All-optical Signal Processing Research Articles

Engineering topological interface states in metal-wire waveguides for broadband terahertz signal processing.

Innovative terahertz waveguides are in high demand to serve as a versatile platform for transporting and manipulating terahertz signals for the full deployment of future six-generation (6G) communication systems. Metal-wire waveguides have emerged as promising candidates, offering the crucial advantage of sustaining low-loss and low-dispersion propagation of broadband terahertz pulses. Recent advances have opened up new avenues for implementing signal-processing functionalities within metal-wire waveguides by directly engraving grooves along the wire surfaces. However, the challenge remains to design novel groove structures to unlock unprecedented signal-processing functionalities. In this study, we report a plasmonic signal processor by engineering topological interface states within a terahertz two-wire waveguide. We construct the interface by connecting two multiscale groove structures with distinct topological invariants, i.e., featuring a π-shift difference in the Zak phases. The existence of this topological interface within the waveguide is experimentally validated by investigating the transmission spectrum, revealing a prominent transmission peak in the center of the topological bandgap. Remarkably, we show that this resonance is highly robust against structural disorders, and its quality factor can be flexibly controlled. This unique feature not only facilitates essential functions such as band filtering and isolating but also promises to serve as a linear differential equation solver. Our approach paves the way for the development of new-generation all-optical analog signal processors tailored for future terahertz networks, featuring remarkable structural simplicity, ultrafast processing speeds, as well as highly reliable performance.

A deep learning method for empirical spectral prediction and inverse design of all-optical nonlinear plasmonic ring resonator switches

All-optical plasmonic switches (AOPSs) utilizing surface plasmon polaritons are well-suited for integration into photonic integrated circuits (PICs) and play a crucial role in advancing all-optical signal processing. The current AOPS design methods still rely on trial-and-error or empirical approaches. In contrast, recent deep learning (DL) advances have proven highly effective as computational tools, offering an alternative means to accelerate nanophotonics simulations. This paper proposes an innovative approach utilizing DL for spectrum prediction and inverse design of AOPS. The switches employ circular nonlinear plasmonic ring resonators (NPRRs) composed of interconnected metal–insulator–metal waveguides with a ring resonator. The NPRR switching performance is shown using the nonlinear Kerr effect. The forward model presented in this study demonstrates superior computational efficiency when compared to the finite-difference time-domain method. The model analyzes various structural parameters to predict transmission spectra with a distinctive dip. Inverse modeling enables the prediction of design parameters for desired transmission spectra. This model provides a rapid estimation of design parameters, offering a clear advantage over time-intensive conventional optimization approaches. The loss of prediction for both the forward and inverse models, when compared to simulations, is exceedingly low and on the order of 10−4. The results confirm the suitability of employing DL for forward and inverse design of AOPSs in PICs.

Fully integrated and broadband Si-rich silicon nitride wavelength converter based on Bragg scattering intermodal four-wave mixing

Intermodal four-wave mixing (FWM) processes have recently attracted significant interest for all-optical signal processing applications thanks to the possibility to control the propagation properties of waves exciting distinct spatial modes of the same waveguide. This allows, in principle, to place signals in different spectral regions and satisfy the phase matching condition over considerably larger bandwidths compared to intramodal processes. However, the demonstrations reported so far have shown a limited bandwidth and suffered from the lack of on-chip components designed for broadband manipulation of different modes. We demonstrate here a silicon-rich silicon nitride wavelength converter based on Bragg scattering intermodal FWM, which integrates mode conversion, multiplexing and de-multiplexing functionalities on-chip. The system enables wavelength conversion between pump waves and a signal located in different telecommunication bands (separated by 60 nm) with a 3 dB bandwidth exceeding 70 nm, which represents, to our knowledge, the widest bandwidth ever achieved in an intermodal FWM-based system.

Wavelength-tunable quasi-unidirectional spatiotemporal mode-locked solitons in an isolator-free Er-doped fiber laser

In this letter, we have demonstrated the realization of the wavelength-tunable spatiotemporal mode-locked (STML) operation in an Er-doped partial multimode fiber laser without the intracavity isolator. The laser adopts an all-fiber structure and the saturable absorption effect is attributed to nonlinear multimode interference (NL-MMI) from the single mode fiber-graded index multimode fiber-single mode fiber (SMF-GIMF-SMF) design. The STML laser can operate in the quasi-unidirectional regime with an emission strength difference of larger than 30 dB for the two counter-propagating lasers. And the switching between the dominating operating directions in the clockwise (CW) or counter-clockwise (CCW) directions can be easily achieved by adjusting the MM polarization controller (PC). Both STML conventional solitons (CSs) and the STML soliton molecules are obtained with a similar tunable wavelength from ∼1.56 μm to ∼1.58 μm for the opposite operating directions. Furthermore, the spatial dynamics of the STML soliton are also investigated in detail. The experimental results are of great help for us to explore nonlinear spatiotemporal dynamics and provide potential applications in fields of all-optical signal processing, fiber sensing and information coding.

