Passive Waveguide Research Articles

Integrated dispersion compensated mode-locked quantum dot laser

Quantum dot lasers are excellent on-chip light sources, offering high defect tolerance, low threshold, low temperature variation, and high feedback insensitivity. Yet a monolithic integration technique combining epitaxial quantum dot lasers with passive waveguides has not been demonstrated and is needed for complex photonic integrated circuits. We present here, for the first time to our knowledge, a monolithc offset quantum dot integration platform that permits formation of a laser cavity utilizing both the robust quantum dot active region and the versatility of passive GaAs waveguide structures. This platform is substrate agnostic and therefore compatible with the quantum dot lasers directly grown on Si. As an illustration of the potential of this platform, we designed and fabricated a 20 GHz mode-locked laser with a dispersion-engineered on-chip waveguide mirror. Due to the dispersion compensation effect of the waveguide mirror, the pulse width of the mode-locked laser is reduced by a factor of 2.8.

Electro-Absorption Optical Modulator Based on Graphene-Buried Polymer Waveguides

By exploiting the electro-absorption characteristic of graphene, we proposed and theoretically demonstrated an electro-absorption optical modulator (EOM) based on the graphene-buried polymer waveguides. By burying the graphene layers inside the polymer waveguide, acquiring a strong graphene-light interaction when the graphene layers buried in the center of core, which bring about an important change of the optical mode and effective mode index in the waveguide. The width of the graphene capacitor was also optimized to enhance the bandwidth of the device. Calculations show that the proposed EOM can realize 29 dB extinction ratio, 7 dB modulation depth and 42 GHz bandwidth with an 800 μm-long active region. The required switching voltage form its minimum to maximum absorption state is 1.6 V and 1.55 pJ/bit energy consumption can be achieved. The proposed modulator can remedy the lack of high-speed optical modulator on the passive polymer waveguide and have a potential application for polymer-based photonic integrated circuits.

III-N/Si₃N₄ Integrated Photonics Platform for Blue Wavelengths

In this paper, we report a low-loss photonic integrated circuits (PICs) platform at blue wavelengths of the visible spectral regime. Silicon nitride (SiN) is a popular passive waveguide material due to its fabrication flexibility, CMOS compatibility and spectral transparency in this wavelength regime. For active devices including lasers, gallium nitride (GaN) and its alloys are considered. Several basic building blocks for the development of a complete integrated platform, including blue diode lasers, in-plane- and out-of-plane light couplers, as well as on-and off-chip coupling between these active and passive components are theoretically investigated. The proposed in-plane and out-of-plane architectures operate through edge- and vertical grating couplers (VGCs), respectively. With edge-coupling, large mode-mismatch between the GaN laser diode and SiN waveguide is alleviated through nanotapers on both the active and passive sections and the calculated peak coupling efficiency is achieved to be 74% at a wavelength of 450 nm. We also separately designed efficient VGCs for coupling light from standard, commercial, off-the-shelf fibers to the SiN chip, and edge couplers for fiber-chip coupling, exhibiting coupling efficiencies of 51% and 83%, respectively. For robust on-chip light-coupling between active and passive circuit elements, with relaxed alignment tolerances, two approaches, i.e., flip-chip based hybrid integration and evanescent coupling based heterogeneous integration are studied. Calculated maximum coupling efficiencies of 40% (-4 dB) are achieved for both the hybrid and heterogeneous schemes. The theoretical work performed is an initial step towards demonstrating complex blue PICs which could offer a comprehensive range of photonic functionalities.

Silicon Photonics: Silicon Photonic Platform for Passive Waveguide Devices: Materials, Fabrication, and Applications (Adv. Mater. Technol. 8/2020)

A silicon photonic chip can integrate multiple waveguide devices. The front cover conceptually illustrates a chip with functional devices. In the image, provided by Yikai Su an co-workers (article number 1901153), the trapezoidal shapes represent the mode and polarization combining/splitting devices, while the ring resonators perform the tunable wavelength (de)multiplexing function. The fabrication process of such photonic chip is generally compatible with that in the CMOS microelectronics industry.

Integration of low loss vertical slot waveguides on SOI photonic platforms for high efficiency carrier accumulation modulators.

