Rib Waveguide Research Articles

Fabrication of low‐loss symmetrical rib waveguides based on x‐cut lithium niobate on insulator for integrated quantum photonics

AbstractLithium niobate on insulator (LNOI) is a promising material platform for applications in integrated quantum photonics. A low optical loss is crucial for preserving fragile quantum states. Therefore, in this study, we have fabricated LNOI rib waveguides with a low optical propagation loss of 0.16 dB/cm by optimizing the etching conditions for various parameters. The symmetry and smoothness of the waveguides on ‐cut LNOI are improved by employing a shallow etching process. The proposed method is expected to facilitate the development of on‐chip quantum photonic devices based on LNOI.

Broadband and widely tunable second harmonic generation in suspended thin-film LiNbO3 rib waveguides

Our study demonstrates second harmonic generation (SHG) in a high confinement LiNbO3 rib waveguide through type-I birefringence phase matching of fundamental modes. The combination of micro-waveguide dispersion and material birefringence reveals unique SHG characteristics that complement the performance of standard LiNbO3 on insulator (LNOI) components. Dual-pump wavelength phase matching in the near-infrared and mid-infrared regions is shown in a given waveguide. These two wavelengths can be positioned in the 1.1–3.5 μm range or converge near 1.5 μm by adjusting the core waveguide size or through temperature tuning. A 25 °C temperature change enables a broad pump tunability band of 300 nm, ranging from 1350 to 1650 nm, with a conversion efficiency exceeding 40 %/W/cm2 within a single waveguide. A temperature tuning range of up to 900 nm is foreseen by tailoring the waveguide core size. In addition, a broadband response of 150 nm within the telecom window is demonstrated experimentally. The nonlinear waveguide, etched in a thin film membrane, is combined with titanium-indiffused waveguides to form a monolithic LiNbO3 component. This configuration provides low coupling losses of 0.8 dB and single-mode operation. It paves the way for a new generation of versatile and cost-effective frequency conversion components with wide-band spectral responses suitable for optical communications, environmental sensing, and quantum information processing.

Tunable narrow band-rejection and band-pass filter based on long-period waveguide grating on a thin-film lithium niobate rib waveguide

We propose and demonstrate experimentally an electro-optic (EO) and thermo-optic (TO) tunable wavelength filter with band-rejection and band-pass dual-function. Our proposed filter is based on a long-period waveguide grating (LPWG) formed on a lithium niobate on insulator (LNOI) rib waveguide with a channel-shaped polymer cladding waveguide. The LPWG formed on the surface of the LNOI core enables efficient mode coupling between the two fundamental modes of the LNOI waveguide and the polymer cladding waveguide and hence dual-function filtering. For the z-/x-polarized input light, our fabricated best filter shows an 18.1-dB/8.2-dB band-rejection contrast and a 15.7-dB/17.4-dB band-pass contrast at 1565.26/1594.24 nm wavelength, with a relatively narrow 3-dB bandwidth of 2.5/2.3 nm, respectively. In addition, the fabricated filter also shows an EO tuning efficiency of ∼32 pm/V and a TO tuning efficiency of 0.465 nm/°C, respectively. Our proposed filter could find applications in wavelength division multiplexing, temperature sensing, and other fields of optical communication and optical information processing.

Numerical methods for modal analysis in optical rib waveguide structures: a review

Numerical methods for modal analysis in optical rib waveguide structures: a review

Scalable transfer printing approach to heterogeneous integration of InP lasers on silicon-on-insulator waveguide platform

InP-based edge-emitting O-band lasers are integrated onto silicon photonics circuit employing micro-transfer printing technology. Blocks of unpatterned InP gain material of typical size 1000 × 60 μ m2 are first transferred onto 400 nm thick silicon rib waveguides with the fabrication steps performed on the target wafer to realize the final lasers. As a result, the InP ridge waveguides are aligned with lithographic accuracy to the underlying Si waveguides resulting in an approach free from any misalignment stemming from the transfer printing process. The fabricated Distributed Bragg Reflector laser shows lasing around 100 mA current injection with minimum 1 mW of output power coupled to a single mode fiber. This integration method paves a reliable route toward scaling-up the integration of active devices such as lasers, modulators, and detectors on 300-mm diameter silicon wafers, which requires high-uniformity across the wafer.

