Excitation Of Higher-order Modes Research Articles

Enhancement of gain and bandwidth of cylindrical dielectric resonator antenna excited by cavity-backed slot

Dielectric resonator antennas have been demonstrated to be an efficient radiator, especially with the possibility of fabricating it at low cost based on 3-D printing technologies. However, they have two main limitations which are: their relatively small gain (about 5dBi) and narrow bandwidth. To enhance the gain of DRA, higher-order mode excitation can be utilized; however, typically this has a limit of a few dB increments besides its negative effects on the bandwidth. This study employs a cavity-backed slot to provide an appropriate feeding method for a cylindrical dielectric resonator antenna (CDRA). This design effectively facilitates the excitation of higher-order modes, resulting in a significant gain of up to 15 dBi, which is essential for many wireless systems that are unable to support larger aperture sizes through the standard use of arrays. Additionally, two modes are excited to keep a relatively broadband of about 15 %. The proposed design consists of a cylindrical dielectric placed on top of a cavity-backed slot that is excited by a coaxial probe soldered to the top patch of the cavity. With this feeding, a good matching is achieved for different dielectric resonator heights, hence, the gain can be adjusted easily without the need of reoptimizing the antenna or the feed structures. The design process is explained in addition to performing a sensitivity analysis of fabrication tolerances which confirms that the 3-D printer provides enough accuracy with a negligible effect on the antenna’s performance. Then a prototype with a medium length is fabricated and measured to provide high realized gain and impedance bandwidth of 11 dBi and 14.5 %, respectively. The measured data demonstrate the good performance of the antenna with great matching compared to the simulated ones. Furthermore, the antenna presents a good performance in terms of sidelobes and polarization. This proposed design outperforms other DRA designs based on higher-order modes.

Spatially Controlled Phase Modulation - Selective Higher-Order Mode Excitation in 3D Nanoprinted on-Chip Hollow-Core Waveguides

Spatially Controlled Phase Modulation - Selective Higher-Order Mode Excitation in 3D Nanoprinted on-Chip Hollow-Core Waveguides

Selective excitation of high-order modes in two-dimensional cavity resonator integrated grating filters.

The selective spatial mode excitation of a bi-dimensional grating-coupled micro-cavity called a cavity resonator integrated grating filter (CRIGF) is reported using an incident beam shaped to reproduce the theoretical emission profiles of the device in one and subsequently two dimensions. In both cases, the selective excitation of modes up to order 10 (per direction) is confirmed by responses exhibiting one (respectively two) spectrally narrowband resonance(s) with a good extinction of the other modes, the latter being shown to depend on the parity and order(s) of the involved modes. These results pave the way toward the demonstration of multi-wavelength spatially selective reflectors or fiber-to-waveguide couplers. Also, subject to an appropriate choice of the materials constituting the CRIGF, this work can be extended to obtain mode-selectable laser emission or nonlinear frequency conversion.

Design of ultrahigh-Q silicon microring resonators based on free-form curves.

A design method for ultrahigh-Q microring resonators (MRRs) based on Bezier free-form curves was proposed and demonstrated. An MRR consisting of a specially designed 180° waveguide bend, a directional coupler, and two low-loss multi-mode strip waveguides was designed. The free-form curves were used to increase the degree of freedom in the design, shaping the waveguide bend with a gradient width and curvature. This design effectively reduced the propagation loss caused by the roughness of waveguide sidewalls and the mode mismatch loss caused by the excitation of high order modes. The small effective radius of only 20µm enabled the MRR to have a large free spectral range (FSR) and a compact and flexible structure. The MRR was manufactured using a standard process provided by foundry and measured to have an ultrahigh loaded Q factor of 1.86 × 106 and a FSR of about 1 nm.

Multipath artifacts enable angular contrast in multimodal endoscopic optical coherence tomography.