Silicon accumulation type modulators offer prospects of high power efficiency, large bandwidth and high voltage phase linearity making them promising candidates for a number of advanced electro-optic applications. A significant challenge in the realisation of such a modulator is the fabrication of the passive waveguide structure which requires a thin dielectric layer to be positioned within the waveguide, i.e. slotted waveguides. Simultaneously, the fabricated slotted waveguide should be integrated with conventional rib waveguides with negligible optical transition losses. Here, successful integration of polysilicon and silicon slot waveguides enabling a low propagation loss 0.4-1.2 dB/mm together with an ultra-small optical mode conversion loss 0.04 dB between rib and slot waveguides is demonstrated. These fabricated slot waveguide with dielectric thermal SiO2 layer thicknesses around 6 nm, 8 nm and 10 nm have been characterized under transmission electron microscopy allowing for strong carrier accumulation effects for MOS-capacitor electro-optic modulators.

Silicon Photonic Platform for Passive Waveguide Devices: Materials, Fabrication, and Applications

AbstractSilicon photonics has attracted tremendous interest from academia and industry, as the fabrication of the silicon family of photonic devices is mostly compatible with the microelectronics process using complementary metal‐oxide semiconductors (CMOS). Herein, three silicon‐family materials are discussed: silicon, silicon nitride, and silica. In addition, hybrid integration with a 2D material, graphene, is examined. First, the material and waveguide properties are reviewed. Second, typical fabrication processes for waveguide devices are introduced. Subsequently, a variety of passive waveguide devices, operating at different physical dimensions covering wavelength, polarization, and mode, are discussed. They correspond to fixed and tunable filters, polarization beam splitters and rotators, and mode conversion and multiplexing devices. These passive waveguide devices play important roles in a wide range of applications including telecom, interconnects, computing, sensing, quantum information processing, bio‐photonics, and energy.

Characterization and Production of PCB Structures With Increased Ratio of Electromagnetic Field in Air

Producing passive printed circuit board (PCB) structures in the millimeter-wave range includes some fundamental challenges, such as low manufacturing costs and high fabrication accuracy. These aspects, in general, have to be combined with low signal attenuation, precise simulation models, and a reliable producibility. This article shows a new method to manufacture different kinds of passive RF waveguides with reduced attenuation and decreased electrical length on low-cost PCB substrates. First, the complex material parameters of the used substrate and the used copper foil are extracted from 10 to 100 GHz to have accurate simulation models in CST Microwave Studio. This is necessary because the parameters provided by the manufacturer are often no longer generally valid in this frequency range. In addition, PCB materials have a significantly higher surface roughness compared with conventional materials for millimeter-wave applications. Therefore, the correct determination and the correct modeling of surface roughness is particularly important. Next, laminate is removed partially on the outer layer of the PCB to increase the electromagnetic field ratio in air. The undesired substrate is removed by a CO2 laser drilling machine by setting narrow drills sequentially. The RF performance of these waveguides is compared with those manufactured in conventional PCB technology according to attenuation and producibility.

Dynamics of on-chip asymmetrically coupled semiconductor lasers

We investigate the dynamics of asymmetrically coupled semiconductor lasers on photonic integrated circuits in experiment and theory. The experimental observations are explained using a rate-equation model for coupled lasers incorporating a saturable coupling waveguide. We perform a bifurcation analysis of the coupled laser dynamics, focusing on the effects of the coupling phase and the dynamical difference between passive and saturable coupling waveguides. For a passive waveguide, we find a bifurcation scenario closely resembling the well-known optical injection setup, which is largely insensitive to the coupling phase. When the coupling waveguide is saturable, the dynamics become increasingly complex and unpredictable, with a strong phase-dependence. Our results show the possibility of a simple layout for reproducible laser dynamics on a chip.

InGaAs Membrane Waveguide: A Promising Platform for Monolithic Integrated Mid-Infrared Optical Gas Sensor.

