Waveguide Coupler Research Articles

Miniature Metamaterial Backward Wave Oscillator With a Coaxial Coupler

A coaxial coupler is proposed to develop a miniature metamaterial (MTM) backward wave oscillator (BWO). With respect to those using waveguide couplers as output structures, the MTM BWO adopting the coaxial coupler exhibits obvious miniaturized characteristics. For the MTM BWO, its high-frequency structure including the MTM slow wave structure (SWS) and the coaxial coupler has longitudinal length of <inline-formula> <tex-math notation="LaTeX">$\sim 1.25\lambda $ </tex-math></inline-formula>. (<inline-formula> <tex-math notation="LaTeX">$\lambda $ </tex-math></inline-formula> is the free-space wavelength at the operating frequencies.) Importantly, the transverse size of <inline-formula> <tex-math notation="LaTeX">$\sim \lambda $ </tex-math></inline-formula>/5 is only &#x007E;1/3 of those based on waveguide couplers. Furthermore, the transmission experiment is carried out. The experimental results show that the coaxial coupler with low loss property can effectively transform the TM-dominant mode in the MTM SWS to the TEM mode. The particle-in-cell simulations predict that the proposed MTM BWO yields MW-level peak output powers at frequencies from 2.85 to 2.903 GHz when the beam current is 30 A, and the beam voltage ranges from 65 to 110 kV. The simulation results confirm the feasibility of miniaturization as well as high output powers of the MTM BWO.

Ultra-thin mid-infrared silicon grating coupler.

Mid-infrared (mid-IR) silicon photonics has been attracting great attention due to its tremendous potential applications in nonlinear optics, ranging, sensing, and spectroscopy. To date, mid-IR silicon devices have usually been developed based on silicon wafers with top-layer silicon thicknesses of hundreds of nanometers. Compared with the thick silicon devices, tens-of-nanometers thin silicon devices can provide giant evanescent-field energy proportions and optical mode areas, being significant for many biochemical sensing and nonlinear optics applications. However, ultra-thin mid-IR silicon devices have seldom been studied due to the difficulty of light coupling. Here, we demonstrated an ultra-thin focusing subwavelength-grating coupler for mid-IR ultra-thin suspended subwavelength-grating-cladding waveguide coupling. The results show that the grating has a maximum coupling efficiency of -7.1 dB at a center wavelength of 2200 nm with a 1-dB bandwidth of ∼115 nm and back reflection of -19.9 dB. We also measured the fiber alignment tolerance of 12 µm for 3-dB coupling efficiency reduction and bending optical loss of 0.25 dB/90°. Our results pave the way to developing mid-IR ultra-thin photonic integrated circuits.

Surface roughness impact on the performance of the 3D metal printed waveguide coupler at millimeterwave band

This paper presents the impact of surface roughness on the performance of a three –dimensional (3D) metal printed waveguide coupler designed at 28 GHz. The surface roughness is a significant factor that may affect 3D printed structures and post processing may be needed. It may degrade the performance of the printed devices in term of the reflection coefficient and increases the insertion loss. Thus, this work analyses the surface roughness impact on the 3D metal printed coupler designed at 28 GHz. The hybrid coupler is a waveguide-based structure with coupled resonators and the coupling is controlled by tuning the iris dimensions. The measurement is done without any post processing procedure to ensure the validation of the printed coupler with simulation results. The surface roughness measurement is performed with six tested areas of coupler structure using advanced 3D optical microscope. Then, the measured surface roughness values are included in CST software to re-simulate and compare with the original and measured results. The analysis shows that the surface roughness has a moderate influence on the reflection coefficient with 7 dB loss and 0.7 dB increased in insertion loss at 28 GHz.

