Waveguide Coupler Research Articles

Fano resonance-boosted topological sensor for next-generation sensing

The rapidly developing field of topological photonics has the potential to revolutionize the design and operation of optical systems. This study presents a novel approach for constructing a resilient sensor based on topological resonance. The coupling of the photonic crystal waveguide (PCW) with the topological corner state (TCS) within the structure forms the proposed sensor. The PCW provides a well-defined propagating mode, while the TCS is a localized mode that is topologically protected against perturbations. The coupling between the two modes contributes growth to a Fano resonance and results in a sharp and narrow spectral feature sensitive to the refractive index variation of the surrounding medium. The proposed sensor possesses a high sensitivity of ∼461.96 nm/RIU with a high Q-factor {10}^{6})$$\\end{document}]]>, high figure of merit {10}^{6}\\:{\ ext{R}\ ext{I}\ ext{U}}^{-1})$$\\end{document}]]>, and has an ideal detection limit value of. The present study gives a new platform for a more productive way of creating highly efficient topological Fano resonance sensors. The proposed sensor is resistant, sensitive, and highly versatile, making it beneficial for different applications.

Cavity–Waveguide Coupling Modulation via an Optical Analogue of Superradiance in Microring Arrays

Cavity–Waveguide Coupling Modulation via an Optical Analogue of Superradiance in Microring Arrays

Theoretical Investigation of Terahertz Spoof Surface-Plasmon-Polariton Devices Based on Ring Resonators

Terahertz is one of the most promising technologies for high-speed communication and large-scale data transmission. As a classical optical component, ring resonators are extensively utilized in the design of band-pass and frequency-selective devices across various wavebands, owing to their unique characteristics, including optical comb generation, compactness, and low manufacturing cost. While substantial progress has been made in the study of ring resonators, their application in terahertz surface wave systems remains less than fully optimized. This paper presents several spoof surface plasmon polariton-based devices, which were realized using ring resonators at terahertz frequencies. The influence of both the radius of the ring resonator and the width of the waveguide coupling gap on the coupling coefficient are investigated. The band-stop filters based on the cascaded ring resonator exhibit a 0.005 THz broader frequency bandwidth compared to the single-ring resonator filter and achieve a minimum stopband attenuation of 28 dB. The add–drop multiplexers based on the asymmetric ring resonator enable selective surface wave outputs at different ports by rotating the ring resonator. The devices designed in this study offer valuable insights for the development of on-chip terahertz components.

Tandem achromatic metasurface for waveguide coupling in full-color AR displays.

Waveguide coupling design is one of the most challenging topics in augmented reality (AR) near-eye displays (NED). The primary challenge stems from the necessity to simultaneously address two competing factors: the overall volume of the AR system and the occurrence of chromatic aberration. To address this issue, what we believe to be a novel tandem trilayer achromatic metasurface is specifically designed for waveguide coupling in AR NEDs, capable of achieving an achromatic effect in a nanometer-thin layer. By analyzing the influence of unit structure parameters on the phase delay of input electromagnetic waves, the optimal parameters are determined and the tandem trilayer achromatic metasurface structure is established. Simulation results show that the incident light can be deflected by 45°, 46°, and 45° at wavelengths of 440 nm ∼ 470 nm, 520 nm ∼ 550 nm, and 620 nm ∼ 660 nm, respectively. The angular deviation error of the three primary colors is maintained lower than 1° in the AR waveguide, ensuring a satisfactory achromatic effect. This design provides a new solution for developing ultra-thin and compact optical systems for full-color AR NEDs.

Polarization-insensitive and compact single-mode filter on a 340 nm silicon-on-insulator platform.

In this paper, we propose and demonstrate an integrated polarization-insensitive single-mode filter (SMF) on a 340 nm silicon-on-insulator (SOI) platform, by introducing two lateral coupling waveguides to couple high-order modes from central single-mode waveguide to lateral waveguides. The experimental results show that the excess loss is <0.29 dB and the extinction ratio is >20 dB with a broad bandwidth of 136 nm for the fabricated SMF with a compact footprint of <13 µm.

MoTe2 Photodetector for Integrated Lithium Niobate Photonics.

The integration of a photodetector that converts optical signals into electrical signals is essential for scalable integrated lithium niobate photonics. Two-dimensional materials provide a potential high-efficiency on-chip detection capability. Here, we demonstrate an efficient on-chip photodetector based on a few layers of MoTe2 on a thin film lithium niobate waveguide and integrate it with a microresonator operating in an optical telecommunication band. The lithium-niobate-on-insulator waveguides and micro-ring resonator are fabricated using the femtosecond laser photolithography-assisted chemical-mechanical etching method. The lithium niobate waveguide-integrated MoTe2 presents an absorption coefficient of 72% and a transmission loss of 0.27 dB µm-1 at 1550 nm. The on-chip photodetector exhibits a responsivity of 1 mA W-1 at a bias voltage of 20 V, a low dark current of 1.6 nA, and a photo-dark current ratio of 108 W-1. Due to effective waveguide coupling and interaction with MoTe2, the generated photocurrent is approximately 160 times higher than that of free-space light irradiation. Furthermore, we demonstrate a wavelength-selective photonic device by integrating the photodetector and micro-ring resonator with a quality factor of 104 on the same chip, suggesting potential applications in the field of on-chip spectrometers and biosensors.

