Waveguide Parameters Research Articles

Numerical analysis of a strip-loaded silicon rich nitride-thin film lithium niobate hybrid waveguides

Hybrid integration of silicon rich nitride and lithium niobate on insulator (SRN-LNOI) is an emerging material platform for photonic integrated circuits (PIC). In this paper, we present a systematic numerical investigation of the mode properties of a strip-loaded SRN-LNOI hybrid waveguide at 1550 nm wavelength using the full-vectorial finite difference method. Considering the anisotropic nature of the lithium niobate (LN) crystal, the effective refractive indices of the transverse electric and transverse magnetic modes of strip-loaded SRN-LN hybrid waveguides were analyzed. The single-mode condition, zero-birefringence, effective mode area, and power distribution in terms of the geometrical parameters of the strip-loaded SRN-LN hybrid waveguide are discussed in detail. Furthermore, the optical power transmission in both straight and bent waveguides, as well as the different characteristics of the optical power confinement of the fundamental modes in the SRN and LN layers were analyzed. This study provides useful information for designing high-performance photonic devices on a hybrid SRN-LNOI platform for future PIC applications.

Pulsed Four-Wave Mixing at Telecom Wavelengths in Si3N4 Waveguides Locally Covered by Graphene.

Recently, the nonlinear optical response of graphene has been widely investigated, as has the integration of this 2D material onto dielectric waveguides so as to enhance the various nonlinear phenomena that underpin all-optical signal processing applications at telecom wavelengths. However, a great disparity continues to exist from these experimental reports, depending on the used conditions or the hybrid devices under test. Most importantly, hybrid graphene-based waveguides were tested under relatively low powers, and/or combined with waveguide materials that already exhibited a nonnegligible nonlinear contribution, thereby limiting the practical use of graphene for nonlinear applications. Here, we experimentally investigate the nonlinear response of Si3N4 waveguides that are locally covered by submillimeter-long graphene patches by means of pulsed degenerate four-wave mixing at telecom wavelength under 7 W peak powers. Our measurements and comparison with simulations allow us to estimate a local change of the nonlinearity sign as well as a moderate increase of the nonlinear waveguide parameter (γ∼-10 m-1W-1) provided by graphene. Our analysis also clarifies the tradeoff associated with the loss penalty and nonlinear benefit afforded by graphene patches integrated onto passive photonic circuits, thereby providing some guidelines for the design of hybrid integrated nonlinear devices, coated with graphene, or, more generally, any other 2D material.

Long-wavelength electromagnetic waves of surface type in circular metal waveguides partially filled by plasma in presence of an axial static magnetic field

The method of successive approximations is applied to solve the Maxwell equations in cylindrical plasma waveguide geometry for electromagnetic waves with arbitrary azimuthal wave index and small axial wavenumber. The theory of surface flute waves is used as zeroth approximation. This study generalizes previous investigations whose results are utilized for verification of newly obtained conclusions. The influences of several plasma waveguide parameters such as magnitude and sign of the azimuthal wave index, the width of the dielectric layer between a plasma and a waveguide wall and the magnitude of its dielectric constant, the radii of the plasma column and the metal wall, and the external axial static magnetic field on the wave dispersion properties are analyzed.

Investigation of Field and Energy in a Weakly-Conducting Optical Fiber with an Arbitrary Degree of Refractive Index Profile

