Waveguide Coupler Research Articles

Higher-order topological sound transport in synthetic spaces

We design a four-dimensional system with straight waveguides composed of coordinate-dependent coupled waveguides. By controlling the phase in synthetic space and the excitation of the corresponding sound source mode, we show that the pumping incorporating adiabatic modulation of discretely varied coupling waveguides along a given orbit in the phason space occurs only in specific space directions. In addition, robust corner-to-corner topological pumping is achieved in finite-sized two-dimensional modulated acoustic structures. Our results provide a general strategy to achieve topological pumping through discrete property modulations and pave new paths for studying higher-dimensional topological phases in discrete systems.

Liquid-Crystal Reconfigurable Coupler for Millimeter-Wave Applications

A Substrate-integrated waveguide (SIW) coupler whose coupling ratio is controllable at millimeter-wave frequency is presented. To realize this unique property, the SIW coupler is loaded with a tunable liquid crystal (LC) in the central region. To increase, translate and shift the tuning range of the LC to the required permittivity, an artificial dielectric slab is introduced and validated. A new approach to locally increase the permittivity of the LC is presented, wherein the relative permittivity is increased over the desired band. Initially, the variation in the relative permittivity of the LC is 2.46 to 3.5; the slab offers an increment in the variation, from 8.82 to 14.7. Varying the permittivity of the slab in the central region enables coupling ratio control. Based on the investigation of the proposed model, the design requirements and physical parameters to achieve a wide range of reconfigurability are obtained at the millimeter-wave band. For experimental/demonstration purposes, two reconfigurable coupler prototypes with angles of 15°and 55°between the arms are fabricated and characterized. The results demonstrate a tunable operating frequency to a certain extend with a controllable coupling ratio between output ports from 0.5 to 15 dB. These results show very good agreement with the values determined via simulation.

Photon propagation control on laser-written photonic chips enabled by composite waveguides

Femtosecond laser direct writing (FsLDW) three-dimensional (3D) photonic integrated circuits (PICs) can realize arbitrary arrangement of waveguide arrays and coupling devices. Thus, they are capable of directly constructing arbitrary Hamiltonians and performing specific computing tasks crucial in quantum simulation and computation. However, the propagation constant β is limited to a narrow range in single-mode waveguides by solely changing the processing parameters, which greatly hinders the design of FsLDW PICs. This study proposes a composite waveguide (CWG) method to increase the range of β, where a new single-mode composite waveguide comprises two adjacent circular waveguides. As a result, the photon propagation can be controlled and the variation range of β can be efficiently enlarged by approximately two times (Δβ∼36 cm−1). With the CWG method, we successfully realize the most compact FsLDW directional couplers with a 9 μm pitch in a straight-line form and achieve the reconstruction of the Hamiltonian of a Hermitian array. Thus, the study represents a step further toward the fine control of the coupling between waveguides and compact integration of FsLDW PICs.

A narrow‐band overmoded rectangular waveguide filter for high‐power microwaves (HPMs)

AbstractAn overmoded rectangular waveguide filter with the advantages of narrowband and high power is presented in this paper. The transmission efficiency of each mode in the filter is analyzed based on mode‐matching techniques and optimized by combining it with the calculation method. Due to the overmoded waveguide and direct coupling topology, the designed structure has characteristics of narrowband and high power. The overall size of the proposed filter is only 17.45 × 36.20 × 126.67 mm3 (0.54λ0 × 1.12λ0 × 3.93λ0, λ0 is the wavelength of the center frequency in free space). The test results match well with the simulated ones and they both show that the transmission efficiency of TE10 mode is higher than 99.77%. Meanwhile, the proposed filter achieves 1% fractional bandwidth and 134.5 kW power capacity, which reveals that the filter has both narrow‐band and high‐power characteristics. The designed filter would be an attractive candidate for HPM systems.

VCSEL-based optical transceiver with a 90°-bent graded-index core polymer waveguide coupler for 25.78-Gb/s transmission

In this paper, we propose a low-loss assembly of optical elements [vertical cavity surface emitting laser (VCSEL)/photodiode (PD)] and a 90°-bent graded-index (GI) core polymer optical waveguide for compact high-speed optical transceivers. The air gap between the 90°-bent GI-core waveguide and VCSEL/PD chips are filled with a high-index resin, and we demonstrate a reduction not only of the Fresnel reflection loss but also of the coupling loss due to the lower divergence angle of VCSEL and fiber output beams. In this paper, the core size and NA of the 90°-bent GI-core in the polymer waveguide are redesigned to match the specific launch conditions realized with the gap filled with resins, resulting in theoretical estimates of insertion losses as low as 0.82 dB and 1.38 dB for the waveguides on the transmitter and receiver sides, respectively. Such low insertion losses are obtained not by step-index core but GI core waveguides. After redesigning the structure, 90°-bent GI-core polymer waveguides with a bending radius of 1 mm are experimentally fabricated using the Mosquito method, and we experimentally demonstrate that the insertion loss of the waveguide can be reduced by about 4 dB when the 100-µm air gap is filled with an optical adhesive (na = 1.51). In addition, an error-free 25.78-Gb/s data transmission is successfully demonstrated over a 100-m MMF link using the fabricated VCSEL based transceivers with 90°-bent GI core polymer optical waveguides.

