Propagation Of Higher Order Modes Research Articles

Design and Performance Analysis of Strip Photonic Waveguide with Coating Layer for Multimode Propagation

Introduction:: Photonic devices play a pivotal role in the realm of high-speed data communication due to their inherent capability to expedite the transfer of information. Historically, research efforts in this domain have predominantly concentrated on investigating the fundamental mode propagation within photonic waveguides. Method:: This study diverges from the conventional approach by delving into the untapped potential of higher-order modes in addition to the fundamental mode of propagation. The exploration of these higher-order modes opens up new possibilities for optimizing and enhancing the performance of photonic devices in high-speed data communication scenarios. As a distinctive aspect of this study, various coating materials were scrutinized for their impact on both fundamental and higher-order mode propagation. The materials under examination included AlN (aluminum nitride), Germanium, and Silicon. These materials were chosen based on their unique optical properties and suitability for influencing different modes of light propagation. The findings from the study reveal that applying a coating of germanium demonstrates advantageous characteristics, particularly in terms of reduced signal loss, even when considering higher-order modes of propagation within photonic devices. Results:: In this context, the results indicate that germanium-coated waveguides exhibit notably low propagation losses, with measurements as minimal as 0.25 dB/cm. This low level of loss is particularly noteworthy, especially when the waveguide has a width of 550 nm and is coated with a thickness of 50 nm. The dimensions and coating specifications play a crucial role in determining the efficiency of light transmission within the waveguide. Conclusion:: The fact that the propagation loss is substantially low under these conditions suggests that the germanium-coated waveguide, even when considering higher-order modes of light propagation, can effectively maintain the integrity of the optical signal.

Approximation method for fast calculation of transmission in multi-mode waveguides.

Freeform dielectric waveguides connect optical chips made of different materials in fully integrated photonic devices. With a spatial extent in the order of 100µm, they constitute a computational challenge and make Maxwell full-wave solvers unhandy for the accelerated design. Therefore, it is of utmost importance to have tools that permit the fast prediction of waveguide loss to enable the rapid optimization of waveguide trajectories. Previously developed methods relied on the assumption that only a single mode propagates in the waveguide. However, the propagation of higher-order modes is not just unavoidable due to the geometry of the waveguides, but also, sometimes, beneficial as it increases the number of channels to transmit information. In this contribution, we present an approximate method for the fast calculation of transmission that accommodates the presence of higher-order waveguide modes, and assess its liability by describing light propagation through selected devices.

Dual-Mode Conical Horn Antenna with 2-D Azimuthal Monopulse Pattern for Millimeter-Wave Applications.

In this paper, a novel concept of a three-dimensional full metal system including a Dual-Mode Converter (DMC) network integrated with a high-gain Conical Horn Antenna (CHA) is presented. This system is designed for 5G millimeter wave applications requiring monopulse operation at K-band (37.5-39 GHz). The DMC integrates two mode converters. They excite either the TE11cir or the TE01cir modes of the circular waveguide of the CHA. The input of the mode converters is the TE10rec mode of two independent WR-28 standard rectangular waveguide ports. By integrating the DMC with the CHA, the whole system, called a Dual-Mode Conical Horn Antenna (DM-CHA), is formed, radiating the sum (Σ) and difference (Δ) patterns associated to the monopulse operation. To adequately prevent the propagation of higher order modes and mode mutual coupling, this integration procedure is carefully designed and fabricated. To prove the performance of the design, the DMC network was fabricated using subtractive manufacturing by Computer Numerical Control (CNC) technology. The CHA was fabricated using additive manufacturing by Direct Metal Laser Sintering (DLMS) technology. Finally, the simulation and measurement results were exhaustively compared, including return loss, isolation, radiation pattern, and gain of the full DM-CHA structure. It is noteworthy that this system provided up to ±11° per beam in the angular of arrival detection to support the high data rate operation for 5G satellite communications in the millimeter-wave band.

Random misalignment and anisotropic deformation of the nested cladding elements in hollow-core anti-resonant fibers.

