Waveguide Attenuation Research Articles

The effect of 3D surface roughness on acoustic wave propagation in a cylindrical waveguide

This paper studies the acoustic wave scattering and attenuation in a cylindrical waveguide (pipe) with wall roughness varying along all three dimensions and roughness height smaller than the acoustic wavelength. Using the decomposition of the acoustic wave field into deterministic and random components, small perturbation method and Fourier transform the analytical solution of a 3-D averaged acoustic wave field is obtained. The correction term describing the mechanism of wave attenuation caused by roughness and determined by the modal cross-talk is also derived. The solution for the plane wave is validated in the frequency range extended well beyond the second cut-off frequency, where the crosstalk between the fundamental and non-axisymmetric modes are observed. The analytical solution is compared with the numerical results obtained with the Monte-Carlo method and Finite Element solver. The numerical study results have demonstrated a close agreement with the analytical solution for the averaged sound field, dispersion curves, and the wave attenuation effect expressed as the wavenumber correction term. A key novelty of this work is a comprehensive analysis of wave dispersion and cut-off frequency changes due to the presence of 3-D wall roughness in a pipe.

Bridging the gap between surface physics and photonics.

Use and performance criteria of photonic devices increase in various application areas such as information and communication, lighting, and photovoltaics. In many current and future photonic devices, surfaces of a semiconductor crystal are a weak part causing significant photo-electric losses and malfunctions in applications. These surface challenges, many of which arise from material defects at semiconductor surfaces, include signal attenuation in waveguides, light absorption in light emitting diodes, non-radiative recombination of carriers in solar cells, leakage (dark) current of photodiodes, and light reflection at solar cell interfaces for instance. To reduce harmful surface effects, the optical and electrical passivation of devices has been developed for several decades, especially with the methods of semiconductor technology. Because atomic scale control and knowledge of surface-related phenomena have become relevant to increase the performance of different devices, it might be useful to enhance the bridging of surface physics to photonics. Toward that target, we review some evolving research subjects with open questions and possible solutions, which hopefully provide example connecting points between photonic device passivation and surface physics. One question is related to the properties of the wet chemically cleaned semiconductor surfaces which are typically utilized in device manufacturing processes, but which appear to be different from crystalline surfaces studied in ultrahigh vacuum by physicists. In devices, a defective semiconductor surface often lies at an embedded interface formed by a thin metal or insulator film grown on the semiconductor crystal, which makes the measurements of its atomic and electronic structures difficult. To understand these interface properties, it is essential to combine quantum mechanical simulation methods. This review also covers metal-semiconductor interfaces which are included in most photonic devices to transmit electric carriers to the semiconductor structure. Low-resistive and passivated contacts with an ultrathin tunneling barrier are an emergent solution to control electrical losses in photonic devices.

Deep learning approach to predict optical attenuation in additively manufactured planar waveguides.

The booming demand for efficient, scalable optical networks has intensified the exploration of innovative strategies that seamlessly connect large-scale fiber networks with miniaturized photonic components. Within this context, our research introduces a neural network, specifically a convolutional neural network (CNN), as a trailblazing method for approximating the nonlinear attenuation function of centimeter-scale multimode waveguides. Informed by a ray tracing model that simulated many flexographically printed waveguide configurations, we cultivated a comprehensive dataset that laid the groundwork for rigorous CNN training. This model demonstrates remarkable adeptness in estimating optical losses due to waveguide curvature, achieving an attenuation standard deviation of 1.5dB for test data over an attenuation range of 50dB. Notably, the CNN model's evaluation speed, at 517µs per waveguide, starkly contrasts the used ray tracing model that demands 5-10min for a similar task. This substantial increase in computational efficiency accentuates the model's paramount significance, especially in scenarios mandating swift waveguide assessments, such as optical network optimization. In a subsequent study, we test the trained model on actual measurements of fabricated waveguides and its optical model. All approaches show excellent agreement in assessing the waveguide's attenuation within measurement accuracy. Our endeavors elucidate the transformative potential of machine learning in revolutionizing optical network design.

Laser Doppler vibrometry measurements of acoustic attenuation in optical fiber waveguides.

