Straight Waveguide Research Articles

Low-loss bent channel waveguides in lithium niobate thin film by proton exchange and dry etching

We propose and demonstrate an efficient method combining proton exchange with dry etching for the fabrication of low-loss bend channel waveguides in lithium niobate (LN) thin film. Our proposed method introduces the chemical etching caused by F+ ion to increase the etching rate. Our fabricated straight and bent channel waveguides have a trapezoid cross section with a top width of ~1.0 µm, a height of ~900 nm, and a slope of ~20° with respect to the vertical direction. To the best of our knowledge, this is the largest etching depth but with a small slope reported up to now. Mode intensity distributions and insertion losses were measured at 1.55 µm wavelength and bending losses were deduced. The results show that our fabricated bent channel waveguide with a radius of 20 μm can achieve low bending losses of 0.455 dB/90° and 0.488 dB/90° for the fundamental quasi-TE (qTE) and quasi-TM (qTM) modes, respectively. Compared with the fabrication methods reported so far, our method can realize a faster etching rate and a larger etching depth while maintaining a high etching quality.

Modeling optical transmissivity of graphene grate in on-chip silicon photonic device

Abstract A three-dimensional (3-D) finite-difference-time-domain (FDTD) analysis was used to simulate a silicon photonic waveguide. We have calculated power and transmission of the graphene used as single or multilayers to study the light transmission behavior. A new technique has been developed to define the straight silicon waveguide integrated with grate graphene layer. The waveguide has a variable grate spacing to be filled by the graphene layer. The number of graphene atomic layers varies between 100 and 1000 (or 380 nm and 3800 nm), the transmitted power obtained varies as ∼30% and ∼80%. The ∼99%, blocking of the light was occurred in 10,000 (or 38,000 nm) atomic layers of the graphene grate.

Analysis of Kerr nonlinearity enhancement in silicon nitride waveguides using one-dimensional slow light structure

A corrugated based slow-light silicon nitride waveguide is studied numerically to enhance Kerr nonlinearity by increasing the group index of the waveguide at the band edge. The structure is optimized taking into account the limitations existing in CMOS-compatible platforms. Bandwidth limitations of the structure are investigated at different wavelengths for various slow-down factors. Finite size realizations of the structure are studied using transfer matrix method to obtain the desired transmission and group velocity at the band edge. Optical loss corresponding to slow-light effect is modeled, and the limitations imposed by the slow-light loss on the nonlinear phase shift saturation are considered. It is shown that by employing slow-down factor of 3, the nonlinear phase shift is improved by a factor of 5. Also, in comparison with a straight waveguide, shorter corrugated waveguides (up to nine times) can provide the same accumulated nonlinear phase shift.

Low Power Consumption VOA Array With Air Trenches and Curved Waveguide

The paper introduces a 24-channel air-trench isolated Silica-based VOA array. The air trenches thermally insulate the VOAs with etched-free waveguide technologies. Reactive ion etching and wet etching were used at different channel locations to suppress thermal crosstalk and reduce power consumption. When thermal crosstalk is declined by 3 dB for the straight waveguide, the power consumption was decreased from 195 to 95 mW because the lightwave radiation can be scattered by the air trench in the dark state. This crosstalk is improved by 9 dB for the curved waveguide. The polarization dependent loss falls down to 0.28 dB at 15 dB attenuation since the air trench and etched-free waveguide also releases the stress.

Reconfigurable Plasmonic Logic Gates

Reconfigurable one-, two-, and three-bit plasmonic logic gate configurations have been proposed, which work by covering a straight slot waveguide with materials with tunable dielectric constants, such as graphene. By encoding the logic states in the values of dielectric constants as opposed to different waveguides, the plasmon excitation problems are minimized and the simplified logic gate configurations could be easily implemented.

GaN microring waveguide resonators bonded to silicon substrate by a two-step polymer process.

Using a polymer bonding technique, GaN microring waveguide resonators were fabricated on a Si substrate for future hybrid integration of GaN and Si photonic devices. The designed GaN microring consisted of a rib waveguide having a core of 510nm in thickness, 1000nm in width, and a clad of 240nm in thickness. A GaN crystalline layer of 1000nm in thickness was grown on a Si(111) substrate by metal organic chemical vapor deposition using a buffer layer of 300nm in thickness for the compensation of lattice constant mismatch between GaN and Si crystals. The GaN/Si wafer was bonded to a Si(100) wafer by a two-step polymer process to prevent it from trapping air bubbles. The bonded GaN layer was thinned from the backside by a fast atom beam etching to remove the buffer layer and to generate the rib waveguides. The transmission characteristics of the GaN microring waveguide resonators were measured. The losses of the straight waveguides were measured to be 4.0±1.7 dB/mm around a wavelength of 1.55μm. The microring radii ranged from 30 to 60μm, where the measured free-spectral ranges varied from 2.58 to 5.30nm. The quality factors of the microring waveguide resonators were from 1710 to 2820.

