Waveguide Refractive Index Research Articles

Low Loss Chip‐to‐Chip Couplers for High‐Density Co‐Packaged Optics

An experimentally demonstrated, vertical chip‐to‐chip evanescent coupler between silicon nitride (SiN) and silicon (Si) is presented with the coupler loss measured to be 0.39 1.06 dB at 1550 nm with a 1‐dB bandwidth of 160 nm extending across the C‐band, S‐band, and L‐band (1480–1640 nm). The average coupling loss is determined to be 0.73 dB for the 1480–1640 nm wavelength range with a 2σ tolerance of 0.92 dB. The 1‐dB lateral alignment tolerance is 1.56 0.14 μm at 1550 nm and the average tolerance is 1.38 0.24 μm across the 1480–1640 nm wavelength regime. In addition, the average coupling loss varies by less than 0.35 dB and the average 1‐dB alignment tolerance varies by less than 30 nm for temperatures varying from 23 to 60 °C. Finally, the average coupling loss range is less than 1.5 dB range across four sets of identically packaged die. This is the first experimental demonstration of an interchip, passively assembled evanescent coupler using standard complementary metal‐oxide‐semiconductor foundry processes for directly coupling between Si and SiN, overcoming a waveguide refractive index difference of = 1.32 without requiring taper tip widths of less than 100 nm.

Reduction in Temperature-Dependent Fiber-Optic Gyroscope Bias Drift by Using Multifunctional Integrated Optical Chip Fabricated on Pre-Annealed LiNbO3

The refractive index change obtained after annealed proton exchange (APE) in lithium niobate (LiNbO3) crystals depends on both the proton exchange process carried out in hot acid and the structure of the crystals. In devices produced by the APE method, dislocations and lattice defects within the crystal structure are considered to be primary contributors to refractive index discontinuities and waveguide instability. In this study, the effects of pre-annealing LiNbO3 crystals at 500 °C on multifunctional integrated optical chips (MIOCs) were investigated through interferometric fiber-optic gyroscope (IFOG) system-level tests. It was observed that the pre-annealing process resulted in an improvement in the optical throughput of MIOCs (from %34 to %51) and the temperature-dependent bias drift stability of the IFOG (from 0.031–0.038°/h to 0.012–0.019°/h). The angle random walk (ARW) was measured as 0.0056 deg/√h.

Design and Simulation of a Three-Channel Plasmonic Demultiplexer in an MIM Waveguide

This study proposes a structure for optical plasmonic demultiplexers based on nanodisk/nanoring resonators in metal-insulator-metal (MIM) plasmonic waveguides. The proposed structure can transmit the desired wavelength spectra, including telecommunication and visible waves. It consists of a bus waveguide, several nanodisks and nanorings, and three drop waveguides, consisting of several filters that can be extended to N×1 channels. The FWHM of the designed structure was calculated to be approximately 18 nm. The transmission spectrum of each demultiplexer channel can be easily adjusted by changing the geometric parameters and refractive index. As the waveguide refractive index increases, the transmission spectrum shifts to higher wavelengths, and its amplitude decreases due to internal structural losses. Therefore, the differences between the waveguide and metal refractive indices gradually decrease, and the light confinement decreases in the waveguide, leading to an increase in FWHM. The outputs of each channel have two sharp resonance modes, resulting from the coupling of two interacting cavities (ring and disk) due to the Fano resonance phenomenon. This study employed a two-dimensional (2D) finite difference time domain (FDTD) numerical method and simulation by FDTD Lumerical software to evaluate the transmission properties.

MIM waveguide refractive index sensor with graphene enhanced three-ring nested resonator Fano resonance

In this paper, a metal-insulator-metal (MIM) waveguide featuring a three-ring nested cavity structure is proposed. We enhance and optimize this design by integrating graphene nanostructures into the ferrule resonance cavity, resulting in a hybrid graphene-MIM waveguide structure. The transmission spectra and electric fields generated by the structure are analyzed by using the finite element method (FEM), shedding light on the formation mechanism of various Fano resonance peaks. The validity of our theoretical analysis is confirmed using multimode interference coupled mode theory (MICMT), and the simulation results are largely aligning with the theoretical verification results. Next, by manipulating the chemical potential of graphene, the Fano resonance can be tuned across a wide range of wavelength bands. And this capability enables the realization of a dynamically tunable refractive index sensor for transmission spectroscopy. Under optimized parameter settings, the refractive index sensor achieves a maximum sensitivity of 1266.67nm/RIU, and a maximum figure of merit (FOM) value of 199.67RIU−1. Compared with conventional MIM waveguide devices, this design overcomes the difficulty of conventional MIM waveguide structures that cannot be dynamically tunable. Notably, the wavelength of the Fano resonance formed by the transmission spectrum can be dramatically altered when the refractive index of the medium in the surrounding environment is slightly changed. Therefore, this structure expands the repertoire of integrated optical devices with potential applications, particularly in the field of optical sensors.