Fiber Bragg grating (FBG) sensors have been widely applied for structural health monitoring applications. In some applications, remote bonding of the optical fiber is applied, where ultrasonic waves are coupled from the structure to the optical fiber and propagated along the fiber to the FBG sensor. The distance that this signal can propagate along the optical fiber without decaying below a threshold value can be critical to the area of the structure that can be monitored per sensor. In this paper, we develop a method to measure the acoustic mode attenuation of fiber waveguides based on laser Doppler vibrometry (LDV) that is independent of the fiber type. In order to validate the method, we compare attenuation measurements on single-mode optical fibers using both the LDV and FBG sensor methods. Once the method is validated, experimental measurements of different coated and uncoated optical fibers are performed to quantify the role of the fiber diameter on the attenuation coefficient. As the radius of the waveguide decreases, the signal attenuation increases exponentially.

Analysis on the seismic wave caused by low frequency sound source in shallow sea based on multi-transmitting formula artificial boundary

The elastic waves propagating in seabed caused by sailing ships are called ship seismic waves, which can be used to identify ship targets. The wave components and the influences of source frequency, source depth and depth of seawater to the seismic waves are important to the application of seismic wave to detect ship targets. Thus, a forward numerical simulation of seismic waves in shallow sea excited by low frequency sources in time domain was carried out with finite element method based on the Multi-Transmitting Formula artificial boundary. The numerical results show that the body waves and conical waves decay faster in space, but the interface wave decays relatively slow. Multi-transmitting Formula (MTF) artificial boundary has achieved good transmitting effect for longitudinal wave, upstream and downstream acoustic wave and transverse wave. When the source frequency is very low and the seawater layer is shallow, due to the effect of low-frequency cutoff, there is no normal mode wave which can propagate without attenuation in the seawater waveguide, and only the interface wave is propagating near the sea bottom surface. When the frequency of the source or the depth of the seawater layer increases, the effect of low frequency cutoff in the shallow water waveguide is weakened, and the normal mode waves propagating in seawater layer are gradually excited. The intensity of the interface wave caused by low-frequency point sound source mainly depends on the distance between the source and the seafloor.

A new model for acoustic attenuation of GHz waveguide induced by metal electrodes in piezoelectric composites

• A new model based on electromagnetic (or optic) property of metal is established. • 3D dispersion curves with energy dissipation are investigated by a novel root-searching method. • A rapid conversion phenomenon in wave mode shapes is identified. • Two new features which significantly change the acoustic attenuation are observed. • Besides the electromagnetic property , mechanical property of electrode also impacts the attenuation in GHz. Acoustic attenuation in piezoelectric composites is an important factor and it can be induced by various sources, including viscoelasticity and semi-conductivity. Since the radio frequency of fifth generation (5G) mobile communication technology has achieved at GHz, the attenuation of the piezoelectric acoustic devices operating at GHz frequency must be addressed. Unfortunately, when frequency is high, the electrode cannot be treated as an electric resistance that follows the traditional Ohm's law. Therefore, in this paper, the acoustic attenuation induced by electrodes, which has been seldom studied in the past, is investigated. A novel multi-disciplinary model is presented which combines the piezoelectric theory and the electromagnetic (or optic) property of metal, instead of the zero-thickness surface electrode model and Ohm's law with a fixed conductivity. Based on this model, the dispersion equations for the composite structure are derived in the case of elastic metal electrode with realistic optical properties, as compared with the ideal short circuit boundary condition case. Three-dimensional (3D) dispersion curves of these two cases are further calculated by two forms of the novel root-searching method introduced recently, accompanying with the wave mode shapes on certain branches. Based on these results, two important features of the wave motion, which significantly change GHz acoustic attenuations, are identified, with their underlying mechanisms. These new features could provide a reference in safe and better design of piezoelectric acoustic devices.

Characterization of PT-symmetric quantum interference based on the coupled mode theory.

In this paper, we propose a comprehensive quantum theoretical framework to formulate the quantum interference inside the parity-time (PT) symmetric waveguide system which is formed by two coupled optical waveguides with unequal losses. Based on the theory, the expression for the well-known Hong-Ou-Mandel (HOM) dip is derived, which is in an exact agreement with the published results. What's more, a novel one-photon quantum interference phenomenon is predicted according to the model, which suggests a quantum interference process similar to the HOM effect can be observed for the one-photon state, while the other photon is lost due to the waveguide attenuation. Such phenomenon cannot occur in a Hermitian system or in the system formed by the waveguides with equal losses.