Introduction. We consider a weakly conductive gradient fiber in the single-mode regime and solve the equation for the electric field in the core of this fiber in a general form in the first approximation. The aim of this study is to study the field and energy in the core of a weakly conductive gradient fiber without taking into account the polarization in the single-mode regime in the case of a power-law (generally) refractive index profile. Materials and Methods. From Maxwell’s equations for dielectric media, there was derived an equation for the field in a fiber with gradient refractive index profile. Making the appropriate substitutions, replacing the zero-order Bessel function with a Gaussian function, and making the necessary approximation of the resulting equation, we arrive at an equation that we solve by the Wentzel – Kramers – Brillouin method and obtain analytical expressions for the field and energy inside waveguide for an arbitrary degree of the refractive index. Results. There was obtained a solution of the equation for the field in fiber with a powerlaw refractive index profile. Numerical calculations were carried out. A graph of the dependence of a dimensionless quantity – “normalized” energy – on the waveguide parameter for the first five powers of the profile (n = 1, 2, 3, 4, 5) was plotted. Discussion and Conclusion. It is shown that the energy increases faster for the profile with n = 1, and after this value, the energy for the profile with n = 1 increases sharply, and for n > 1, the energy growth decreases with increasing n. The results obtained in this work can be used for creating an energy-efficient core, for carrying out a possible analysis of information transmission, and for designing waveguides taking into account specific applications.

Promotion of Specific Single-Transverse-Mode Beam Characteristics for GaSb-Based Narrow Ridge Waveguide Lasers via Customized Parameter Design

GaSb-based single-transverse-mode narrow ridge waveguide (RW) lasers with high power and simultaneous good beam quality have broad application prospects in the mid-infrared wavelength region. Yet its design and formation have not been investigated systematically, while the beam characteristics that affect their suitability for specific applications remain rarely analyzed and optimized. The present work addresses these issues by theoretically establishing a waveguide parameter domain that generalizes the overall possible combinations of ridge widths and etch depths that support single-transverse-mode operation for GaSb-based RW lasers. These results are applied to develop two distinct and representative waveguide designs derived from two proposed major optimization routes of model gain expansion and index-guiding enhancement. The designs were evaluated experimentally based on prototype 1-mm cavity-length RW lasers in the 1950 nm wavelength range, which were fabricated with waveguides having perpendicular ridge and smooth side-walls realized through optimized dry etching conditions. The model gain expanded RW laser design with a relatively shallow-etched (i.e., 1.55 upmum) and wide ridge (i.e., 7 upmum) yielded the highest single-transverse-mode power to date of 258 mW with a narrow lateral divergence angle of 11.1^circ full width at half maximum at 800 mA under room-temperature continuous-wave operation, which offers promising prospects in pumping and coupling applications. Meanwhile, the index-guiding enhanced RW laser design with a relatively deeply etched (i.e., 2.05 upmum) and narrow ridge (i.e., 4 upmum) provided a highly stable and nearly astigmatism-free fundamental mode emission with an excellent beam quality of M^2 factor around 1.5 over the entire operating current range, which is preferable for seeding external cavity applications and complex optical systems.

Sensitivity and Evanescent Field Study of a Uniaxial Anisotropic Planar Waveguide Based Optical Sensor

The effect of source, geometrical and physical parameters of slab waveguide on the sensitivity of optical sensor and its evanescent field have been investigated. The waveguiding film of LiNbO3 was fabricated and the observations revealed that the maximal sensitivities of Transverse Magnetic (TM) modes and their corresponding frequencies are greater than those for Transverse Electric (TE) modes. Furthermore, the optimal source parameters improve the maximal sensitivity and evanescent field in the cover. However, the increment in the core thickness reduces the sensitivity of sensor due to reduction in evanescent field in the cover. The sensitivity of sensor was observed as a function of refractive indices of cover, core and the substrate. The increase in refractive indices of cover and core, directly affect the sensitivity while an inverse relation has been observed regarding increase in the refractive index of the substrate. It is worth noting that, any changes in the physical parameters of waveguide sensor show an insignificant effect on the evanescent fields.

Design and simulation of widely tunable picosecond synchronously pumped terahertz parametric oscillator based on silicon waveguide platform

AbstractA picosecond synchronously pumped terahertz optical parametric oscillator based on silicon membrane waveguide (Si‐based WTPO) is proposed and numerically simulated. Through designing the structure parameters of the silicon membrane waveguide, a broadband terahertz frequency range is obtained by satisfying the phase matching condition. The parametric oscillation process and output characteristics of Si‐based WTPO are numerically analyzed by solving the four‐wave mixing coupled‐wave equations, and the result has indicated that the parametric oscillation within the resonant cavity can apparently enhance the output terahertz wave power. Meanwhile, high bandwidth and efficiency output terahertz wave can be obtained, and too high pump power will lead to a reaction of reverse conversion. Finally, the threshold pump power is calculated. The Si‐based WTPO system we proposed is expected to make some contributions to the study of compact and efficient terahertz wave sources.

