Capacity Optical Transmission Research Articles

Net gain in C+L band from LED pumped broadband emission in Er3+-doped oxyhalide tellurite glass

Expanding gain bandwidth into the L-band for erbium-doped fiber amplifiers (EDFAs) is a crucial strategy to overcome limitations in prevalent C-band focused commercial silica-based EDFAs, significantly boosting optical network transmission capacity. Herein, we synthesized Er3+-doped broadband near-infrared emission tellurite glasses utilizing the melt quenching process, systematically exploring the impact of ZnCl2 on the glass structure and optical properties. The 82TeO2–3Nb2O5–15ZnCl2–3Er2O3 (TNZ15) glass emerged as a standout candidate for broadband amplifiers around wavelength at 1.55 μm. With escalating ZnCl2 concentration, a discernible red shift in the emission peak wave lines was observed, accompanied by an expansion of the full width at half maximum (FWHM) from 98 nm to 106 nm. Noteworthy characteristics of all samples included exceptional thermal stability and elevated near-infrared (NIR) transmittance. Application of the Judd-Ofelt (J-O) theory facilitated the calculation of J-O parameter intensity, providing deeper insights into the optical behavior of the materials. Finally, the evaluation of TNZ15 planar optical waveguides, employing the vertical top pumping mode of two 980 nm LEDs, encompassed both the C-band (1530–1565 nm) and the L-band (1565–1625 nm). The results underscore the potential of TNZ15 glass as a promising gain material for broadband optical amplifiers and lasers, offering valuable insights into the development of next-generation optical amplification technologies.

TE/TM mode electro-optic conversion based on a titanium diffusion lithium niobate waveguide with a polarization-maintained fiber structure.

For the development of photonic integrated circuits and lithium niobate (L i N b O 3, LN) optical waveguide technology, the implementation and application of polarization devices based on LN are also becoming more widespread, where titanium (Ti)-diffused LN waveguides form the basis of many important electro-optic (EO) integrated optical devices. Moreover, utilizing polarization conversion has the potential to enhance both the effectiveness and capacity of optical transmission. Thus, we have presented an EO polarization mode converter packaging with PANDA polarization-maintaining optical fibers (PMFs) in the broadband wavelength range (1440-1620nm) to obtain the multiwavelength modulation, featuring the wavelength tunability. Additionally, the fabricated device is able to achieve transverse electric (TE) to transverse magnetic (TM) mode conversion efficiently with the applied voltage of ±, which provides high conversion efficiency. Importantly, our device also features a high-frequency response of about 600MHz with overall insertion loss below 5dB. The rapid development of LN-based polarization devices holds great promise for chip-integrated systems in the field of polarization telecommunication.

Design of Silicon Photonics Integrated Bulk Zigzag and Sinusoidal Structured Mode Conversion Devices Using Genetic Algorithm (GA) Optimization

To increase the optical interconnect transmission capacity, different multiplexing technologies, including wavelength division multiplexing (WDM), polarization division multiplexing (PolDM) and mode division multiplexing (MDM), can be utilized. Among them, MDM is a promising technique in silicon photonics (SiPh) integrated optical interconnects since higher order modes can be easily generated and preserved in SiPh waveguides. In this work, we propose and demonstrate the designs of SiPh-based bulk zigzag and sinusoidal structured MDM mode conversion devices using genetic algorithm (GA) optimization. A traditional periodic zigzag structured mode converter design has many sharp zigzag angles in the periodic structure, which are very sensitive to the fabrication error. Here, first of all, we propose and demonstrate a bulk zigzag structure to achieve MDM mode conversion. The proposed bulk zigzag structure can reduce the zigzag angle error as a large number of zigzag angles in the periodic structure are eliminated. Moreover, we further improve our device by proposing a bulk sinusoidal structure to further eliminate the zigzag angle. Results show that both the proposed bulk zigzag and sinusoidal MDM mode converters can still maintain high transmissions of >86%, while the mode conversion lengths of both devices can be significantly reduced by >60% in the C-band wavelength window. In addition, as there are many degrees of freedom (DOFs) during the design of the SiPh mode converter, including the waveguide width, length, period, zigzag angle, etch depth, duty cycle, etc., the GA optimization algorithm is employed. Here, detailed implementation of the GA optimization is discussed.