Low-Loss Si3N4 TriPleX Optical Waveguides: Technology and Applications Overview

An overview of the most recent developments and improvements to the low-loss TriPleX Si3N4 waveguide technology is presented in this paper. The TriPleX platform provides a suite of waveguide geometries (box, double stripe, symmetric single stripe, and asymmetric double stripe) that can be combined to design complex functional circuits, but more important are manufactured in a single monolithic process flow to create a compact photonic integrated circuit. All functionalities of the integrated circuit are constructed using standard basic building blocks, namely straight and bent waveguides, splitters/combiners and couplers, spot size converters, and phase tuning elements. The basic functionalities that have been realized are: ring resonators and Mach–Zehnder interferometer filters, tunable delay elements, and waveguide switches. Combination of these basic functionalities evolves into more complex functions such as higher order filters, beamforming networks, and fully programmable architectures. Introduction of the active InP chip platform in a combination with the TriPleX will introduce light generation, modulation, and detection to the low-loss platform. This hybrid integration strategy enables fabrication of tunable lasers, fully integrated filters, and optical beamforming networks.

Fully suspended slot waveguide platform

A fully suspended slot waveguide (FSSWG) platform, including straight slot waveguides, 90° bends, high-Q racetrack resonators, and strip-to-slot mode converters, is demonstrated for broadband and low-loss operation in the mid-infrared spectral region. The proposed FSSWG platform has inherent merits of a broad spectral range of transparency which is limited only by the absorption of silicon, strong light–analyte interaction, good mechanical stability, and single lithography step fabrication process. By using asymmetric FSSWGs, the propagation loss, bending loss, and intrinsic optical Q factor are demonstrated to be 2.8 dB/cm, 0.15 dB/90°, and 12 600, respectively. The average conversion efficiency of a mode converter is 95.4% over a bandwidth of 170 nm and 97.0% at 2231 nm. The FSSWG platform would be promising for a long-range and cavity-enhanced light–analyte interaction.

GaN-based integrated photonics chip with suspended LED and waveguide

We propose a GaN-based integrated photonics chip with suspended LED and straight waveguide with different geometric parameters. The integrated photonics chip is prepared by double-side process. Light transmission performance of the integrated chip verse current is quantitatively analyzed by capturing light transmitted to waveguide tip and BPM (beam propagation method) simulation. Reduction of the waveguide width from 8 μm to 4 μm results in an over linear reduction of the light output power while a doubling of the length from 250 μm to 500 μm only results in under linear decrease of the output power. Free-space data transmission with 80 Mbps random binary sequence of the integrated chip is capable of achieving high speed data transmission via visible light. This study provides a potential approach for GaN-based integrated photonics chip as micro light source and passive optical device in VLC (visible light communication).

Asymmetric double QW superluminescent diodes for broadband and high‐power use

AbstractWe report the use of broadband and high‐power InGaAs/GaAs asymmetric double quantum‐well (ADQW) superluminescent diodes (SLDs), which consist of a tilted, bent, and straight waveguide. To examine the effects of the arrangement of the ADQW and the gain provided by a straight waveguide on the output performances, we fabricated SLDs with different lengths for two types of ADQW, that is, a wide QW at the p‐side for Sample 1, and a narrow QW at the p‐side for Sample 2, and tested their spectral width and output power. The fabricated SLDs with a straight length (L3) of 80 μm show −3 dB spectral widths of 122 and 116 nm and output powers of 20 and 16 mW at an injection current of 250 mA for Samples 1 and 2, respectively. For both samples, it appears that, as the L3 increases, the spectral width decreases and the output power increases. From this result, we confirmed that both the spectral width and the output power can be adjusted and optimized at a given structure for various applications.