Multipath artifacts are inherent to double-clad fiber based optical coherence tomography (OCT), appearing as ghost images blurred in the A-line direction. They result from the excitation of higher-order inner-cladding modes in the OCT sample arm which cross-couple into the fundamental mode at discontinuities and thus are detected in single-mode fiber-based interferometers. Historically, multipath artifacts have been regarded as a drawback in single fiber endoscopic multimodal OCT systems as they degrade OCT quality. In this work, we reveal that multipath artifacts can be projected into high-quality two-dimensional en face images which encode high angle backscattering features. Using a combination of experiment and simulation, we characterize the coupling of Mie-range scatterers into the fundamental image (LP01 mode) and higher-order image (multipath artifact). This is validated experimentally through imaging of microspheres with an endoscopic multimodal OCT system. The angular dependence of the fundamental image and higher order image generated by the multipath artifact lays the basis for multipath contrast, a ratiometric measurement of differential coupling which provides information regarding the angular diversity of a sample. Multipath contrast images can be generated from OCT data where multipath artifacts are present, meaning that a wealth of clinical data can be retrospectively examined.

Highly sensitive temperature and refractive index sensor based on no-core fiber cascaded with a balloon-shaped bent single-mode fiber

A highly sensitive temperature and refractive index (RI) sensor based on no-core fiber (NCF) cascaded with a balloon-shaped bent single-mode fiber (BSBSF) is proposed and demonstrated. The NCF can excite higher-order modes which will be concentrated and transmitted into the BSBSF due to the characteristic of self-imaging effect. The BSBSF has an excellent temperature performance due to the high thermo-optical coefficient and thermal expansion coefficient of the polymer coating. The NCF and BSBSF are both conducive to the excitation of higher-order modes, which induces the sensitivity of the sensor with an efficiency improvement. The experimental results show that the maximum temperature sensitivity is −3.19nm/∘C in the range of 22°C–83°C, which is the highest temperature sensitivity in the cascaded BSBSF structure to our best knowledge. In addition, the maximum RI sensitivity is 232.16 nm/RIU when the RI changes from 1.3234 to 1.3512. Compared with other cascaded BSBSF structures, this sensor has a higher temperature sensitivity and can be applicated in the prospects of food, biology, and environmental monitoring.

Multipole engineering by displacement resonance: a new degree of freedom of Mie resonance

The canonical studies on Mie scattering unravel strong electric/magnetic optical responses in nanostructures, laying foundation for emerging meta-photonic applications. Conventionally, the morphology-sensitive resonances hinge on the normalized frequency, i.e. particle size over wavelength, but non-paraxial incidence symmetry is overlooked. Here, through confocal reflection microscopy with a tight focus scanning over silicon nanostructures, the scattering point spread functions unveil distinctive spatial patterns featuring that linear scattering efficiency is maximal when the focus is misaligned. The underlying physical mechanism is the excitation of higher-order multipolar modes, not accessible by plane wave irradiation, via displacement resonance, which showcases a significant reduction of nonlinear response threshold, sign flip in all-optical switching, and spatial resolution enhancement. Our result fundamentally extends the century-old light scattering theory, and suggests new dimensions to tailor Mie resonances.

Tuning nonlocal effects in grooved cylindrical plasmons by hydrodynamic model: A competing mechanism

Using the linearized hydrodynamic equations, we explore the optical characteristics of grooved cylinders of different groove numbers and widths, which have a strong impact on the corresponding resonance peaks. The number of grooves is an important new variable parameter to tune the energy of the dipolar plasmon without appreciable broadening of the plasmonic peak. As the groove number and width increase, the excitation and interesting anomalous redshifts of plasmonic higher-order modes are found, which can be unraveled by a competing mechanism between nonlocal effects and surface electron scattering. These findings are of importance for further apprehending the optical belongings of nanoscale complex structures and play a significant guiding role in the design of tunable plasmonic devices.

