we propose and experimentally demonstrate a novel demultiplexing-free dual-mode 3×3 thermal-optical switch based on micro-rings (MRs) and mode exchangers (MEs). All MRs are designed to handle TE1 mode, while the ME converts TE0 mode to TE1 mode, enabling separate routing of both modes. Bezier curves are employed to optimize not only the ME, but also the dual-mode 45° and 90° waveguide bends, which facilitate the flexible and compact layout design. Moreover, our structure can support multiple wavelength channels and spacings by adding pairs of MRs, exhibiting strong WDM compatibility. The switch has an ultra-compact footprint of 0.87×0.52 mm2. Under both "all-bar" and "all-cross" configurations, its insertion losses (ILs) remain below 8.7 dB at 1,551 nm, with optical signal-to-noise ratios (OSNRs) exceeding 13.0 dB. Also, 32 Gbps data transmission experiments validate the switch's high-speed transmission capability.

Compact silicon photonic-lantern mode (de)multiplexer based on tilt slot waveguide.

As the key component in on-chip mode-division multiplexing systems, a compact silicon photonic-lantern mode (de)multiplexer is proposed and demonstrated using the shallow-etched tilt slot waveguide. The proposed six-mode (de)multiplexer is designed as a constant coupling length of 11.7 µm for each mode conversion and eliminates the adiabatic transition tapers for cascaded asymmetric directional couplers, which have an ultra-short total length of 69 µm. The measured peak insertion losses of the fabricated device for all mode channels are less than 1.2 dB, and the crosstalk is below -12.6 dB in a 60 nm waveband. Additionally, the simulation results indicate that the device has a good fabrication tolerance. The proposed mode (de)multiplexer is scalable and could provide a feasible solution for the dense integration of on-chip mode division multiplexing systems.

Physics to system-level modeling of silicon-organic-hybrid nanophotonic devices

The continuous growth in data volume has sparked interest in silicon-organic-hybrid (SOH) nanophotonic devices integrated into silicon photonic integrated circuits (PICs). SOH devices offer improved speed and energy efficiency compared to silicon photonics devices. However, a comprehensive and accurate modeling methodology of SOH devices, such as modulators corroborating experimental results, is lacking. While some preliminary modeling approaches for SOH devices exist, their reliance on theoretical and numerical methodologies, along with a lack of compatibility with electronic design automation (EDA), hinders their seamless and rapid integration with silicon PICs. Here, we develop a phenomenological, building-block-based SOH PICs simulation methodology that spans from the physics to the system level, offering high accuracy, comprehensiveness, and EDA-style compatibility. Our model is also readily integrable and scalable, lending itself to the design of large-scale silicon PICs. Our proposed modeling methodology is agnostic and compatible with any photonics-electronics co-simulation software. We validate this methodology by comparing the characteristics of experimentally demonstrated SOH microring modulators (MRMs) and Mach Zehnder modulators with those obtained through simulation, demonstrating its ability to model various modulator topologies. We also show our methodology's ease and speed in modeling large-scale systems. As an illustrative example, we use our methodology to design and study a 3-channel SOH MRM-based wavelength-division (de)multiplexer, a widely used component in various applications, including neuromorphic computing, data center interconnects, communications, sensing, and switching networks. Our modeling approach is also compatible with other materials exhibiting the Pockels and Kerr effects. To our knowledge, this represents the first comprehensive physics-to-system-level EDA-compatible simulation methodology for SOH modulators.

Multidimensional Fiber‐to‐Chip Optical Processing Using Photonic Integrated Circuits

AbstractMultidimensional multiplexing technologies have been proven to be a promising scheme to meet the demands of high‐capacity optical interconnects and optical processing networks. However, challenges remain to realize multidimensional hybrid multiplexing data transmission and signal processing between few‐mode fiber transmission links and on‐chip optical processing networks, since the optical coupling between fiber and chip is almost exclusively in the single‐mode regime. Here, a multidimensional fiber‐to‐chip optical processing system is proposed and demonstrated for hybrid wavelength‐, mode‐, and polarization‐division multiplexing signals, where multidimensional coupling between few‐mode fiber transmission link and on‐chip multi‐mode processing network is realized by using a 3D photonic integrated (de)multiplexers on a glass chip as well as 2D photonic integrated (de)multiplexers on a silicon chip, and the on‐chip multidimensional optical processing network composed by parallel cascaded micro‐ring resonator array performs a function of reconfigurable optical add/drop multiplexer. By operating wavelength‐division multiplexing in conjunction with mode‐division multiplexing and polarization‐division multiplexing, the communication bandwidth of fiber‐to‐chip optical processing systems can be further scaled.

On-chip integrated few-mode erbium–ytterbium co-doped waveguide amplifiers

We propose for the first time, to the best of our knowledge, an on-chip integrated few-mode erbium–ytterbium co-doped waveguide amplifier based on an 800 nm thick Si3N4 platform, which demonstrates high amplification gains and low differential modal gains (DMGs) simultaneously. An eccentric waveguide structure and a co-propagating pumping scheme are adopted to balance the gain of each mode. A hybrid mode/polarization/wavelength-division (de)multiplexer with low insertion loss and crosstalk is used for multiplexing and demultiplexing in two operation wavebands centered at 1550 nm and 980 nm, where the light in these two bands serves as the signal light and pump light of the amplifier, respectively. The results demonstrate that with an input signal power of 0.1 mW, TE0 mode pump power of 300 mW, and TE1 mode pump power of 500 mW, the three signal modes (TE0/TM0/TE1) all exhibit amplification gains exceeding 30 dB, while maintaining a DMG of less than 0.1 dB.

Thermo-optic reconfigurable three-mode (de)multiplexer based on an asymmetrical horizontal directional coupler

In this paper, we propose a thermo-optic reconfigurable three-mode (de)multiplexer based on an asymmetrical horizontal three-waveguide directional coupler that includes two identical single-mode waveguides and a three-mode waveguide. Over the whole wavelength range of 1540–1560 nm, and for the TE (TM) polarization, our typical fabricated device with polymer material shows coupling efficiencies as high as 94% (93%) and 93% (92%) for the mode conversions of LP01−LP11a and LP01−LP11b, with the heating powers of 53.57 mW and 71.19 mW, respectively. Our proposed device can be employed in the fields of reconfigurable mode-division multiplexing systems.