Waveguide Design Research Articles

Design and 3D printing of soft optical waveguides towards monolithic perceptive systems

Design and 3D printing of soft optical waveguides towards monolithic perceptive systems

Additive manufacturing of planar waveguides for terahertz applications

The emerging planar photonic crystal (PC) and effective medium (EM) waveguides are considered promising technical platforms for terahertz communications. Beyond short-scale connections and various signal processing functionalities, versatile terahertz on-chip systems could be achievable by their physical integration; however, the inefficient coupling poses challenges. In this work, we propose an interface design between planar PC and EM waveguides with minimal insertion loss. Stereolithography 3D printing was validated as a cost-effective alternative to the microfabrication technologies for the demonstration of these two waveguide designs. Experiments found that optimized PC and EM waveguides in photosensitive resin exhibited transmission losses of 3.4 dB/cm and 2.1 dB/cm at 140 GHz, respectively. Furthermore, by adjusting the radius and positioning of certain air holes adjacent to the waveguide core, we reduced the insertion loss associated with the waveguide interface to 0.4 dB; significantly lower compared with direct butt coupling without transitions. The ∼30% increase in power coupling efficiency enabled terahertz signal transmission with higher data rates and lower bit error rate for terahertz communications. We believe that the proposed terahertz planar waveguide fabrication routes and structural designs could hold huge potential to offer efficient rapid-prototyping and inter-waveguide integration solutions for multifunctional terahertz circuits.

Inverse design of waveguide grating mode converters using artificial neural networks

Abstract Machine learning techniques, notably various deep neural network methods, are instrumental in processing extensive and intricate data sets in engineering and scientific fields. This paper shows how deep neural networks can inversely design cascaded-mode converting systems, particularly the waveguide gratings that implement selective mode conversion upon reflection. Neural networks can map the grating's physical features to scattering parameters of the modes reflected from the grating. The trained networks can then be utilized to inversely design the waveguide gratings mode converters based on the desired values of the scattering parameters. The process of the inverse design involves using the technique of gradient descent of a defined loss function. Minimizing this loss function leads to calculating more accurate features fulfilling the desired scattering parameters.

Single-pass superluminescent diodes with grazing stripe waveguide.

We report on an investigation of InGaAs/GaAs quantum well-dot superluminescent diodes (SLDs) based on what we believe to be a novel and simple design of stripe waveguides. The design employing a chip side facet as a component of the SLD structure allows effective suppression of optical feedback, thus increasing the optical power. The test SLDs under study emitting in 950-1150 nm spectral range show CW optical power as high as 150 mW in combination with broad emission spectra of 20 nm full width at half maximum (FWHM).

Atmospheric-level carbon dioxide gas sensing using low-loss mid-IR silicon waveguides.

Interest in carbon dioxide (CO2) sensors is growing rapidly due to the increasing awareness of the link between air quality and health. Indoor, high CO2 levels signal poor ventilation, and outdoor the burning of fossil fuels and its associated pollution. CO2 gas sensors based on integrated optical waveguides are a promising solution due to their excellent gas sensing selectivity, compact size, and potential for mass manufacturing large volumes at low cost. However, previous demonstrations have not shown adequate performance for atmospheric-level sensing on a scalable platform. Here, we report the clearly resolved detection of 500 ppm CO2 gas at 1 s integration time and an extrapolated 1σ detection limit of 73 ppm at 61 s integration time using an integrated suspended silicon waveguide at a wavelength of 4.2 µm. Our waveguide design enables suspended strip waveguides with bottom anchors while maintaining a constant waveguide core cross-sectional geometry. This unique design results in a low propagation loss of 2.20 dB/cm. The waveguides were implemented in a 150 mm silicon on insulator (SOI) platform using standard optical lithography, providing a clear path to low-cost mass manufacturing. The low CO2 detection limit of our proposed waveguide, combined with its compatibility for high-volume production, creates substantial opportunities for waveguide sensing technology in CO2 sensing applications such as fossil fuel combustion monitoring and indoor air quality monitoring for ventilation and air conditioning systems.