Prototype design of a digital Low-Level RF system for S3FEL S-band Transverse Deflecting Cavities

Transverse Deflecting Cavities (TDCs) are generally adopted for electron beam diagnosis. Three sets of S-band and two sets of X-band TDCs are planned at Shenzhen Superconducting Soft X-ray Free Electron Laser (S3FEL) to accurately measure the temporal distribution of ultra-short electron bunches. The microwave system of one TDC consisting of a Low-Level Radio-Frequency system (LLRF), a solid-state amplifier, a klystron, and several waveguide couplers is operated in pulse mode with a maximum repetition rate of 50 Hz. Its microwave stabilities for amplitude and phase are required to be better than 0.05%/0.05∘ (RMS). This article will introduce the prototype design of the hardware, firmware, and software of the digital LLRF system for S-band TDCs. We use a homemade local oscillator and commercial cards based on the MicroTCA standard in hardware design. The firmware design will use an IQ demodulation and a reference-tracking algorithm to eliminate the measurement noise and drift. The software design is based on the Experimental Physics and Industrial Control System (EPICS), achieving data acquisition, slow control, and interface display functions. This technical report will also show some preliminary test results.

Photonic Supercoupling in Silicon Topological Waveguides.

Waveguide interconnect coupling control is essential for enhancing the chip density of photonic integrated circuits to incorporate a growing number of components. However, a critical engineering challenge is to achieve both strong waveguide isolation and efficient long-range coupling on a single chip. Here, a novel photonic supercoupling phenomenon is demonstrated for waveguide coupling over separation distances from a quarter to five wavelengths (λ), leveraging the tunable mode tails and the vortex energy flow in topological valley Hall system. A supercoupled integrated chip is developed, realizing a 91% coupling ratio and a -30dB isolation over 2.8λ waveguide separations simultaneously. Supercoupled devices are further showcased including a waveguide-cavity system with 3.2λ excitation distance, and a waveguide directional supercoupler with a compact coupling area of nearly λ2/4, which outperform conventional devices. Supercoupling provides new degrees of freedom for optimizing coupling and isolation between photonic integrated components, facilitating new applications in on-chip sensing, lasing, and telecommunications.

Denoising-autoencoder-facilitated MEMS computational spectrometer with enhanced resolution on a silicon photonic chip

Silicon photonics enables the construction of chip-scale spectrometers, in which those using a single tunable interferometer provide a simple and cost-effective solution. Among various tuning mechanisms, electrostatic MEMS reconfiguration stands out as an ideal candidate, given its high tuning efficiency and ultra-low power consumption. Nonetheless, MEMS devices face significant noise challenges arising from their susceptible minuscule components, adversely impacting spectral resolution. Here, we propose a distinct paradigm of spectrometers through synergizing an easily-fabricated MEMS-reconfigurable low-loss waveguide coupler on a silicon photonic chip and a convolutional autoencoder denoising (CAED) mechanism. The spectrometer offers a 300 nm bandwidth and a reconstruction resolution of 0.3 nm in a noise-free condition. In a noisy environment with a signal-to-noise ratio as low as 30 dB, the reconstruction resolution of the interferograms processed by the CAED exhibits an enhancement from 1.2 to 0.4 nm, approaching the noise-free value. Our technology is envisaged to provide a powerful and cost-effective solution for applications requiring accurate, broadband, and energy-efficient spectral analysis.

Low-Cost and Affordable Thermistor-Based Wideband Sub-THz Detector with Dielectric Waveguide Coupling.

Bolometric detection of electromagnetic radiation is a well-established method in a wide frequency range, from millimeter waves through the terahertz region up to infrared. Fabrication of such a detector is often an expensive and demanding process. We propose a simple device based on a commercially available thermistor as a sensing element. To direct radiation to the sensor, we designed and fabricated a 3D-printed optical element integrated with the dielectric waveguide. An electronic setup was prepared to measure the sensor response. The described device is an affordable detector with acceptable detection parameters such as SNR or responsivity at a hundreds of volts per watt level.