Hollow-core anti-resonant fibers (HC-ARFs) are en route to compete with and surpass the transmission performance of standard single-mode fibers (SSMFs). Recently, nested cladding elements emerged as a key enabler in reaching ultra-low transmission losses over a wide bandwidth. However, implementing nested geometry features poses a great challenge even in the current state-of-the-art fiber fabrication technology, often leading to structural imperfections, which ultimately worsen overall fiber performance. This article provides insights into the impact of fabrication-based perturbations of the cladding elements on the transmission performance and identifies areas of highest susceptibility. The impact of random outer and nested cladding tube misalignments as well as their anisotropic deformation on the propagation loss is analyzed based on observations of experimentally fabricated fibers. A dominance of the deformation effect over the misalignment effect is observed, with higher-order modes (HOMs) being affected one order of magnitude stronger than the fundamental mode (FM). The impact on propagation loss by structural perturbations is highly wavelength dependent, ranging from negligibly small values up to loss increases of 65% and 850% for FM and HOM propagation, respectively. The investigations are directly linked to fabrication metrics and therefore pave the way for assessing, predicting, and improving the transmission quality of fabricated hollow-core fibers.

All-Fiber Fourth-Order OAM Mode Generation Employing a Long Period Fiber Grating Written By Preset Twist

There are great challenges in the generation of higher-order modes for long period fiber grating (LPFG). The cross-coupling coefficient between the fundamental mode and the higher-order azimuthal modes is extremely small due to the difficulty of angular modulation with traditional methods. Therefore, how to generate higher-order orbital angular momentum (OAM) modes with a high efficiency and low insertion loss is an urgent problem to be solved. In this paper, an efficient all-fiber approach to generate fourth-order OAM mode by employing a preset twisted LPFG (PT-LPFG) in a few-mode fiber (FMF) is presented. The difficulty of generating fourth-order mode utilizing traditional single-side exposure carbon dioxide laser manufacturing method and the advantages of the PT-LPFG in enhancing the coupling coefficient are theoretically analyzed in detail. In experiment, the conversion from the fundamental mode to the fourth-order mode is achieved, and the fourth-order interference patterns demonstrated the successful generation of the OAM <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">±4</sub> modes. The conversion efficiency of the OAM modes is over 99% and the purity is measured to be more than 90%. To the best of our knowledge, this is the first time to generate the fourth-order OAM mode in an all-fiber system using single long-period fiber grating.

Efficient generation of Second-order beam Based on angular modulated Long-period fiber grating

Higher-order mode generation using long-period fiber grating (LPFG) has a lot of advantages such as simple structure and compatibility with optical fiber system. However, due to the large effective refractive index difference and very small cross-coupling coefficient between fundamental core mode and higher-order core modes in a few-mode fiber, it is difficult to achieve a high coupling efficiency and low insertion loss at the same time. In this paper, we demonstrated a simple model to illustrate the relationship between refractive index (RI) modulation and coupling coefficient in a CO2 laser LPFG fabrication system. Theoretical analysis shows that changing the RI modulation area from semicircle to sector with certain angle can significantly enhance the mode coupling between LP01 and LP21 modes. We also proposed an easy way who can achieve an angular modulation to fabricate LPFG which could generate second-order modes with a high efficiency as well as a low insertion loss. Experimental results are in good agreement with theoretical analysis. Finally, we get a LPFG mode converter who has a conversion efficiency of over 99% and an insertion loss of only 0.27 dB. Furthermore, the CO2 laser has almost no damage on the fiber structure. This fabrication method and analyzation model has a great guiding significance to regular the coupling coefficient and realize higher-order mode excitation in the future.

Large Mode Area Single Mode Fiber with a Twisted Polygon-Shaped Core

We present numerical studies on twisted polygon-shaped core fibers for achieving single mode operation with a large mode area propagation in this paper. These large mode area fibers can achieve single mode operation due to twisting-induced high losses to higher order modes and coupling higher order modes with leaky modes. A tail-like profile will appear in higher order modes when they couple with leaky modes, and the tail-like profile also indicates that these modes carry angular momentum. Numerical simulations show that twisted octagon-shaped core fiber with a core diameter of 80 μm can obtain first higher order mode losses larger than 19 dB/m while maintaining fundamental mode losses at about 0.15 dB/m. In the case of twisted heptagon-shaped core, an 80 μm diameter fiber design exists with more than 17 dB/m first higher order modes propagation losses and less than 0.14 dB/m fundamental mode losses.

