Optical Signal Processing Applications Research Articles

Broadband second-harmonic generation in an angle-cut lithium niobate-on-insulator waveguide by a temperature gradient.

Frequency conversion via nonlinear wave mixing is an important technology to broaden the spectral range of lasers, propelling their applications in optical communication, spectroscopy, signal processing, and quantum information. Many applications require not only a high conversion efficiency but also a broad phase matching bandwidth. Here, we demonstrate broadband birefringence phase matching (BPM) second-harmonic generation (SHG) in angle-cut lithium niobate-on-insulator (LNOI) ridge waveguides based on a temperature gradient scheme. The bandwidth and shift of the phase matching spectrum can be effectively tuned by controlling the temperature gradient of the waveguide. Broadband SHG of a telecom C-band femtosecond laser is also demonstrated. The approach may open a new avenue for tunable broadband nonlinear frequency conversion in various integrated photonics platforms.

Axicon lenses with chiral-focusing properties modeling by means of analytical functions

Axicon lenses exhibit a large focal depth that generates an energy distribution, resulting in non-diffracting beams’ generation. These light structures are called Bessel beams and show numerous important applications in optical signal processing, imaging, transparent material processing, and even in metal drilling/cutting manufacturers. Asymmetrical and twisted axicon-lenses are boundary conditions on the Maxwell equation able to generate complex-structured light fields with specially designed intensity, phase, and polarization distributions. This paper presents a set of analytical functions that allows the definition of such surfaces with absolute freedom in relation to the lens profile. Obviously, before manufacturing any lens, it is necessary to compute its electromagnetic output in order to save resources. In this sense, we have studied some of these lenses by simulation using the well-known finite difference method in the time domain. Some necessary verification related to conservative magnitudes, such as the Poynting vector, have been done analytically and numerically. The numerical results show the distribution of iso-values corresponding to the electric field and 3D-vectorial representation of the chirality flux and the Poynting vector. Finally, by means of discrete fast Fourier transform, it is shown the field distributions as well as the power before and after the lens per frequency.

Reconfigurable optical filter based on microring resonator assisted by tunable Sagnac reflector

To meet the demands for various applications in optical filtering and microwave signal processing, integrated silicon photonic filters are required to be multifunctional, reconfigurable and tunable. In this work, an integrated multi-functional optical filter is proposed, which is designed based on a tunable Sagnac loop reflector and a microring resonator. The through port and drop port of an add-drop microring resonator are connected with the two ports of a tunable reflector. By controlling the thermal phase shifters in different scenarios, the device can be reconfigured into a reflective-type asymmetric Mach-Zehnder interferometer filter, a reflective-type all-pass microring resonator filter and self-interference microring resonator filters. An analytical model is established based on the transfer matrix. The simulation results show that the device can achieve the following functions: sinusoidal spectral filtering with four different free spectral ranges, Lorentzian spectral filtering toggling between band pass and band stop, and spectral reconfigurations of Fano resonance, electromagnetically induced transparency, and electromagnetically induced absorption. Each spectrum mentioned above can be tuned fast and widely. Reflection provides a new degree of freedom in design, breaks through the inherent footprint limit, and achieves a wide range of free spectral ranges. Our proposed tunable Sagnac loop reflector assisted microring resonator provides a new scheme for realizing flexible, tunable and multi-functional reconfigurable integrated photonic filters, and has broad applications in the integrated photonic analog signal processing and microwave photonics.