Polarization Volume Hologram for On‐Chip Wavefront Engineering

AbstractLiquid crystal (LC) planar optics have advanced wavefront engineering toward ultrathin designs, capturing widespread attention. However, most wavefront control in LC planar optics remains constrained to freespace due to limitations in the precision of freely controllable units. Here, LC on‐chip wavefront engineering is proposed and confirmed. By controlling the initial azimuth angle of the polarization grating, the initial phase can be engineered, as theoretically predicted by rigorous coupled‐wave analysis. Experimentally, the initial azimuth angle of a polarization volume hologram grating, used as a waveguide coupler, is ingeniously modulated using a holographic template. Consequently, several on‐chip optical elements, including lenses, vortex beam generators, and holograms, are demonstrated. Furthermore, exit pupil expansion and multiexposure technologies are adopted to enhance off‐chip functionality and enable multifunctional, highly integrated LC on‐chip photonic systems. The proposed LC on‐chip wavefront engineering may find applications in freeform optics, near‐eye displays, LIDAR, and integrated photonic systems.

High-performance and fabrication-tolerant edge coupler on thin film lithium niobate based on a three-dimensional inverse taper

Thin film lithium niobate has been widely considered as a promising photonic integration platform. However, due to the non-vertical sidewall of lithium niobate etching, it is challenging to design an efficient fiber coupler for the submicron-sized lithium niobate waveguide. Here, a high-performance edge coupler to a cleaved fiber is introduced on the thin film lithium niobate platform using a sharp three-dimensional taper. The proposed taper structure, which ensures a nearly adiabatical transition of both transverse-electric (TE) and transverse-magnetic (TM) modes, is formed by two standard patterning steps for lithium niobate. A silica ridge waveguide is used as the intermediate transition structure between the taper and a high-numerical-aperture fiber. The simulated coupling losses are as low as −0.1 dB per facet, while the experimental mean values reach −0.29 and −0.24 dB for TE and TM modes, respectively. The coupling spectra exhibit a flat wavelength response, which almost coincides for both polarizations. The 0.5-dB coupling bandwidth is beyond 180 nm. Within this bandwidth, the worst coupling losses are −0.61 and −0.56 dB for TE and TM modes, respectively, and the polarization-dependent loss is below 0.17 dB. The present edge coupler also exhibits a good fabrication tolerance, and its fabrication can also facilitate wafer-scale processing without wafer dicing and end-facet polishing.

Reconfigurable dual-mode meta-waveguide

With the rapid growth of information flux, integrating more channels in limited spaces has been a critical requirement for highly-integrated long-range parallel transmissions. Although meta-waveguides, such as spoof surface plasmon polariton (SSPP) waveguides, can suppress crosstalk between adjacent channels, it is still urgent to seek new methods to further increase the number of channels without occupying additional space. Here, we propose a reconfigurable dual-mode meta-waveguide (RDMMW) by integrating diodes into meta-units, which supports two independent channels using two orthogonal modes. The RDMMW can be switched among dual-mode, even-mode, odd-mode and cutoff states by controlling the diodes, which provides more freedom to manipulate the channel features. The RDMMW possess two outstanding merits in highly-integrated long-range parallel transmissions, including lower coupling between two channels than conventional single-mode waveguides and constant mode coupling in longer waveguides. Thus the proposed RDMMW has advantages of multi-mode, reconfiguration, low coupling and high integration density, providing a new avenue to realize highly-integrated multi-functional systems.

A High-Balanced Interdigital Hybrid Metallic Waveguide Coupler Based on SSPPs- Coupling for WR-4 Ultra-Full- Band Applications

A High-Balanced Interdigital Hybrid Metallic Waveguide Coupler Based on SSPPs- Coupling for WR-4 Ultra-Full- Band Applications

Chiral sensing via dielectric waveguide-nanoparticle array interactions.

Identifying the handedness of chiral materials in small quantities remains a significant challenge in biochemistry. Nanophotonic structures offer a promising solution by enhancing weak chiroptical responses through increased optical chirality. Utilizing a silicon-based approach for chiral sensing on a photonic integrated platform is highly desirable. In this study, we explore the interaction between a dielectric waveguide and silicon nanoparticles for detecting the handedness of chiral analytes. A chiral core induces polarization rotation of wavefields traveling along a dielectric waveguide with a square cross-section. This polarization rotation affects waveguide coupling differently depending on the left- or right-handed arrangement of nanoparticles around the waveguide, enabling enantiomer detection through discernible transmission differences. From a basic design to more practical structures, we investigate configurations that maintain the same working principles. Theoretical results based on the transfer matrix method corroborate the numerical simulations.