Triple-layered optical interconnecting integrated waveguide chip based on epoxy cross-linking fluorinated polymer photonic platform.

In this study, a triple-layered optical interconnecting integrated waveguide chip was designed and fabricated using an epoxy cross-linking polymer photonic platform. Fluorinated photopolymers FSU-8 and AF-Z-PC EP were self-synthesized as waveguide cores and cladding materials, respectively. The triple-layered optical interconnecting waveguide device comprised 4 × 4 arrayed waveguide grating (AWG) -based wavelength-selective switching (WSS) arrays, 4 × 4 multi-mode interference (MMI) -cascaded channel-selective switching (CSS) arrays, and 3 × 3 direct-coupling (DC) interlayered switching arrays. The overall optical polymer waveguide module was fabricated by direct UV writing. For the multilayered WSS arrays, the wavelength-shifting sensitivity was ∼0.48 nm/°C. For the multilayered CSS arrays, the average switching time was ∼280 µs, and the maximum power consumption was <30 mW. For interlayered switching arrays, the extinction ratio approximated 15.2 dB. The transmission loss for the triple-layered optical waveguide chip was measured as 10.0-12.1 dB. The flexible multilayered photonic integrated circuits (PIC) can be used in high-density integrated optical interconnecting systems with a large-volume optical information transmission capacity.

Compact asymmetric directional-coupler-based two-mode optical switch utilizing low loss Sb2Se3 phase-change material

Mode division multiplexing (MDM) has provided a new trend in high capacity optical transmission systems. Some of the most important devices in MDM systems are multimode switches that can route signals simultaneously into different paths. In this work, a two-mode 1 × 2 optical switch based on an asymmetric directional coupler and using Sb2Se3 phase-change material is proposed with a compact size of 45 µ m × 6 µ m . This switch can route the fundamental and first transverse magnetic modes in a wide bandwidth in the third telecommunication window from 1535 to 1565 nm. According to the 3D finite-difference time-domain simulation results, this switch has a low insertion loss of 0.83 dB and cross talk of − 7.93 d B across the bandwidth in the “on” state. In addition, when the switch is off, it has suitable specifications in terms of insertion loss and cross talk with values lower than 0.29 dB and − 11.98 d B across the bandwidth, respectively.

Multi-Associated Parameters Aggregation-Based Routing and Resources Allocation in Multi-Core Elastic Optical Networks

Space division multiplexing (SDM), as a potential means of enhancing the capacity of optical transmission systems, has attracted widespread attention. However, the adoption of SDM technology has also additionally increased resource dimensions, introduced complex crosstalk, and made it difficult to integrate multi-dimensional fragments. These factors force the transmission constraints to be more complicated. Especially, some factors have a mutual restraint relationship, and excessive consideration of certain factors will cause the deterioration of other ones. Therefore, how to comprehensively consider the associated factors to achieve trade-offs and improve network performance is a problem worthy of study. This paper exploits the advantages of self-organizing feature mapping (SOFM) model to process multi-dimensional data with relevant features. Firstly, multiple constraints will be input into SOFM as mode vectors from the core level. Then, by judging the similarity between the competition layer neuron and the pattern vector, the position of the winning neuron is located, which determines the transmission level of each core. Finally, a routing, core, and spectrum allocation scheme is proposed by preferentially locating the core with higher transmission quality. Along the selected core, the available slots will be classified twice respectively by the number of adjacent cores and crosstalk direction to quickly find the spectrum blocks with relatively small crosstalk. Results indicate the scheme can reduce blocking probability and the resource fragmentation. Further, it can increase the resource utilization within tested network load.