Simultaneous detection and ranging of baleen whale impulsive vocalizations using a temporal convolutional network

In shallow-water environments, low-frequency acoustic signals exhibit dispersive propagation due to interactions with the sea surface and seabed. The received signal can then be modeled as a set of propagating modes. Single-hydrophone modal dispersion has been used to range baleen whale vocalizations and estimate shallow-water geoacoustic properties. However, these algorithms require preliminary signal detection and human labor to estimate the modal dispersion. Here, we apply a temporal convolutional network (TCN) to time-frequency representations of baleen whale gunshots (impulsive calls) to simultaneously detect and range them in large single-hydrophone passive acoustic monitoring datasets. The TCN jointly learns ranging and detection by training using both synthetic gunshots simulated across multiple environments/ranges and experimental noise. The synthetic data is informed by only the experimental dataset’s water column depth, sound speed, and density, while other waveguide parameters vary within empirically observed bounds. The method is applied to an experimental North Pacific right whale dataset collected in the Bering Sea using a single hydrophone. To evaluate model performance, 50 calls are manually ranged using a state-of-the-art physics-based inversion method. The TCN closely matches the physics-based range estimations and detects dispersive gunshots among noise-only examples with high precision and recall. [Work supported by the ONR].

Recent advances in phase-array based wideband multimodal dispersion curves extraction and plate-waveguide parameter inversion

Multimodal ultrasonic guided waves with rich dispersion information have been widely applied for waveguide evaluation. However, there are still some challenges to extract multimode dispersion curves using the traditional pitch-catch method, including lack of spatial information and poor signal-to-noise ratio. Phase array has been recently developed for measuring ultrasonic guided waves, which brings many advantages for wideband multimodal dispersion curves extraction and parameter inversion. In the last years, basing on a multi-emitter and multi-receiver measurement, we proposed some array dispersive signal processing strategies for analyzing guided waves in long cortical bone, including the sparse singular vector decomposition (sparse-SVD) method for enhancing wavenumber estimation resolution and the low-amplitude mode extraction, Radon transform and dispersive Radon transform (DRT) with the capability to project temporal array dispersive signals on the space of parameters of interest for solving the inverse problem, and our recent work of deep neural networks for solving the intractable multiparameter inverse problem from the array signals to the waveguide elasticity. In the talk, we present (1) recent advances in array signal processing for extracting wideband multimodal dispersion curves; (2) some new perspectives to retrieve the waveguide parameters; (3) some pilot clinical results of long cortical bone evaluation using ultrasonic guided waves.

Experimental and theoretical investigation of waveguided plasmonic surface lattice resonances.

Plasmonic nanostructures are good candidates for refractive index sensing applications through the surface plasmon resonance due to their strong dependence on the surrounding dielectric media. However, typically low quality-factor limits their application in sensing devices. To improve the quality-factor, we have experimentally and theoretically investigated two-dimensional gold nanoparticle gratings situated on top of a waveguide. The coupling between the localized surface plasmon and waveguide modes results in Fano-type resonances, with high quality-factors, very similar to plasmonic surface lattice resonances. By combining plasmonic surface lattice resonance and waveguide theory, we present a theoretical framework describing the structures. By immersing the fabricated samples in three different media we find a sensitivity of ∼50 nm/RIU and figure of merit of 8.9, and demonstrate good agreement with the theory presented. Further analysis show that the sensitivity is very dependent on the waveguide parameters, grating constant and the dielectric environment, and by tuning these parameters we obtain a theoretical sensitivity of 887 nm/RIU.

