Transverse Mode Research Articles

Response of n-Heptane and Dodecane Swirl-Stabilized Spray Flames to Transverse Forcing Characterized By Flame Describing Functions Based on Downstream Acoustic Pressure or Velocity Signals

Abstract Predicting thermoacoustic instabilities in annular combustors requires knowledge of the impact of acoustic oscillations on heat release rate oscillations. Flame Describing Functions (FDF) measured at the burner exit using acoustic forcing are key elements of thermoacoustic instability analyses. FDFs based on acoustic pressure measurements, FP' or on axial velocity measurements FU', are compared here. This study is done on the TACC-Spray bench, an original linear array of spray flames stabilized by a strong swirling flow, representing an unfolded sector of a self-unstable annular combustor. Acoustic forcing of a standing transverse chamber mode is applied downstream of the injectors. Experiments are conducted with liquid n-heptane or dodecane, with the flames placed at a pressure or an intensity antinode of the transverse mode. FP' does not depend on the measurement location for acoustically-compact flames provided that this location remains in the flame vicinity. FU' can lead to significant discrepancies, as swirling flows present strong velocity gradients, which can be minimized by carefully choosing the measurement location. The injector admittance linking the two FDFs is shown to be quasi-independent of the forcing amplitude here. Consequently, both FDFs show a similar dependence on the forcing amplitude. FP' indicates constructive combustion-acoustics interference whatever the flame location in the acoustic field and the fuel, consistent with self-sustained instabilities observed in the annular combustor. An analysis using the Rayleigh criterion corroborates the results derived from the FDFs. So, FP' appears as a powerful and practical tool for characterizing combustion dynamics.

High-efficient, polarization-insensitive, wide-angle, compact metamaterial energy harvester for S-band and C-band applications

This article proposes a polarization-insensitive compact Metamaterial (MM) energy harvester that can be used seamlessly in the S-band and C-band frequencies. It is important to focus on the limitations of many current designs of harvesters, which need to be overcome. The existing devices are large and often operate in a single frequency band, while their Energy Harvesting (EH) efficiency is low. The proposed harvester solves these problems using smart technology, using a rectangle strip with two gaps, each containing 50 Ω resistors for efficient energy collecting. Also, four hexagonal ring resonators are embedded into the cross-dumbbell configuration, connecting them with strip lines. Smaller rectangular rings surround these hexagonal rings, each with gaps labelled g1–g4. Despite its sophisticated design, the size of this harvester is (10 × 10) mm2 only. This harvester operates at frequencies of 3.5 GHz and 5.5 GHz, demonstrating remarkable absorption responses across varying polarizations and incident angles in both transverse electric (TE) and transverse magnetic (TM) modes. The simulation results indicated impressive energy harvesting efficiencies of 97% at 3.5 GHz and 98% at 5.5 GHz. In addition, experiments in an anechoic chamber with a 3 × 3 array (30 × 30) mm2 were used to confirm the efficiencies empirically. The simulated and measured results showed a strong correlation, confirming the reliability of the proposed design. The proposed MM harvester is distinguished by its high efficiency, polarization-insensitive behaviour, and compactness, making it very promising for many applications in EH.

Ultrabroadband Photodetection with MoS2 and Gold Nanorod Using Gap-Mode Plasmon Effect.

Plasmon resonance using metal nanostructures enables the realization of high-performance optoelectronic devices via field enhancements in the vicinity of the metal nanostructure. This study proposes an ultrabroadband MoS2 photodetector based on the gap-mode plasmon of gold nanorods. The use of MoS2 as a gap spacer for the gap-mode plasmon effect and as a channel material for the photodetector is demonstrated. The proposed photodetector demonstrates superior performance, a high photoresponsivity of 2.8 A/W at a near-infrared wavelength of 1100 nm, and a fast response time of 17 μs. In addition to the gap-mode plasmon effect of the gold nanorod enhancing the photoresponsivity from 1.13 to 8.7 A/W, the lateral surface plasmon resonance of the gold nanorods enhances the absorption of the longitudinal mode of the gold nanorods (near-infrared to infrared range). Further, the gap-mode plasmon effect of the gold nanorods enhances the absorption of the transverse mode of the gold nanorods (visible range), thus realizing ultrabroadband photodetection. Therefore, the proposed device design strategy contributes significantly to overcoming the trade-off between photoresponsivity and response time.