Mid-infrared (mid-IR) absorption spectroscopy based on integrated photonic circuits has shown great promise in trace-gas sensing applications in which the mid-IR radiation directly interacts with the targeted analyte. In this paper, considering monolithic integrated circuits with quantum cascade lasers (QCLs) and quantum cascade detectors (QCDs), the InGaAs-InP platform is chosen to fabricate passive waveguide gas sensing devices. Fully suspended InGaAs waveguide devices with holey photonic crystal waveguides (HPCWs) and subwavelength grating cladding waveguides (SWWs) are designed and fabricated for mid-infrared sensing at λ = 6.15 μm in the low-index contrast InGaAs-InP platform. We experimentally detect 5 ppm ammonia with a 1 mm long suspended HPCW and separately with a 3 mm long suspended SWW, with propagation losses of 39.1 and 4.1 dB/cm, respectively. Furthermore, based on the Beer-Lambert infrared absorption law and the experimental results of discrete components, we estimated the minimum detectable gas concentration of 84 ppb from a QCL/QCD integrated SWW sensor. To the best of our knowledge, this is the first demonstration of suspended InGaAs membrane waveguides in the InGaAs-InP platform at such a long wavelength with gas sensing results. Also, this result emphasizes the advantage of SWWs to reduce the total transmission loss and the size of the fully integrated device's footprint by virtue of its low propagation loss and TM mode compatibility in comparison to HPCWs. This study enables the possibility of monolithic integration of quantum cascade devices with TM polarized characteristics and passive waveguide sensing devices for on-chip mid-IR absorption spectroscopy.

Comparing Performances of Five Distinct Automatic Classifiers for Fin Whale Vocalizations in Beamformed Spectrograms of Coherent Hydrophone Array

A large variety of sound sources in the ocean, including biological, geophysical, and man-made, can be simultaneously monitored over instantaneous continental-shelf scale regions via the passive ocean acoustic waveguide remote sensing (POAWRS) technique by employing a large-aperture densely-populated coherent hydrophone array system. Millions of acoustic signals received on the POAWRS system per day can make it challenging to identify individual sound sources. An automated classification system is necessary to enable sound sources to be recognized. Here, the objectives are to (i) gather a large training and test data set of fin whale vocalization and other acoustic signal detections; (ii) build multiple fin whale vocalization classifiers, including a logistic regression, support vector machine (SVM), decision tree, convolutional neural network (CNN), and long short-term memory (LSTM) network; (iii) evaluate and compare performance of these classifiers using multiple metrics including accuracy, precision, recall and F1-score; and (iv) integrate one of the classifiers into the existing POAWRS array and signal processing software. The findings presented here will (1) provide an automatic classifier for near real-time fin whale vocalization detection and recognition, useful in marine mammal monitoring applications; and (2) lay the foundation for building an automatic classifier applied for near real-time detection and recognition of a wide variety of biological, geophysical, and man-made sound sources typically detected by the POAWRS system in the ocean.

Tuning SPP propagation length of hybrid plasmonic waveguide by manipulating evanescent field

Exploring hybrid plasmonic waveguide is a major breakthrough for achieving low-loss lightwave propagation at subwavelength size. Various types of passive hybrid plasmonic waveguides have been reported, but research on active hybrid plasmonic waveguides is very limited. In this study, we propose a tunable hybrid plasmonic waveguide in which its modal characteristics could be dynamically tuned by manipulating the evanescent field of a dielectric waveguide using a closely suspended tuning waveguide. The device can obtain a long-distance lightwave propagation at centimeter scale while maintaining the propagation mode at deep-subwavelength size (2.44λ2/105). Using the proposed tuning method, the propagation length can be tuned more than 100% at the same propagation mode area. The proposed device with a propagation length that can be tuned in a wide range is a promising candidate for building active photonic components.

High Absorption Contrast Quantum Confined Stark Effect in Ultra-Thin Ge/SiGe Quantum Well Stacks Grown on Si

We report on the performance of the quantum confined Stark effect (QCSE) in ultra-thin (~350 nm) Ge/SiGe quantum well stacks grown on Si. We demonstrate an absorption contrast $\Delta \alpha /\alpha $ of 2.1 at 1 Vpp swing in QCSE stacks grown on ultra-thin (100 nm) strain relaxed GeSi buffer layers on 300 mm Si wafers. Such ultra-thin QCSE stacks will enable future integration of highly efficient QCSE electro-absorption modulators with low optical coupling loss to passive Si waveguides in a sub-micron silicon photonics platform.