Beam Duplicator Consisting of a Waveguide Coupler and Display on a Head Mounted Display

Beam Duplicator Consisting of a Waveguide Coupler and Display on a Head Mounted Display

Interlayer Slope Waveguide Coupler for Multilayer Chalcogenide Photonics

The interlayer coupler is one of the critical building blocks for optical interconnect based on multilayer photonic integration to realize light coupling between stacked optical waveguides. However, commonly used coupling strategies, such as evanescent field coupling, usually require a close distance, which could cause undesired interlayer crosstalk. This work presents a novel interlayer slope waveguide coupler based on a multilayer chalcogenide glass photonic platform, enabling light to be directly guided from one layer to another with a large interlayer gap (1 µm), a small footprint (6 × 1 × 0.8 µm3), low propagation loss (0.2 dB at 1520 nm), low device processing temperature, and a high bandwidth, similar to that in a straight waveguide. The proposed interlayer slope waveguide coupler could further promote the development of advanced multilayer integration in 3D optical communications systems.

Circular TE₀ₙMode Content Analysis Technique for High-Power Gyro-TWT System

A circular TE_0<i>n</i> mode content analysis technique based on the multi-mode coupler with high mode selectivity is proposed. To improve the mode selectivity, a deformed waveguide, entitled the quad-polar waveguide, which discriminates the azimuthally symmetric TE-mode on the waveguide wall, is introduced. The influence of deformation on the TE₀₁ and TE₀₂ modes&#x2019; field pattern, the corresponding transmission loss, and the power capacity in the quad-polar waveguide are analyzed. Based on the analysis, a dual-mode coupler of the circular TE₀₁ and TE₀₂ modes, consisting of an oversized deformed circular waveguide coupler and a pair of circular-to-quad-polar waveguide transitions, is designed for a high-power <i>Ka</i>-band gyrotron traveling-wave tube system. To verify the high mode selectivity of a quad-polar waveguide, a simple coupling with equal-radius and equal-interval holes are used. A prototype is fabricated and measured, and the results show that the coupling coefficient is &#x2212;40 dB with the directivity of 27 dB for the TE₀₁ mode, suppressing the unwanted mode TE₀₂ mode with 53 dB. For the TE₀₂ mode, the coupling coefficient is &#x2212;40 dB with the directivity of 25 dB and suppresses the unwanted mode TE₀₁ mode with 75 dB from 30 to 40 GHz.

Terahertz-wave detector on silicon carbide platform

We developed a novel terahertz-wave detector fabricated on a SiC platform implementing an InP/InGaAs Fermi-level managed barrier (FMB) diode. The FMB diode epi-layers were transferred on a SiC substrate, and a waveguide coupler and filters were monolithically integrated with an FMB diode. Then, the fabricated detector chip was assembled in a fundamental mixer module with a WR-3 rectangular-waveguide-input port. It exhibited a minimum noise equivalent power as low as 3 × 10–19 W Hz−1 at around 300 GHz for a local oscillator power of only 30 μW.

Compact and efficient three-mode (de)multiplexer based on horizontal polymer waveguide couplers.

A compact and efficient polymer three-mode (de)multiplexer with two cascaded waveguide directional couplers fabricated on the same substrate along the horizontal direction is proposed. Three waveguides formed two couplers, where two narrower waveguides were placed on either side of the central waveguide. By optimizing the core height and width, the two couplers can ensure that the E11x mode of the two narrower waveguides are highly coupled into the E21x and E31x modes of the central waveguide at a wavelength of 1310 nm. The structural size of the fabricated three-mode (de)multiplexer using ultraviolet (UV) lithography technology is in agreement with the designed value. The fabricated device, which is 35 mm long, exhibits coupling ratios of 98.07% and 95.43% for the two couplers, respectively. The insertion losses of the three waveguides are 5.23 dB, 8.58 dB, and 14.39 dB, respectively. The device can achieve the multiplexing of three modes in two dimensions, which can increase the channel capacity of optical communication.

Waveguide-type Maxwellian near-eye display using a pin-mirror holographic optical element array.

We propose a novel, to the best of our knowledge, waveguide-type optical see-through Maxwellian near-eye display for augmented reality. A pin-mirror holographic optical element (HOE) array enables the Maxwellian view and eye-box replication. Virtual images with deep depth of field are presented by each pin-mirror HOE, alleviating the discrepancy between vergence and accommodation distance. The compact form factor is achieved by the thin waveguide and HOE couplers.