Investigation of Modal Characteristics of Silicon Nitride Ridge Waveguides for Enhanced Refractive Index Sensing

This paper investigates the wavelength-dependent sensitivity of a ridge waveguide based on a silicon nitride (Si3N4) platform, combining numerical analysis and experimental validation. In the first part, the modal characteristics of a Si3N4 ridge waveguide are analyzed in detail, focusing on the effective refractive index (neff), evanescent field ratio (EFR), and propagation losses (αprop). These parameters are critical for understanding the interplay of guided light with the surrounding medium and optimizing waveguide design for sensing applications. In the second part, the wavelength-dependent sensitivity of a racetrack ring resonator (RTRR) based on the Si3N4 waveguide is experimentally demonstrated. The results demonstrate a clear increase in the sensitivity of the RTRR, rising from 116.3 nm/RIU to 143.3 nm/RIU as the wavelength shifts from 1520 nm to 1600 nm. This trend provides valuable insights into the device’s enhanced performance at longer wavelengths, underscoring its potential for applications requiring high sensitivity in this spectral range.

A novel active switchable multi-channel waveguide based on the Bragg scattering mechanism and the force-magnetic coupling effect

PurposeThis study aims to propose a novel acoustic metamaterial waveguide with active switchable channels by changing the magnetic field strength.Design/methodology/approachBased on the Bragg scattering mechanism and the force-magnetic coupling effect of magnetorheological elastomer (MRE), an acoustic metamaterial waveguide structure containing lead scatterers and an MRE/rubber matrix is constructed. By changing the external magnetic field strength, the bandgap of the acoustic metamaterial can be adjusted, and then the channels of the proposed acoustic metamaterial waveguide can be actively switched. The bandgap ranges of acoustic metamaterials containing scatterers with different sizes are different and by designing the size of the scatterers, an acoustic metamaterial waveguide can be formed. The design and control method of this study will be useful for the design of waveguides and active control of bandgaps.FindingsThe proposed switchable multi-channel waveguide and active control method can effectively control the elastic wave propagation, and the opening and closing of the channel are achieved.Practical implicationsThis study provides a new control method for waveguides and expands the application range of MRE. The proposed design concept of adjustable waveguides can be extended for the design of waveguides, metamaterials and vibration reduction structures.Originality/valueThis article proposes a waveguide structure controlled by an external magnetic field in a non-contact manner based on the principle of Bragg scattering and the force-magnetic coupling effect. The model is established, and its feasibility is demonstrated through numerical simulations.

A phononic crystal waveguide using surface waves below the sound cone

Surface acoustic waves are commonly used in a variety of radio-frequency electrical devices as a result of their operation at high frequencies and robust nature. For devices based on Rayleigh-like plane waves, functionality is based on the fact that the Rayleigh wave mode is confined at the solid–air interface. However, to create advanced functionality through the use of phononic crystal structures, standard cylindrical inclusions have been shown to couple Rayleigh modes to the shear horizontal bulk modes and provide a significant pathway to energy loss. We introduce alternative inclusion shapes with a reduced two-fold symmetry that lowers the speed of the Rayleigh-like surface acoustic wave to below that of the shear horizontal mode. With an eigenfrequency below the sound line, the mode is confined to the surface with limited coupling and loss to the bulk. Based on these inclusions, an acoustic waveguide design is proposed, which demonstrates a strong confinement of wave energy both at the surface and within the waveguide.

Fabrication of periodically poled lithium niobate waveguides for broadband nonlinear photonics

Nonlinear optics is the precursor for many of the modern-day applications of photonics, including femtosecond pulse synthesis, precision spectroscopy, and metrology. In the last decade, nanophotonic waveguides have not only boosted the efficiencies of nonlinear effects but also unlocked new degrees of freedom in the design process and enabled the monolithic integration of multiple nonlinear devices. Now, the advent of thin-film variants of platforms with a strong second-order nonlinearity such as lithium niobate-on-insulator (LNOI) enables entirely new applications while further improving efficiency for the existing ones. However, suitable fabrication processes are needed to exploit the full potential of these new platforms. Here, we introduce a process for fabricating high-confinement lithium niobate waveguides with periodic poling. Our waveguide designs enable both third-order nonlinear χ(3) broadening and sum frequency generation (SFG) up to the fourth harmonic through a quasi-phase-matched χ(2) section. In supercontinuum (SC) experiments, our devices produce multi-octave SC spectra when pumped with an 80 fs mode-locked laser at 1560 nm.