Recent Progress of Supercontinuum Generation in Nanophotonic Waveguides

AbstractSupercontinuum (SC) has opened up possibilities for numerous applications in optical communications, signal processing, metrology, and spectroscopy. Supercontinuum generation (SCG) in the integrated nonlinear platforms has attracted much interest recently, due to its fundamental advantages in terms of complementary metal oxide semiconductor compatibility, low power consumption, compact size, and cost‐effectiveness. In this paper, the latest progress on various types of nanophotonic waveguides for SCG is reviewed. The material properties of silicon, germanium, silicon–germanium alloy, silicon nitride, silica, chalcogenide, III–V materials, lithium niobate, and other materials, which are used as nonlinear media for SCG are discussed. The wavelength‐dependent nonlinear Kerr index, material, and nonlinear loss of the waveguides are taken into account. This review mainly focuses on the SCG resulting from the cubic χ(3) nonlinearity processes pumped by the femtosecond pulse. The recent representative SCG works based on the integrated optical waveguides are summarized, and further classified according to the dispersion characteristics. Furthermore, different types of spectra broadening mechanisms in details according to the classifications are analyzed. Perspectives on the SC spectral coverage, the realization of various dispersion curves, and the novel materials, which are the key aspects of the SCG in nanophotonic waveguides are provided.

Dynamic power and chirp measurements of a quantum dash semiconductor optical amplifier amplified picosecond pulses using a linear pulse characterization technique

Quantum Dash (QDash) SOAs are of interest as an alternative to quantum dot SOAs, since they have some dot-like properties and are easier to fabricate such as to operate in the 1550 nm region, albeit with longer gain recovery times in the range of hundreds of picoseconds. QDash-SOAs are promising core component is optical subsystems for applications in next generation coherent communications and optical signal processing, which often involve pulse amplification. It is of interest to measure both the dynamic power, phase and associated chirp and thereby the spectrum of typical QDash-SOA amplified pulses. Non-linear measurement techniques are appropriate for pulsewidths less than 5 ps but have low sensitivity for wider pulsewidths often encountered in practice. This paper describes the use of a modified linear pulse characterization technique to measure the dynamic power and phase of the identical pulses of a QDash-SOA amplified 20 GHz repetition rate 19 ps pulsewidth pulse train. The pulse chirp and power from which the pulse chirp and pulse train power spectrum is calculated. The amplified pulse structure is strongly dependent on the amplifier gain and the input pulse shape and energy.

Electrically reconfigurable waveguide Bragg grating filters.

We propose and demonstrate an electrically reconfigurable waveguide Bragg grating filters in silicon-on-insulator using a multiple-contact heater element. There are six electrical pads connected to the heater element in an equidistant manner. These electrical pads allow to create different heat, and corresponding refractive index, distributions across the grating so that the local Bragg wavelength corresponding to the heated segments can be controlled. In turn, this control over the heat distribution allows the device to be reconfigured to implement different filter spectral responses. These filters are applicable for both wavelength division multiplexing systems and optical signal processing applications. As a verification, we demonstrate the generation of two (or more) separate filter bands with a spacing up to 35 nm or a Fabry-Pérot cavity with a 1.6 nm free-spectral range. Moreover, we explain a firm and accurate simulation framework of the proposed device based on COMSOL Multiphysics and the transfer matrix method, which is in excellent agreement with our experimental measurements.

Silicon–Organic Hybrid Tunable Fabry–Perot Filter for Electro-Optic Modulation

Recently, electro-optic integrated photonic elements have attracted considerable interest owing to their widespread application in optical communication and signal processing. The strong electro-optical properties and the complementary metal–oxide semiconductor (CMOS) technology compatibility of silicon–organic hybrid structures makes them promising photonic platforms. This study investigated a novel implementation of a high-speed electrically tuned bandpass optical filter using finite-difference time-domain and eigenmode expansion numerical methods. The proposed device was based on a Fabry–Perot cavity and was realized via an electro-optic organic molecular glass filled slot waveguide with Bragg gratings. Furthermore, the calculation considered the plasma dispersion effect, free-carrier absorption in the silicon component of the slot waveguide for different applied voltages, and doping profiles corresponding to ion implantation. It is shown that the spatial distribution of charge carrier density formed by the surface states accumulation/depletion layers has a significant effect on the optical properties of the slot waveguide. Lithographic distortions in the geometry of the proposed structures during fabrication were also considered. Therefore, this paper presents an optical filter designed for fabrication using CMOS-compatible processes on silicon-on-insulator wafers. The filter produced a 3 nm tunable range for a 10 V bias voltage swing, a 0.3 nm full width at half maximum (FWHM) passband with a center at 1550 nm at zero bias voltage, and a 12.5 nm free spectral range with a 27 dB extinction ratio. The proposed device outperforms many narrow passband analogs in terms of sensitivity (tunability: 0.3 nm/V) and can be applied extensively in the fields of optical communications and photonic spectral signal processing.