Study on Acoustic Sensing Characteristics of a Novel Grooved SiO2 Waveguide Resonator

Photoacoustic sensor has become a research hotspot in the field of acoustic sensing in recent years due to its high sensitivity and anti-electromagnetic interference. A novel high- <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> grooved silica waveguide acoustic sensor (GSWAS) is proposed, which is etched and filled the Polydimethylsiloxane (PDMS) above the waveguide coupling region. The <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${Q}$ </tex-math></inline-formula> factor of the sensor is of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.82\times 10^{{6}}$ </tex-math></inline-formula> . In order to detect the sound signal easily and intuitively, an acoustic sensing system was built by using Pound–Drever–Hall (PDH) frequency locking technology, and the weak sound signal was demodulated by using the method of synchronous demodulation of high frequency modulation. Experimental results show that the designed sensor has a flat response in the frequency range of 0.4–15 kHz and can accurately detect the acoustic signal, with a high sensitivity of 1143 mV/Pa and a signal-to-noise ratio (SNR) of up to 45.19 dB under the action of acoustic signal of 8 kHz. This article explores a new method of frequency bandwidth and high sensitivity sound detection, laying a foundation in the field of acoustic signal detection such as aeroacoustics, photoacoustic imaging, and all that.

Enhancing Evanescent Wave Coupling of Near-Surface Waveguides with Plasmonic Nanoparticles.

Evanescent field excitation is a powerful means to achieve a high surface-to-bulk signal ratio for bioimaging and sensing applications. However, standard evanescent wave techniques such as TIRF and SNOM require complex microscopy setups. Additionally, the precise positioning of the source relative to the analytes of interest is required, as the evanescent wave is critically distance-dependent. In this work, we present a detailed investigation of evanescent field excitation of near-surface waveguides written using femtosecond laser in glass. We studied the waveguide-to-surface distance and refractive index change to attain a high coupling efficiency between evanescent waves and organic fluorophores. First, our study demonstrated a reduction in sensing efficiency for waveguides written at their minimum distance to the surface without ablation as the refractive index contrast of the waveguide increased. While this result was anticipated, it had not been previously demonstrated in the literature. Moreover, we found that fluorescence excitation by waveguides can be enhanced using plasmonic silver nanoparticles. The nanoparticles were also organized in linear assemblies, perpendicular to the waveguide, with a wrinkled PDMS stamp technique, which resulted in an excitation enhancement of over 20 times compared to the setup without nanoparticles.

Plasmonic nanoparticle doped medium for frequency selective waveguide coupling

We present a powerful technique for frequency selective coupling of dielectric waveguides by incorporating plasmonic nanoparticles within the structure. This is achieved by the inclusion of an optimal density of subwavelength plasmonic nanoparticles within the cladding layer between the coupling waveguides. Frequency selectivity of the coupling is possible by careful manipulation of the nanoparticle characteristics that govern the effective refractive index of the doped layer that supports plasmonic excitations within a narrow frequency range. The efficient coupling arises from inducing modes within the nanoparticle doped cladding layer of the waveguide in the spectral and spatial vicinities of the selected waveguide modes. Our observations are modeled with the transfer matrix method and corroborated with full-wave simulations.

On‐chip terahertz whispering gallery mode resonator with high‐Q based on multislot waveguide

AbstractA whispering gallery mode resonator (WGMR) with a high‐quality factor and large coupling spectrum width can enhance the interaction of matter and lightwave by several orders of magnitude in a large bandwidth range. The performance of the resonator determines the accuracy and range of terahertz (THz) sensing. However, the quality factors of the reported THz WGMRs are highly limited by the high absorption loss of materials within THz spectral range. The coupling spectrum width is usually limited by the coupling structure. In this paper, an on‐chip THz whispering gallery mode resonator based on multislot waveguide and gradual coupling structure is proposed. By increasing the evanescent field of guided wave mode, the absorption loss in the resonator is significantly reduced. Through gradual coupling structure, resonance curves are more uniform within a wide spectrum. The simulation results show that the structure can improve the instinct quality factor of the THz resonator to the order of when the bending radius is as small as 2mm. At the same time, sensitivity is increased by more than six times and uniform resonance curves are achieved from THz.