Quasi-Phase-Matched Four-Wave Mixing Enabled by Grating-Assisted Coupling in a Hybrid Silicon Waveguide

Phase matching must be performed to realize four-wave mixing (FWM) in a silicon waveguide, which is challenging when a signal wavelength is highly deviated from an idler wavelength. To solve this problem, quasi-phase-matching (QPM) based on grating-assisted directional coupling (GADC) in a hybrid structure is theoretically investigated. The proposed system consists of a silicon strip with periodic width modulation as the grating and a silicon nitride strip that is vertically aligned to the silicon strip. GADC occurs between the TE_S mode (mainly confined in the silicon strip) and TE_N mode (mainly confined in the silicon nitride strip) at an idler wavelength. This phenomenon compensates for the phase mismatch occurring in FWM among the TE_S modes at the pump, signal, and idler wavelengths. Results of analysis of the hybrid structure show that the TE_S and TE_N modes at an idler wavelength of 2.1177 &#x03BC;m are efficiently generated with the TE_S modes at a pump wavelength of 1.58 &#x03BC;m and signal wavelength of 1.2601 &#x03BC;m. Moreover, the signal TE_S mode can be efficiently implemented with the pump TE_S mode and idler TE_N mode. Owing to the GADC characteristics, the conversion bandwidth of the signal is 0.8 nm; however, the signal wavelength can be thermally tuned, with a temperature change of 50 &#x00B0;C corresponding to a signal wavelength change of 3.6 nm. The hybrid structure with the GADC-based QPM can be used to generate and detect mid-infrared light with well-developed O-band and L-band devices.

Characteristics and mechanism of coupling effects in parallel-cladded acoustic waveguides

The characteristics and mechanism of coupling effects between parallel cladded acoustic waveguides (PCAWs) are essential when considering their applications in acoustic wave control and signal processing. We investigated its characteristics and revealed the nature of the coupling effect using a theoretical model of two-dimensional PCAWs and simulation experiments. We derived the eigenmode equation describing the behavior of a single waveguide based on the wave acoustic theory and derived analytic expressions for the coupling effects in the PCAWs using the coupled mode theory. Using the finite-element method, we analyzed the waveguide coupling exhibited by this structure given different configurational and acoustic parameter settings. Both theoretical and simulated results indicate that the input wave directed into one of four ports of this structure propagates and tunnels alternately between the two waveguides. Our theoretical model established yields analytic relations between the coupling lengths as well as the dependence on parameters of the evanescent wave and the structure. Analyses indicate wave coupling in the two PCAWs is essentially mediated by the evanescent wave. The unique evolution of the acoustic wave in PCAWs can be employed to develop pure acoustic devices such as frequency-selective filters, directional couplers, and acoustic switches.

Detection of cylindrical vector beams with chiral plasmonic lens

The cylindrical vector beam (CVB) has been extensively studied in recent years, but detection of CVBs with on-chip photonic devices is a challenge. Here, we propose and theoretically study a chiral plasmonic lens structure for CVB detection. The structure illuminated by a CVB can generate single plasmonic focus, whose focal position depends on the incident angle and the polarization order of CVB. Thus, the incident CVB can be detected according to the focal position and incident angle and with a coupling waveguide to avoid the imaging of the whole plasmonic field. It shows great potential in applications including CVB-multiplexing integrated communication systems.