Designing a Facility for Measuring Higher-Order Acoustic Modes in Uniform Heated Flows

An experimental facility was recently developed at the Georgia Tech Research Institute to measure the acoustic scattering coefficients of a test article under mean flow, high-temperature, high-amplitude, and higher-order mode propagation conditions. The experimental facility acts like a two-sided impedance tube, in which the measurement section is a circular duct with 16 acoustic drivers and 16 microphones mounted on each side of the test article. Such a facility can be configured for grazing and bias flow measurements of acoustic liners with sound traveling in and against the direction of the flow. The new facility is designed to maximize the accuracy of acoustic measurements of higher-order modes in heated flows. The influence of experimental uncertainties on the acoustic measurements is analyzed using a Monte Carlo method. Measurements of half-wave resonators in quiescent air and heated flows demonstrate that the experimental uncertainty decreases as the absorption coefficient increases. High-sensitivity microphones produce extremely low uncertainty, and even lower-sensitivity pressure transducers are sufficiently accurate for many applications. This paper is intended to serve as a reference for designing and analyzing an experimental facility to measure the acoustic scattering coefficients in heated-flow and higher-order mode propagation conditions.

Methodology for Measuring Higher-Order Acoustic Modes in Uniform Heated Flows

In this paper, the complete methodology for measuring the acoustic properties of a test article under uniform heated flow and higher-order mode propagation conditions is developed from the theory of higher-order mode propagation in circular ducts. The major steps in the methodology include modal decomposition based on multiple acoustic pressure measurements; determination of the acoustic scattering matrix using a multiple-source method; and calculation of the sound power reflection, transmission, dissipation, and absorption coefficients from the scattering matrix. These steps are based on contributions by a number of authors spanning several decades, but the purpose of this paper is to unify the various techniques and nomenclature used by these authors into a single, cohesive methodology. Thus, this paper is intended to provide a comprehensive reference on the methodology for measuring the acoustic scattering coefficients under the conditions of uniform heated flows and higher-order mode propagation.

Waveguide Bends for Suppressed Mode Coupling

Waveguide bends are key components of photonic integrated circuits, enabling complex layouts and high packing densities. Often waveguides at bends are wide and consequently multi-mode for lower loss propagation. Higher order modes, which are excited by the bend curvature, may degrade performance for devices downstream, such as extinction ratio in modulators. It is demonstrated that bend curvature functions inspired by window functions used in digital signal processing can greatly suppress the undesirable generation of higher order modes. The proposed curvature functions include sine to the power of $n$ , and Dolph-Chebyshev window. Coupled mode theory provides the insight that the suppression of higher order modes in a bend is related to the side-lobe suppression of the bend curvature Fourier transform. A particular application for the proposed bend geometries is mode division multiplexing, where inter-mode crosstalk may be significantly reduced.

Terahertz broadband spoof surface plasmon polaritons using high-order mode developed from ultra-compact split-ring grooves.

Terahertz (THz) broadband spoof surface plasmon polaritons (SSPPs) using new structure of ultra-compact split-ring grooves are proposed. The high-order mode propagation is highly concentrated around the proposed structure with lower radiation loss implying improved operating bandwidth. More importantly, a size reduction of 83.5% can be realized as compared to the traditional grounded SSPP structure with the same high-order asymptotic frequency. To further verify the proposed idea, a similar structure in microwave regime is designed and measured, where the excitation is easily achieved by directly connecting the microstrip line to the proposed SSPP waveguide. The gradient transition section, such as flaring ground, can be avoided, which decreases the waveguide's longitudinal and transversal lengths and simplifies the design procedure. The measured results of the microwave prototype illustrate that it has good lowpass filtering performance, in which the reflection coefficient is better than -10 dB up to 13 GHz, with the smallest and worst insertion losses of 0.5 dB and 4.5 dB, respectively. To the best of the authors' knowledge, this work presents THz high-order broadband SSPP propagation for the first time, having significant potential for plasmonic integrated circuits application at microwave/THz frequencies.