Detection of damage in laminated composite structures using transverse shear mode

Detection of damage in laminated composite structures using transverse shear mode

Random Raman Lasing in Diode-Pumped Multi-Mode Graded-Index Fiber with Femtosecond Laser-Inscribed Random Refractive Index Structures of Various Shapes

Diode-pumped multi-mode graded-index (GRIN) fiber Raman lasers provide prominent brightness enhancement both in linear and half-open cavities with random distributed feedback via natural Rayleigh backscattering. Femtosecond laser-inscribed random refractive index structures allow for the sufficient reduction in the Raman threshold by means of Rayleigh backscattering signal enhancement by +50 + 66 dB relative to the intrinsic fiber level. At the same time, they offer an opportunity to generate Stokes beams with a shape close to fundamental transverse mode (LP01), as well as to select higher-order modes such as LP11 with a near-1D longitudinal random structure shifted off the fiber axis. Further development of the inscription technology includes the fabrication of 3D ring-shaped random structures using a spatial light modulator (SLM) in a 100/140 μm GRIN multi-mode fiber. This allows for the generation of a multi-mode diode-pumped GRIN fiber random Raman laser at 976 nm with a ring-shaped output beam at a relatively low pumping threshold (~160 W), demonstrated for the first time to our knowledge.

The symmetry change of the transverse modes in ring-LD end-pumped Nd:YVO[formula omitted] laser with an étalon

A straightforward method for converting the symmetry of a diode end-pumped laser with an annular pump beam has been proposed. By introducing an étalon into the cavity, we observed a change in the spatial symmetry of the transverse mode profile from rectangular to cylindrical both in degenerate and non-degenerate ranges. A simulation of the transmittance distribution of the beam through the étalon after filtering is given. Our experimental results indicate that this method of altering the symmetry maintains relative stability during the tilting of the étalon.

Generation of High-Quality Cylindrical Vector Beams from All-Few-Mode Fiber Laser

Transverse mode control of laser intracavity oscillation is crucial for generating high-purity cylindrical vector beams (CVBs). We utilized the mode conversion and mode selection properties of two-mode long-period fiber gratings (TM-LPFGs) and two-mode fiber Bragg gratings (TM-FBGs) to achieve intracavity hybrid-mode oscillations of LP01 and LP11 from an all-few-mode fiber laser. A mode-locked pulse output with a repetition rate of 12.46 MHz and a signal-to-noise ratio of 53 dB was achieved with a semiconductor saturable absorber mirror (SESAM) for mode-locking, at a wavelength of 1550.32 nm. The 30 dB spectrum bandwidth of the mode-locked pulse was 0.13 nm. Furthermore, a high-purity CVB containing radially polarized and azimuthally polarized LP11 modes was generated. The purity of the obtained CVB was greater than 99%. The high-purity CVB pulses have great potential for applications in optical tweezers, high-speed mode-division multiplexing communication, and more.

A spurious-free SAW filter employing a novel transducer structure on bulk LiNbO3 substrates