Coupling light into a guided Bloch surface wave using an inversely designed nanophotonic cavity

Controlling the propagation of light in the form of surface modes on miniaturized platforms is crucial for multiple applications. For dielectric multilayers that sustain Bloch surface waves at their interface to an isotropic dielectric medium, a conventional approach to manipulate them exploits shallow surface topographies fabricated on top of the truncated stack. However, such structures typically exhibit low index contrasts, making it challenging to confine, steer, and guide the Bloch surface waves. Here, we theoretically and experimentally demonstrate a device for a Bloch surface wave platform that resonantly couples light from a cavity to a straight waveguide. The structure is designed using topology optimization in a 2D geometry under the effective index approximation. In particular, the cavity–waveguide coupling efficiency of the radiation emitted by an individual source in the cavity center is optimized. The cavity is experimentally found to exhibit a narrow resonant peak that can be tuned by scaling the structure. The waveguide is shown to guide only light that resonates in the cavity. Fully three-dimensional simulations of the entire device validate the experimental observations.

A fixed phase tunable directional coupler based on coupling tuning

The field of photonic integrated circuits has witnessed significant progress in recent years, with a growing demand for devices that offer high-performance reconfigurability. Due to the inability of conventional tunable directional couplers (TDCs) to maintain a fixed phase while tuning the reflectivity, Mach-Zehnder interferometers (MZIs) are employed as the primary building blocks for reflectivity tuning in constructing large-scale circuits. However, MZIs are prone to fabrication errors due to the need for perfect balanced directional couplers to achieve 0-1 reflectivity, which hinders their scalability. In this study, we introduce a design of a TDC based on coupling constant tuning in the thin film Lithium Niobate platform and present an optimized design. Our optimized TDC design enables arbitrary reflectivity tuning while ensuring a consistent phase across a wide range of operating wavelengths. Furthermore, it exhibits fewer bending sections than MZIs and is inherently resilient to fabrication errors in waveguide geometry and coupling length compared to both MZIs and conventional TDCs. Our work contributes to developing high-performance photonic integrated circuits with implications for various fields, including optical communication systems and quantum information processing.

Selecting robust silicon photonic designs after Bayesian optimization without extra simulations.

Optimization methods are frequently exploited in the design of silicon photonic devices. In this paper, we demonstrate that pushing the objective function to its minimum during optimization often results in devices that gradually become more sensitive to perturbations of design variables. The dominant strategy of selecting the design with the smallest objective function can lead to fabrication failure or yield loss due to manufacturing process variations. To address this issue, we propose an intuitive selection criterion that can identify designs not only possessing small objective functions but that are also robust to variations. Our simulation results on the Y-splitter, direction coupler, and bent waveguide designs demonstrate that the proposed method can achieve 2x higher coverage of robust designs with almost negligible run time, compared to the two baseline methods.

Waveguide adapter with contactless flange for operation in the millimeter wavelength range

Methods of increasing accuracy of measurement results of S-parameters of waveguide devices operating in the upper part of extremely high frequency range are reviewed. It is shown that accuracy and reliability of measurements of S-parameters of waveguide devices depend on the quality of contact between waveguide flanges: leakage of electromagnetic radiation through gaps poses a significant problem and can lead to unreliable results in measuring insertion loss and voltage standing wave ratio. Therefore, particular attention is paid to quality of contact between the measurement system and the device under test when operating in the millimeter wavelength range. In this paper, a contactless waveguide adapter flange for millimeter wavelength range insensitive to any air gap when connected to the device under test is designed. The waveguide adapter with contactless flange constitutes a section of a standard WR10 rectangular waveguide terminated on one side by a standard UG-387 flange and on the other side by the proposed contactless flange with a pin-like structure. Use of an adapter with a contactless flange allows for minimizing issues related to measuring insertion loss and return loss caused by incomplete contact or lack of it when connecting two waveguide flanges. This type of flange connection can be used for rapid measurements because no flange screw fastening is required. A comparative analysis of a standard adapter and an adapter with a contactless flange is performed in a WR10 waveguide channel. Efficiency of contactless coupling is experimentally confirmed within the entire operating range of 75 to 110 GHz with a 100 μm gap at the edge of the waveguide coupling. Use of a waveguide adapter with a contactless flange allows for faster and at the same time more accurate measurement of S-parameters of the devices under test. This adapter is indispensable for quick certification of millimeter-wave waveguide devices by modern metrological tools (vector network analyzer and scalar network analyzer).

A methodical approach to design adiabatic waveguide couplers for heterogeneous integrated photonics

Abstract We present an elegant and effective approach for the design of adiabatic waveguide couplers tailored for the heterogeneous integration of photonic building blocks. This method empowers users to incorporate the shortest taper(s) in their designs, while upholding optimal coupling efficiency. The technique assesses mode overlap between a minimum of two waveguides within the cross-section of any heterogeneous material stack, determining the necessary waveguide cross-sectional dimension to achieve optimal coupling efficiency. Two illustrative design applications are showcased and compared to a linear, concave, and convex taper for reference: a SiN-to-polymer structure exhibiting a 40% coupling improvement and a Si-to-GeSi structure having a 2.2 up to 5 times shorter length.