Single-mode Propagation Research Articles

300-nm-thick, ultralow-loss silicon nitride photonic integrated circuits by 8-in. foundry production

Silicon nitride (Si3N4) photonic integrated circuits are rapidly developing in recent decades. The low loss of Si3N4 attracts significant attention and facilitates a wide range of applications in integrated photonics. In this work, we demonstrate the foundry fabrication of a 300-nm-thick 8-in. wafer-scale Si3N4 platform, with a microresonator intrinsic quality factor of up to 15×106, corresponding to an ultralow loss of 2.2 dB/m. Leveraging this platform, we develop a mature process design kit, achieving a single-mode waveguide propagation loss of less than 5 dB/m, an edge coupler loss of 1.3 dB, and an insertion loss of 0.07 dB for multimode interference couplers. Utilizing the processed Si3N4 chip, we realize a hybrid integrated tunable external cavity laser with a tuning range from 1534 to 1602 nm, a record-high side-mode suppression ratio of up to 76 dB, an optical power of 26 mW, and an intrinsic linewidth of down to 314 Hz. Our work lays a solid foundation for the further development of applications, including nonlinear optics, quantum optics, optical communications, and ranging.

A novel 3D topological metamaterial for controllability of polarization-dependent multilayer elastic waves

The achievement of high-quality wave manipulation and energy concentration has always been considered as state-of-the-art technologies, especially for integrated photonics, acoustics, and mechanics. The exploration of the topological phase of matter provides abundant design tools for robust waveguiding that is immune to backscattering at small defects and sharp bends. Recent research has extended the elastic wave manipulation from 2D edge waveguiding to 3D planar waveguiding. However, most of them are limited to single-mode and single-frequency wave propagation along the designed plane. This paper introduces a novel 3D topological metamaterial structure whose geometrical parameters are specifically configured to obtain dual-mode topological states at distinct frequencies. Parametric studies are presented to demonstrate the controllability of bandgaps and to provide a design principle for preventing the effects of unwanted modes. Topological interface modes with either high group velocity or near-zero group velocity along the z direction are found. Full-scale finite element simulations are presented to uncover the elastic wave propagation behavior. The interesting layer-locked and layer-unlocked waveguiding based on excitation polarization and frequency for both straight path and zig-zag path are demonstrated. The outcomes of this work suggest abundant potential applications related to elastic wave control such as wave filters, energy harvesters, mechanical computers, and the like. This work may also help inspire future research on more complex and sophisticated multi-mode waveguiding in 3D spaces.

Semi-Analytical Approach versus Finite Element Method for Analysis of Propagation Properties in Rectangular Waveguides: Silica-Titania Technological Platform

This work explicitly demonstrates a semi-analytical effective index approximation (EIA) approach for the description of the propagation properties of rib and ridge waveguides. By using the example of waveguides realized on a low-cost silica-titania (SiO2:TiO2) technological platform, we present that EIA may be successfully applied for the approximate determination of modal effective indices and single mode propagation conditions. All obtained results have been confirmed to be convergent with the finite element method (FEM) simulations at low relative error. Due to the tremendously fast execution time of EIA simulations in comparison with the FEM solver, we believe that the presented approach may be applied in a preliminary step of designing functional blocks in new and existing photonic integrated circuit technologies, which often require complex and multi-parameter calculations.

High-confinement alumina waveguides with sub-dB/cm propagation losses at 450 nm

Amorphous alumina is highly transparent across the visible spectrum, making it a promising candidate for low-loss waveguiding at short wavelengths. However, previous alumina waveguide demonstrations in the visible region have focused on low- to moderate-confinement waveguides, where the diffuse mode reduces the design flexibility and integration density of photonic integrated circuits. Here, we have developed a high-quality etch mask and a highly selective BCl3 plasma etch, allowing etching of amorphous alumina waveguides up to 800 nm thick. Using this process, we have fabricated waveguides using an alumina film grown by atomic layer deposition (ALD) which are the lowest-loss high-confinement waveguides for blue light to date: we achieve single-mode propagation losses of 0.8 dB/cm at a propagation wavelength of 450 nm.