Effective and group refractive index extraction and cross-sectional dimension estimation for silicon-on-insulator rib waveguides

Effective and group refractive index extraction and cross-sectional dimension estimation for silicon-on-insulator rib waveguides

Reversible Laser Imprinting of Phase Change Photonic Structures in Integrated Waveguides.

Formation of laser-induced periodic surface structures (LIPSS) is known as a fast and robust method of functionalization of material surfaces. Of particular interest are LIPSS that manifest as periodic modulation of phase state of the material, as it implies reversibility of phase modification that constitute rewritable LIPSS, and recently was demonstrated for chalcogenide phase change materials (PCMs). Due to remarkable properties of chalcogenide PCMs─nonvolatality, prominent optical contrast and ns switching speed─such novel phase change LIPSS hold potential for exciting applications in all-optical tunable photonics. In this work we explore phase change LIPSS formation in thin films of Ge2Sb2Te5 (GST) integrated with planar and rib waveguides. We demonstrate that by fine-tuning laser radiation, the morphology of phase change LIPSS can be controlled, including their period and fill factor, and investigate the limitations of multicycle rewriting of the structures. We also demonstrate the formation of phase change LIPSS on a 1D waveguide, which has potential for use as tunable Bragg filters or structures for on-demand light decoupling into the far-field. The presented concept of applying phase change LIPSS offers a promising approach to enable fast and simple tuning in integrated photonic devices.

Distributed Bragg reflector based ASE noise removal pump wavelength filters for futuristic chip-scale quantum photonic circuits

This article reports a novel design of a compact tunable resonance filter with a highly extinguished and ultra-broad out-of-band rejection for on-chip amplified spontaneous noise suppression from pump lasers highly demanding for generating pure/entangled photon pairs via χ(3) process in a CMOS compatible silicon photonics technology platform. The proposed device is designed with two identically apodized distributed grating structures for guided Fabry-Perot resonant transmissions in a silicon-on-insulator rib waveguide structure. The device design parameters are optimized by theoretical simulation for a low insertion loss singly-resonant transmission peak at a desired wavelength. We observed that a device length of as low as ∼ 35 µm exhibits a rejection band as large as ∼ 60 nm with an extinction of ∼ 40 dB with respect to the resonant wavelength peak at λ r ∼ 1550 nm (FWHM ∼ 80 pm, IL ∼ 2 dB). The experimental results have been shown to be closely matching to our theoretical simulation and modeling results in terms of its stop bandwidth and resonance wavelength for noise suppressed pump laser wavelength filtering. As expected from the theoretical prediction, the trend pertaining to the trade-off between passive insertion loss and Q-value of the resonances has been observed depending on the device parameters. The thermo-optic tuning characteristics of resonant wavelengths have been obtained by integrating microheaters. The resonance peak could be tuned at a rate of 96 pm per mW of consumed thermal power. Noise associated with an amplified pump wavelength (λ P ∼ 1550 nm) has been shown to be suppressed (∼ 40-dB), up to the detector noise floor.

Octave spanning and flat-top supercontinuum generation in standard foundry-compatible process-made Ge–Sb–Se chalcogenide glass waveguide

We reported near octave-spanning supercontinuum generation in Ge28Sb12Se60 chalcogenide glass rib waveguides. The waveguides were fabricated using foundry-compatible deep ultra-violet lithography, followed by plasma etching. We demonstrated an average propagation loss of ∼1.3 dB/cm at the 1550 nm telecommunication wavelength. With dispersion engineering, optimizing waveguide geometry, and pumping by a multi-wavelength femtosecond soliton fiber laser, we achieved a flat-top supercontinuum generation spanning from 1290 to 2400 nm and beyond . It has a 3 dB bandwidth of 436 nm and a 20 dB bandwidth of more than 900 nm. The implementation of such waveguides provides a practical broadband light source solution for on-chip spectroscopy and sensing systems.

Estimation of losses caused by sidewall roughness in thin-film lithium niobate rib and strip waveguides

Samples of dielectric optical waveguides of rib or strip type in thin-film lithium niobate (TFLN) technology are characterized with respect to their optical loss using the Fabry-Pérot method. Attributing the losses mainly to sidewall roughness, we employ a simple perturbational procedure, based on rigorously computed mode profiles of idealized channels, to estimate the attenuation for waveguides with different cross sections. A single fit parameter suffices for an adequate modelling of the effect of the waveguide geometry on the loss levels.