Supercontinuum generation and IR image transportation using soft glass optical fibers: a review [Invited

Soft glass optical fibers, especially highly nonlinear optical fibers, have expanded their application fields. We have been engaged in soft glass optical fibers, such fluoride, tellurite and chalcogenide glass, for optical signal processing, lightwave generation and waveguide applications. Furthermore, we have challenged the research on novel waveguides, for example, transverse Anderson localization of mid-infrared light using transversely disordered optical fiber. Here we report our achievements on SC generation and novel waveguides research using the soft glass highly nonlinear optical fibers.

Collective Coupling of 3D Confined Optical Modes in Monolithic Twin Microtube Cavities Formed by Nanomembrane Origami.

We report the monolithic fabrication of twin microtube cavities by a nanomembrane origami method for achieving collective coupling of 3D confined optical modes. Owing to the well-aligned twin geometries, two sets of 3D confined optical modes in twin microtubes are spectrally and spatially matched, by which both the fundamental and higher-order axial modes are respectively coupled with each other. Multiple groups of the coupling modes provide multiple effective channels for energy exchange between coupled microcavities illustrated by the measured spatial optical field distributions. The spectral anticrossing and changing-over features of each group of coupled modes are revealed in experiments and calculations, indicating the occurrence of strong coupling. In addition, the simulated 3D mode profiles of twin microcavities confirm the collective strong coupling behavior, which shows good agreement with experiments. The collective coupling of 3D confined resonant modes promises broad applications in multichannel optical signal processing, nanophotonics, and 3D non-Hermitian systems.

Ultrafast intramolecular charge transfer dynamics and nonlinear optical properties of phenothiazine-based push–pull zinc porphyrin

Comprehensive investigations of the ultrafast photophysical properties and charge injection dynamics (adsorbed on mesoporous TiO2) of phenothiazine-functionalized push–pull Zinc porphyrin (LG6) utilizing a benzothiadiazole (BTD) as an acceptor unit is presented herewith. This includes density functional theory (DFT) calculations, fluorescence measurements, time-dependent fluorescence measurements, and femtosecond transient absorption (fs-TA) spectroscopy measurements. The charge injection dynamics of LG6 were compared with another porphyrin, LG5, with a similar molecular structure but thiophene as the acceptor unit instead of BTD. The multicomponent electron injection analysis from different excited electronic energy levels reveals ultrafast electron injection times of 434 fs and 336 fs for LG6 and LG5, respectively, revealing the impact of acceptor units on charge injection times. Further, femtosecond nonlinear optical (NLO) properties were characterized using the femtosecond Z-scan measurements with 70 fs, 800 nm pulses. The analysis of Z-scan open aperture data has been performed by solving three-level rate equations incorporating excited-state lifetimes calculated from the target/global analysis of fs-TA data set. The estimated NLO parameters and ultrafast NLO response times of LG6 indicated the strong potential for optical limiting and optical signal processing applications. The second hyperpolarizability was estimated to be γ=2.5×10-31esu, which was found to be in close agreement with the theoretically calculated value of 0.4×10-31e.s.u.

Y-branch wavelength demultiplexer based on topological valley photonic crystals

Inspired by topological insulators in the condensed matter physics, topological photonics is a new paradigm, which is free of light backscattering at sharp waveguide bends due to the topological protection, and shows unidirectional light propagation based on the valley-chiral locking. Herein, we propose a two-channel Y-branch wavelength demultiplexer operating at the telecom band using the topological photonic integrated circuits for the first time. The proposed Y-branch wavelength demultiplexer is designed as adjoining and separating of two different topological interfaces at its input and output ports, respectively. Topological edge states with spectral interleaving can be excited in the two different topological interfaces, and be split into two output ports of the demultiplexer in short and long wavelengths. The Y-branch topological waveguide can be used as a dichroic mirror or a wavelength filter for separating light at two different wavelengths. The proposed topological integrated circuit is immune to structural defects. It is promising for the practical applications of optical interconnects and signal processing on a silicon photonic chip.