A high-order TE50 mode waveguide with single mode transmission property and its application in slot array antenna

Conventional array antenna techniques contain various feeding networks, such as T-junction power dividers, phase dividers, and baluns. To feed a relatively large array, one can cascade the traditional T-junction power dividers, resulting in the increase of design complexity. In this paper, an overmoded waveguide supporting the single mode propagation of TE50 mode, is presented. Since TE50 mode can be separated into five TE10 modes with equi-amplitude and opposite phase properties, it is advisable to distribute the input signal flexibly and decrease the cascading of the T-junctions for the traditional feeding networks. To actualize a TE50 mode waveguide, several rows of metallic blocks acting as metamaterial are periodically loaded inside an overmoded waveguide to tailor the dispersion of the waveguide modes and yield the proposed mode bandgaps. Considering the Bloch theory, parts of the undesired waveguide modes are not supported inside the metamaterial and will be removed by these bandgaps. In addition, central feeding method is used to suppress the unwanted modes further in light of mode mismatch. As a proof-of-concept, we drilled the slot array on the top of the overmoded waveguide directly and give rise to a pencil beam radiation along the broadside with 15–17 dBi realized gain among 9.4–10.6 GHz, which demonstrates the stable feeding ability of TE50 mode.

Van der Waals Materials for Overcoming Fundamental Limitations in Photonic Integrated Circuitry.

With the advance of on-chip nanophotonics, there is a high demand for high-refractive-index and low-loss materials. Currently, this technology is dominated by silicon, but van der Waals (vdW) materials with a high refractive index can offer a very advanced alternative. Still, up to now, it was not clear if the optical anisotropy perpendicular to the layers might be a hindering factor for the development of vdW nanophotonics. Here, we studied WS2-based waveguides in terms of their optical properties and, particularly, in terms of possible crosstalk distance. Surprisingly, we discovered that the low refractive index in the direction perpendicular to the atomic layers improves the characteristics of such devices, mainly due to expanding the range of parameters at which single-mode propagation can be achieved. Thus, using anisotropic materials offers new opportunities and novel control knobs when designing nanophotonic devices.

Design and Analysis of Si/CaF<sub>2</sub> Near-Infrared (λ∼1.7µm) DFB Quantum Cascade Laser for Silicon Photonics

We have designed a near-infrared wavelength Si/CaF2 DFB quantum cascade laser and investigated the possibility of single-mode laser oscillation by analysis of the propagation mode, gain, scattering time of Si quantum well, and threshold current density. As the waveguide and resonator, a slab-type waveguide structure with a Si/CaF2 active layer sandwiched by SiO2 on a Si (111) substrate and a grating structure in an n-Si conducting layer were assumed. From the results of optical propagation mode analysis, by assuming a λ/4-shifted bragg waveguide structure, it was found that the single vertical and horizontal TM mode propagation is possible at the designed wavelength of 1.70 μm. In addition, a design of the active layer is proposed and its current injection capability is roughly estimated to be 25.1 kA/cm2, which is larger than required threshold current density of 1.4 kA/cm2 calculated by combining analysis results of the scattering time, population inversion, gain of quantum cascade lasers, and coupling theory of a Bragg waveguide. The results strongly indicate the possibility of single-mode laser oscillation.

Evaluation of the Impedance Variability Effect of Acoustic Liner on Aircraft Engine Fan Noise in Calculation of Far Field Sound Modes Propagation

The influence of the impedance variability of the acoustic liner of an air intake on aircraft engine fan noise in the far field has been studied. The calculations were carried out in an axisymmetric statement for single modes based on the finite element solution to the Blokhintsev equation for the acoustic potential. The acoustic liner impedance was determined from a well-developed semiempirical model that takes into account the influence of a high sound pressure level (SPL) and grazing flow. A procedure has been developed for calculating the propagation of single sound modes through the air intake of an aircraft engine with a variable liner impedance depending on the flow velocity and SPL at a local point. The calculation procedure has been validated. Calculations were made for single mode propagation into the forward hemisphere for an air intake on the wall of which the following conditions were set: rigid wall, constant impedance (the impedance does not change along the liner) and variable impedance (the impedance changes along the liner depending on the changes in the velocity of the grazing flow and SPL). According to the calculation results, it was found that constant and variable impedances yield different SPL values in the far field; as well, taking into account the impedance variability changes not only the SPL values in the far field, but also the direction of the maximum mode radiation.