Topological Bound Modes With Orbital Angular Momentum in Optical Waveguide Arrays

Coupled optical waveguide arrays, in which the wave tunnelling dynamics resembles that of electrons in crystals, have been utilized as a versatile tool to demonstrate topological phenomena. So far, the study of topological phases in waveguide arrays mostly focused on the supermodes constituted by the fundamental modes of single waveguide while the high-order modes with orbital angular momentum (OAM) are rarely considered. Here the topological bound modes are investigated in a zigzag chain composed of evanescently coupled waveguides that support OAM modes. We show the topological propagation is aroused by OAM without discovering any bound modes generated by fundamental modes. The reason lies in the coupling of OAM modes between adjacent waveguides naturally gives rise to synthetical gauge fields, significantly different from fundamental modes. Furthermore, we study the coupling mechanism of elliptical waveguides and show the orientation angle of ellipses serves as gauge fields as well, which provides an additional degree of freedom to control topological bound modes.

A Novel Injection-Locked S-Band Oven Magnetron System Without Waveguide Isolators

A novel injection-locked <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">${S}$ </tex-math></inline-formula> -band microwave oven magnetron system is proposed, analyzed, and experimented. The magnetron is considered as a two-port oscillator, and the filament structures of it are used as a port for injection locking. To our knowledge, it is the first time that microwaves have been injected through magnetron filaments. The microwave is injected into the magnetron filter box using a self-designed injection structure, entering the resonant cavity through filament leads. The intrinsic isolation between the magnetrons’ output and filament is utilized as an isolator. No waveguide circulators or couplers between the injection source and the magnetron were used in the system, resulting in low cost and compact injection locking. Two types of injection structures were performed. The maximum locking bandwidth of 0.7 MHz was achieved at an injection ratio of 0.2. The weight and volume of the proposed injection-locked magnetron system were reduced to 28% and 16%, respectively. The magnetron’s output is successfully locked by an external signal. The experiment results reveal that the proposed low-cost injection-locked oven magnetron system works reliably. It has significant potential applications for future microwave ovens with frequency-selective heating to improve both efficiency and uniformity.

A high-precision silicon-on-insulator position sensor

Integrated photonic devices provide significant advantages over their conventional counterparts, such as a drastically reduced footprint as well as compatibility with other photonic or electronic circuitry. In this work, we present a high-precision optical position sensor fabricated on a silicon-on-insulator platform. The sensor relies on the principle of position-dependent directional waveguide coupling upon excitation of a monolithically integrated scatterer with a tightly focused polarization-tailored beam. We demonstrate a spatial resolution of 7.2 nm, corresponding to approximately λ/200.

Electrostatic Epitaxy of Orientational Perovskites for Microlasers.

Orientational growth of single-crystalline structures is pivotal in the semiconductor industry, which is achievable by epitaxy for producing thin films, heterostructures, quantum wells, and superlattices. Beyond silicon and III-V semiconductors, solution-processible semiconductors, such as metal-halide perovskites, are emerging for scalable and cost-effective manufacture of optoelectronic devices, whereas the polycrystalline nature of fabricated structures restricts their application toward integrated devices. Here, electrostatic epitaxy, a process sustained by strong electrostatic interactions between self-assembled surfactants (octanoate anions) and Pb2+ , is developed to realize orientational growth of single-crystalline CsPbBr3 microwires. Strong electrostatic interactions localized at the air-liquid interface not only support preferential nucleation for single crystallinity, but also select the crystal facet with the highest Pb2+ areal density for pure crystallographic orientation. Due to the epitaxy at the air-liquid interface, direct growth of oriented single-crystalline microwires onto different substrates without the processes of lift-off and transfer is realized. Photonic lasing emission, waveguide coupling, and on-chip propagation of coherent light are demonstrated based on these single-crystalline microwires. These findings open an avenue for on-chip integration of single-crystalline materials.

Raman-induced mode coupling in temporal waveguides formed by short solitons

We study the propagation of optical pulses trapped inside a temporal waveguide formed by two solitons in a dispersive nonlinear medium such as an optical fiber. The solitons are short enough that they decelerate as their spectra shift continuously toward the red side because of intrapulse Raman scattering. We show that the shape of a probe pulse trapped between the two solitons evolves in a periodic fashion, while its spectrum shifts toward the blue side. We develop a coupled-mode theory showing that such changes occur because of mode coupling induced by the deceleration of the short solitons, resulting in a curved waveguide. A simplified two-mode model is used to introduce a single-parameter governing modal coupling and to find the condition under which coupling becomes weak enough that the probe pulse blueshifts its spectrum without changes in its pulse shape.