High-order mode direct oscillation of few-mode fiber laser for high-quality cylindrical vector beams

Generation of high-order modes with high quality is important for the application of cylindrical vector beams in fibers. We experimentally demonstrated high-order LP11 mode generation and amplification with a broad bandwidth in an all few-mode fiber laser. A wavelength-division-multiplexing (WDM) mode selective coupler (MSC) is proposed to achieve efficient mode conversion from LP01 mode to LP11 mode, but also combine high-order LP11 modes at the wavelengths of 980/1550 nm. To the best of our knowledge, this is the first report on the high-order mode oscillation in an all few-mode fiber laser. LP11 mode and cylindrical vector beams including radially and azimuthally polarized beams are obtained with high modal purity. The purity of the generated high-order modes are all in excess of 95%.

Femtosecond inscription and thermal testing of Bragg gratings in high concentration (40 mol%) germania-doped optical fibre

Using point-by-point infrared femtosecond laser inscription, a fibre Bragg grating with a resonance wavelength of 1550.28±0.02 nm, 25 dB extinction ratio, and 1.10±0.02 nm bandwidth (measured at first minima) was inscribed in a high concentration (40 mol%) germania-doped silica fibre. At this wavelength, the high concentration germania doped silica fibre had a normalized frequency of V ≃ 3.06 permitting higher order mode propagation. Subsequently, two additional Bragg resonances were recorded at 1534.40 ± 0.02 and 1535.78 ± 0.02 nm, corresponding to the coupling of the forward propagating fundamental mode and counter propagating HE21 and TM01/TE01, respectively. Thermal tests revealed the grating was stable up to 800 °C for 30 minutes. Analysis determined the grating had first and second order coefficients dλB/dT = 11.1 pm/°C and d2λB/dT2 = 8.37 × 10−3 pm2/°C2 respectively.

Geometric phase due to orbit–orbit interaction: rotating LP11 modes in a two-mode fiber

Accumulation of geometric phase due to non-coplanar propagation of higher-order modes in an optical fiber is experimentally demonstrated. Vertically-polarized LP11 fiber mode, excited in a horizontally-held, torsion-free, step-index, two-mode optical fiber, rotates due to asymmetry in the propagating k-vectors, arising due to off-centered beam location at the fiber input. Perceiving the process as due to rotation of the fiber about the off-axis launch position, the orbital Berry phase accumulation upon scanning the launch position in a closed-loop around the fiber axis manifests as rotational Doppler effect, a consequence of orbit–orbit interaction. The anticipated phase accumulation as a function of the input launch position, observed through interferometry is connected to the mode rotation angle, quantified using the autocorrelation method.

An Investigation of Resonant Modes in Rectangular Dielectric Resonator Antenna Using Transcendental Equation

This article presents an analysis of resonant modes in a rectangular dielectric resonator antenna and investigates the generation of higher order modes. Mathematical analysis of resonant frequency and transcendental equation using MATLAB has been performed. A prototype model for rectangular DRA has been developed. Resonant frequency of higher modes based on delta (ź) variation (where delta = 0.1---0.9) on Eigen vector has been investigated. The paper demonstrates that the design of a compact size dielectric resonator antenna (DRA) covering wide range of frequency with multiple modes is possible by selecting the value of p less than 1 (p < 1), where p is number of half-wave variations in the z-direction of TEmnp modes. An orthogonal feed has been used to cancel out the undesired modes. Simulations and experimental measurements were carried out to verify the proposed idea. Theoretical, simulated, and experimental results of resonant frequency and gain of the antenna have been compared and are found to be in agreement with each other. The results presented should have useful potential applications in wireless communication systems and satellite applications.

Influence of vocal tract geometry simplifications on the numerical simulation of vowel sounds.