Design and Modeling of the Photonic Crystal Waveguide Structure for Heat‐Assisted Magnetic Recording

The application of the photonic crystal (PC) waveguide (WG) as the light delivery system in the heat‐assisted magnetic recording (HAMR) system is demonstrated. The structure consists of a 90° bending PC waveguide and a ridge dielectric waveguide taper coupler. Three‐dimensional (3D) models of structures are built and simulated in order to determine light coupling and transmission efficiencies. Geometric parameters including the taper length (LTP), coupler inlet width (WFW), and PC waveguide width (WWG) are investigated. The initial simulation shows that the transmission efficiency of over 90% can be achieved with the coupler integrated with the straight PC waveguide. However, the overall transmission efficiency is substantially reduced to 53.8% when the coupler is attached to the 90° bending PC waveguide. Our analysis shows that the wave mode matching and light decay rate in the waveguide cavity are significant contributing factors. The transmission efficiency increases to around 60.8% after some modification of the bending region.

Widely tunable fractional-order photonic differentiator using a Mach–Zenhder interferometer coupled microring resonator

We demonstrate a tunable temporal photonic fractional-order differentiator based on a Mach–Zenhder interferometer (MZI) coupled microring. An MZI structure is designed to replace conventional coupling structure of straight waveguide and ring waveguide. When the refractive index of one arm of the MZI is adjusted by thermo-optic effect, the coupling coefficient between the straight waveguide and the microring can be changed and a tunable photonic differentiator is implemented. In the experiment, the device presents a tunable differentiation order range from 0.25 to 1.75, which is the largest tuning range of a tunable photonic differentiator using a single microring resonator to the best of our knowledge.

Terahertz spoof surface-plasmon-polariton subwavelength waveguide

Surface plasmon polaritons (SPPs) with the features of subwavelength confinement and strong enhancements have sparked enormous interest. However, in the terahertz regime, due to the perfect conductivities of most metals, it is hard to realize the strong confinement of SPPs, even though the propagation loss could be sufficiently low. One main approach to circumvent this problem is to exploit spoof SPPs, which are expected to exhibit useful subwavelength confinement and relative low propagation loss at terahertz frequencies. Here we report the design, fabrication, and characterization of terahertz spoof SPP waveguides based on corrugated metal surfaces. The various waveguide components, including a straight waveguide, an S-bend waveguide, a Y-splitter, and a directional coupler, were experimentally demonstrated using scanning near-field terahertz microscopy. The proposed waveguide indeed enables propagation, bending, splitting, and coupling of terahertz SPPs and thus paves a new way for the development of flexible and compact plasmonic circuits operating at terahertz frequencies.

Investigating the Characteristics of a Double Circular Ring Resonators Slow Light Device Based on the Plasmonics-Induced Transparency Coupled with Metal-Dielectric-Metal Waveguide System

We have numerically investigated an analog of electromagnetically induced transparency (EIT) in a metal-dielectric-metal (MDM) waveguide bend. The geometry consists of two asymmetrical stubs extending parallel to an arm of a straight MDM waveguide bend. Finite-difference time-domain simulations show that a transparent window is located at 1550 nm, which is the phenomenon of plasmonic-induced transparency (PIT). Signal wavelength is assumed to be 820 nm. The velocity of the plasmonic mode can be largely slowed down while propagating along the MDM bends. Multiple-peak plasmon-induced transparency can be realized by cascading multiple cavities with different lengths and suitable cavity-cavity separations. Large group index up to 73 can be obtained at the PIT window. Our proposed configuration may thus be applied to storing and stopping light in plasmonic waveguide bends. In addition, the relationship between the transmission characteristics and the geometric parameters including the radius of the nano-ring, the coupling distance, and the deviation length between the stub and the nano-ring is studied in a step further. The velocity of the plasmonic mode can be largely slowed down while propagating along the MDM bends. For indirect coupling, formation of transparency window is determined by resonance detuning, but, evolution of transparency is mainly attributed to the change of the coupling distance. Theoretical results may provide a guideline for control of light in highly integrated optical circuits. The characteristics of our plasmonic system indicate a significant potential application in integrated optical circuits such as optical storage, ultrafast plasmonic switch, highly performance filter, and slow light devices.

Two-dimensional interferometric characterization of laser-induced refractive index profiles in bulk Topas polymer

In this paper we precisely determine laser-induced refractive index profiles created in cyclic olefin copolymer Topas 6017 employing a sophisticated phase shifting Mach-Zehnder interferometry approach. Beyond the usual one-dimensional modification depth measurement we highlight that for straight waveguide structures also a two-dimensional refractive index distribution can be directly obtained providing full information of a waveguide's exact cross section and its gradient refractive index contrast. Deployed as direct data input in optical waveguide simulation, the evaluated 2D refractive index profiles permit a detailed calculation of the waveguides' actual mode profiles. Furthermore, conventional one-dimensional interferometric measurements for refractive index depth profiles with varying total imposed laser fluence of a 248 nm KrF excimer laser are included to investigate the effect on refractive index modification depth. Maximum surface refractive index increase turns out to attain up to 1.86·10−3 enabling laser-written optical waveguide channels. Additionally, a comprehensive optical material characterization in terms of dispersion, thermo-optic coefficient and absorption measurement of unmodified and UV-modified Topas 6017 is carried out.