Research of Large Capacity Optical Transmission System based on S/C/L Band

Research of Large Capacity Optical Transmission System based on S/C/L Band

Optimal Slicing of Virtualized Passive Optical Networks to Support Dense Deployment of Cloud-RAN and Multi-Access Edge Computing

The commercialization of Cloud-RAN, and Open-RAN in particular, is a key factor to enable 5G cell densification by providing lower cost and more agile deployment of small cells. In addition, the adoption of multi-access edge computing (MEC) is important to support the ultra-low latency and high reliability required by mission-critical applications, which constitute a milestone of the 5G and beyond vision of a fully connected society. However, connecting antenna site, C-RAN processing, and MEC at low cost is challenging, as it requires high-capacity, low-latency connectivity delivered through a highly inter-connected topology. While passive optical networks (PONs) are being considered as a solution for providing low-cost connectivity to C-RAN, they only allow data transmission from the endpoints (e.g., hosting the radio unit, RU, at the antenna site) toward a central node (e.g., the central office, hosting computing equipment), and thus cannot support traffic from RUs toward MEC end nodes that could host the distributed unit (DU) and possibly central unit (CU) and network core. This has led to research into the evolution of PON architectures with the ability to provide direct communications between endpoints, thus supporting mesh traffic patterns required by MEC installations. In this context, virtualization plays a key role in enabling efficient resource allocation (i.e., optical transmission capacity) to endpoints, according to their Communication patterns. In this article, we address the challenge of dynamic allocation of virtual PON slices over mesh-PON architectures to support C-RAN and MEC nodes. We make use of a mixed analytical-iterative model to compute optimal virtual PON slice allocation, with the objective of minimizing the use of MEC node resources, while meeting a target latency threshold (100 µs in our scenario). Our method is particularly effective in reducing computation time, enabling virtual PON slice allocation in timescales compatible with real-time or near real-time operations.

Octagonal polarization-maintaining supermode fiber for mode division multiplexing system

We present a new type of polarization-maintaining supermode fiber which can separate adjacent modes from each other. The proposed polarization-maintaining supermode fiber features an octagonal core composed of multiple high-refractive-index circular holes. By using high refractive index cores and setting the angle subtly between the cores, the fiber proposed can support 22 modes with different polarization states under two parameters in the entire C-band. Through the finite element method simulation, we conducted simulation study to analyze the influence of the angle between the cores and the core diameter on the performance of the optical fiber. In addition, we analyzed effective refractive index difference (Δneff) and effective modal area (Aeff) of the fiber for each characteristic mode. The differential mode delay (DMD) under different cores angle is also calculated. Numerical results present that the minimum Δ neff of the octagonal polarization-maintaining supermode fiber in the entire C-band under the two parameters is greater than 1.1 × 10−4. It was measured to have good tolerance under the condition of unsatisfactory structure. These results indicate that the proposed fiber is superior candidate for improving the optical transmission capacity in the transmission of the mode division multiplexing system.

Design of an ultrasharp silicon multimode waveguide bend assisted by multiregional subwavelength grating structures

Sharp multimode waveguide bends are beneficial to improve the integration density of the on-chip mode-division multiplexing (MDM) systems, which can be used to increase the optical information transmission capacity. A method of designing a multimode bending structure based on subwavelength gratings is proposed in this paper, through the mode conversion and matching between the straight and bending waveguide. The proposed bending structure is composed of a 90-deg arc-bend with a radius of 5 μm combined with the shallow-etched grating structures on the top, and two-mode converters on the input and output straight waveguide realized also with the help of nonuniform grating-like structures, achieving a total effective bending radius of 8.25 μm and high transmission efficiencies of the first three TE modes (TE0 − TE2) in a wide wavelength range of 1500 to 1600 nm. At the center wavelength of 1550 nm, the calculated excess losses of the three modes TE0, TE1, and TE2 are 0.21, 0.19, and 0.59 dB, respectively, and the crosstalks for all modes are less than −26.58 dB. More importantly, we add a non-uniform grating converter in the straight waveguide region which pre-deviates the mode fields inside the straight waveguide to help further reduce the size of the bending part to only 5 μm, which will be helpful for large scale integration when a great number of bends are cascaded.

Thermally Tunable Orbital Angular Momentum Mode Generator Based on Dual-Core Photonic Crystal Fibers.

The combination of mode division multiplexing (MDM) based on orbital angular momentum (OAM) modes with wavelength division multiplexing (WDM) has attracted considerable attention due to its ability to increase optical transmission capacity. However, the switching of the multi-wavelength and multi-order OAM mode in an all-fiber structure has always been a challenge. As a solution, a thermally tunable dual-core photonic crystal fiber (DC-PCF) is proposed to achieve multi-order and multi-wavelength switching of the OAM mode. The results show that the OAM mode with topological charge m = ±1 can be excited with the linear polarization fundamental mode (LPFM) and circular polarization fundamental mode (CPFM). In addition, the device can effectively excite a high-purity ±1st order OAM mode with wavelengths ranging from 1520 to 1575 nm by thermal tuning. The purity of the mode is in excess of 99%, and the energy conversion efficiency (ECE) is above 95%. The proposed design is expected to be applied in all-fiber communication systems combined with MDM and WDM.