Simultaneous Efficient Excitation of Multiple Higher-Order OAM Modes Using a Single Long-Period Fiber Grating

In this paper, a method for the simultaneous generation of multiple higher-order orbital angular momentum (OAM) modes in a few-mode fiber (FMF) using a single long-period fiber grating (LPFG) is proposed and demonstrated. The high diffraction order of the grating is used to achieve the resonant coupling of multiple modes, and the coupling coefficients of different order modes are balanced by a preset-twist method. This solves the two coupling problems of over-coupling for lower-order modes and difficult excitation of higher-order modes faced by multi-order mode generation, and overcomes the challenge of balanced excitation of multiple modes with high coupling efficiency. In the experiment, the simultaneous generation and flexible combination of first-order, second-order and third-order OAM modes are successfully achieved in a six-mode fiber, in which the mode order, resonant wavelength and wavelength interval can be flexibly controlled. By fabricating with grating periods of 2300 μm, 782 μm and 990 μm, respectively, high conversion efficiency of first-order and second-order modes, first-order and third-order modes, second-order and third-order modes can be generated simultaneously. In addition, for the first time, the first-order, second-order and third-order modes are generated simultaneously at a grating period of 1630 μm with conversion efficiencies of 98.61%, 99.11% and 98.89%, respectively. The mode profiles and interference patterns of different order modes show that multiple higher-order OAM modes with insertion loss less than 1 dB and mode purity over 95% are successfully generated.

Higher order mode conversion induced by discontinuities in waveguides

In this paper, we provide a theoretical prediction about the likelihood of producing high-order modes using, as far as we know, the simplest mode converter. The mode converter is a simple discontinuous waveguide for which reflection and transmission have recently been reported. As a result of the scattering of the fundamental guided mode (TE0), we have found that the high-order mode excitation is highly dependent on the position of the discontinuity. On the one hand, we discovered that in the presence of a discontinuity in the propagation direction, only even modes (TE0 and TE2) are excited, skipping the odd mode (TE1). When a lateral shift is considered, however, both even and odd higher-order modes (TE1 and TE2) are generated. Furthermore, after some lateral shifting, we found that only the pure TE2 mode is propagated.

Suppression of high order mode oscillation in a C-band long pulse high efficiency relativistic backward wave oscillator

In order to realize mode control and long pulse operation of the proposed C-band high efficiency relativistic backward wave oscillator, the asymmetric high order mode oscillation and its influences on the working mode operation are investigated in the 3D particle-in-cell simulation. The simulation shows that the quasi-TM21 mode is the dominant competing mode, which is mainly located in the two-period extraction cavity of the device. Accordingly, a simplified model of a two-period extraction cavity is established to investigate the mechanism of the high order mode excitation. It is indicated that the distance between two gaps of the two-period extraction cavity has great impact on the startup time of the competing mode. By optimizing the gap distance to delay the startup of the competing mode, as well as shortening the rising time of the input pulsed power pulse, the competing quasi-TM21 mode can be suppressed sufficiently.

Study on the Vibration Characteristics of Marine Riser Based on Flume Experiment and Numerical Simulation

This study investigated vortex-induced vibrations of marine risers in Ocean Thermal Energy Conversion systems. Flume experiments were conducted under two conditions: Condition 1 was with a fixed riser bottom, and Condition 2 was with a fixed bottom on a mooring platform. The cross-flow acceleration of the riser was measured at different current velocities, and corresponding vibration responses were analyzed. Numerical simulations based on the flume experiments were employed to validate the reliability of the simulation method. Results from the flume experiments revealed vortex-induced resonance in Condition 1 when the flow velocity approached the riser’s natural frequency. In Condition 2, similar vibration responses were observed, with maximum acceleration occurring during flow velocity-induced vortex-induced vibrations. However, at higher flow velocities, the acceleration response showed a decrease followed by an increase, indicating the excitation of higher-order modes. The numerical simulations matched the flume experiments in Condition 1. In Condition 2, while the acceleration response and frequency agreed during vortex-induced resonance, discrepancies arose in the flow velocity that caused vortex-induced vibrations compared to the flume experiments. This study demonstrated the accuracy of numerical simulations in reflecting vortex-induced vibrations of risers, providing a foundation for further research on complex riser systems.

Ultrahigh-Q lithium niobate microring resonator with multimode waveguide.