Measuring Cryogenic Waveguide Loss in the Terahertz Regime

Waveguide attenuation due to conductor loss increases significantly in the terahertz regime. There are a variety of models to estimate this loss but the accuracy is unverified at cryogenic temperatures above 100 GHz. In this article, we present a new technique for measuring the conductor loss of these waveguides. The technique is based on measurements of a waveguide cavity resonator and uses a frequency- to distance-domain transform to isolate the different orders of transmission through the cavity. By calculating the relative loss between successive orders, we can estimate the loss of the resonator and calculate the waveguide attenuation constant. This technique is less susceptible to calibration errors and standing waves in the measurement system, making it suitable for cryogenic high-frequency measurements. For the purposes of this study, we measured the attenuation constant of several WR-2.8 and WR-3.0 waveguides made from aluminum, gold-plated aluminum, and tellurium copper. At room temperature, the measured attenuation constants were between 0.25 and 0.40 dB/cm in the 260- to 350-GHz frequency range. At cryogenic temperatures (4 K), the attenuation range reduced to between 0.18 and 0.26 dB/cm.

Sapphire Photonic Crystal Waveguides with Integrated Bragg Grating Structure

This contribution demonstrates photonic crystal waveguides generated within bulk planar sapphire substrates. A femtosecond laser is used to modify the refractive index in a hexagonal pattern around the pristine waveguide core. Near-field measurements reveal single-mode behavior at a wavelength of 1550 nm and the possibility to adapt the mode-field diameter. Based on far-field examinations, the effective refractive index contrast between the pristine waveguide core and depressed cladding is estimated to 3·10−4. Additionally, Bragg gratings are generated within the waveguide core. Due to the inherent birefringence of Al2O3, the gratings exhibit two distinct wavelengths of main reflection. Each reflection peak exhibits a narrow spectral full width at a half maximum of 130 pm and can be selectively addressed by exciting the birefringent waveguide with appropriately polarized light. Furthermore, a waveguide attenuation of 1 dB cm−1 is determined.

A supersymmetric approach to the problem of micro-bending attenuation in optical waveguides

Micro-bending is a well-known source of loss in optical waveguides. By treating the micro-bending as a stochastic process, the problem of loss mitigation can be modeled in terms of a Fokker–Planck equation. Given an initial refractive index profile, and taking micro-bending into account, we develop a formalism to derive a new refractive index profile which potentially results in less loss. Our formalism is based on applying the techniques of Supersymmetric Quantum Mechanics to a Fokker–Planck equation that is associated with a particular refractive index profile. We derive a non-linear differential equation, whose solutions determine whether an index profile can undergo a supersymmetric transformation that results in less loss. As an explicit example, we consider a monomial index profile. We show that there exists a range of values for the monomial exponent which results in the new index profile having less loss.

The Effect of Attenuation from Fish on Passive Detection of Sound Sources in Ocean Waveguide Environments

Attenuation from fish can reduce the intensity of acoustic signals and significantly decrease detection range for long-range passive sensing of manmade vehicles, geophysical phenomena, and vocalizing marine life. The effect of attenuation from herring shoals on the Passive Ocean Acoustic Waveguide Remote Sensing (POAWRS) of surface vessels is investigated here, where concurrent wide-area active Ocean Acoustic Waveguide Remote Sensing (OAWRS) is used to confirm that herring shoals occluding the propagation path are responsible for measured reductions in ship radiated sound and corresponding detection losses. Reductions in the intensity of ship-radiated sound are predicted using a formulation for acoustic attenuation through inhomogeneities in an ocean waveguide that has been previously shown to be consistent with experimental measurements of attenuation from fish in active OAWRS transmissions. The predictions of the waveguide attenuation formulation are in agreement with measured reductions from attenuation, where the position, size, and population density of the fish groups are characterized using OAWRS imagery as well as in situ echosounder measurements of the specific shoals occluding the propagation path. Experimental measurements of attenuation presented here confirm previous theoretical predictions that common heuristic formulations employing free space scattering assumptions can be in significant error. Waveguide scattering and propagation theory is found to be necessary for accurate predictions.