InAs/InP quantum dot mode-locked laser with an aggregate 12.544 Tbit/s transmission capacity.

Chip-scale optical frequency comb sources are ideal compact solutions to generate high speed optical pulses for applications in wavelength division multiplexing (WDM) and high-speed optical signal processing. Our previous studies have concentrated on the use of quantum dash based lasers, but here we present results from an InAs/InP quantum dot (QDot) C-band passively mode-locked laser (MLL) for frequency comb generation. By using this single-section QDot-MLL we demonstrate an aggregate line rate of 12.544 Tbit/s 16QAM data transmission capacity for both back-to-back (B2B) and over 100-km of standard single mode fiber (SSMF). This finding highlights the viability for InAs/InP QDot lasers to be used as a low-cost optical source for large-scale networks.

Ultra-compact plasmonic unidirectional wavelength multiplexer/demultiplexer based on slot cavities

An ultra-compact plasmonic unidirectional wavelength multiplexer/demultiplexer based on slot cavities is proposed and numerically simulated. The structure consists of slot cavities which are etched on either sides of a metal–insulator–metal (MIM) bus waveguide. The cavities capture surface plasmon polariton (SPP) waves at their resonant wavelengths and unidirectionally couples them to the drop waveguide etched parallel to the bus. The structure basically relies on resonance and interference of SPP waves and its functioning is validated through finite element method (FEM) simulations. The simulation results show that the proposed structure functions as expected with a full width at half maximum (FWHM) bandwidth of less than 50 nm, extinction ratio (ER) more than 10 dB, and crosstalk (CT) of less than − 10 dB for the designed wavelengths. The proposed structure holds lot of potential to enhance the miniaturization of ultra-compact integrated photonic circuits for optical signal processing and other related applications.

Elliptic vortex beam in a fractional complex Ginzburg–Landau model

We have investigated the elliptical vortex structures in dissipative media governed by the fractional complex cubic-quintic Ginzburg–Landau model. Stable regions of generating dissipative fundamental, dipole, tripole and fourpole solitons are found for relatively small elliptical coefficient (e ) at values of (LI) , which can produce critical or supercritical collapse in free space. By adjusting the system parameters, the elliptical vortex can evolve into the circular ones and remain stable circular vortex structure with given topological charge of the vorticity S (from 1 to 4). Effects of the value of the respective LI on stability regions of generation stable circular vortex solitons have been investigated in detail. These results have potential applications in optical signal processing.

Design of Broadband Flat Optical Frequency Comb Based on Cascaded Sign-Alternated Dispersion Tellurite Microstructure Fiber.

We designed a tellurite microstructure fiber (TMF) and proposed a broadband optical frequency comb generation scheme that was based on electro-optical modulation and cascaded sign-alternated dispersion TMF (CSAD-TMF). In addition, the influence of different nonlinear effects, the ultrashort pulse evolution in the CSAD-TMF with the anomalous dispersion (AD) zones and the normal dispersion (ND) zones were analyzed based on the generalized nonlinear Schrodinger equations (GNLSE) modelling. According to the simulations, when the input seed comb had a repetition rate of 20 GHz and had an input pulse peak power of 30 W, the generation scheme could generate optical frequency combs with a 6 dB spectral bandwidth spanning over 170 nm centered at 1550 nm. Furthermore, the generated combs showed good coherence in performance over the whole 6 dB spectral bandwidth. The highly coherent optical frequency combs can be used as high-repetition-rate, multi-wavelength light sources for various integrated microwave photonics and ultrafast optical signal processing applications.