Optical waveguide switch based on a negative-index metamaterial load.

An optical switch concept is presented which involves moving a negative-index metamaterial (NIM) load, possibly with loss, close to one waveguide in a two-waveguide directional coupler. The NIM load limits the number of optical modes in the switch, creating a system where the single waveguide mode propagation constant is far from other mode propagation constants. The wide spacing in propagation constants, not possible with positive-index media (PIM) loads, permits a small switch size. Three-dimensional (3D) finite-difference time-domain (FDTD) simulations confirm results from a simple one-dimensional (1D) model. Importantly, realistic NIM losses do not impede switch operation.

Brillouin Interaction between Two Optical Modes Selectively Excited in Weakly Guiding Multimode Optical Fibers.

A multimode optical fiber supports excitation and propagation of a pure single optical mode, i.e., the field pattern that satisfies the boundary conditions and does not change along the fiber. When two counterpropagating pure optical modes are excited, they could interact through the stimulated Brillouin scattering (SBS) process. Here, we present a simple theoretical formalism describing SBS interaction between two individual optical modes selectively excited in an acoustically isotropic multimode optical fiber. Employing a weakly guiding step-index fiber approach, we have built an analytical expression for the spatial distribution of the sound field amplitude in the fiber core and explored the features of SBS gain spectra, describing the interaction between modes of different orders. In this way, we give a clear insight into the sound propagation effects accompanying SBS in multimode optical fibers, and demonstrate their specific contributions to the SBS gain spectrum.

Adiabatic guided modes of a three-layer integral optical waveguide

The numerical solution of the problem of guided propagation of polarized light in a smooth junction of a planar waveguide is considered. Within the framework of the model of adiabatic guided modes, the system of Maxwell equations is reduced to a system of four ordinary differential equations and two algebraic equations for six components of the electromagnetic field in the zeroth approximation and the same number of equations in the first approximation. The multilayer structure of waveguides makes it possible to reduce the problem to a homogeneous system of linear algebraic equations, whose nontrivial solvability condition yields the dispersion equation. Auxiliary eigenvalue problems for describing the adiabatic modes of the waveguide are solved. Keywords: smoothly irregular integrated-optical multilayer waveguides, eigenvalue and eigenvector problems, single-mode propagation of adiabatic waveguide modes.

On the Correct Generation of a Single Sound Mode in a Duct with Flow in COMSOL Multiphysics

The paper considers the statement of a single-mode sound generation in a duct with flow in the COMSOL Multiphysics finite element analysis software. Verification of the COMSOL Multiphysics solution is based on a comparison with the analytical solution of a single-mode propagation in an annular duct with uniform flow. The verified statement of a single-mode generation in COMSOL Multiphysics is then used to predict the noise of a JT15D turbofan engine in the far field. The results of computations are compared with JT15D static test data and known numerical simulation results.

Strip waveguides in Yb3+-doped silicate glass formed by combination of He+ ion implantation and precise ultrashort pulse laser ablation

Abstract Strip optical waveguides were realized in Yb3+-doped silicate glass with ultrashort pulse laser ablation assisted He+ ion implantation. Planar waveguides were first prepared near the glass surface by He+ ion implantation (450 keV + 500 keV + 550 keV), followed by annealing at 260℃. After that, under the processing parameters of 3 μJ energy and 50 μm/s ablation velocity, two parallel tracks with separation of 15, 20, and 25 μm were, respectively, inscribed on the sample, which confine the light in lateral direction to form a strip waveguide. The near-field intensity measurement indicates that the strip waveguides maintain the single-mode propagation characteristics with 976 nm laser injection, and present the multi-mode characteristics with 632.8 nm laser injection, showing that the guided modes are well supported in the strip waveguides. The minimum propagation loss of strip waveguide is 1.35 dB/cm. Fluorescence emission spectra indicate that the gain properties of waveguide core were maintained well after waveguide preparation, revealing that the strip waveguide device in Yb3+-doped silicate glass has the potential to become an active device as waveguide laser or waveguide amplifier.