Characterizing mid-infrared micro-ring resonator with frequency conversion

Due to the high cost, low-performance lasers and detectors in the mid-infrared (MIR) band, the development of MIR-integrated devices is very slow. Here, we demonstrate an effective method to characterize the parameters of MIR devices by using frequency conversion technology. We designed and fabricated rib waveguides and the micro-ring resonators (MRRs) on a silicon-on-sapphire platform. The MIR laser for the test is generated by difference frequency generation, and the transmission spectrum of the MIR-MRRs is detected by sum frequency generation. The experimental results show that the waveguide transmission loss is 4.5 dB/cm and the quality factor of the micro-ring reaches 38000, which is in good agreement with the numerical simulations. This work provides a useful method to characterize MIR integrated devices based on the frequency conversion technique, which can boost the development of MIR integrated optics in the future.

Silicon rib waveguide with palladium coated gold nanorods overlayer for hexane and N-methylaniline detection

The adsorption of hexane and N-methylaniline was investigated by on-chip vibrational spectroscopy. The interaction with the Pd surface significantly perturbs the vibrational modes of hexane. Near-infrared absorption spectroscopy on the waveguide detected a redshifted CH stretching vibration. We showed that while rapid evaporation of one component may occur, the modification facilitates the sensing for large molecules and mixtures of substances. Rib waveguides were modified with palladium to exhibit an enhanced electromagnetic field effect due to the electrostatic interaction of Pd and hydrogen, showing the drop in the amine overtone absorption of 7.6 dB and the overtone related to the bond of carbon to hydrogen of 6.8 dB. The absorption of hexane bonds in water is 5.5 dB. The obtained values are 100 times higher than those predicted by the electromagnetic simulator for pure rib waveguides. This may be explained by CH bonds in hexane that can be activated through a hydrogen bond-like interaction with metal substrates. This achievement holds promising potential for advancing hexane and N-Methylailine detection and analysis technologies, with implications for diverse applications in various industries, such as environmental monitoring, industrial processes, and safety measures.

Rational design of an integrated directional coupler for wideband operation

We consider a design procedure for directional couplers for which the coupling length is approximately wavelength-independent over a wide bandwidth. We show analytically that two coupled planar waveguides exhibit a maximum in the coupling strength, which ensures both wideband transmission and minimal device footprint. This acts as a starting point for mapping out the relevant part of phase space. This analysis is then generalized to the fully three-dimensional geometry of rib waveguides using an effective medium approximation. This forms an excellent starting point for fully numerical calculations and leads to designs with unprecedented bandwidths and compactness.

Design of enlarged-asymmetric-power single-mode silicon evanescent lasers with asymmetric distributed feedback gratings

Great achievements in the bonded III–V lasers have pushed the productization process of silicon photonic chips. In this work, we report the design of silicon evanescent lasers with asymmetric gratings, which can provide high output power and stable single-longitudinal-mode operation. The high coupling efficiency and low threshold current density are achieved by linearly expanding the width of rib waveguide from 0.4 µm to 2 µm through 150 µm taper coupler. By optimizing the λ/4 phase-shift position ratio from conventional 0.50 to 0.64, the ratio of output power at both sides of the bonded lasers (P2/P1) is improved from1.0 to 5.0, and the normalized grating coupling coefficient (κL) remains at 2.78. By optimizing the duty cycle at one side of λ/4 phase-shift from 0.5 to 0.8 and another side remains at 0.5, P2/P1 is 3.3, and κL lowers from 2.78 to 2.21. Besides, keeping the phase-shift position ratio at 0.64 while changing the duty cycle to 0.7, P2/P1 increases significantly to 7.5 with κL of 2.59. This work provides a step forward towards the development of high-power silicon-based light sources.

On-chip photothermal gas sensor based on a lithium niobate rib waveguide

On-chip optical gas sensors have attracted a lot of attention in many fields such as the internet of things and point-of-care diagnosis due to their compactness and high scalability. However, the current laser-based on-chip absorption gas sensors have limited performance caused by weak gas absorbance and strong interference fringe noise. Here, we report sensitive on-chip photothermal gas detection on an integrated lithium niobate photonic platform. The evanescent wave of the frequency-modulated pump light (2004 nm) on a nanophotonic lithium niobate rib waveguide (length of 91.2 mm) is absorbed by the target gas molecules (carbon dioxide, CO2), leading to the significant refractive index modulation of the rib waveguide due to the photothermal effect. A probe light (1550 nm) transmitted through the same waveguide undergoes the photothermal-induced phase modulation, which is sensitively detected by a heterodyne interferometer. As a proof of concept, we demonstrate the on-chip photothermal detection of CO2 with a minimum detection limit of 870 ppm. This work provides new insight for future on-chip gas sensor development with high sensitivity and robustness.