Free-space group-velocity dispersion induced in space-time wave packets by V-shaped spectra

The authors introduce a space-time wave-packet structure that propagates in free space with a group velocity fixed at the speed of light in vacuum and large anomalous dispersion in free space. This is a noteworthy addition to the family of known optical wave packets and could have applications in optical signal processing and nonlinear optics.

Photonic Microresonators Created by Slow Optical Cooking

Silica and water are known as exceptionally inert chemical materials whose interaction is not completely understood. Here we show that the effect of this interaction can be significantly enhanced by optical whispering gallery modes (WGMs) propagating in a silica microcapillary filled with water. Our experiments demonstrate that WGMs, which evanescently heat liquid water over several hours, induce permanent alterations in silica material characterized by the subnanometer variation of the WGM spectrum. We use the discovered effect to fabricate optical WGM microresonators having potential applications in optical signal processing and microfluidic sensing. Our results pave the way for the ultraprecise fabrication of resonant optical microdevices and the ultra-accurate characterization of physical and chemical processes at solid–liquid interfaces.

Dual-Mode GVD Tailoring in a Convex Waveguide

Mode division multiplexing (MDM) silicon photonic integrated circuits (PICs) have been widely developed for achieving high-speed optical interconnects and communications. As an excellent nonlinear optical platform, silicon PICs also receive great attention in applications of optical parametric devices and nonlinear optical signal processing. However, it is still challenging to develop MDM optical parametric devices due to the strong mode dependence of the group velocity dispersion (GVD) in a silicon waveguide. Here, we theoretically design a convex waveguide exhibiting almost the same GVD profiles for quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> and quasi-TE <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sub> modes based on the standard fabrication flow of silicon photonic foundries. Specifically, flat GVD curves varying from -1500 ps/nm/km to -1000 ps/nm/km are obtained for the two modes in a convex waveguide within a spectral region of 1.37 μm to 1.75 μm covering from E-band to U-band. The study is expected to open an avenue for exploring unprecedented MDM nonlinear applications.

Reconfigurable Rectangular Filter With Continuously Tunable Bandwidth and Wavelength

We propose and demonstrate a rectangular optical lattice filter with reconfigurable bandwidth and wavelength. The proposed reconfigurable rectangular lattice filter consists of three-stage cascaded ring-assisted Mach-Zehnder interferometers (RAMZIs), and each RAMZI unit serves as an approximate optical elliptical filter with optimized shape factor. The scheme and design principle are presented with the method of zero-pole theory. As a proof of concept, the proposed structure is fabricated on the Si $_3$ N $_4$ waveguide platform. In the experiment, the 3-dB bandwidth of the proposed filter can be continuously tuned from 14.1 GHz to 4.1 GHz, maintaining a rectangular transmission response, and the wavelength can also be continuously tuned. This reconfigurable rectangular optical lattice filter shows the potential in many practical applications for reconfigurable optical signal processing due to flexibility and reconfigurability.

Tunable narrowband shortwave-infrared absorber made of a nanodisk-based metasurface and a phase-change material Ge2Sb2Te5 layer.

A tunable absorber made of a nanodisk-based metasurface is proposed to realize a narrowband shortwave-infrared (SWIR) perfect absorption. By introducing a phase-change material Ge2Sb2Te5 (GST) layer, we produce a selective and active control of the optical response. It is found that the narrowband absorption of 99.9% can be achieved for amorphous GST (aGST) with a modulation depth of 54.6% at 1931 nm, which is attributed to the strong electric dipole resonance in the germanium nanodisks. Moreover, under the aGST state, the full width at half-maximum of 22 nm can be acquired for a normal TM-polarized wave, and such a nanodisk-based absorber enables a tunable operating wavelength by adjusting the geometrical parameters to realize the spectral selectivity. In addition, the nanodisk-based metasurface nanostructure, combined with a dielectric Bragg reflector with alternately stacked SiO2 and TiO2 layers, can realize the SWIR dual-band absorption for aGST and single-band absorption for crystalline GST through the adjustment of electric and magnetic resonances. The designed absorbers have the potential applications in tunable absorption filter, thermal sensing, and optical signal processing.