In order to cope with the ubiquitous wireless connections and the rapid transmission of data in communication systems, surface acoustic wave filters are required to have high performance. With a large electromechanical coupling coefficient (K2), conventional bulk 64°YX-LiNbO3 substrates are attractive for low-cost mass production of surface acoustic wave (SAW) resonators. However, the influence of the transverse modes restricts their practical application. In this work, a novel and simple tilted transducer is fabricated, and successfully achieved spurious-free passband SAW filters based on 64°YX-LN piezoelectric substrates. In this work, the transverse modes suppression method is theoretically studied and designed, and the effectiveness of the energy concentrated structure is verified. The optimal tilt IDT angle for eliminating the influence of transverse modes in Al/64 °YX-LN structure is about 6°. Subsequently, based on the above research, three types of tilted resonators were designed and fabricated, which not only verified the reliability of the aforementioned results but also enhanced the performance of the devices, compensating for the deficiency caused by the reduction of the Q factor due to the tilted structure. Finally, based on the double busbar tilted structure, a SAW filter was designed, which has a large 3 dB fractional bandwidth (FBW) and a spurious-free passband. The SAW filter designed in this work has low cost and excellent performance, and can be mass produced. In the field of low-cost RF devices, this work contributes to maintaining the competitive advantage of SAW technology.

Ultracompact polarization beam splitter based on hybrid plasmonic waveguide

A compact polarization beam splitter (PBS) based on a three-waveguide plasmonic directional coupler was proposed and modeled. The directional coupler comprised two common silicon-on-insulator (SOI) ridge waveguides, with a metal-capped plasmonic SOI waveguide sandwiched between them. The geometrical structure of the three-waveguide directional coupled waveguide was optimized, realizing the phase matching of transverse magnetic (TM) modes but not of transverse electric (TE) modes. The structure and performance of the PBS was optimized using a finite-difference time-domain method. Simulation results showed that the birefringence comparison of TE and TM modes in a hybrid plasmonic waveguide was enhanced using a dielectric loading waveguide to obtain an ultra-compact structure; the final optimized coupling length of the device was 6.1 µm. At the center wavelength of 1.55 µm, the polarization extinction ratio of the TE and TM modes was 34.57 dB and 23.24 dB, and the insertion loss was 0.01 dB and 0.13 dB, respectively.

Deep-learning-assisted optical communication with discretized state space of structured light

Abstract The rich structure of transverse spatial modes of structured light has facilitated their extensive applications in quantum information and optical communication. The Laguerre-Gaussian (LG) modes, which carry a well-defined orbital angular momentum (OAM), consist of a complete orthogonal basis describing the transverse spatial modes of light. The application of OAM in free-space optical communication is restricted due to the experimentally limited OAM numbers and the complex OAM recognition methods. Here, we present a novel method that uses the advanced deep learning technique for LG modes recognition. By discretizing the spatial modes of structured light, we turn the OAM state regression into classification. A proof-of-principle experiment is also performed, showing that our method effectively categorizes OAM states with small training samples and the accuracy exceeds 99% from 3-dimensional to 15-dimensional space. By assigning each category a classical information, we further apply our approach to an image transmission task, achieving a transmission accuracy of 99.58%, which demonstrates the ability to encode large data with low OAM number. This work opens up a new avenue for achieving high-capacity optical communication with low OAM number based on structured light.

Ultra-compact silicon nitride dual-polarization mode converter for visible light demonstrated on a thinned 320 nm silicon wafer

A dual-polarization higher-order mode converter is proposed for visible light, which operates within the (400–700) nm range, to enhance data capacity in on-chip visible light communication systems. The proposed structure is optimized at wavelengths of 410 nm, 560 nm, and 660 nm to cover the entire visible spectrum. These three optimized designs, in particular, have an intrinsic property wherein they can be optimized to simultaneously convert both polarized fundamental (TE0) and fundamental (TM0) modes to the second higher-order (TE2) and (TM2) modes, providing significant advantages for hybrid multiplexing systems like PDM-MDM and PDM-WDM schemes. The mode converters are constructed on a silicon nitride waveguide. This waveguide is etched and filled with silicon dioxide material to create two dielectric substrips, followed by an additional rectangle shape etched using the same process into the propagating waveguide. This alteration enhances insertion loss and diminishes crosstalk to the fundamental mode. The devices achieve a broad operating bandwidth of approximately 100 nm while maintaining a compact footprint of only 1µm×1.754µm for the entire device at the center wavelength of 560 nm. Upon optimizing the suggested structure, a TE0-to-TE2 mode converter with a modal conversion efficiency of 94% is designed at the wavelength of 410 nm. The insertion loss is 0.6025 dB, and the crosstalk with the transverse electric fundamental mode TE0 is 35 dB. The reported devices feature a straightforward structure with low insertion loss and minimal crosstalk.