Liquid-crystal clad tunable wavelength de-multiplexer and coupler/filter in silicon nitride rib waveguide structure

We report long-period waveguide grating (LPWG) based tunable wavelength de-multiplexer and tunable coupler/filter in liquid crystal (LC) clad rib waveguides on silicon nitride on insulator platform. High birefringence and electrically tunable refractive index of the MLC-9200-000 LC have been used to achieve a tuning range covering most of the S, C and L bands of the optical communication system. The structure consists of two parallel rib waveguides with an elevated slab. We have considered a new concept of injecting power into a higher-order slab mode. The power from the slab mode is coupled to the rib modes using LPWGs inscribed on the sidewalls of the ribs. The structure works as a tunable wavelength de-multiplexer when the grating periods in both ribs are different. The wavelength tunability of the de-multiplexer is achieved by an external voltage applied across the liquid crystal cell. Our numerical results show wavelength tuning of the de-multiplexer in the range 1.437 - 1.627 μm. The structure works as an electrically tunable coupler/filter when the grating period of both the gratings are the same. We numerically demonstrate the tuning of the coupler in the wavelength range 1.487 - 1.572 μm and that of the filter in the range 1.484 - 1.577 μm with 20-dB rejection.

Ultra-broadband on-chip power splitters for arbitrary ratios on silicon-on-insulator.

We propose and demonstrate on-chip power splitters based on adiabatic rib waveguide enabling arbitrary splitting ratios on a monolithic silicon photonic platform. The devices are elaborately engineered based on adiabatic directional couplers with a trapezoid-structure in the longitudinal direction in the mode evolution region. The measurement results indicate that the proposed devices can achieve over 150 nm bandwidth for arbitrary splitting ratios of 50%:50%, 70%:30% and 90%:10%. The mode evolution footprint is greatly narrowed to below 79 µm with an insertion loss of less than 0.22 dB. The demonstrated arbitrary ratio power splitters offer a promising application prospect in high-density photonic integrated circuits.

Mitigating the bending losses of the silica-titania-based rib waveguide structure

Mitigating the bending losses of the silica-titania-based rib waveguide structure

Surface Bragg gratings of proteins patterned on integrated waveguides for (bio)chemical analysis.

The incorporation of biomacromolecules onto silicon waveguiding microstructures constitutes a growing trend that pushes towards compact and miniaturized biosensing systems. This paper presents the integration of one-dimensional periodic nanostructures of proteins on the surface of micrometric silicon waveguides for transducing binding events between biomacromolecules. The study demonstrates this new bioanalytical principle by experimental results and theoretical calculations, and proves that rib waveguides (1--1.6-µm width) together with protein gratings (495--515-nm period) display suitable spectral responses for this optical biosensing system. Protein assemblies of bovine serum albumin are fabricated on the surface of silicon nitride waveguides, characterized by electron microscopy, and their response is measured by optical frequency domain reflectometry along the fabrication process and the subsequent stages of the biorecognition assays. Detection and quantification limits of 0.3 and 3.7 µg·mL-1, respectively,of specific antibodies are inferred from experimental dose-response curves. Among other interesting features, the results of this study point towards new miniaturized and integrated sensors for label-free bioanalysis.

Semi-Analytical Approach versus Finite Element Method for Analysis of Propagation Properties in Rectangular Waveguides: Silica-Titania Technological Platform

This work explicitly demonstrates a semi-analytical effective index approximation (EIA) approach for the description of the propagation properties of rib and ridge waveguides. By using the example of waveguides realized on a low-cost silica-titania (SiO2:TiO2) technological platform, we present that EIA may be successfully applied for the approximate determination of modal effective indices and single mode propagation conditions. All obtained results have been confirmed to be convergent with the finite element method (FEM) simulations at low relative error. Due to the tremendously fast execution time of EIA simulations in comparison with the FEM solver, we believe that the presented approach may be applied in a preliminary step of designing functional blocks in new and existing photonic integrated circuit technologies, which often require complex and multi-parameter calculations.