Silicon Photonic Devices Research Articles

Fabrication of vertical-taper structures for silicon photonic devices by using local-thickness-thinning process

This paper describes a simple fabrication process of vertical-taper structures which can locally tune the thickness of silicon photonic devices. For low-loss spot-size conversion, taper angles less than 10° are required. To fabricate the gradual-slope shape of the vertical tapers, we have developed a step-and-exposure lithography process, which is realized by repeated light exposure to photoresist and movement of the wafer stage by using commercial steppers. The process is conducted at a lower temperature (∼120 °C) than the conventional process and is compatible with the complementary metal-oxide-semiconductor process. Also, we have made a model of the lithography to predict the angle of the taper. Theoretical angles are consistent with the experimental results. We demonstrate the conversion of a 400 nm thick silicon waveguide to 220 nm, whose length was 2.4 μm and insertion loss was measured to be less than 0.3 dB. The process enables us to choose the optimal thickness for each silicon-photonic device.

Feature Extraction From Images Using Integrated Photonic Convolutional Kernel

Optical neural networks are expected to solve the problems of computational efficiency and energy consumption in neural networks. Herein, we experimentally implemented a 2 × 2 photonic convolutional kernel (PCK) using four on-chip micro-ring resonators (MRRs) and demonstrated feature extraction for images with different convolutional kernels. We trained a simple convolutional neural network model to recognize the MNIST dataset and used our PCK devices for processing in the first convolutional layer, achieving a recognition rate of 91%, which further verified the feasibility of MRRs for convolution operations. In addition to the source, all silicon photonic devices used can be monolithically integrated and feature good scalability, which is important for realizing large-scale, low-cost optical neural networks.

Inducing optical self-pulsation by electrically tuning graphene on a silicon microring.

A mechanism for self-pulsation in a proposed graphene-on-silicon microring device is studied. The relevant nonlinear effects of two photon absorption, Kerr effect, saturable absorption, free carrier absorption, and dispersion are included in a coupled mode theory framework. We look at the electrical tunability of absorption and the Kerr effect in graphene. We show that the microring can switch from a stable rest state to a self-pulsation state by electrically tuning the graphene under constant illumination. This switching is indicative of a supercritical Hopf bifurcation since the frequency of the pulses is approximately constant at 7 GHz and the amplitudes initial grow with increasing Fermi level. The CMOS compatibility of graphene and the opto-electronic mechanism allows this to device to be fairly easily integrated with other silicon photonic devices.

Radiation-hardened silicon photonic passive devices on a 3 µm waveguide platform under gamma and proton irradiation.

Silicon photonics is considered to be an ideal solution as optical interconnect in radiation environments. Our previous study has demonstrated experimentally that radiation responses of device are related to waveguide size, and devices with thick top silicon waveguide layers are expected to be less sensitive to irradiation. Here, we design radiation-resistant arrayed waveguide gratings and Mach-Zehnder interferometers based on silicon-on-insulator with 3 µm-thick silicon optical waveguide platform. The devices are exposed to 60Co γ-ray irradiation up to 41 Mrad(Si) and 170-keV proton irradiation with total fluences from 1×1013 to 1×1016 p/cm2 to evaluate performance after irradiation. The results show that these devices can function well and have potential application in harsh radiation environments.

Third harmonic generation from the gold/amorphous silicon hybrid metasurface

Abstract The nonlinear optical properties of silicon have great potentials for developing all-optical switches and modulators, etc. Strategies based on all dielectric silicon photonic crystals and resonators have been proposed to design the nanophotonic devices with high nonlinearity. Nevertheless, the lack of compatible configuration with the mature CMOS technology may hinder the practical applications of the all dielectric devices. In this work, we proposed a metal–silicon hybrid metasurface to generate strong third harmonic signals from an amorphous silicon (α-Si) film. By integrating the α-Si film into a gold plasmonic nanocavity and controlling the periods of the gold meta-atoms, the efficiency of the THG process is expected to be greatly boosted. Compared to the planar α-Si film, the THG signal is enhanced by a factor of ∼370. The presented results in this work may open new routes for developing various silicon photonic devices with high optical nonlinearity.