Hybrid WDM-MDM transmitter with an integrated Si modulator array and a micro-resonator comb source.

We demonstrate a multi-channel silicon photonic transmitter based on wavelength division multiplexing (WDM) and mode division multiplexing (MDM). The light source is realized by a silicon nitride (Si3N4) Kerr frequency comb and optical modulation is realized by silicon electro-optic modulators. Three wavelengths and two modes are employed to increase the optical transmission capacity. The accumulated data rate reaches 150 Gb/s. The dense integration of WDM and MDM components with a compact optical comb source opens new avenues for the future high-capacity multi-dimensional optical transmission.

Coal Fly Ash in Vietnam and its Application as a Lightweight Material

Each year in Vietnam, to operate the country's thermal power plants, it is necessary to utilize a large quantity of coal. The power plants then generate a large amount of coal fly ash (FA), which is a hazardous solid waste that can seriously affect the environment. There is an urgent need to develop appropriate solutions for treating or reusing the generated FA. In this study, the layout of coal-fired thermal power plants in Vietnam, as well as the country's coal use and FA emissions, are discussed. Research is reviewed on the potential applications of FA in Vietnam, along with related research from the past 20 y worldwide, and is found to demonstrate that the main application of FA is that of a geopolymer in construction, acting as a low-cost adsorbent that removes compounds, organic matter, emissions, metals, light aggregates, backfills, and auxiliary baselines, and synthesizes zeolites. On that basis, FA samples collected in the northern and southern regions of Vietnam were analyzed for their chemical compositions. The results determined that FA throughout Vietnam has the chemical composition, SiO2 + Al2O3 + Fe2O3 over 70 wt%. This is FA of classification F, according to TCVN 10302: 2014 (2014) and ASTM C618. This study concludes that the fly ash originating in Vietnam is suited to the production of porous, super-light materials with a high technical value, such as aerogels or aerogel composites. Such materials as these possess special properties such as a low density, high specific surface area or porosity, low thermal conductivity, sound insulation, low dielectric constant, low optical refractive index, or a high optical transmission capacity, elasticity, durability, or flexibility. The application of Vietnamese FA in the production of materials such as these is recommended by this paper, as a means of countering the environmental pollution problem of coal-fired thermal power plants and promoting the development of advanced materials in Vietnam.

Enhancing Lightpath QoT Computation With Machine Learning in Partially Disaggregated Optical Networks

Increasing traffic demands are causing network operators to adopt disaggregated and open networking solutions to better exploit optical transmission capacity, and consequently enable a software-defined networking (SDN) approach to control and management that encompasses the WDM data transport layer. In these frameworks, a quality of transmission estimator (QoT-E) that gives the generalized signal-to-noise ratio (GSNR) is commonly used to compute the feasibility of transparent lightpaths (LP)s, taking into account the amplified spontaneous emission (ASE) noise and the nonlinear interference (NLI). In general, the ASE noise is the main contributor to the GSNR and is also the most challenging noise component to evaluate in a scenario with varying spectral loads, due to fluctuations in the optical amplifier responses. In this work, we propose a machine learning (ML) algorithm that is trained using different ASE-shaped spectral loads in order to predict the OSNR component of the GSNR; this methodology is subsequently used in combination with a QoT-E in the lightpath computation engine (L-PCE). We present an experiment on a point-to-point optical line system (OLS), including 9 commercial erbium-doped fiber amplifiers (EDFA)s used as black-boxes, each with variable gain and tilt values, and 8 fibers that are characterized by distinct physical parameters. Within this experiment, we receive the signal at the end of the OLS, measuring the bit-error-rate (BER) and the power spectrum, over 2520 different spectral loads. From this dataset, we extract the expected GSNRs and their linear and nonlinear components. Through joint application of a ML algorithm and the open-source GNPy library, we obtain a complete QoT-E, demonstrating that a reliable and accurate LP feasibility predictor may be implemented.