Mode coalescence and the Green’s function in a two-dimensional waveguide with arbitrary admittance boundary conditions

This study focuses on sound attenuation in a two-dimensional waveguide with arbitrary admittance boundary conditions on both sides of the guide. The emphasis is on understanding the formation and potential applications of the exceptional points (EPs) which arise when two (EP2) or three (EP3) modes degenerate into a single mode. A perturbation approach is used to obtain asymptotic expressions for the trajectories of the axial wavenumbers in the complex plane as they coalesce to form an EP. The numerical results presented herein suggest that the first triple root (EP3) assures maximum modal attenuation along the waveguide. Further, it is demonstrated that the classical Green’s function is degenerate at an EP. Modified Green’s functions which are valid at EP2 and EP3 are presented.

The fluid bottom approximation and its implications for sound attenuation in underwater waveguides

The fluid-bottom model makes forward and inverse problems more amenable to theoretical analysis and numerical simulations but has some well-known limitations. The accuracy and weaknesses of the fluid approximation are elucidated in this paper by quantifying the effects of weak shear rigidity on acoustic normal modes. Mathematically, appearance of finite shear rigidity and attendant slow shear waves is a singular perturbation of the fluid-bottom model, which standard perturbation theories fail to capture correctly. The problem is addressed here by combination of an asymptotic technique and an analysis of exact solutions for idealized seabed models and placed in the context of earlier studies of the effects of shear rigidity. Due to coupling between compressional and shear waves at the seafloor and within the seabed, shear rigidity affects phase and group speeds of normal modes. By transferring the energy from compressional to shear waves, wave coupling makes a significant contribution to sound attenuation from low up to mid-frequencies. It is found that the effects of shear rigidity on the acoustic field in water are magnified by seabed stratification and especially by density variations. Implications of the results for seabed parameterization in geoacoustic inversions will be discussed. [Work supported by ONR.]

Supersymmetric analysis of stochastic micro-bending in optical waveguides

Micro-bending attenuation in an optical waveguide can be modeled by a Fokker-Planck equation. It is shown that a supersymmetric transformation applied to the Fokker-Planck equation is equivalent to a change in the refractive index profile, resulting in a larger or smaller attenuation. For a broad class of monomial index profiles, it is always possible to obtain an index profile with a larger micro-bending attenuation using a supersymmetric transformation. However, obtaining a smaller attenuation is not always possible and is restricted to a subset of index profiles.

Multi-layered parallel plate waveguide with electrically and magnetically biased graphene sheets

The performance of a multi-layered parallel plate waveguide, supported by graphene plates, is evaluated here. The graphene layers were biased with both magnetic and electric fields. The graphene plates weremodelled as an anisotropic surface derived from the Kubo formula. Maxwell's equations were solved for these waveguides; and it is shown when both magnetic and electric biases were applied to the graphene, hybrid modes propagated inside the waveguides. The intensity of each TM and TE mode could be adjusted with an applied external field bias. This study of wave confinement has shown that, by varying the bias, full control can be exerted over the wave's propagation within a waveguide; whereby, increasing the bias causes added phase changes over the propagation length, while increasing the waveguide's attenuation. It must be emphasized that the presented procedure is also applicable to other guiding structures that have boundary with isotropic or anisotropic conductivities.

Shear waves and sound attenuation in underwater waveguides.

In addition to dissipation of acoustic energy in the seabed, bottom-interacting normal modes are attenuated by radiation of shear waves into soft sediments, where shear speed is small compared to the sound speed in water. The shear-wave contribution and the dissipation have distinct frequency dependencies, and their relative magnitude affects the observed frequency dependence of mode attenuation. Previous studies suggested that the shear-wave contribution to the attenuation is proportional to the cube of the small ratio of the shear and sound speeds. Here, coupling of compressional and shear waves in layered soft sediments is analyzed. Besides the well-known, third-order contribution to the attenuation due to shear-wave generation at the water-sediment interface, a stronger, first-order, contribution is found to occur due to compressional-to-shear wave conversion at interfaces within the sediment. First-order effects of weak shear on mode travel times are also identified. Stratification of the sediment density and interference of shear waves reflected within the seabed control the frequency dependence of the shear-wave contribution to sound attenuation. With the shear-wave contribution being larger than previously estimated, its effect on the experimentally measured frequency dependence of the sound dissipation may need to be re-assessed.