Investigation of modal electromagnetic propagation across magnetic chiral interfaces without and with dielectric loss

Fresnel coefficients (FCs) corresponding to a lossless magnetic achiral/chiral (ACC) interface under variable angles of incidence (of s-polarized light) and permittivity/permeability ratios are derived including the nonmagnetic-magnetic case, examining possible anomalous Brewster and critical angle conditions. Bimodal and single-mode propagation (including also cases where both characteristic modes (right- and left-circular) in the chiral region enter evanescence) across a magnetic chiral/achiral (CAC) interface are then examined for possible anomalies. Simulation results are presented along with interpretations, supported extensively by verified tabular data. Finally, the concepts behind examining magnetic chiral interfaces in the presence of realistic dielectric loss are introduced for the ACC interface, indicating how the Snell’s laws are modified for each mode, and the decay behavior of the fields has to be rederived to accommodate the effect of loss. To introduce possible application areas, the characteristics of lossy, magnetic, and chiral slab resonators are presented, specifically relative to transverse resonances. Finally, a magnetic, chiral Fresnel zone plate is introduced via a methodology for examining its imaging and tuning properties relative to the two circular modes.

DESIGN AND INVESTIGATION OF THREE-DIMENSIONAL POLARIZATION-INDEPENDENT POLYMER MACH-ZEHNDER INTERFEROMETERS AT 1550 NM

We propose a Mach-Zehnder interferometer (MZI) which consists of a taper, a bending waveguide, and two symmetrical arms. The structure is realized with organic-inorganic hybrid polymer materials and analyzed by a commercially available RSoft CAD BeamPROP solver. We set the core width for input and output of the Mach-Zehnder interferometer at 6 µm for single-mode propagation. This interferometer shows an excess loss of 0.162 dB and has an efficiency of almost 96.4%. We also investigate the polarization dependency of this interferometer varying the beam incident angles from 0-30 degrees and this investigation suggests that this MZI shows polarization dependency when the light incident on the launched side of this MZI at for transverse electric (TE) and transverse magnetic (TM) modes.

A nonlinear ZBLAN-based photonic crystal fiber with ultra-high birefringence and flattened dispersion in short-wavelength infrared region

In this paper, a high nonlinear ZBLAN-based photonic crystal fiber (PCF) with ultra-high birefringence and flattened dispersion is numerically proposed in short-wavelength infrared region based on finite element method. Eight elliptical-hole arrays are arranged surrounding the fiber core to separate x-polarized and y-polarized fundamental modes. The birefringence can reach to an excellent level of 4.46 × 10−2 at wavelength 2.5 µm and interestingly shows a hopping phenomenon as the ellipticity increases from 2.2 to 3.1. In addition, the design also exhibits a single-mode propagation, short beat length (< 0.12 µm), high nonlinear coefficient 131 W−1·km−1, low confinement loss 10−9 dB/m and flattened dispersion with a maximum fluctuation 50 ps·km−1nm−1 in wavelength range from 1.5 µm to 2.5 µm. The effect of the structure parameters (air hole diameter, lattice constant and ellipticity of elliptical holes) on the optical properties is symmetrically investigated. This ZBLAN-based PCF has great potential in the nonlinear applications of fiber laser and supercontinuum generation in infrared region.

Integratable electro-optic modulator based on a polymer-embedded silicon racetrack resonator with high electro-optic wavelength tuning.

Electro-optic (EO) modulators based on polymer-embedded silicon racetrack resonators (EOM-PSRR) are investigated. To obtain the single-mode propagation condition, the mode and transmission characteristics of the polymer-embedded silicon waveguide are simulated by the finite element method (FEM). By adding a static bias voltage, the EO modulation performances of EOM-PSRR embedded with lithium niobate (LiNbO3), EO polymer (AJ309), and hybrid EO polymer/TiO2 material (HEOT) are studied. The results show that the EOM-PSRR embedded with LiNbO3 achieves a high modulation depth (MD) of ∼27.6dB with a low EO wavelength tuning (λEO) of 10 pm/V. However, the EOM-PSRR embedded with HEOT has a high λEO of 100 pm/V but a low MD of ∼6.2dB with an extinction ratio of ∼5.2dB. The EOM-PSRR has potential application prospects in optical communication, optical signal processing, and optical network links. It can be produced as an optical frequency comb generator in a dense wavelength division multiplexing system, an EO frequency shifter for laser beams, an optical soliton former, and a photon time-delay device in a phased array radar.