High-Q slow light and its localization in a photonic crystal microring

We introduce a photonic crystal ring cavity that resembles an internal gear and unites photonic crystal (PhC) and whispering gallery mode (WGM) concepts. This `microgear' photonic crystal ring (MPhCR) is created by applying a periodic modulation to the inside boundary of a microring resonator to open a large bandgap, as in a PhC cavity, while maintaining the ring's circularly symmetric outside boundary and high quality factor ($Q$), as in a WGM cavity. The MPhCR targets a specific WGM to open a large PhC bandgap up to tens of free spectral ranges, compressing the mode spectrum while maintaining the high-$Q$, angular momenta, and waveguide coupling properties of the WGM modes. In particular, near the dielectric band-edge, we observe modes whose group velocity is slowed down by 10 times relative to conventional microring modes while supporting $Q~=~(1.1\pm0.1)\times10^6$. This $Q$ is $\approx$50$\times$ that of the previous record in slow light devices. Using the slow light design as a starting point, we further demonstrate the ability to localize WGMs into photonic crystal defect (dPhC) modes for the first time, enabling a more than 10$\times$ reduction of mode volume compared to conventional WGMs while maintaining high-$Q$ up to (5.6$\pm$0.1)$\times$10$^5$. Importantly, this additional dPhC localization is achievable without requiring detailed electromagnetic design. Moreover, controlling their frequencies and waveguide coupling is straightforward in the MPhCR, thanks to its WGM heritage. By using a PhC to strongly modify fundamental properties of WGMs, such as group velocity and localization, the MPhCR provides an exciting platform for a broad range of photonics applications, including sensing/metrology, nonlinear optics, and cavity quantum electrodynamics.

Exact mobility edges and topological phase transition in two-dimensional non-Hermitian quasicrystals

The emergence of the mobility edge (ME) has been recognized as an important characteristic of Anderson localization. The difficulty in understanding the physics of the MEs in three-dimensional (3D) systems from a microscopic image encourages the development of models in lower-dimensional systems that have exact MEs. While most of the previous studies are concerned with one-dimensional (1D) quasiperiodic systems, the analytic results that allow for an accurate understanding of two-dimensional (2D) cases are rare. In this work, we disclose an exactly solvable 2D quasicrystal model with parity-time ( $$\cal{P}\cal{T}$$ ) symmetry displaying exact MEs. In the thermodynamic limit, we unveil that the extended-localized transition point, observed at the $$\cal{P}\cal{T}$$ symmetry breaking point, is topologically characterized by a hidden winding number defined in the dual space. The coupling waveguide platform can be used to realize the 2D non-Hermitian quasicrystal model, and the excitation dynamics can be used to detect the localization features.

E-Plane Waveguide Filtering Six-Port Junction

A novel E-plane filtering six-port junction (SPJ) based on waveguide is presented. Different from the traditional designs, the proposed SPJ is formed by two identical filtering 180° couplers and two waveguide couplers with 180° and 90° phase shifts. A coupling-matrix (CM) technique, combining both resonant and nonresonant structures, has been developed to represent the entire SPJ. This has been used to predict the theoretical response as well as to investigate the effect of the couplers on the performance of the SPJ. A demonstrator has been designed to work at 10 GHz with a bandwidth of 0.4 GHz. Stub-loaded waveguide resonators have been used to keep a consistent profile and ease the layout of the complicated waveguide network. A good agreement between the CM-based theoretical responses, simulations, and measurement has been achieved. The measured average insertion loss is 0.4 dB, and the maximum in-band phase error is 10°. The measured in-band isolation between two input ports is higher than 26 dB.

Tunable laser induced periodic surface structures in Ge2Sb2Te5 thin films

Planar photonic structures, such as gratings and metasurfaces, are routinely used for beam steering, waveguide coupling, and light localization. However, conventional fabrication techniques that involve lithography are demanding in terms of time and cost. Much cheaper and simpler methods for surface patterning and formation of periodic surface structures are enabled by direct laser processing. Here, we demonstrate low-cost rapid fabrication of high-quality phase gratings based on the formation of laser induced periodic surface structures (LIPSS, or ripples) in Ge2Sb2Te5 (GST) thin films. Due to unique phase change properties of GST, the structures demonstrate strong modulation of refractive index with period controlled by the wavelength of laser irradiation. We study the formation of phase change LIPSS in a broad range of excitation wavelengths and observe transition between regimes with different orientations of generated ripples with respect to laser polarization.