Achieving optimal attenuation in acoustic waveguides with 3D printed Helmholtz resonators

A double root, commonly referred to as an exceptional point (EP), for modal frequencies in a 2D or 3D waveguide can exhibit optimal attenuation rates over a relatively broad frequency range. The key to the phenomenon is that the wall impedance is such that modes coalesce at a complex-valued frequency. In this talk we consider a novel approach to feasibly achieve the aforementioned wall impedance with the use of simple resonators, which can be shown to exhibit mode coalescence at distinct frequencies when treated as a unit cell component of a larger metasurface. Numerical results of several unit cell designs are given to test the boundaries on the following constraints: realizable resonator dimensions, scale separation, and target attenuation goals. To validate the analytical work, experimental setups consisting of additively manufactured Helmholtz resonators incorporated onto a custom-made impedance tube are tested. The dependence of printed resonator quality on theoretical inertial, loss, and stiffness values are discussed in tandem with printing repeatability. Simulated finite element model results will be used as comparative backdrops with the numerical and experimental results. Work supported by NSF.

A Tunable Graphene Filtering Attenuator Based on Effective Spoof Surface Plasmon Polariton Waveguide

In this article, to reduce the operating frequency and the size of the spoof surface plasmon polariton (SSPP) attenuator and introduce the filtering function into the graphene attenuator, we propose a tunable graphene filtering attenuator, namely the graphene-based effective SSPP (ESSPP) attenuator. It consists of an ESSPP waveguide and a graphene sandwich structure (GSS). The effect of GSS on the attenuation and dimensions of the ESSPP waveguide on the passband are analyzed in detail, and the closed form of the attenuation is proposed. Finally, five prototypes of the graphene filtering attenuator are fabricated and measured. The experimental results show that not only the attenuation of this attenuator can be tuned but also that the upper and lower cutoff frequencies can be adjusted independently, which means that this filtering attenuator demonstrates expected filtering and attenuation performance.

Optical Properties of InAs Quantum Dots/GaAs Waveguides for Ultra-fast Scintillators

InAs Quantum Dots (QDs) embedded in a GaAs matrix have unique scintillation properties, valuable for high-energy physics and medical applications. Temperature-dependent photoluminescence, waveguide attenuation and alpha particle response measurements were employed to analyze the optical properties of a 25 μm thick waveguiding scintillator. Optimizing the electrostatics of the QD layered structure with p-type modulation doping resulted in QD photoluminescence (PL) efficiency as high as 60% at room temperature. Analysis of attenuation of the QD waveguide showed surface scattering predominated over the first 2–3 mm of light propagation and low (~1 cm−1) self-absorption was more significant at longer distances, after the decay of high order modes (high angle light rays). Responses to 5.5 MeV alpha particles from the integrated photodiode on top of the QD scintillator/waveguide (QD/WG) show an extremely fast (300 ps) decay constant, and a 70 ps time resolution (limited by circuit noise and bandwidth) with a collection efficiency of 17,000 photons per 1 MeV of deposited energy.

Optimized microwave delivery in dDNP.

Dissolution dynamic nuclear polarization (dDNP) has permitted the production of highly polarized liquid-state samples, enabling real-time imaging of metabolic processes non-invasively in vivo. The desire for higher magnetic resonance sensitivity has led to the development of multiple home-built and commercial dDNP polarizers employing solid-state microwave sources. Providing efficient microwave delivery that avoids unwanted heating of the sample is a crucial step to achieve high nuclear polarization. Consequently, a process is described to reduce waveguide attenuation due to resistive loss thereby doubling the delivered power. A mirror and reflector are designed and tested to increase the microwave field density across the sample volume resulting in a 2.3 dB increase of delivered power. Thermal considerations with regards to waveguide geometry and dDNP probe design are discussed. A thermal model of the dDNP probe is computed and experimentally verified.