3D Laser Writing of Low-Loss Cross-Section-Variable Type-I Optical Waveguide Passive/Active Integrated Devices in Single Crystals.

Optical waveguides fabricated in single crystals offer crucial passive/active optical components for photonic integrated circuits. Single crystals possess inherent advantages over their amorphous counterpart, such as lower optical losses in visible-to-mid-infrared band, larger peak emission cross-section, higher doping concentration. However, the writing of Type-I positive refractive index modified waveguides in single crystals using femtosecond laser technology presents significant challenges. Herein, this work introduces a novel femtosecond laser direct writing technique that combines slit-shaping with an immersion oil objective to fabricate low-loss Type-I waveguides in single crystals. This approach allows for precise control of waveguide shape, size, mode-field, and refractive index distribution, with a spatial resolution as high as 700nm and a high positive refractive index variation on the order of 10-2, introducing new degrees of freedom to design and fabricate passive/active optical waveguide devices. As a proof-of-concept, this work successfully produces a 7mm-long circular-shaped gain waveguide (≈10µm in diameter) in an Er3+-doped YAG single crystal, exhibiting a propagation loss as low as 0.23dBcm-1, a net gain of ≈3dB and a polarization-insensitive character. The newly-developed technique is theoretically applicable to arbitrary single crystals, holding promising potential for various applications in integrated optics, optical communication, and photonic quantum circuits.

3‐5: Distinguished Paper: Full‐color, Wide FoV Single‐layer Waveguide for AR Displays

We analyze the field‐of‐view (FoV) limitations in a single‐layer, full‐color waveguide‐based augmented reality display, revealing key influences from the waveguide's refractive index, exit pupil expansion (EPE) scheme, and combiner's angular response. Based on these analyses, we propose an optimized butterfly EPE scheme with gradient‐pitch polarization volume gratings (PVGs), achieving a theoretical diagonal FoV of 54.06° with a 16:10 aspect ratio.

Full‐color, wide field‐of‐view single‐layer waveguide for augmented reality displays

AbstractIn the quest for more compact and efficient augmented reality (AR) displays, the standard approach often necessitates the use of multiple layers to facilitate a large full‐color field of view (FoV). Here, we delve into the constraints of FoV in single‐layer, full‐color waveguide‐based AR displays, uncovering the critical roles played by the waveguide's refractive index, the exit pupil expansion (EPE) scheme, and the combiner's angular response in dictating these limitations. Through detailed analysis, we introduce an innovative approach, featuring an optimized butterfly EPE scheme coupled with gradient‐pitch polarization volume gratings (PVGs). This novel configuration successfully achieves a theoretical diagonal FoV of 54.06° while maintaining a 16:10 aspect ratio.

Chirp-induced chaotic self-trapped patterns and power controllable interactions in nonlocal nonlinear system with oscillatory responses

We introduced a class of chaotic self-trapped patterns (CSTPs) in nonlocally nonlinear system with sine-oscillatory response, which are induced by the linear chirps. Such chirp-induced CSTPs can exist under a general nonlocality condition, and can be produced by optical beams of any profiles. The CSTPs exhibit both the soliton property and the chaotic property. The mechanism for generating the CSTPs in the nonlinear system with sine-oscillatory response is the multi-reflections by the optical self-induced nonlinear refractive index waveguide, and the multi-interferences between the beams and their reflected counterparts. The interactions of two linearly chirped beams were investigated, which can be controlled by the optical power. We can realize the “ON” and the “OFF” states of light fields by only changing the optical power, and could present a method in the optical switching.

Metal‐Insulator‐Metal Waveguide Plasmonic Sensor System for Refractive Index Sensing Applications

Plasmonic sensors are well known for their miniaturized size and high sensitivity. Herein, a numerical analysis of a metal‐insulator‐metal (MIM) waveguide (WG) for refractive index (RI) sensing applications is proposed. The sensing device is composed of a MIM bus WG with a semicircular formation side coupled to a semicircular cavity. This configuration provides a transmission dip with a high extinction ratio as compared to the standard MIM WG side‐coupled to a semicircular cavity. Moreover, the mode converters are also embedded in the sensing system solving the obstacle of light coupling to the MIM plasmonic WG. The sensitivity of the proposed device is ≈941.33 nm RIU−1, making it a promising candidate to be employed in point‐of‐care (POC) testing. This typically involves the use of portable or handheld diagnostic devices that can quickly analyze samples of blood, urine, or other bodily fluids to diagnose diseases or conditions such as infections, diabetes, or heart disease.