Improving the tolerances in high-performance silicon photonics modulators

A racetrack-shaped modulator, based on Double Injection method, has been recently presented able to provide multiple response shapes and exhibiting enhanced robustness to deviations in parameters. Fabrication process deviations that influence sensitive ingredients of ring-based modulators such as the ring's waveguide, couplers, and heat that is generated in Silicon Photonics devices due to carrier transport, can degrade the modulator extinction-ratio (ER) and optical modulation depth (OMD). The sensitivity of the, so-called, PIR20 configuration (Parameters-Insensitive Response with ER of 20 dB) to fabrication and temperature deviations is analyzed in detail. Comparison to other known modulators based on either a ring, MZI or Fabry-Pérot resonators, reveals that the PIR20 device is considerably less sensitive to deviations of these types.

High-Capacity Space-Division Multiplexing Communications With Silicon Photonic Blind Source Separation

Space-division multiplexing is a widely used technique to improve data-carrying capacities in both wireless and optical communication systems. However, tightly packed spatial channels cause severe crosstalk. High data rates and large channel counts impose severe constraints on resolving the crosstalk using traditional digital signal processing algorithms and electronic circuits. In order to solve these issues, this paper presents a silicon photonic system combining high-speed silicon photonic devices with a novel blind source separation (BSS) algorithm. We first demonstrate using photonic BSS to undo modal crosstalk in a short-reach multimode optical fiber interconnect for intra-data-center communications. The proposed photonic BSS system inherits the advantages of photonic matrix processor and the “blindness” of BSS, leading to superior energy and cost efficiency and reduced latency, while allowing to recover the signals using a sub-Nyquist sampling rate and in a free-running mode, and offering unmatched agility in signal format and data rate. The feasibility of using photonic processors for mode crosstalk equalization has been recently demonstrated, assisted with training sequences. Our approach, photonic BSS, in contrast, can tackle the more difficult problem of making the receiver transparent to any data rate and modulation format, and workable with slow and cost-effective electronics. In addition, we find that photonic BSS has a much better scaling law for space-division multiplexing (SDM)-based communication systems than digital signal processing (DSP). When compared to state-of-the-art DSP, photonic BSS can reduce system power consumption, speed, and latency by several orders of magnitude, particularly for high-capacity communications with high data rates per channel and a large number of channels. Photonic BSS has the added advantages of being agnostic to transmission content, making it exceptional at protecting communication privacy. This paper also discusses our previous work in demonstrating photonic BSS for privacy protection in wireless multiple-in multiple-out (MIMO) communications using silicon photonic micoring resonator (MRR) weight banks.

Side-lobe reduction by cascading Bragg grating filters on a Si-photonic chip.

Bragg-grating based cavities and coupler designs present opportunities for flexible allocation of bandwidth and spectrum in silicon photonic devices. Integrated silicon photonic devices are moving toward mainstream, mass adoption, leading to the need for compact Bragg grating based designs. In this work we present a design and experimental validation of a cascaded contra-directional Bragg-grating coupler with a measured main lobe to side-lobe contrast of 12.93 dB. This level of performance is achieved in a more compact size as compared to conventional apodized gratings, and a similar design philosophy can be used to improve side-lobe reduction in grating-based mirror design for on-chip lasers and other cavity-based designs as well.

Recent Progress in Silicon-Based Slow-Light Electro-Optic Modulators.