40 Gb/s multimode all-optical regenerator based on the low-loss silicon-based nanowaveguide.

With the increasing demand for communication capacity, all-optical regeneration of multimode signals is a helpful technology of network nodes and optical signal processors. However, the difficulty of regenerating signal in higher-order modes hinders the practical application of multimode all-optical regenerators. In this study, we experimentally demonstrate the 40 Gb/s all-optical regeneration of NRZ-OOK signal in TE0 and TE1 modes via four-wave mixing (FWM) in the low-loss silicon-based nanowaveguide. By optimizing the parameters of waveguide section to enhance FWM conversion efficiency of two modes, and introducing Euler bending to reduce crosstalk between modes, the transmission loss of the silicon waveguide is 0.3 dB/cm, and the FWM conversion efficiency of the multimode regenerator is as high as -9.6 dB (TE0) and -13.0 dB (TE1). Both modes achieve extinction ratio enhancement of about 6 dB after regeneration. This silicon-based all-optical regenerator has great application potential in all-optical signal processing systems.

Ultralow switching threshold optical bistable devices based on epsilon-near-zero Ga<sub>2</sub>O<sub>3</sub>-SiC-Ag multilayer structures

Optical bistability has attracted much attention due to its enormous potential applications in all-optical operation and signal processing. However, the weak nonlinear responses typically require huge pump power to reach the threshold of the optical bistability, thus hindering the real applications. In this study, we propose an efficient optical bistable metamaterial, which is composed of multilayer Ga<sub>2</sub>O<sub>3</sub>-SiC-Ag metal-dielectric nanostructures. We not only use the epsilon-near-zero (ENZ) with SiC-Ag thin layers to enhance the substantial field, but also incorporate the SiC material to increase its significant optical nonlinear coefficient. In the structural design, the introduction of Ga<sub>2</sub>O<sub>3</sub> layer facilitates the light field concentration, contributing to the further reduction in threshold power for optical bistability, and also conducing to the improvement of the physical and chemical stability of the device. The influences of the thickness and length of the ENZ layer on the optical bistability are systematically investigated by using the finite element method. The results demonstrate that optical bistability becomes more pronounced with the increase of the thickness and length of ENZ layer, exhibiting a bistability switching threshold as low as ~10<sup>–6</sup> W/cm<sup>2</sup> in the telecommunication band. Comparing with the previously reported optical bistability based on ENZ mechanism, the threshold shows a significant reduction by 9 orders of magnitude, demonstrating great application potential in the fields of semiconductor devices and photonic integrated circuits.

Novel Optical Kerr Switching Photonic Device Based on Nonlinear Carbon Material.

In the context of current communication systems, there is an urgent demand for more efficient and higher-speed optical signal processing technologies. Researchers are actively exploring new materials and devices to harness nonlinear optical phenomena, seeking advancements in this field. Nonlinear carbon materials, especially promising 2D materials, have garnered attention for their potential interaction with light and have become integral to the development of all-optical signal processing devices. This study focuses on utilizing a photonic device based on a nonlinear Au/CB composite material for optical Kerr switching. The application of Au/CB as a nonlinear material in the Kerr switch represents a noteworthy advancement, demonstrating its capability to modulate optical signals. By appropriately applying a pump light, the study achieves optical Kerr switching with an extinction ratio of approximately 15 dB in the fully off state of the signal light carrying a 10 GHz analog signal, marking a pioneering achievement in the field to the best of our knowledge. The experimental results, encompassing extinction ratios, signal control, and stability, not only validate the feasibility of this technology but also underscore its potential applicability within optical communication systems. The successful modulation and control of a 10 GHz analog signal showcase the practicality and effectiveness of the Au/CB-based optical Kerr switch. This progress contributes to the continuous evolution of optical Kerr switching, a crucial component in modern optical communication systems. Therefore, we believe that the Au/CB-based optical Kerr switch is an exceptionally promising and stable all-optical signal processing device. As the contemporary communication landscape evolves, the integration of this technology holds the potential to enhance the efficiency and speed of optical signal processing.