Research on sensing characteristics of third-order runway series symmetric microring resonator based on hybrid plasma waveguide and metal insulator metal.

In this paper, a high-sensitivity refractive index sensor based on a hybrid plasma waveguide and metal-insulator-metal waveguide combined third-order runway series mosaic microring resonator is proposed. In this structure, a GaAs waveguide ring surrounds a gold waveguide ring in the middle, and the innermost layer is a disk made of gold material. The outer groove waveguide is composed of GaAs-air-alloy, and the inner groove waveguide is made of the Gold-Air-Gold material disc. By filling different substances in the groove, the change of refractive index will affect the optical signal strength of the output spectrum. The finite element method simulates the transmission spectrum and electric field distribution of the sensor structure. The amplitude coupling coefficient and attenuation factor affecting the resonator's performance are analyzed, and the structural parameters of the slot waveguide are optimized. The numerical simulation results show that the sensor quality factor of this structure is 1.54 × 104 , the sensitivity is 1.2 × 103 nm/RIU which is about 1.5 times higher than that of the Si ring with the same structure, the detection limit can reach 8.1892 × 10-7 RIU, and the free spectral range can reach 109 nm. Compared with the traditional microring structure, this microring has higher design freedom and free spectral range and is more suitable for producing biosensors with high sensitivity, low detection limit, and multi-parameter measurement.

Nonlinear spectral broadening of a dual-carrier electro-optic frequency comb in a graphene oxide cladded silicon-rich nitride hybrid waveguide.

We demonstrate a detailed theoretical analysis describing the generation of an electro-optic comb (EOC) in the near-IR range through discrete phase and amplitude modulation driven by radio frequency (RF) signal generators. Furthermore, the generated EOC spectra suffer nonlinear spectral broadening while propagating through a hybrid Si-rich nitride (SRN) waveguide structure integrated with two-dimensional (2D) layered graphene oxide (GO) films. We perform a detailed analysis to investigate the influence of GO layers, pump wavelength detuning, and other waveguide parameters on the evolution of comb spectra propagating through the hybrid waveguide structure. Owing to the strong modal overlapping between the SRN waveguides and the highly nonlinear GO films, the nonlinearity of the system is enhanced effectively, and broadband comb spectra have been achieved in the near-IR range. Furthermore, we investigate the spectral coherence of the generated comb spectra under different input conditions. The results exhibit strong potential to generate a tunable frequency comb with high spectral coherence in the near-IR range by employing the hybrid waveguide structure.

MODIFICATION OF THE COUPLED INTEGRAL EQUATIONS METHOD FOR CALCULATION OF THE ACCELERATING STRUCTURE CHARACTERISTICS

In this paper we present modification of coupled integral equations method (CIEM) for calculating the characteristics of the accelerating structures. In earlier developed CIEM schemes the coupled integral equations are derived for the unknown electrical fields at interfaces that divide the adjacent volumes. In addition to the standard division of the structured waveguide by interfaces between the adjacent cells, we propose to introduce new interfaces in places where electric field has the simplest transverse structure. Moreover, the system of coupled integral equations is formulated for longitudinal electrical fields in contrast to the standard approach where the transverse electrical fields are unknowns. The final vector equations contain expansion coefficients of the longitudinal electric field at these additional interfaces. This modification makes it possible to deal with a physical quantity that plays an important role in the acceleration of particles (a longitudinal electric field), and to obtain approximate equations for the case of a slow change in the waveguide parameters.