Design of few-mode optical fibers with air-hole structure for MIMO-less data transmission

Rapidly space-division multiplexing (SDM) techniques have drawn attention because the traditional available physical dimensions to orthogonally multiplex data have been almost exhausted. Few-mode fiber becomes a viable option to realize a high-density SDM system. Thus, we propose an air-hole assisted elliptical ring fiber. Four air-holes are introduced surrounding the elliptical ring fiber, and one air-hole is in the core, which can prevent higher-order modes to be cutoff and improve the degeneracy between modes to achieve multiple-input multiple-output-free data transmission. This fiber can support 14 modes in a wide wavelength range from 1.52 to 1.58 μm (h = 5.0 μm, dx = 0.3 μm, and dy = 1.1 μm), and the effective index difference of its adjacent modes is larger than 1.25 × 10 − 4. The fiber we designed would be a strong candidate for SDM to improve optical transmission capacity.

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

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.

Design of solid-core Bragg few-mode fiber for short-reach MDM networks in [formula omitted] band

We propose a solid-core Bragg few-mode fiber with alternating layers of high/low refractive indices. The refractive index of the low-index layer in the cladding is the same as that of the fiber core. The influences of filling ratio, periodic structure width and core size on neff, Δneff and Aeff are investigated. Bending loss of each guided mode and differential mode delay (DMD) between adjacent spatial modes are also studied at 1550 nm. With appropriate geometrical parameters, the minimum effective refractive index difference (minΔneff) between adjacent modes is 2×10−3 at 1550 nm. Wavelength-dependent performance is analyzed subsequently ranging from 1180 nm to 1630 nm. Numerical simulations indicate that such fiber could support 7 linearly polarized (LP) modes and have good bending insensibility performance. These results illustrate that the proposed fiber could be a promising candidate for mode division multiplexing to enhance optical transmission capacity.

THz-to-optical conversion in wireless communications using an ultra-broadband plasmonic modulator

Future wireless communication networks will need to handle data rates of tens or even hundreds of Gbit s−1 per link, requiring carrier frequencies in the unallocated THz spectrum1,2. In this context, seamless integration of THz links into existing fibre-optic infrastructures3 is of great importance to complement the inherent portability and flexibility advantages of wireless networks and the reliable and virtually unlimited capacity of optical transmission systems. On the technological level, this requires novel device and signal processing concepts for direct conversion of data streams between the THz and optical domains. Here, we demonstrate a THz link that is seamlessly integrated into a fibre-optic network using direct THz-to-optical (T/O) conversion at the wireless receiver. We exploit an ultra-broadband silicon-plasmonic modulator having a 3 dB bandwidth in excess of 0.36 THz for T/O conversion of a 50 Gbit s−1 data stream that is transmitted on a 0.2885 THz carrier over a 16-m-long wireless link. Optical-to-THz (O/T) conversion at the wireless transmitter relies on photomixing in a uni-travelling-carrier photodiode. A high-speed wireless THz communication link is seamlessly integrated into a fibre-optic network. The demonstration relies on an ultra-broadband modulator exploiting two-dimensionally localized gap plasmons for direct conversion of the THz signals to the optical domain.

Mode Division Multiplexing Based on Ring Core Optical Fibers

The unique modal characteristics of ring core fibers (RCFs) potentially enable the implementation of mode-division multiplexing (MDM) schemes that can increase optical data transmission capacity with either low-complexity modular multi-input multi-output (MIMO) equalization or no MIMO equalization. This paper attempts to present a comprehensive review of recent research on the key aspects of RCF-based MDM transmission. Starting from fundamental fiber modal structures, a theoretical comparison between RCFs and conventional step-index and graded-index multi-mode fibers in terms of their MDM capacity and the associated MIMO complexity is given first as the underlining rationale behind RCF-MDM. This is followed by a discussion of RCF design considerations for achieving high-mode channel count and low crosstalk performances in either MIMO-free or modular MIMO transmission schemes. The principles and implementations of RCF mode (de-)multiplexing devices are discussed in detail, followed by RCF-based optical amplifiers culminating in MIMO-free or modular-MIMO RCF-MDM data transmission schemes. A discussion on further research directions is also given.