Y-shaped phononic demultiplexer based on Fano and acoustically induced transparency resonances in one dimensional asymmetric/symmetric loops structure

In this work, we present an analytical demonstration of the possibility to realize a simple phononic demultiplexer based on Fano and Acoustically Induced Transparency (AIT) resonances. The demultiplexer is a Y-shaped waveguide with an input line and two output lines (called also channels). The first output line is composed of a segment (guide) of length d1 and two symmetric/asymmetric loops of lengths d2 and d3, the second output line contains a segment of length d4 and two symmetric/asymmetric loops of lengths d5 and d6. We derive the analytical expressions for a selective transfer of a single propagation mode through one line by keeping the other line unaffected. This AIT resonance is characterized by a maximum transmission peak wedged between two transmission zeros. The existence of a complete transmission beside a transmission zero (Fano resonance) enables the selection of a given frequency in the first output line, by canceling the transmission in the second output line and the reflection in the input line. The position and the quality factor of these resonances are sensitive to the geometrical parameters of the segments and loops which should be chosen appropriately. The effect of the symmetric and asymmetric loops on the transmission spectra of our proposed demultiplexer is also discussed. The theoretical results are obtained out using the continuous medium interface response theory, which makes it possible to calculate the Green function of any composite material. This allows us to determine the two transmission rates T1, T2 and the reflection rate R. This structure can be used as high performance and high transmittance acoustic guiding, demultiplexing and noise filtering.

Design of a Highly Sensitive Photonic Crystal Fiber Sensor for Sulfuric Acid Detection.

In this research, a photonic crystal fiber (PCF)-based sulfuric acid detector is proposed and investigated to identify the exact concentration of sulfuric acid in a mixture with water. In order to calculate the sensing and propagation characteristics, a finite element method (FEM) based on COMSOL Multiphysics software is employed. The extensive simulation results verified that the proposed optical detector could achieve an ultra-high sensitivity of around 97.8% at optimum structural and operating conditions. Furthermore, the proposed sensor exhibited negligible loss with suitable numerical aperture and single-mode propagation at fixed operating conditions. In addition, the circular air holes in the core and cladding reduce fabrication complexity and can be easily produced using the current technology. Therefore, we strongly believe that the proposed detector will soon find its use in numerous industrial applications.

Design, fabrication, and characterization of a single-polarization single-mode flexible hollow waveguide for low loss millimeter wave propagation.

A flexible metallic waveguide with elliptical core that achieves single-polarization single-mode (SPSM) propagation at millimeter wave was designed, fabricated, and characterized. In order to achieve SPSM propagation, optimization of the lengths of major/minor axes of elliptical core was conducted to cut off one of the two orthogonally polarized fundamental modes and all high-order modes. A one-meter long hollow elliptical waveguide (HEW) with major/minor axis length of 1.5/2.7 mm was fabricated. The substrate tube was a flexible elliptical polycarbonate (PC) tube, which was fabricated through glass-draw technique. Silver film was then coated on the inner surface of the tube. Simulation results show that the 1.5/2.7 mm HEW maintains SPSM propagation in the frequency band from 66.5 to 114 GHz. The SPSM operation was experimentally discussed in detail at 100 GHz. The measured loss of 2.58 dB/m and the output polarization ratio of 99.9% was obtained after propagating one meter. Furthermore, the waveguide was robust to bending and twisting. The additional loss was as small as 0.2 dB/m even when the waveguide was coiled into a circle. The potential application of HEWs as polarizers was demonstrated by using a 10 cm long waveguide for polarization detection and extinction ratio of 22.3 dB was achieved at 100 GHz.