A proposal for wide range wavelength switching process using optical force

Optomechanical wavelength up-conversion based on optical force and core–shell scattering effects are used to control light coupling between two waveguides. This system consists of two parallel optical waveguides with 20 μm lengths suspended on a silica substrate embedded with Ag/Si/SiO2 core–shell nanoparticles. By mid-IR plane wave illumination with different intensities and different wavelengths on nanoparticles, scattering would increase and result in an improvement in attractive gradient optical force exerted on waveguides. Via bending waveguides toward each other, visible light propagating in the first waveguide would couple to another. PDMS as a polymer is used to reduce the required power for bending waveguides. Results reveal that when waveguides’ gap equilibrium is 400 nm and wavelengths of control and probe lights are 4.5 μm and 0.45 μm respectively, about 10.75 mW μm−2 power is needed to bend waveguides for total coupling of light between waveguides. The efficiency of the coupled waveguides system is %43.

ESD mm-Wave-Circuit Protection: 3-dB Couplers

This article presents an innovative architecture for the implementation of an electrostatic discharge (ESD) protection for millimeter-wave (mm-wave) devices. The proposed architecture is based on the use of 3-dB couplers, whose coupled outputs have been shorted to the ground node. In order to reduce the physical dimensions of the 3-dB coupler and to reach the high coupling required for this application, the Coupled Slow wave CoPlanar Waveguides (CS-CPWs) architecture was favored. The demonstrator of the proposed system was fabricated in the STMicroelectronics 55-nm bipolar-cmos (BiCMOS) technology, demonstrating a return loss greater than 10 dB across the 50–160-GHz frequency range, an attenuation greater than 18 dB below 5 GHz, and a minimum insertion loss at 90 GHz of 0.55 dB. To prove the performance of the proposed solution, human body model (HBM) and charge device model (CDM) events were simulated at the input of the 3-dB coupler, showing that the proposed architecture effectively protects against these events while minimally impacting the RF path.

Differential Refractive Index Sensor Based on Coupled Plasmon Waveguide Resonance in the C-Band.

We proposed a differential fiber-optic refractive index sensor based on coupled plasmon waveguide resonance (CPWR) in the C-band. The sensor head is a BK7 prism coated with ITO/Au/ITO/TiO2 film. CPWR is excited on the film by the S-polarized components of an incident light. The narrow absorption peak of CPWR makes it possible to realize dual-wavelength differential intensity (DI) interrogation by using only one incident point. To implement DI interrogation, we used a DWDM component to sample the lights with central wavelengths of 1529.55 and 1561.42 nm from the lights reflected back by the sensor head. The intensities of the dual-wavelength lights varied oppositely within the measurement range of refractive index, thus, a steep slope was produced as the refractive index of the sample increased. The experimental results show that the sensitivity is 32.15/RIUs within the measurement range from 1.3584 to 1.3689 and the resolution reaches 9.3 × 10−6 RIUs. Benefiting from the single incident point scheme, the proposed sensor would be easier to calibrate in bio-chemical sensing applications. Moreover, this sensing method is expected to be applied to retro-reflecting SPR sensors with tapered fiber tip to achieve better resolution than wavelength interrogation.

A six‐port network based on substrate integrated waveguide coupler with metal strips

In this study, a novel coupling structure based on half-mode substrate-integrated waveguide (HMSIW) is proposed with a small size and broad bandwidth. The HMSIW coupler is realised by connecting their equivalent magnetic walls with metal strips, which avoids exposing the semi-enclosed magnetic walls outside. Hence, it has greater anti-interference capability than the existing HMSIW couplers. The directional coupling principle is introduced by analysing the proposed single connecting strip coupler. In order to lower return loss and higher isolation, a 3 dB directional coupler working from 10.8 to 16.8 GHz is designed with two connecting strips. Finally, a six-port circuit is achieved by utilising this compact 3 dB directional coupler. Furthermore, a broadband HMSIW power divider is optimised to satisfy the performance of this six-port circuit. The proposed coupler with metal strips helps to increase bandwidth and reduce size, which makes it more applicable to demanding systems with complex requirements such as detecting and transceiver systems. The 3 dB coupler and six-port circuit are fabricated and measured. The measured results are in accordance with the simulated data.