Metasurface-Based Phosphor-Converted Micro-LED Architecture for Displays─Creating Guided Modes for Enhanced Directionality.

Phosphor-converted micro-light emitting diodes (micro-LEDs) are a crucial technology for display applications but face significant challenges in light extraction because of the high refractive index of the blue pump die chip. In this study, we design and experimentally demonstrate a nanophotonic approach that overcomes this issue, achieving up to a 3-fold increase in light extraction efficiency. Our approach involves engineering the local density of optical states (LDOS) to generate quasi-guided modes within the phosphor layer by strategically inserting a thin low-index spacer in combination with a metasurface for mode extraction. We demonstrate the trade-offs between blue light pumping, LDOS enhancement at the converted emission wavelength, and radiation pattern control using a stratified system solver for dipole emission. Experimentally, the integration of plasmonic antennas and a silica spacer resulted in a 3-fold overall brightness enhancement, with nearly a 4-fold increase in forward emission. This nanophotonic metasurface waveguide design is a critical advancement for producing bright, directional micro-LEDs, particularly in augmented/virtual reality (AR/VR) devices and smartwatch displays, without the need for bulky secondary optics or reflectors.

Ultrabroadband tunable difference frequency generation in a standardized thin-film lithium niobate platform

Thin-film lithium niobate (TFLN) on an insulator is a promising platform for nonlinear photonic integrated circuits (PICs) due to its strong light confinement, high second-order nonlinearity, and flexible quasi-phase-matching for three-wave mixing processes via periodic polling. Among the three-wave mixing processes of interest, difference frequency generation (DFG) can produce long-wave infrared (IR) light from readily available near IR inputs. While broadband DFG is well studied for mid-IR frequencies, achieving broadband idler generation within the telecom window (near C-band) and the short-wave infrared (near 2 micron) is more challenging due to stringent dispersion profile requirements, especially when using standardized TFLN thicknesses. In this paper, we investigate various standard waveguide designs to pinpoint favorable conditions for broadband DFG operation covering several telecom bands. Our simulations identify viable designs with a possible 3-dB conversion efficiency bandwidth (CE-BW) of 300 nm and our measurements show idler generation from 1418 nm to 1740 nm, limited by our available sources, experimentally confirming our design approach. Furthermore, temperature tuning allows a further shift of the idler towards the mid-IR, up to 1819 nm. We also achieve a stretched wavelength range of idler generation by leveraging the longitudinal variation of the waveguide in addition to poling. Finally, our numerical simulations show the possibility of extending the CE-BW up to 780 nm while focusing on waveguide cross-sections that are available for fabrication within a foundry. Our work provides a methodology that bridges the deviations between fabricated and designed cross-sections, paving a way for standardized broadband DFG building blocks.

Broadband polarization-maintaining anti-resonant fiber design via swarm intelligence.

In this study, we utilized a discrete point configuration method in conjunction with genetic algorithm (GA) and particle swarm optimization (PSO) to design broadband polarization-maintaining anti-resonant fibers (PM-ARFs). The resulting structure features a confinement loss (CL) below 0.17 dB/m, birefringence of approximately 8.2 × 10-5, and a polarization loss ratio (PLR) close to 297 at 1550 nm, providing a usable bandwidth of 340 nm across a range from 1.27 μm to 1.61 μm. This design approach provides additional design freedom beyond parameter optimization and component stacking typical of intelligent design methods. Our study offers a robust design methodology for broadband PM-ARFs, with significant implications for the design of other non-uniform optical waveguides.