Graphene-integrated hybrid plasmonic waveguide for Kerr nonlinear application

Graphene, with its broadband optical properties and high optical nonlinearity, shows promise as an ultrathin material compatible with CMOS technology for all-optical signal processing in integrated photonics. This paper presents simulation, analysis, and design of a novel graphene-integrated nonlinear hybrid plasmonic waveguide. Finite element simulations study the Kerr nonlinear effect of extra graphene layers. The enhanced nonlinear parameter of [Formula: see text][Formula: see text]W[Formula: see text][Formula: see text]Km[Formula: see text] and nonlinear figure-of-merit (FOM) of 2.10 achieved at 1,550[Formula: see text]nm exceed those of silicon-on-insulator (SOI) hybrid plasmonic waveguides. The proposed graphene-integrated hybrid plasmonic waveguide (GHPW) demonstrates high Kerr nonlinearity and FOM, indicating potential for nonlinear all-optical signal processing.

Integration of Plasmonic Structures in Photonic Waveguides Enables Novel Electromagnetic Functionalities in Photonic Circuits

The development of integrated, compact, and multifunctional photonic circuits is crucial in increasing the capacity of all-optical signal processing for communications, data management, and microsystems. Plasmonics brings compactness to numerous photonic functions, but its integration into circuits is not straightforward due to insertion losses and poor mode matching. The purpose of this article is to detail the integration strategies of plasmonic structures on dielectric waveguides, and to show through some examples the variety and the application prospect of integrated plasmonic functions.

Nanomaterials in Nanophotonics Structure for Performing All-Optical 2 × 1 Multiplexer Based on Elliptical IMI-Plasmonic Waveguides

In this study, an all-optical multiplexer (Mux) based on elliptical insulator-metal-insulator (IMI) plasmonic waveguides is designed. The area of the proposed structure is very small (400 nm × 400 nm) which operates at a wavelength of 1,550 nm. The developed device utilizes constructive and destructive interferences between the input signals and the selector signal. This structure is less complex and has lower loss compared to the previous works. Transmission (T), contrast ratio (CR), modulation depth (MD), insertion loss (IL), and contrast loss (CL) are the five parameters that describe the performance of the plasmonic Mux. The transmission threshold between logic 0 and logic 1 is 0.5. Moreover, the maximum transmission efficiency of the device is 163%. Moreover, based on the MD value of 95.09%, the dimensions of the proposed structure are excellent and optimal. The proposed plasmonic Mux structure contributes substantially to developing an all-optical arithmetic logic unit (ALU) and all-optical signal processing nanocircuits. The finite element method (FEM) simulates the proposed plasmonic multiplexer with COMSOL Multiphysics 5.4 software.

Engineering Nonlinear Effects of Quantum-Well Semiconductor Optical Amplifiers for Applications in Optical Signal Processing

Quantum-well semiconductor optical amplifiers (QW-SOAs) have been widely used in all-optical signal processing functions because of their various nonlinear effects. For different optical signal processing functions, the requirements for performance of the QW-SOAs are different, and nonlinear effects of the QW-SOAs should be controlled selectively. Engineering nonlinear effects of QW-SOA is very important for different applications in optical signal processing. Here, a comprehensive QW-SOA model by combining the QW band structure calculation with the dynamic model is established to connect the structure parameters with the nonlinear effects of QW-SOA. Those characterized parameters of the QW-SOAs, such as gain coefficient, differential gain coefficient, refractive index change, differential refractive index change, linewidth enhancement factor, third-order susceptibility and polarization dependent gain, are used to characterize the nonlinear effects, and are calculated with different structure parameters based on this developed model. Take the wavelength conversion based on cross gain modulation effect and four wave mixing effect as examples, the structure parameters of the QW-SOAs are optimized for engineering the nonlinear effects and achieving the best output performance. This theoretical work presents a guide for choosing the optimized structure parameters while fabricating the QW-SOAs for different optical signal processing functions

S-Band WDM Transmission Using PPLN-Based Wavelength Converters and 400-Gb/s C-Band Real-Time Transceivers