Optical Waveguide Refractive Index Sensor for Biochemical Sensing

This study describes the basic principles of optical waveguide refractive index sensing and the various design structures of refractive index sensors. These waveguides generate different optical resonances, which cause changes in the sensing refractive index and temperature and are subsequently used to detect the concentration in the analyses. First, the structural characteristics and performance indices of the microring sensor and interferometer are studied based on the refractive index of the optical waveguide. Second, the principle and sensing detection mechanism of the two types of refractive index sensing employed in these sensors are analyzed. Then, the two sensors are classified and discussed from the perspective of the waveguide materials and structures, as well as the substances to be measured. Simultaneously, performance indicators such as sensitivity and detection range are compared and summarized. The comparison results show that there is a compromise between the sensitivity and quality factor of the optical waveguide refractive index sensor. Finally, applications of refractive index sensing in the biochemical field for material detection are discussed, showing that the optical waveguide refractive index sensor has significant advantages over other types of biochemical optical sensors.

Plasmonic Sensors beyond the Phase Matching Condition: A Simplified Approach.

The conventional approach to optimising plasmonic sensors is typically based entirely on ensuring phase matching between the excitation wave and the surface plasmon supported by the metallic structure. However, this leads to suboptimal performance, even in the simplest sensor configuration based on the Otto geometry. We present a simplified coupled mode theory approach for evaluating and optimizing the sensing properties of plasmonic waveguide refractive index sensors. It only requires the calculation of propagation constants, without the need for calculating mode overlap integrals. We apply our method by evaluating the wavelength-, device length- and refractive index-dependent transmission spectra for an example silicon-on-insulator-based sensor of finite length. This reveals all salient spectral features which are consistent with full-field finite element calculations. This work provides a rapid and convenient framework for designing dielectric-plasmonic sensor prototypes-its applicability to the case of fibre plasmonic sensors is also discussed.

Simple yet effective analysis of waveguide mode symmetry: generalized eigenvalue approach based on Maxwell's equations.

Particular waveguide structures and refractive index distribution can lead to specified degeneracy of eigenmodes. To obtain an accurate understanding of this phenomenon, we propose a simple yet effective approach, i.e., generalized eigenvalue approach based on Maxwell's equations, for the analysis of waveguide mode symmetry. In this method, Maxwell's equations are reformulated into generalized eigenvalue problems. The waveguide eigenmodes are completely determined by the generalized eigenvalue problem given by two matrices (M, N), where M is 6 × 6 waveguide Hamiltonian and N is a constant singular matrix. Close examination shows that N usually commute with the corresponding matrix of a certain symmetry operation, thus the waveguide eigenmode symmetry is essentially determined by M, in contrast to the tedious and complex procedure given in the previous work [Opt. Express25, 29822 (2017)10.1364/OE.25.029822]. Based on this new approach, we discuss several symmetry operations and the corresponding symmetries including chiral, parity-time reversal, rotation symmetry, wherein the constraints of symmetry requirements on material parameters are derived in a much simpler way. In several waveguides with balanced gain and loss, anisotropy, and geometrical symmetry, the analysis of waveguide mode symmetry based on our simple yet effective approach is consistent with previous results, and shows perfect agreement with full-wave simulations.

Strong coupling between colloidal quantum dots and a microcavity with hybrid structure at room temperature

The interaction between light and matter has always been the focus of quantum science, and the realization of truly strong coupling between an exciton and the optical cavity is a basis of quantum information systems. As a special semiconductor material, colloidal quantum dots have fascinating optical properties. In this study, the photoluminescence spectra of colloidal quantum dots are measured at different collection angles in microcavities based on hybrid refractive-index waveguides. The photon bound states in the continuum are found in the low–high–low refractive-index hybrid waveguides in the appropriate waveguide width region, where the photoluminescence spectra of colloidal quantum dots split into two or more peaks. The upper polaritons and lower polaritons avoid resonance crossings in the systems. The Rabi splitting energy of 96.0 meV can be obtained. The observed phenomenon of vacuum Rabi splitting at room temperature is attributed to the strong coupling between quantum dots and the bound states in the continuum.

A Voyage from Plasmonic to Hybrid Waveguide Refractive Index Sensors Based on Wavelength Interrogation Technique: a Review

A Voyage from Plasmonic to Hybrid Waveguide Refractive Index Sensors Based on Wavelength Interrogation Technique: a Review

Complex Waveguide Supermode Analysis of Coherently-Coupled Microcavity Laser Arrays

Coherently coupled microcavity lasers have desirable properties for emerging applications. We use 2-dimensional complex refractive index waveguide modeling of 2-element photonic crystal vertical cavity surface emitting laser arrays to analyze their supermodes. The complex modal effective indices are used in turn to calculate the complex coupling coefficient between the laser array elements. An analysis of the effects of array design parameters, such as photonic crystal period, fill-factor, or confinement, to engineer the coupling coefficient for the desired properties is given and example designs for 850 nm arrays are presented.