Design and FDTD simulation of a remote-controllable visible-light plasmonic waveguide and refractive-index-sensitive switch

In this paper, a plasmonic switch based on metal-insulator-metal (MIM) waveguide structure is proposed, whose transmission characteristics can be remotely controlled by a rake switch design. The distance from the remote control unit to the bus waveguide is more than 1 um and still maintains a very high efficiency. The refractive-index-dependent transmission spectrum of this filter was simulated using the finite-difference time-domain method. The results show that the on/off switching of wave propagation in the bus waveguide can be achieved by changing the refractive index of the control unit 1 µm away from the bus waveguide. A change in refractive index of only 0.2 is required to simultaneously control the switching off and on of four waves in the waveguide in the visible band, with corresponding switching ratios of 40.5, 74.3, 48.6, and 15.1, showing great potential for applications in refractive index sensors and remotely controllable band stop filters.

Anti-resonant reflecting acoustic rib waveguides for strong opto-acoustic interaction

Few known material systems can simultaneously guide optical and elastic fields through total internal reflection. This natural limit has restricted the realization of strong optoacoustic effects to highly specialized and purpose-built platforms, which employ either exotic materials or complex waveguide designs. Here, we apply the concept of Anti-Resonant Reflecting Acoustic Waveguides (ARRAWs) as a potential solution to this issue. ARRAWs confine the elastic field to a high-elastic-velocity core via the anti-resonances of a cladding layer of lower elastic velocity. We numerically study the appearance and dispersion of ARRAW-guided modes in a conventional silicon-on-insulator rib waveguide geometry. Applying the technique to the problem of efficient backward stimulated Brillouin scattering, we predict that ARRAW guidance, in conjunction with conventional optical confinement, can produce Brillouin gains comparable with those of more exotic geometries.

Advancing pure ray tracing for the simulation of volume holographic optical elements: innovations in diffractive waveguide-based augmented reality systems

As augmented reality (AR) glasses technology evolves, volume holographic diffractive waveguide designs are increasingly adopted to enhance portability and performance. Traditionally, these systems require separate geometric and wave optics approaches to handle ray propagation and holographic element diffraction, adding significant complexity to the design process. This study presents an innovative pure ray tracing simulation method that integrates geometric and wave optics seamlessly. By incorporating Kogelnik's coupled wave theory, our model accurately predicts the diffraction behavior of volume holographic optical elements (VHOEs) and converts this information into ray data for tracing, enabling exact AR imaging simulations. Applied to the design of volume holographic waveguide AR glasses, human vision simulations, and experiments validated this method's reliability, demonstrating its versatility and effectiveness. This model improves design efficiency and promotes innovative advancements in cross-theoretical optical system design, positioning it as a crucial tool for future AR glasses development.

Miniaturized Arrow-Shaped Flexible Filter-Embedded Antenna for Industrial and Medical Applications

This paper presents the design and characterization of a coplanar waveguide (CPW) fed, low-profile, and flexible arrow-shaped filtenna for ISM band applications at 2.45 GHz. The antenna design involves an innovative approach incorporating etching slots to achieve miniaturization by 34%, contrasting with a traditional quadrilateral-shaped antenna. After the attainment of desired miniaturization, the unwanted harmonics are also mitigated by deploying simple filtering methodology. A perpendicular rectangular stub is strategically introduced to the feedline, effectively minimizing harmonics across a broad frequency range of 3.3–11.0 GHz. Through simulations and measurements, the results indicate that the antenna’s operational band spans from 2.276 to 2.75 GHz, encompassing the entire ISM band (2.4–2.5 GHz). Notably, the antenna demonstrates promising radiation characteristics, including omnidirectional gain of approximately 2.2 dBi and a radiation efficiency exceeding 95%. With a compact overall size of 0.24λ × 0.20λ × 0.0005λ (where λ is the free-space wavelength at 2.45 GHz), coupled with wide harmonic rejection property, the proposed arrow-shaped flitenna emerges as a compelling candidate for ISM band applications.