For many years, the vocal tract shape has been approximated by one-dimensional (1D) area functions to study the production of voice. More recently, 3D approaches allow one to deal with the complex 3D vocal tract, although area-based 3D geometries of circular cross-section are still in use. However, little is known about the influence of performing such a simplification, and some alternatives may exist between these two extreme options. To this aim, several vocal tract geometry simplifications for vowels [ɑ], [i], and [u] are investigated in this work. Six cases are considered, consisting of realistic, elliptical, and circular cross-sections interpolated through a bent or straight midline. For frequencies below 4-5 kHz, the influence of bending and cross-sectional shape has been found weak, while above these values simplified bent vocal tracts with realistic cross-sections are necessary to correctly emulate higher-order mode propagation. To perform this study, the finite element method (FEM) has been used. FEM results have also been compared to a 3D multimodal method and to a classical 1D frequency domain model.

Acoustic properties of multiple cavity resonance liner for absorbing higher-order duct modes.

This paper describes analytical and experimental studies conducted to investigate the acoustic properties of axially non-uniform multiple cavity resonance liner for absorbing higher-order duct modes. A three-dimensional analytical model is proposed based upon transfer element method. The model is assessed by making a comparison with results of a liner performance experiment concerning higher-order modes propagation, and the agreement is good. According to the present results, it is found that the performance of multiple cavity resonance liner is related to the incident sound waves. Moreover, an analysis of the corresponding response of liner perforated panel-cavity system is performed, in which the features of resonance frequency and dissipation of the system under grazing or oblique incidence condition are revealed. The conclusions can be extended to typical non-locally reacting liners with single large back-cavity, and it would be beneficial for future non-locally reacting liner design to some extent.

Reduction of Intensity Noise in Hollow Core Optical Fiber Using Angle-Cleaved Splices

When spliced to solid core optical fiber, a hollow core optical fiber (HCF) exhibits multipath interference. Reflections at the air/glass interfaces add coherently in transmission, producing intensity fluctuations. This can lead to excess intensity noise at the output of a fiber segment. In this letter, angle-cleaved splices were performed on HCFs to reduce this noise without significant alteration of the fiber’s optical properties. Cleave angles ranging from 7° to 12° produced splices with return losses less than −50 dB and excess insertion losses typically around 1 or 2 dB. A 37-dB reduction in intensity noise over a flat-cleaved splice was achieved with negligible increase in higher order mode generation or polarization-dependent loss.

Coherent light transmission properties of commercial photonic crystal hollow core optical fiber

Photonic crystal hollow core fiber (PC-HCF) has enabled many exciting new applications in nonlinear optics and spectroscopy. However, to date there has been less impact in coherent applications where preservation of optical phase over long fiber lengths is crucial. This paper presents characteristics of three commercially available PC-HCFs relevant to coherent applications including higher-order mode analysis, birefringence and polarization-dependent loss, and their impact on coherent light transmission in PC-HCF. Multipath interference due to higher-order mode propagation and Fresnel reflection is shown to generate excess intensity noise in transmission, which can be suppressed by up to 20 dB through high frequency phase modulation of the source laser. To demonstrate the potential of PC-HCF in high performance sensing, a Mach-Zehnder interferometer (MZI) incorporating 10 m of PC-HCF in each arm is characterized and demonstrates a phase resolution (59×10(-9) rad/Hz(1/2) at 30 kHz) close to the shot noise limit, which is better than can be achieved in a MZI made with the same length of single mode solid core fiber because of the limit set by fundamental thermodynamic noise (74×10(-9) rad/Hz(1/2) at 30 kHz).

Rayleigh scattering in an optical nanofiber as a probe of higher-order mode propagation

Optical nanofibers provide a rich platform for exploring atomic and optical phenomena even when they support only a single spatial mode. Nanofibers supporting higher-order modes provide additional degrees of freedom to enable complex evanescent field profiles for interaction with the surrounding medium, but local control of these profiles requires nondestructive evaluation of the propagating fields. Here, we use Rayleigh scattering for rapid measurement of the propagation of light in few-mode optical nanofibers. Imaging the Rayleigh scattered light provides direct visualization of the spatial evolution of propagating fields throughout the entire fiber, including the transition from core-cladding guidance to cladding-air guidance. We resolve the interference between higher-order modes to determine local beat lengths and modal content along the fiber, and show that the modal superposition in the waist can be systematically controlled by adjusting the input superposition. With this diagnostic we can measure variations in the radius of the fiber waist to below 3 nm in situ using purely optical means. This nondestructive technique also provides useful insight into light propagation in optical nanofibers.