Tailoring the Microstructure and Porosity of Porous Piezoelectric Ceramics for Ultrasonic Transducer Application.

Porous piezoelectric ceramics and composites are advantageous for ultrasonic transducers due to their capability to decouple longitudinal and transverse modes, their improved voltage sensitivity, and their enhanced acoustic matching. However, the design and fabrication of porous piezoelectric ultrasonic transducers with excellent electromechanical properties are still challenging. In this work, porous lead zirconate titanate (PZT) ceramics with an aligned pore structure were prepared using the freeze-cast technique, and the effect of porous structure and porosity on the electromechanical parameters was investigated. The introduction of an aligned pore structure is beneficial to enhance the electromechanical properties and reduce the acoustic impedance. A high d33 (∼530 pC/N), a higher kt (∼0.676), and a lower acoustic impedance (∼10.4 MRalys) were achieved in the porous PZT ceramic with the porosity of 44 vol %. The effect of porosity and pore structure on the decoupling degree of vibration modes and ferroelectric polarization was considered to correct the homogeneous medium model, which can quantify the relationship between the kt and porosity of the porous structure. A demonstration of a piezoelectric ultrasonic transducer based on freeze-cast porous PZT ceramics was presented, which exhibits a -6 dB bandwidth of 52% and a theoretical axial resolution of 520 μm. This work therefore provides a potential alternative of piezoelectric ultrasonic transducers for nondestructive testing and imaging applications.

Numerical modeling and analysis of multistage gear transmission system in wind turbine gearbox under load uncertainty

A multistage gear transmission system (MGTS) is widely used in wind turbine gearboxes. Its dynamic responses are closely related to stochastic aerodynamic torque that has a wide interval range between cut-in and cut-out wind speeds. However, most of the dynamic models of MGTS lack attention to dynamic responses caused by uncertainties of multiple parameters under varied input torque. In this study, a dynamic modeling method in conjunction with interval uncertain analysis of MGTS caused by multiple load-related parameters due to varied input torques is proposed. A comprehensive dynamic model of MGTS is developed considering time-varying gear mesh transmission error, load-related gear mesh stiffness and bearing stiffness as well as flexible shafts. The reasonability of simulation is validated, and then the effects of multiple load-related parameters on uncertain dynamic responses are discussed in-depth. Results show that both gear mesh stiffness and bearing stiffness are enlarged with increasing input torque to cause uncertain dynamic responses. These uncertainties would propagate along the flexible shaft and be suppressed by system rigidity. The dominant transverse vibration modes of shafts are sensitive to bearing stiffness, thus, uncertain bearing stiffness has remarkable effects on dynamic responses than gear mesh stiffness.

Theoretical Study on Transverse Mode Instability in Raman Fiber Amplifiers Considering Mode Excitation.

Raman fiber lasers (RFLs), which are based on the stimulated Raman scattering effect, generate laser beams and offer distinct advantages such as flexibility in wavelength, low quantum defects, and absence from photo-darkening. However, as the power of the RFLs increases, heat generation emerges as a critical constraint on further power scaling. This escalating thermal load might result in transverse mode instability (TMI), thereby posing a significant challenge to the development of RFLs. In this work, a static model of the TMI effect in a high-power Raman fiber amplifier based on stimulated thermal Rayleigh scattering is established considering higher-order mode excitation. The variations of TMI threshold power with different seed power levels, fundamental mode purities, higher-order mode losses, and fiber lengths are investigated, while a TMI threshold formula with fundamental mode pumping is derived. This work will enrich the theoretical model of TMI and extend its application scope in TMI mitigation strategies, providing guidance for understanding and suppressing TMI in the RFLs.