Difficulty in etching lithium niobate (LN) results in a relatively high propagation loss, which necessitates sophisticated processes to fabricate high-quality factor (Q) microresonators. Here, we fabricate a multimode microring resonator with an intrinsic Q of 6 × 106, which exhibits a propagation loss 50 times lower than that of a single-mode LN microring fabricated under the same process. Notably, the excitation of higher-order modes in the multimode microring is effectively suppressed by utilizing the Euler bend. The highly regular transmission spectrum of the resonator demonstrates a free spectral range (FSR) of 56 GHz. Based on this microresonator, we implement a bandpass microwave photonic filter with an ultra-narrow 3 dB bandwidth of 47.5 MHz and a large tuning range of 2-26.5 GHz. It can be anticipated that the combination of existing advanced etching techniques with this work will drive the propagation loss of a LN waveguide closer to the material absorption loss, significantly facilitating the optimization of performance in applications requiring ultrahigh-Q LN microresonators, such as frequency combs, frequency conversion, electro-optic modulation, and quantum photonics.

Evolution of gain enhancement techniques in dielectric resonator antenna: applications and challenges

AbstractDielectric resonator antennas (DRA) are advantageous due to their small size, high radiation efficiency, ease of excitation, and minimal metallic losses. Enhancing the gain of DRA will make them suitable for a variety of applications such as wireless communication technology. High-gain antennas are required for wireless communication to overcome the difficulties of high path loss. The researchers have presented numerous techniques, including stacked DRA, DRA array, and higher-order mode excitation for enhancing the gain. This article provides a comprehensive summary of the state of art techniques and geometries including stacking, frequency-selective surfaces superstrate, electromagnetic band gap structures, hybrid techniques, arrays, etc., for increasing DRA gain. Techniques to improve other parameters like bandwidth and radiation efficiency along with gain are also discussed. The challenges in each technique and the possible applications are presented. Based on the studies, it is observed that the DRA array technique is suitable for obtaining gains up to 19 dBi for radar-based applications. For low-profile DRA requirements, hybrid DRAs prove prolific in enhancing gain up to 16 dBi.

Mode-Modulation Structure Based on 650 nm Ridge Waveguide Edge-Emitting Laser

Traditional laser diodes operating at 650 nm are more prone to high-order mode excitation, resulting in poorer beam quality. In this paper, we designed GaInP–AlGaInP laser diodes (LD) with a 650 nm range and a trench mode-modulation structure based on the structure of edge-emitting laser (EEL) diodes. The effect of the three-trench structure was investigated theoretically and experimentally. The right trench structure laser chips demonstrated good beam quality while maintaining a high power output. An electro-optical conversion efficiency of 56% was demonstrated with a slope efficiency of 1.32 W/A at a 40 mA current. The maximum optical output power reached 40.8 mW.

Mid-infrared laser heterodyne spectrometer byhollow optical fiber and its newly designedcoupler.

We demonstrate a newly designed, to the best of our knowledge, hollow optical fiber coupler for a mid-infrared (IR) laser heterodyne spectrometer that mixes a targeted light source with local oscillator (LO) light. The hollow fiber achieves a high transmission efficiency ∼80-90%/m, not only for a coherent laser source but also for an incoherent blackbody source. The branching characteristics of the hollow optical fiber coupler are found to be strongly dependent on the curvature and length of the input port fiber, indicating that the branching ratio could be designed independently for each input port. Our laboratory measurements demonstrate that the branching ratio and transmittance of the coupler can be varied by coupling a flexible fiber to the input side owing to the excitation of higher-order modes. Using the hollow optical fiber coupler, a high-resolution emission spectrum of the quantum cascade laser at 10.3µm for our C O 2 laser-based heterodyne spectrometer is successfully achieved. Using a C O 2 laser with a hollow fiber and a blackbody as a direct input signal in free space, we obtain the sensitivity performance of IR laser heterodyne spectrometer as 2000-3000K of the system noise temperature. This suggests that the transmission of a coherent LO laser through a hollow optical fiber has almost the same sensitivity for the IR heterodyne detection as that without a fiber.

Process Variation-Aware Compact Model of Strip Waveguides for Photonic Circuit Simulation

We report a novel process variation-aware compact model of strip waveguides that is suitable for circuit-level simulation of waveguide-based process design kit (PDK) elements. The model is shown to describe both loss and -- using a novel expression for the thermo-optic effect in high index contrast materials -- the thermo-optic behavior of strip waveguides. A novel group extraction method enables modeling the effective index's ($n_{\mathrm{eff}}$) sensitivity to local process variations without the presumption of variation source. Use of Euler-bend Mach-Zehnder interferometers (MZIs) fabricated in a 300~mm wafer run allow model parameter extraction at widths up to 2.5~$\mu$m (highly multi-mode) with strong suppression of higher-order mode excitation. Experimental results prove the reported model can self-consistently describe waveguide phase, loss, and thermo-optic behavior across all measured devices over an unprecedented range of optical bandwidth, waveguide widths, and temperatures.