Analysis and design of weak coupling coupler based on the half mode substrate integrated waveguide

AbstractSubstrate integrated waveguide (SIW) technology represents a good solution for the design of couplers. Coupler structures proposed in most relevant reports cannot achieve excellent performance in the case of weak coupling. This work proposes a new weak coupling coupler architecture, similar to the branch line coupler. The metal via arrays is used to reshape the electric field distribution of the SIW structure, making the overall structure achieve weak coupling characteristics. The even-odd mode decomposition method analyzes this structure's equivalent transmission line model. For this purpose, a systematic design procedure is deployed to achieve several coupling values over a wide frequency bandwidth. A novel half-mode substrate integrated waveguide (HMSIW) coupler with a 29 dB coupling is designed and fabricated for verification based on the proposed method. Good agreements between the calculated and simulated results are observed. The proposed coupler has the advantage of high directivity within the broadband and can be used for SIW-based circuits and power detection in the Ku-band.

Design of low profile wideband high power waveguide to substrate integrated waveguide coupler

AbstractThis paper presents a new wideband waveguide substrate integrated waveguide (SIW) coupler with maximum deviation of ±0.5 dB in a 7.5–12.25 GHz bandwidth (48% fractional bandwidth). Proposed coupler consists of the standard WR90 waveguide and single layer/double clad SIW printed circuit board (PCB) which is placed crosswise on the waveguide. Proposed three port coupler (isolated port was ended with match load) has a two coupling level (average of coupling in 8–12 GHz frequency range) of 43.2 and 58.3 dB, which is correspond with the hole diameter of 3.5 and 3 mm, respectively. The main contribution of this work is based on the hybrid techniques of SIW and RWG for coupler design in the high power microwave application. Validation of the simulation results with measurement one proves that the proposed X‐band waveguide to SIW coupler is a good candidate for use in high power industrial/commercial applications.

Low-loss fiber-to-chip edge coupler for silicon nitride integrated circuits.

Silicon nitride (SiN) integrated optical waveguides have found a wide range of applications due to their low loss, broad wavelength transmission band and high nonlinearity. However, the large mode mismatch between the single-mode fiber and the SiN waveguide creates a challenge of fiber coupling to these waveguides. Here, we propose a coupling approach between fiber and SiN waveguides by utilizing the high-index doped silica glass (HDSG) waveguide as the intermediary to smooth out the mode transition. We achieved fiber-to-SiN waveguide coupling efficiency of lower than 0.8 dB/facet across the full C and L bands with high fabrication and alignment tolerances.

Fabrication-tolerant directional couplers on thin-film lithium niobate.

Directional couplers (DCs) are essential components in integrated photonics. A symmetric DC with a large fabrication tolerance on thin-film lithium niobate is demonstrated here. The principle is based on the beat length compensation of the opposite trend of width and gap fabrication errors in the DC. The tolerance is greater than ±100 nm for an optimized structure. The experimental results support the simulated ones. The principle can be applied to DC-based devices, such as 3-dB couplers and waveguide array couplers with a high yield.

Propagating and Localized Surface Plasmon Co-Enhanced Raman Scattering Based on a Waveguide Coupling Surface Plasmon Resonance Structure

We propose a waveguide-coupled surface plasmon resonance (WCSPR) structure with MgF2 as the waveguide layer to enhance Raman scattering. The WCSPR was a five-layer resonance structure composed of prism/Ag film/MgF2 film/outer Ag film layers. The electromagnetic field was stronger with the WCSPR structure than with a traditional SPR structure. Furthermore, localized and propagating surface plasmon co-enhanced Raman scattering of 4-mercaptopyridine was observed on the WCSPR/4-Mpy/silver-nanoparticle sandwich structure. The most effective coupling of the localized and propagating surface plasmons resulted in a 60-fold enhancement in the signal intensity relative to that obtained with a conventional SPR structure. The enhancement factor of the WCSPR/4-Mpy/silver-nanoparticle configuration was 3.6 × 107.

Field programmable spin arrays for scalable quantum repeaters

The large scale control over thousands of quantum emitters desired by quantum network technology is limited by the power consumption and cross-talk inherent in current microwave techniques. Here we propose a quantum repeater architecture based on densely-packed diamond color centers (CCs) in a programmable electrode array, with quantum gates driven by electric or strain fields. This ‘field programmable spin array’ (FPSA) enables high-speed spin control of individual CCs with low cross-talk and power dissipation. Integrated in a slow-light waveguide for efficient optical coupling, the FPSA serves as a quantum interface for optically-mediated entanglement. We evaluate the performance of the FPSA architecture in comparison to a routing-tree design and show an increased entanglement generation rate scaling into the thousand-qubit regime. Our results enable high fidelity control of dense quantum emitter arrays for scalable networking.