Extending the spectral operation of multimode and polarization-independent power splitters through subwavelength nanotechnology

Power splitters play a crucial role in virtually all photonic circuits, enabling precise control of on-chip signal distribution. However, state-of-the-art solutions typically present trade-offs in terms of loss, bandwidth, and fabrication robustness, especially when targeting multimode operation. Here, we present a novel multimode 3-dB power splitter based on a symmetric Y-junction assisted by two regions of subwavelength grating metamaterials with different geometries. The proposed device demonstrates high-performance for multimode and dual-polarization operation with relaxed fabrication tolerances by leveraging the additional degrees of freedom offered by two distinct geometries of subwavelength metamaterials to control mode evolution. Our design achieves, to the best of our knowledge, the widest operational bandwidth reported to date for a nanophotonic multimode silicon power splitter. Simulations for a standard 220-nm-thick silicon-on-insulator platform predict minimal excess loss (< 0.2 dB) for the fundamental and the first-order transverse-electric modes over an ultra-broad 700 nm bandwidth (1300 – 2000 nm). For the fundamental transverse–magnetic mode, losses are less than 0.3 dB in the 1300 – 1800 nm range. Experimental measurements validate these predictions in the 1430 – 1630 nm wavelength range, demonstrating losses < 0.4 dB for all three modes, even in the presence of fabrication deviations of up to ± 10 nm. We believe that this device is suitable for the implementation of advanced photonic applications requiring high–-performance distribution of optical signals, such as programmable photonics, multi-target spectroscopy and quantum key distribution.

Generation of sub-100 fs vortices from a Kerr-lens mode-locked Hermite-Gaussian Yb:CALGO oscillator.

Mode-locked oscillators with high-order transverse modes are excellent platforms for generating femtosecond optical vortices with high average power and good propagation stability. These have important applications in diverse fields such as optical communication, strong-field physics, and laser processing. So far, generating vortex pulses with ultrashort pulse duration remains a challenge. In this Letter, we report a Kerr-lens mode-locked Yb:CALGO laser oscillator delivering Hermite-Gaussian (HG) pulses with a pulse duration of 86 fs using a non-collinear pumping technique. 91 fs optical vortex pulses were generated by using a cylindrical-lens mode converter. To the best of our knowledge, this is the shortest pulse duration ever obtained from a diode pumped solid-state mode-locked oscillator with a pure high-order Hermite-Gaussian mode. The phase structures of the generated femtosecond vortices are characterized.

Halide Perovskite Thin Film Lasing Mode Control.

Metal halide perovskite semiconductors have emerged as highly efficient amplifying materials; to ensure practical applicability, it is important to have precise control over the spectral and polarization characteristics of lasing emission. In this study, we present effective strategies for manipulating single- and multimode lasing from surface-emitting and optically pumped perovskite distributed feedback lasers. We show that cladding structures can be made to modify the optical properties of guided transverse electric and magnetic modes within the gain medium. This leads to spectral tuning of multipeak lasing emission and the generation of linear polarization modes. Our results are supported with a comprehensive multilayer slab waveguide model and confirmed with two different perovskite materials: methylammonium lead iodide and cesium lead bromide.

Turbulence effects mitigation for twin-photons generated by high order transverse modes

Turbulence presents challenges for free-space light-based protocols. In this work, we investigated the effect of turbulence on the correlations of twin photons prepared in Spontaneous Parametric Down Conversion (SPDC) by a pump beam with different transverse modes. We demonstrate that the strategy of coordinate inversion for mitigating turbulence-induced distortions on transverse coincidence profiles is effective for twin photons generated by HG/LG modes. The conservation of orbital angular momentum in SPDC enables us to encode information in the transverse degrees of freedom of the down-converted photon pairs. This information can have its correlation protected by the coordinate inversion technique against weak turbulence effects in free-space links.