The Influence of the Different Gain Material on the Loss Compensation of Hybrid Plasmonic Waveguide

The influence of different gain materials and waveguide parameters on propagation properties are analyzed in detail. In this paper, three materials including Al2 O3 , Si3 N4 , and Polymethylmethacrylate (PMMA) as low index materials are introduced instead of SiO2 which is used in many experiments. The results obtained via simulations showed that by increasing the width of metal, field confinement increased and for widths larger than 450 nm the filed profile was completely confined in below the metal (zone I). In addition to, simulation results showed that Al2 O3 and Si3 N4 are more efficient than SiO2 whereas PMMA is completely similar to SiO2 in loss compensation. It was attributed to the fact that the refractive index of PMMA is close to the refractive index of SiO2 . Moreover, the effect of InGaAsP as high index gain material instead of Si is analyzed. In this case, InGaAsP also reveals a loss compensation effect similar to Si due to their close refractive indexes. Therefore, the configuration with new different gain materials potentially offers an appealing solution to provide lossless light transport via low compensation gain, hence opening venues for ultra-compact active plasmonic devices.

Fano Resonance in Directly Coupled Microresonators and Its High-Sensitivity Refractometric Sensing

Resonant optical phenomena related to Fano resonances have elegantly applied to highly sensitive sensor. Here, we explore the photonic Fano formula based on the model of two directly coupled loss-loss and gain-loss optical microcavities using the temporal coupled-mode theory. The dependence of the Fano formula (the asymmetric parameter and the reduced frequency) and excitation criterion on the property parameters of the optical waveguide based microresonators is studied. In the meanwhile, the excitation criterion of Fano resonance is systematically analyzed. We found that the sensitivity of refractive index of the Fano-like resonator can be magnified by <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$7.27 \times 10^{6}$ </tex-math></inline-formula> compared with the loss-loss cavity by adjusting the device parameters.

Phase-matched second-harmonic generation in hybrid polymer-LN waveguides

Here we propose a hybrid polymer-LN waveguide for achieving phase-matched second-harmonic generation (SHG). From the aspect of super-mode theory, the geometric parameters of the hybrid semi-nonlinear waveguide were optimized to utilize both symmetric (even) and antisymmetric (odd) modes of the pump and SHG waves so as to facilitate phase matching with large modal overlap. Phase matching between a fundamental even (TE00-like) mode at 1320 nm and a fundamental odd (TE01-like) mode at 660 nm was found with a calculated modal overlap integral of 0.299, while utilizing the largest nonlinear coefficient d 33, and achieving an efficient calculated normalized conversion efficiency of 148% W−1⋅cm−2. Considering the fabrication feasibility of such hybrid waveguide with features including etchless, large dimension, and low structural sensitivity, we believe our findings would provide a useful reference for future on-chip efficient nonlinear conversion devices.

Theoretical and experimental investigation of on-chip mid-infrared chalcogenide waveguide CH4 sensor based on wavelength modulation spectroscopy

Compared with direct absorption spectroscopy (DAS), wavelength modulation spectroscopy (WMS) with good ability of suppressing noise is rarely used in waveguide sensor. The influence of waveguide parameters on WMS sensing performance is required to clarify in both theory and experiment. Three mid-infrared waveguide methane (CH4) sensors (trapezoid, rectangular, suspended) were proposed and optimized for WMS simulation. Theoretical formulation and the experimentally derived sensing noise were used to numerically evaluate the key figure of merit more accurately. Simulation results were presented to show the influence of waveguide parameters on sensing performance. Two chalcogenide on magnesium fluoride (ChG-on-MgF2) trapezoid waveguide CH4 sensors were fabricated using lift-off method, and CH4 measurement was carried out using WMS. The waveguide loss was measured to be 1.52 dB/cm, leading to an optimal sensor length of 2.7 cm. The limit of detection (LoD) of CH4 for the 1cm- and 2cm-long waveguide sensors are 0.68% and 0.17% with an averaging time of 0.2 s. The experimentally achieved second harmonic (2f) signal amplitude and LoD show good agreement with the theoretical results, verifying the feasibility of the design and analysis model of the WMS-based waveguide sensor. Due to the use of WMS and a reduction of waveguide loss from 3.6 to 1.52 dB/cm, the LoD of the 2cm-long ChG-on-MgF2 sensor is 24 times lower than our previously reported 2cm-long ChG-on-SiO2 sensor based on DAS. This work provides a systematic guidance for the design of waveguide gas sensor based on WMS and contributes to improving the sensitivity of on-chip gas sensing.