Ultra-broadband and highly efficient silicon nitride bi-layer grating couplers

The silicon nitride (Si3N4) waveguide platform becomes prominent in silicon photonics due to its ultralow propagation loss, negligible two-photon absorption, tolerance to phase error, and so on. Particularly, Si3N4 photonic circuits provide nonlinear opportunities as thick waveguides enable dispersion engineering, which is fundamental for frequency comb and supercontinuum generation. However, grating couplers based on thick Si3N4 waveguides, as the building block for any device, are prone to higher-order Bloch mode excitation, restricting the coupling efficiency dramatically, thus precluding practical applications that large bandwidth and are power restricted. We propose and demonstrate an 800 nm-thick Si3N4bi-layer fully etched grating coupler which is ultra-broadband and highly efficient. By applying the apodization technique, the bi-layer grating shows a conversion efficiency of −1.44 dB and a 1 dB bandwidth of 135 nm. Further integrated with a bottom reflector, a conversion efficiency of −1.01 dB with a 1-dB bandwidth of 117 nm is achieved. The proposed structures are highly fabrication tolerant and remain great performance regardless large structural parameter variations. It paves the way for a large-volume high-yield production towards fully-integrated frequency comb chip, optical beam forming network, Lidar, etc.

Experimental investigation of multimodal noise generation by ducted low Mach number flows through orifice plates

Noise generation by low Mach number air flows through circular orifices in rectangular ducts is investigated. In particular, the influence of the number and position of the orifices maintaining a constant flow area is addressed. A review of the available theories suggests a certain importance of such parameters in the excitation of higher-order acoustic duct modes. A qualitative coefficient is proposed for a first characterization of the ability to enhance or lessen a given higher-order acoustic mode by the plate geometry. An experimental campaign is performed to measure the total emitted acoustic power by different plate geometries as well as its modal composition. It is found that the orifices' numbers and positions greatly influence the acoustic emissions while the flow pressure drop caused by the obstacles is similar. The proposed qualitative coefficient shows good agreement with the experimental results. A particle image velocimetry measurement campaign is performed to visualize the near-field average flow behavior upstream and downstream of the orifice plates. An increase in the turbulent velocity fluctuations in the vicinity of the orifices is observed on both sides, further validating previous studies on the subject.

Minimizing the influence of higher-order transverse modes on the precision of cavity ring-down measurements

In cavity ring-down (CRD) measurements, the excitation of higher-order transverse modes and their random occurrence together with the fundamental mode inside the ring-down cavity (RDC) inevitably induce measurement errors of the decay time constant, which lead to a reduction in the measurement precision of the CRD instrument. To analyze and minimize the influence of higher-order transverse mode excitations on the CRD measurement precision, in this paper we report on the experimental observations of the impact of mode matching lens, aperture inside the cavity and trigger threshold on the excitation probability of fundamental and higher-order transverse mode excitations. The theoretical analysis and experimental results showed that when the optical misalignment is negligibly small, the occurrence probability of higher-order transverse mode excitations can be effectively suppressed by using a single lens between the laser and RDC to match preferably the fundamental model of the laser beam into the RDC, or inserting an aperture of an appropriate diameter inside the RDC, or using a high trigger threshold to record only CRD events with high amplitudes, or the combination of these approaches. For example, when the mode-matching lens is located at the optimal experimental position, and combined with the 2.2 mm diameter aperture, a 100% suppression of the excitation of high-order transverse modes can be achieved. Our results demonstrated that these methods effectively eliminated the decay time constant determination errors caused by the higher-order transverse mode excitations and therefore improved the precision of CRD experiments for high reflectance measurements. In practice the combination of mode-matching and appropriate trigger threshold setting is preferable for highly precise and yet fast CRD measurements.