Automated machine learning approaches for humpback whale vocalization classification

The vocalization behavior of humpback whales was previously studied and mapped over instantaneous wide areas of the Gulf of Maine (GOM), spanning more than 100 km in diameter, using the passive ocean acoustic waveguide remote sensing technique during their Fall feeding season. Acoustic signals were received on a 160-element hydrophone array system where beamforming was employed to significantly improve signal-to-noise ratio of the received vocalizations. The humpback whale vocalizations can be divided into two classes, song and non-song calls. Song vocalizations are composed of repeatable set of phrases with consistently short inter-pulse intervals. The non-song vocalizations, such as ‘bow-shaped’ and ‘downsweep’ moans, have large and highly variable inter-pulse intervals and no repeatable pattern. Here we employ machine learning approaches to classify humpback whale vocalizations into song and non-song calls. Preprocessing methods including frequency filtering, wavelet denoising, beamforming, and spectral smoothing are applied. Several automated classification methods including Support Vector Machines, Gaussian Naive Bayes, and Neural Networks are explored. Implementation of these algorithms on the GOM dataset results in over 88% classification accuracy implying the machine learning approaches can be used in field studies for real-time classification.

Simulation and design of a compact GaAs based tunable dual-wavelength diode laser system

We present our design of a compact, integrated and tunable dual-wavelength diode laser system emitting around 785 nm, which is of interest for several applications like Raman spectroscopy and the generation of THz radiation. To achieve a more compact device compared to previous GaAs based designs two etch depths are realized, leading to shallowly etched ridge waveguides in regions were optical gain is applied and deeply etched waveguides used to enable compact integrated waveguide components. The device parameters are optimized using a numerically efficient simulation tool for passive waveguides. Subsequently, the entire laser system is further analyzed applying a sophisticated traveling-wave equation based model for active devices giving access to internal intensity and carrier density distributions. It is shown that active laser simulations are crucial to deduce critical and performance limiting design aspects not accessible via an all-passive simulation.

Tunable Acoustic Nonreciprocity in Strongly Nonlinear Waveguides with Asymmetry

While there has been exciting progress in the emerging area of nonlinear acoustic nonreciprocity, one of the main challenges is to realize $p\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}s\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}v\phantom{\rule{0}{0ex}}e$ nonreciprocal acoustic waveguides with predictable, reliable broadband operation. Here the authors demonstrate an entirely passive nonreciprocal acoustic waveguide that enables transmission of acoustic waves in a preferred direction. Moreover, due to its strong nonlinearity, this nonreciprocity is passively adaptive to energy and can be predictively tuned by the intensity of the applied broadband excitation. Related applications span telecommunication, optics, and biomedical imaging.

Spatial Photocontrol of the Optical Output from an Organic Crystal Waveguide.

The versatility in mechanical properties and the capability of optical waveguiding of molecular crystals have attracted research on the potential application of these materials in optomechanical transduction. Here, we demonstrate spatial photocontrol over the optical output from slender single crystals of an azo compound, 3',4'-dimethyl-4-(dimethylamino)azobenzene that can be used as a crystalline optical waveguide. The position of the free end of a single crystal can be controlled through reversible photoswitching between the trans and cis isomers at the irradiated crystal surface. The passive optical waveguiding capability of the crystal remains unaffected by its deformation induced by exposure to UV light. Moreover, the response time of the material by bending upon irradiation can be thermally regulated to control the positioning of the tip of the crystal. These single-crystal organic actuators with dual (optical and photomechanical) response deliver on the long sought for dynamic all-organic optical elements to be incorporated in microcircuits.

Integrated and Interconnected Array of Light-Emitting Transistors and Transistor Lasers on Silicon for Photonic Logic