Multi-band WDM transmission beyond the C+L-band is a promising technology for achieving larger capacity transmission by a limited number of installed fibers. In addition to the C- and L-band, we can expect use the S-band as the next band. Although the development of optical components for new bands, particularly transceivers, entails resource dispersion, which is one of the barriers to the realization of multi-band systems, wavelength conversion by transparent all-optical signal processing enables new wavelength band transmission using existing components. Therefore, we proposed a transmission system including a new wavelength band such as the S-band and made it possible to use a transceiver for the existing band by performing the whole-band wavelength conversion without using a transceiver for the new band. As a preliminary verification to demonstrate multi-band WDM transmission including S-band, we investigated the application of a novel wavelength converter between C-band and S-band, which consists of periodically poled lithium niobate waveguide, to the proposed system. We first characterized the conversion efficiency and noise figure of the wavelength converter and estimated the transmission performance of the system through the wavelength converter. Using the evaluated wavelength converters and test signals of 64 channels arranged in the C-band at 75-GHz intervals, we constructed an experimental setup for S-band transmission through an 80-km standard single-mode fiber. We then demonstrated error-free transmission of real-time 400-Gb/s DP-16QAM signals after forward error correction decoding. From the experimental results, it was clarified that thewavelength converter which realizes the uniformlossless conversion covering the whole C-band effectively achieves the S-band WDM transmission, and it was verified that the capacity improvement of the multi-band WDM system including the S-band can be expected by applying it in combination with the C+L-band WDM system.

Integrated multi-wavelength lasers for all-optical processing of ultra-high frequency signals

Semiconductor lasers are nowadays simply unavoidable and essential light sources. While their complexity and dynamical behavior have attracted some attention from a fundamental viewpoint, these special properties remain largely left aside in applications outside the lab. The development of multi-wavelength or multi-color lasers may be a turning point in this regard. On the one hand, multi-color lasers allow for simultaneous emission at multiple and controllable modes, thus adding extra versatility to the lasers. On the other hand, the coupling between the different modes may lead to exciting new functionalities and applications exploiting directly the intrinsic dynamical response of the laser itself. In this perspective letter, we describe the role that multi-wavelength lasers may, in our opinion, play in the future in signal processing applications, especially at the mm-wave and subterahertz frequencies.

Stimulated intermodal Brillouin scattering in a hybrid photonic-phononic silicon waveguide

On-chip stimulated Brillouin scattering (SBS) has attracted extensive attention by introducing acousto-optic coupling interactions in all-optical signal processing systems. In this article, we demonstrate stimulated intermodal Brillouin scattering (SIMBS) through a hybrid photonic-phononic silicon waveguide on the silicon-on-insulator (SOI) platform. The designed photonic-phononic waveguide can independently control the optical and acoustic fields by introducing the ridge waveguide as a line defect into the honeycomb lattice phononic crystal slab. Small-signal Stokes gain of 0.7 dB is achieved in a 1-cm long straight waveguide device with an on-chip pump power of 42.5 mW. Positive net Brillouin amplification is also expected by effectively reducing the linear loss. Our proposed device offers a potential approach to implementing Brillouin amplifiers, nonreciprocal devices, and signal processing in planar photonic integrated circuits.

All optical modulation in vertically coupled indium tin oxide ring resonator employing epsilon near zero state

We present an innovative approach to achieve all-optical modulation within an ITO-based vertically coupled ring resonator. This method leverages the material's enhanced nonlinear response in the near-infrared wavelengths, particularly within the epsilon-near-zero (ENZ) state. To enhance the interaction between light and the material while minimizing scattering losses, our approach employs an ITO-based vertically connected ring resonator. The vertical arrangement eliminates the need for etching fine gaps to separate the ring and bus waveguide. The novel waveguide design addresses the necessity of high sensitivity, non-linear effects and compact size opening the possibilities for all-optical signal processing. This unique resonator structure effectively facilitates the coupling of a high-intensity pump wavelength into the ITO-based micro-ring resonator. Consequently, this optical pumping induces electron heating within the ITO material, leading to a significant increase in its nonlinear optical properties. This, in turn, results in a noteworthy alteration of ITO's refractive index, specifically in the unity order, thereby modifying the complex effective index of the optical beam propagating at 1550 nm. Our experimental findings demonstrate an impressive extinction ratio of 18 dB for a 30 µm long device, which highlights the efficiency of our approach in achieving all-optical modulation through the optical pumping of an ITO-based vertically coupled ring resonator. The proposed all-optical modulator has outperformed as compared to conventional waveguide-based modulators in terms of extinction ratio and footprint. This novel technique holds immense potential for advancing high-speed data communication systems in the future. As the demand for advanced processing capabilities, such as artificial intelligence, continues to grow, all-optical modulation emerges as a groundbreaking technology poised to revolutionize the next generation of computing and communication systems.