Si photonic crystal optical antenna serial array and frequency-modulated continuous-wave light detection and ranging action

Photonic crystal waveguide slow-light grating emits a free-space optical beam and steers it widely by changing the optical wavelength or waveguide refractive index. In the reverse process, returned light is coupled into the device again. We have proposed to use this optical transmission and reception antenna as a beam scanner for light detection and ranging (LiDAR). Ideally, a large-aperture antenna can narrow the transmission beam and enhance the reception efficiency. Actually, however, the transmission and reception performance is not scalable owing to waveguide loss even though the waveguide is simply lengthened. A serial array configuration in which the waveguide is divided into multiple antennas is effective for mitigating this problem. In this study, we fabricated such a device using Si photonics technology and obtained a small beam divergence of 0.02° at a telecom wavelength. Then, we observed the ranging operation by adding an optical setup of frequency-modulated continuous-wave (FMCW) LiDAR and confirmed that the divided antenna device improved the reception intensity by 12 dB. Moreover, we fabricated a FMCW LiDAR chip in which the serial array antennas were integrated in parallel with switch trees and Ge photodiodes and obtained point cloud images by two-dimensional beam scanning.

Waveguide-based total internal reflection fluorescence microscope enabling cellular imaging under cryogenic conditions.

Total internal reflection fluorescence (TIRF) microscopy is an important imaging tool for the investigation of biological structures, especially the study on cellular events near the plasma membrane. Imaging at cryogenic temperatures not only enables observing structures in a near-native and fixed state but also suppresses irreversible photo-bleaching rates, resulting in increased photo-stability of fluorophores. Traditional TIRF microscopes produce an evanescent field based on high numerical aperture immersion objective lenses with high magnification, which results in a limited field of view and is incompatible with cryogenic conditions. Here, we present a waveguide-based TIRF microscope, which is able to generate a uniform evanescent field using high refractive index waveguides on photonic chips and to obtain cellular observation at cryogenic temperatures. Our method provides an inexpensive way to achieve total-internal-reflection fluorescence imaging under cryogenic conditions.

Low-threshold-current (~85 mA) of AlGaN-based UV-B laser diode with refractive-index waveguide structure

We report on a refractive-index waveguide AlGaN-based ultraviolet-B band (UV-B) laser diode grown on a sapphire substrate, which achieves a laser oscillation at a low threshold current (I th), ∼85 mA. The refractive index waveguide structure in a ridge-type structure is fabricated by a unique method combining dry inductively coupled plasma reactive ion etching and wet etching with tetramethylammonium hydroxide solution. Using this structure, the longitudinal-mode transverse optical confinement is successfully achieved, in which the ridge width is 3 μm, demonstrating a low I th of UV-B laser diode.

Designer Glasses—Future of Photonic Device Platforms

AbstractThe intentional inclusion of key atomic elements in a purpose designed glass helps to achieve unprecedented control over the ultrafast laser written circular waveguide morphology and refractive index change. Behavioral response of glass constituents to ultrafast laser in 14 different commercial silicate glasses having various compositions are studied. Viscosity, aluminum to alkaline earth+alkali ratio, and total silicon content within the glass are the prime control factors for producing waveguides with high circularity and refractive index change. Drawing on this knowledge, the designer glass is successfully fabricated from an empirical formula that facilitates maintaining circular waveguide morphology, high refractive index over fast feed rates, and amorphous composition.

Highly Sensitive Sensor Structure Based on Sol-Gel Waveguide Films and Grating Couplers

The technologies of optical planar evanescent wave chemical and biochemical sensors require chemically resistant, high refractive index waveguide films having very good optical transmission properties. In this paper we present such two-compound SiOx:TiOy waveguide films fabricated by using the sol-gel method and the dip-coating technique. These films not only have high optical quality and low propagation losses but also an extremely high refractive index of >1.90 (λ = 632.8 nm). Further we demonstrate efficient and simple sensing structures, designed and fabricated based on these films. For this purpose, grating couplers with a period of Λ = 417 nm were fabricated on the interface between a waveguide film and cover using the single-step nanoimprint method. These sensing structures were tested as planar refractometers. The results of the theoretical analysis on the basis of which the structures were designed as well as results of their experimental characterization are presented in this work. Consequently, the relationship between parameters and the sensitivity of investigated sensing structures is discussed. As a result, the profitable properties of the designed grating coupler sensors are verified and excellent consistency between the results of the theoretical analysis and experimental results is achieved.