A scalable method of integrating III-V epitaxial structures onto silicon and producing a large-area array of interconnected three-terminal photonic devices in the integrated material is demonstrated. This work establishes CMOS-compatible techniques for GaAs-based active photonic material (n-InGaP/p-GaAs/n-GaAs) to be converted into a distribution of epitaxial islands with direct thermal and electrical connection to a silicon host wafer. A temporary epitaxial bonding process orients and etches modified heterojunction bipolar transistor (HBT) material into a gridded mesh of islands (collector-side up) on a silicon-based carrier wafer with device-level alignment. A subsequent epitaxial transfer creates a permanent metal-eutectic bond between the III-V islands and the silicon host wafer that serves as a uniform electrical interconnect between front-end-of-line processed CMOS and the collector terminals of a photonic chip layer. Thermal, electrical, and mechanical characterization of the integrated material is performed to ensure that the interconnect is suitable for fabrication of photonic devices into the larger-area transferred material. A separate epitaxial process for uniting passive waveguide materials to the same host wafer is then similarly performed. The result is a closely-aligned grid where each III-V island is surrounded by passive material islands. The integrated III-V islands (emitter-side up) are then patterned in unison into an array of light-emitting transistors (LET) and edge-emitting transistor lasers (TL) that are designed for efficient optical coupling into waveguides formed in the passive material, functioning together as active and passive devices in a single network. Device processing parameters are inspected and optimized to account for variations in processing on integrated (thin-film) III-V materials as compared to monolithic device processing, as well as to adhere to CMOS-compatible methods and metallization. The resulting array of three-terminal photonic devices are linked by means of a vertical electrical interconnect through the collector contact and a lateral optical interconnect aligned to the carrier-recombination quantum well (QW) in the base region. Optical and electrical testing (DC) is performed on the integrated devices both individually and as an integrated network for electrical and optical output characteristics, while a comparison to fabricated monolithic three-terminal photonic devices is utilized to relate performance of integrated active photonic devices. Initial work on high-speed testing is performed to benchmark the thermal and electrical improvements in integrated photonic devices. Simulation of the device characteristics is further employed to investigate methods to optimize optical output in relation to electrical gain and beta suppression. The techniques and processes established enable a scalable network of photonic devices for novel heterogeneously integrated electronic-photonic circuitry. This heterogeneous integration produces a platform for pairing a photonic chip layer with a CMOS host wafer such that the interconnects for electrical and optical signals allow for photonic logic designs. Figure 1

Homogeneous photonic integration of mid-infrared quantum cascade lasers with low-loss passive waveguides on an InP platform

Mid-infrared (mid-IR, λ≈3–12 μm) photonic integrated circuits on low-loss passive waveguide platforms are of significant interest for a wide range of mid-IR applications. Quantum cascade lasers (QCLs) are currently the only room-temperature electrically pumped semiconductor light sources that can operate in a continuous-wave at room temperature over the entire mid-IR spectral range. Given very high thermal dissipation in QCL active regions, achieving long-term reliability and continuous-wave operation of heterogeneously integrated devices on silicon platforms is challenging. Here we experimentally demonstrate homogeneous integration of mid-IR QCLs with low-loss In0.53Ga0.47As passive waveguides epitaxially grown on InP substrates. The homogeneous integration approach uses materials, growth, and processing steps nearly identical to those used for conventional high-performance mid-IR QCLs, which offers superior reliability and performance of photonic integrated circuits. Over 0.57 W of peak-pulsed optical power was coupled to the passive waveguide from a homogeneously integrated λ≈4.6 μm QCL, which represents an order of magnitude improvement in optical power compared to the best results obtained with heterogeneously integrated QCLs.

Snakelike similaritons in tapered grating dual-core waveguide amplifiers

We present a physical model that describes bright and dark snakelike similaritons propagating in an asymmetric dual-core waveguide amplifier by considering self-focusing and self-defocusing Kerr nonlinearities. The asymmetric dual-core waveguide is composed of two adjoining, closely spaced, upper and lower cores, in which the upper one is an active tapered graded-index waveguide with a long-period grating along the medium while the lower one is a passive step-index waveguide. For different choices of tapered index profile, novel exact analytical similariton solutions undergoing a self-compression are obtained. Our results show that these optical structures can be effectively controlled by adjusting the long-period grating and amplitude of the source. Besides, the stability and interaction of two neighboring bright similaritons are discussed by means of numerical simulations. These results may simulate the possibility of experiments related to tapered grating dual-core waveguide amplifiers.

Anomalous optical forces in PT-symmetric waveguides.

Evanescently coupled passive waveguides experience optical forces of attractive or repulsive nature, depending on the mode of operation. Here we explore the optical forces between parity-time-symmetric coupled waveguides, with balanced levels of gain and loss. We find that, besides the diagonal stress components that result in a pressure normal to the surface of the waveguides, this system exhibits an off-diagonal stress component that creates a shear along the propagation direction. In addition, for a critical value of balanced gain and loss, the normal pressure can be reduced to zero. These anomalous optical forces are related to the unusual power flow in coupled active-passive channels, and open interesting opportunities for microfluidics and micro-optomechanical systems.