As an important optoelectronic integration platform, silicon photonics has achieved significant progress in recent years, demonstrating the advantages on low power consumption, low cost, and complementary metal–oxide–semiconductor (CMOS) compatibility. Among the different silicon photonics devices, the silicon electro-optic modulator is a key active component to implement the conversion of electric signal to optical signal. However, conventional silicon Mach–Zehnder modulators and silicon micro-ring modulators both have their own limitations, which will limit their use in future systems. For example, the conventional silicon Mach–Zehnder modulators are hindered by large footprint, while the silicon micro-ring modulators have narrow optical bandwidth and high temperature sensitivity. Therefore, developing a new structure for silicon modulators to improve the performance is a crucial research direction in silicon photonics. Meanwhile, slow-light effect is an important physical phenomenon that can reduce the group velocity of light. Applying slow-light effect on silicon modulators through photonics crystal and waveguide grating structures is an attractive research point, especially in the aspect of reducing the device footprint. In this paper, we review the recent progress of silicon-based slow-light electro-optic modulators towards future communication requirements. Beginning from the principle of slow-light effect, we summarize the research of silicon photonic crystal modulators and silicon waveguide grating modulators in detail. Simultaneously, the experimental results of representative silicon slow-light modulators are compared and analyzed. Finally, we discuss the existing challenges and development directions of silicon-based slow-light electro-optic modulators for the practical applications.

Reconfigurable hybrid silicon waveguide Bragg filter using ultralow-loss phase-change material.

Reconfigurable silicon photonic devices attract much research attention, and hybrid integration with tunable phase-change materials (PCMs) exhibiting large refractive index contrast between amorphous (Am) and crystalline (Cr) states is a promising way to achieve this goal. Here, we propose and numerically investigate a Sb2Se3-Si hybrid waveguide Bragg filter operating in the telecom C-band on the silicon-on-insulator (SOI) platform. The proposed device consists of a Bragg grating (BG) with a thin top layer of ultralow-loss Sb2Se3 PCM interacting with evanescent field of the silicon waveguide mode. By harnessing the ultralow-loss and reversible index change of Sb2Se3 film, the spectral response of the hybrid BGs could be dynamically tuned. We also theoretically investigate the reversible phase transitions between Am and Cr states of Sb2Se3 film that could be attained by applying voltage pulses on the indium-tin-oxide (ITO) strip heater covered on Sb2Se3 film. Thermal simulations show that a 2 V (4.5 V) pulse with a duration of 400 ns (55 ns) applied to electric contacts would produce crystallization (or amorphization). The proposed structure may find great potential for on-chip phase tunable devices on a silicon platform.

Ultralow‐Loss Silicon Photonics beyond the Singlemode Regime

AbstractSilicon photonics beyond the singlemode regime is applied for enabling ultralow‐loss waveguide propagation for the fundamental mode even without any special fabrication process. Here a micro‐racetrack resonator is fabricated with a standard 220‐nm‐SOI (silicon‐on‐insulator) multiproject‐wafer foundry and shows a record high intrinsic quality factor of 1.02×107, corresponding to an ultralow propagation loss of only 0.065 dB cm−1, which is about 20 times less than that of regular 450‐nm‐wide waveguides on the same chip. A state‐of‐the‐art microwave photonic filter on silicon is then realized with an ultranarrow 3‐dB bandwidth of 20.6 MHz and a tuning range of ≈20 GHz for the first time. A 100‐cm‐long delayline employed the broadened waveguides is also demonstrated with compact 90° Euler‐curve bends, and the measured average propagation loss is about 0.14 dB cm−1. The concept of silicon photonics beyond the singlemode regime helps solve the issue of high propagation loss significantly. In particular, it enables silicon photonic devices with enhanced performances, which paves the way for realizing large‐scale silicon photonic integration. This concept can be extended further to any other material platforms, such as silicon nitride and lithium niobate. This also brings numerous new opportunities for various applications such as nonlinear photonics, large‐scale photonic integration, quantum photonics, microwave photonics, etc.

High energy irradiation effects on silicon photonic passive devices.