Silicon nitride directional coupler-based polarization beam splitter utilizing shallow ridge waveguides for improved fabrication tolerance

This study presents the design and fabrication of a silicon nitride (Si3N4) asymmetrical directional coupler (DC)-based polarization beam splitter (PBS) utilizing a shallow ridge waveguide structure. The use of Si3N4 with a moderate refractive index contrast and the design of a shallow ridge waveguide enables the DC structure to have a wider coupling gap of approximately 600 nm, which can be readily achieved with standard I-line lithography. The simulation results indicate that the polarization splitting is highly efficient with minimal insertion loss. This is corroborated by the experimental measurements, which show consistent broadband performance. The fabricated polarization beam splitter (PBS) exhibits a high polarization extinction ratio (PER) of over 20 dB for both transverse electric (TE) and transverse magnetic (TM) modes across a broad bandwidth of 120 nm (1500-1620 nm). This work demonstrates the potential of shallow ridge Si3N4 waveguides to enhance the fabrication tolerance and performance of integrated photonic devices.

Finite-element Adaptive Meshing Statistics of Liquid Crystal Coaxial Phase Shifters for mmW Electronics and THz Photonics Beyond Display: A Comparative Study

Sub-mmW and THz frequencies offer ultra-high-speed communications for next-generation 5G/6G networks, wherein research advancing the understanding will benefit the electronics and photonics community. Specifically, meshing resolutions across varying frequency ranges and material properties are central to the solution accuracy, reliability, and cost (memory and time) of computational mmW and THz simulations, particularly for the emerging reconfigurable coaxial phase shifters employing nematic liquid crystals (LCs) as tunable media. In the present study, a comparative meshing statistics analysis is conducted for two devices designed for 60 GHz and 0.3 THz, respectively. Each design features two distinct tuning states (LC permittivity of 2.754 and 3.3, respectively), all pertinent to the coaxial TEM (Transverse Electromagnetic) mode. By quantifying the broadband meshing and solution statistics of diverse frequencies and dielectric tuning states for the first time, we establish memory-conserving computational metrology involving reconfigurable coaxial devices operationalized with LC-filled tunable dielectrics tailored for mmW electronics and THz photonics. Full Text: PDF References M. Makowski, I. Ducin, K. Kakarenko, J. Suszek, A. Kowalczyk, "Performance of the 4k phase-only spatial light modulator in image projection by computer-generated holography", Phot. Lett. Poland 8, 1 (2016). CrossRef J.F. Li, H.R. Li, "Liquid Crystal-Filled 60 GHz Coaxially Structured Phase Shifter Design and Simulation with Enhanced Figure of Merit by Novel Permittivity-Dependent Impedance Matching", Electronics 13, 3 (2024). CrossRef J.F. Li, H.R. Li, "Modeling 0.3 THz Coaxial Single-Mode Phase Shifter Designs in Liquid Crystals with Constitutive Loss Quantifications", Crystals 14, 4 (2024). CrossRef J.F. Li, D.P. Chu, "Liquid Crystal-Based Enclosed Coplanar Waveguide Phase Shifter for 54–66 GHz Applications", Crystals 9, 12 (2019). CrossRef Z. Cendes, "The development of HFSS", USNC-URSI Radio Science Meeting (Fajardo, IEEE 2016). CrossRef T. Vaupel, "Accuracy And Modeling Improvements For An Integral Equation Framework Applied To Thin Layer Microstrip And Substrate Integrated Waveguide (Siw) Structures", EuCAP (Copenhagen, IEEE 2020). CrossRef L.J. Jiang, W.C. Chew, "Low-frequency fast inhomogeneous plane-wave algorithm (LF-FIPWA)", Microwave Opt. Tech. Lett. 40, 2 (2004). CrossRef J. Zhu, D. Jiao, "Eliminating the Low-Frequency Breakdown Problem in 3-D Full-Wave Finite-Element-Based Analysis of Integrated Circuits", IEEE Trans. Microw. Theory Tech. 58, 10 (2010). CrossRef