Characterizing THz Scattering Loss in Nano-Scale SOI Waveguides Exhibiting Stochastic Surface Roughness with Exponential Autocorrelation

Electromagnetic (EM) scattering may be a significant source of degradation in signal and power integrity of high-contrast silicon-on-insulator (SOI) nano-scale interconnects, such as opto-electronic or optical interconnects operating at 100 s of THz where two-dimensional (2D) analytical models of dielectric slab waveguides are often used to approximate scattering loss. In this work, a formulation is presented to relate the scattering (propagation) loss to the scattering parameters (S-parameters) for the smooth waveguide; the results are correlated with results from the finite-difference time-domain (FDTD) method in 2D space. We propose a normalization factor to the previous 2D analytical formulation for the stochastic scattering loss based on physical parameters of waveguides exhibiting random surface roughness under the exponential autocorrelation function (ACF), and validate the results by comparing against numerical experiments via the 2D FDTD method, through simulation of hundreds of rough waveguides; additionally, results are compared to other 2D analytical and previous 3D experimental results. The FDTD environment is described and validated by comparing results of the smooth waveguide against analytical solutions for wave impedance, propagation constant, and S-parameters. Results show that the FDTD model is in agreement with the analytical solution for the smooth waveguide and is a reasonable approximation of the stochastic scattering loss for the rough waveguide.

Polarization-independent photon up-conversion with a single lithium niobate waveguide.

We propose a polarization-independent up-conversion protocol for single-photon detection at telecom band with a single thin-film periodically poled lithium niobate waveguide. By choosing the proper waveguide parameters, the waveguide dispersion can compensate the crystal birefringence so that quasi-phase-matching conditions for transverse electric and transverse magnetic modes can be simultaneously fulfilled with single poling period. With this scheme, randomly-polarized single photons at 1550 nm can be up-converted with a normalized conversion efficiency of 163.8%/W cm2.

Cadmium telluride waveguide for coherent MIR supercontinuum generation covering 3.5-20 µm.

The mid-infrared (MIR) wavelength coincides with various molecular resonances. In particular, a 13-20 µm wavelength window has fingerprints of unique groups such as organometallic, halogenated, and aromatic bonds. In this work, for the first time, to the best of our knowledge, an on-chip supercontinuum generation (SCG) source based on cadmium telluride (CdTe)/ cadmium sulfide (CdS)/ silicon heterostructure is proposed to extend the on-chip SCG beyond 13 µm (spanning 3.5 to 20 µm). CdTe has an ultra-broad transparent spectral range up to 25 µm, and almost the largest third-order nonlinear coefficient (n2∼ 5×10-17 m2/W at 1.55 µm, 1.3×10-17 m2/W at 9 µm, several times larger than that of silicon) among the MIR materials, making CdTe an excellent candidate for long-wavelength MIR on-chip SCG. The waveguide structure is designed with CdS as the intermediate cladding layer to achieve a low waveguide loss and high mode confinement. A large-core CdTe waveguide is tailored to generate a low and flat dispersion (< 30 ps/nm/km) in a spectral range spanning from 5 to 20 µm, while balancing the large effective nonlinearity and the convenience of coupling. The simulation results solved by the nonlinear Schrödinger equation manifest that the engineered large cross-section waveguide with only 2.5-mm propagation distance broadens the MIR spectrum covering 3.5 to 20 µm pumped by a 9 µm femtosecond laser. Moreover, it is found that good coherence is achieved from the designed MIR waveguide, before severe soliton fission breaks the temporal profile. 5-fold self-compression of the pump pulse down to 1.6 optical cycles is observed while propagating inside the CdTe waveguide. The detailed simulation of the CdTe/CdS/Si waveguide design with the various waveguide parameters, polarizations, pump wavelengths, and pump power are provided. With the SC spectrum covering almost the entire fingerprint regime and the excellent coherence generated from the designed CdTe waveguide, it provides abundant new opportunities for MIR microphotonics.