Fabrication of optical waveguides inside transparent silica xerogels containing PbS quantum dots using a femtosecond laser

The fabrication and photoluminescence properties of waveguide structures inside transparent silica xerogels containing PbS quantum dots (QDs) have been investigated. Stable dispersion of water-soluble PbS QDs, which emit in the wavelength region of 1.3 μm, in silica xerogels was conducted by a sol–gel method using 3-aminopropyltrimethoxysilane. It was found that both the photoluminescence intensity and the dispersion stability are highly sensitive to catalysts, reaction temperature and time, and annealing temperature. Continuous and straight waveguide structures with refractive-index change were obtained by choosing appropriate irradiation conditions. The fabricated waveguides led approximately 60% increase in the intensity of the photoluminescence generated from the PbS QDs.

Efficient, High Brightness 1030 nm DBR Tapered Diode Lasers With Optimized Lateral Layout

Efficient, high brightness 1030 nm distributed Bragg reflector (DBR) tapered diode lasers with optimized lateral waveguide and grating designs are presented in this paper. The layout changes were carried out in order to enhance the diffraction efficiency of the manufactured DBR gratings and improve the spatial mode filtering for these lasers. In this study, lasers with straight waveguide and grating, straight waveguide and tapered grating, and tapered waveguide and straight grating are compared. The lasers provide an optical output power up to 15.5 W and narrowband emission over the whole power range. Within the studied parameter range, the highest central lobe output power of 10.1 W is obtained for a laser with a tapered grating. The layout comparison also shows that the influence of the applied ridge waveguide injection current density on the central lobe power is significantly different. Here, the lateral design with a tapered grating is the most robust layout. Compared to previous approaches for DBR tapered diode lasers, optimized waveguide and grating designs and their impact on laser performance may help to increase the application potential of such devices.

Directionally induced quasi-phase matching in homogeneous AlGaAs waveguides.

We report on the experimental observation of quasi-phase matching in a homogeneous waveguide. By fabricating a monolithic snake-shaped suspended AlGaAs nanowire on a (001) GaAs wafer, we demonstrate the unraveled version of a χ(2) whispering-gallery-mode microdisk, obtaining second-harmonic generation in the optical telecom wavelength range. With a radius of curvature of 50μm and four spatial oscillations along the (110) average direction, a splitting of the second-harmonic spectrum occurs around the phase-matching wavelength of the corresponding straight waveguide. This splitting, which increases as the radius of curvature decreases, provides a useful degree of freedom for the design of small-footprint nonlinear photonic devices on-chip.

Experimental demonstration of a broadband two-mode multi/demultiplexer based on asymmetric Y-junctions

A broadband two-mode multi/demultiplexer using asymmetric Y-junctions is designed and experimentally demonstrated on a silicon-on-insulator platform for on-chip mode-division multiplexing applications. Within a bandwidth from 1513 to 1619 nm, the fabricated device, which consists of a two-mode multiplexer, a multimode straight waveguide, and a two-mode demultiplexer, exhibits demultiplexing crosstalk of less than −9.1 dB. The demultiplexing crosstalk as low as −42.1 dB, lower than −12.8 dB over the C band can be obtained. The measured insertion loss varies from 0.40 to 0.56 dB at a wavelength of 1550 nm. A transmission experiment of 10 Gbit/s electrical signals carried on TE0 and TE1 modes is successfully achieved with open and clear eye diagrams.

Full-vectorial propagation model and modified effective mode area of four-wave mixing in straight waveguides.

We derive from Maxwell's equations full-vectorial nonlinear propagation equations of four-wave mixing valid in straight semiconductor-on-insulator waveguides. Special attention is given to the resulting effective mode area, which takes a convenient form known from studies in photonic crystal fibers, but has not been introduced in the context of integrated waveguides. We show that the difference between our full-vectorial effective mode area and the scalar equivalent often referred to in the literature may lead to mistakes when evaluating the nonlinear refractive index and optimizing designs of new waveguides. We verify the results of our derivation by comparing it to experimental measurements in a silicon-on-insulator waveguide, taking tolerances on fabrication parameters into account.