All-optical group velocity manipulation in Mach-Zehnder fiber interferometers incorporating with Brillouin random lasing resonance

We proposed and experimentally demonstrated a novel scheme of light group velocity control via a Mach-Zehnder fiber interferometer incorporated with Brillouin random lasing resonance in a half-open linear cavity. A theoretical model of the proposed Brillouin random lasing resonance-based Mach-Zehnder interferometer (BRLR-MZI) for the light group velocity manipulation was derived, predicting optical power-dependent transmittance spectrum shift as well as the enhancement of the temporal advancement. Thanks to Brillouin random lasing-induced fast light effect, the proposed scheme not only achieves a flexible all-optical manipulation of its transmittance spectrum but also overcomes the saturation effect for the light speed control based on a traditional MZI. In experimental validation, sinusoidally modulated signals with the modulation frequency of 160 kHz ultimately experienced a maximum advancement of 4031 ns (corresponding to the group index of 0.875), which exhibits an advancement improvement of 1364 ns compared with the traditional MZI-based scheme. It suggests a promising platform for exploring applications in fields of all-optical signal processing, microwave photonic, and ultra-sensitive fiber sensing.

Fiber-Integrated All-Optical Signal Processing Device for Storage and Computing

Fiber-Integrated All-Optical Signal Processing Device for Storage and Computing

TiN/Ti3C2 Heterojunction Microfiber-Enhanced Four-Wave Mixing-Based All-Optical Wavelength Converter

As a novel nanomaterial, the TiN/Ti3C2 heterojunction has been demonstrated to possess exceptional optoelectronic properties, offering significant potential for applications in fields such as communication, optical sensors, and image processing. The rapid evolution of the internet demands higher communication capacity and information processing speed. In this context, all-optical wavelength conversion, a pivotal technique in all-optical signal processing, holds paramount importance in overcoming electronic bottlenecks, enhancing wavelength utilization, resolving wavelength competition, and mitigating network congestion. Utilizing the idle light generated through the four-wave mixing (FWM) process accurately mimics the bit patterns of signal channels. This process is inherently rapid and theoretically capable of surpassing electronic bottlenecks with ease. By placing an optical filter at the fiber output end to allow idle light passage while blocking pump and signal light, the output becomes a wavelength-converted replica of the original bitstream. It has been verified that TiN/Ti3C2 heterojunction-coated microfiber (THM) exhibits outstanding third-order nonlinear coefficients. Building upon this, we achieved a THM-enhanced FWM all-optical wavelength converter, resulting in a ~4.48 dB improvement in conversion efficiency. Compared to conventional high-nonlinear fibers, this compact device significantly reduces fiber length and can be easily integrated into current high-speed optical communication networks. It demonstrates broad prospects in the realms of all-optical signal processing, robotic applications, ultra-high-speed communication, and beyond.

Evolution and dynamics of spatial self-phase modulation in pyrromethene 567 for all-optical photonic diode applications

Spatial self-phase modulation has emerged as a novel and ubiquitous nonlinear optical effect, revealing potential application in all-optical devices for signal processing. The work investigates the evolution and dynamics of SSPM patterns in pyrromethene 567 (PM 567). A temperature-dependent refractive index gradient which induces a phase shift in the incident beam is responsible for the formation of spatial self-phase modulation. The non-linear response of PM 567 in different solvents and concentrations is investigated and tabulated. The effect of wavefront curvature, path length, and mechanical chopper on the formation of the SSPM patterns was studied. To our knowledge, an optical-diode-based unidirectional generation of spatial self-phase modulation in dyes has been reported for the first time. Simply cascading PM 567 dissolved in DMSO and Methanol having distinct nonlinear optical responses, an all-optical diode for unidirectional light propagation based on spatial self-phase modulation in PM 567 was achieved. The work reveals the potential application of PM 567 for achieving spatial asymmetry in light propagation based on SSPM in photonic devices for all-optical signal processing.

Light wave propagation and diffraction inhibition in three-dimensional photonic quasicrystal lattices

We have investigated the linear propagation of light wave in three-dimensional photonic quasicrystal lattices by numerical simulation. The diffraction characteristics of the light waves in the longitudinal non-modulated, in-phase modulated and out-of-phase modulated quasicrystal lattices have been compared and discussed. We have demonstrated the diffraction inhibition of light wave in three-dimensional photonic quasicrystals. Linear transverse light localization has been realized in the longitudinal out-of-phase modulated three-dimensional quasicrystal lattices by adjusting the structural parameters appropriately. Furthermore, the results also show that the longitudinal out-of-phase modulated photonic quasicrystals are easier to produce diffraction inhibition than the longitudinal out-of-phase modulated periodic lattices under low transverse refractive index contrast. Distortionless dot image transmission has numerically demonstrated in out-of-phase modulated photonic quasicrystals array. It provides a new way for distortion-free image transmission, which has promising applications in optical information transmission and all-optical signal processing.