Analytical formulae for design of one-dimensional sonic crystals with smooth geometry based on symbolic regression

Even though locally periodic structures have been studied for more than three decades, the known analytical expressions relating the waveguide geometry and the acoustic transmission are limited to a few special cases. Having an access to numerical model is a great opportunity for data-driven discovery. Our choice of cubic splines to parametrize the waveguide unit cell geometry offers enough variability for waveguide design. Using Webster equation for unit cell and Floquet–Bloch theory for periodic structures, a dataset of numerical solutions was prepared. Employing the methods of physics-informed machine learning, we have extracted analytical formulae relating the waveguide geometry and the corresponding dispersion relation or directly the bandgap widths. The results contribute to the overall readability of the system and enable a deeper understanding of the underlying principles. Specifically, it allows for assessing the influence of the waveguide geometry, offering more efficient alternative to computationally demanding numerical optimization.

Subwavelength-modulated silicon photonics for low-energy free-electron-photon interactions.

We investigate silicon waveguides with subwavelength-scale modulation for applications in free-electron-photon interactions. The modulation enables velocity matching and efficient interactions between low-energy electrons and co-propagating photons. Specifically, we design a subwavelength-grating (SWG) waveguide for interactions between 23-keV free electrons and ≈1500-nm photons. The SWG waveguide and electron system exhibit a coupling coefficient of |gQu| = 0.23, and as we corroborate with time-domain, particle-in-cell simulations, the system operates as a backward-wave oscillator. Overall, our results show that modulated waveguides could open the door to strong, extended interactions between photons and low-energy (10-keV-scale) electrons, like those typically present in scanning electron microscopes. Additionally, our SWG waveguide design suggests that periodic waveguides could offer intriguing dispersion engineering opportunities for tailoring these interactions.

In Situ Multiphysical Metrology for Photonic Wire Bonding by Two-Photon Polymerization.

Femtosecond laser two-photon polymerization (TPP) technology, known for its high precision and its ability to fabricate arbitrary 3D structures, has been widely applied in the production of various micro/nano optical devices, achieving significant advancements, particularly in the field of photonic wire bonding (PWB) for optical interconnects. Currently, research on optimizing both the optical loss and production reliability of polymeric photonic wires is still in its early stages. One of the key challenges is that inadequate metrology methods cannot meet the demand for multiphysical measurements in practical scenarios. This study utilizes novel in situ scanning electron microscopy (SEM) to monitor the working PWBs fabricated by TPP technology at the microscale. Optical and mechanical measurements are made simultaneously to evaluate the production qualities and to study the multiphysical coupling effects of PWBs. The results reveal that photonic wires with larger local curvature radii are more prone to plastic failure, while those with smaller local curvature radii recover elastically. Furthermore, larger cross-sectional dimensions contribute dominantly to the improved mechanical robustness. The optical-loss deterioration of the elastically deformed photonic wire is only temporary, and can be fully recovered when the load is removed. After further optimization based on the results of multiphysical metrology, the PWBs fabricated in this work achieve a minimum insertion loss of 0.6 dB. In this study, the multiphysical analysis of PWBs carried out by in situ SEM metrology offers a novel perspective for optimizing the design and performance of microscale polymeric waveguides, which could potentially promote the mass production reliability of TPP technology in the field of chip-level optical interconnection.

Monolithic dispersion engineered mid-infrared quantum cascade laser frequency comb

The high-power quantum cascade laser (QCL) frequency comb capable of room temperature operation is of great interest to high-precision measurement and low-noise molecular spectroscopy. While a significant amount of research is devoted to the longwave spectral range, shortwave 3–5 μm QCL combs are still relatively underdeveloped due to the excessive material dispersion. In this work, we propose a monolithic integrated multimode waveguide scheme for effective dispersion engineering and high-power-efficiency operation. Over watt-level output power at room temperature with a wall plug efficiency of 7% and robust dispersion reduction is achieved from a quantum cascade laser frequency comb at a wavelength approximately 4.6 μm. Narrow beatnote linewidth less than 1 kHz and clear dual-comb multiheterodyne comb lines manifest the coherent phase relation among the comb modes which is crucial to fast molecular spectroscopy. This monolithic dispersion engineered waveguide design is also compatible to an efficient active–passive optical coupling scheme and would open up a new research playground for ring comb and on-chip dual-comb spectroscopy.