In this work, the radiation responses of silicon photonic passive devices built in silicon-on-insulator (SOI) technology are investigated through high energy neutron and 60Co γ-ray irradiation. The wavelengths of both micro-ring resonators (MRRs) and Mach-Zehnder interferometers (MZIs) exhibit blue shifts after high-energy neutron irradiation to a fluence of 1×1012 n/cm2; the blue shift is smaller in MZI devices than in MRRs due to different waveguide widths. Devices with SiO2 upper cladding layer show strong tolerance to irradiation. Neutron irradiation leads to slight changes in the crystal symmetry in the Si cores of the optical devices and accelerated oxidization for devices without SiO2 cladding. A 2-µm top cladding of SiO2 layer significantly improves the radiation tolerance of these passive photonic devices.

Characterization and Optimization of Coherent MZI-Based Nanophotonic Neural Networks Under Fabrication Non-Uniformity

Silicon-photonic neural networks (SPNNs) and artificial intelligence (AI) accelerators have emerged as promising successors to electronic accelerators by offering orders of magnitude lower latency and higher energy efficiency. Nevertheless, the underlying silicon photonic devices in SPNNs are sensitive to inevitable fabrication-process variations (FPVs) stemming from optical lithography imperfections. Consequently, the inferencing accuracy in an SPNN can be highly impacted by FPVs—e.g., can drop to below 10%—the impact of which is yet to be fully studied. In this paper, we, for the first time, model and explore the impact of optical phase noise due to FPVs in the waveguide width, silicon-on-insulator (SOI) thickness, and etch depth in coherent SPNNs that use Mach–Zehnder interferometers (MZIs). Leveraging such models, we propose a novel variation-aware, design-time optimization solution to improve MZI tolerance to different FPVs in SPNNs. Simulation results for two example SPNNs of different scales under realistic and correlated FPVs indicate that using the optimized MZIs can lead to significant improvements in the network inferencing accuracy. The proposed one-time optimization imposes low area overhead and hence is applicable even to resource-constrained designs.

Graphene on Silicon Photonics: Light Modulation and Detection for Cutting-Edge Communication Technologies

Graphene—a two-dimensional allotrope of carbon in a single-layer honeycomb lattice nanostructure—has several distinctive optoelectronic properties that are highly desirable in advanced optical communication systems. Meanwhile, silicon photonics is a promising solution for the next-generation integrated photonics, owing to its low cost, low propagation loss and compatibility with CMOS fabrication processes. Unfortunately, silicon’s photodetection responsivity and operation bandwidth are intrinsically limited by its material characteristics. Graphene, with its extraordinary optoelectronic properties has been widely applied in silicon photonics to break this performance bottleneck, with significant progress reported. In this review, we focus on the application of graphene in high-performance silicon photonic devices, including modulators and photodetectors. Moreover, we explore the trend of development and discuss the future challenges of silicon-graphene hybrid photonic devices.

Broadband graphene-based optical power coupler and polarization beam splitter using a directional coupler

The utilization of directional couplers (DCs) as power splitters and combiners is ubiquitous for the realization of integrated silicon photonic devices in the silicon-on-insulator (SOI) platform. Benefiting from the excellent modal modulation performance of graphene in silicon waveguides, we present its high flexibility to engineer and reduce the wavelength dependence of devices based on the DC. In this regard, a wavelength-insensitive 3-dB optical power coupler (3-dB OPC) is firstly proposed, in which various power splitting ratios are realized for the TE mode. Next, we design a broadband polarization beam splitter (PBS) consisting of a silicon waveguide (SW) and a graphene-silicon vertical slot waveguide (GSVSW). The simulation results show that the proposed PBS has high polarization extinction ratios (PERs) of 20.93 and 20.4 dB and low insertion losses (ILs) of 0.12 and 0.21 dB at 1550 nm for the thru and cross ports, respectively. In addition, the designed PBS demonstrates the ability to work over a wide range of wavelengths by changing the chemical potential of graphene.

Simulation Study of Stress Effect on Performance and Design Methodology of Proposed Si/SiGe Integrated Bragg Grating Filter

The stress-induced due to thermal and intrinsic mismatch between different layers in silicon photonic devices critically affect the device performance. This makes the inclusion of stress while designing such devices absolutely necessary to ensure reliable and predictable operation of the eventually fabricated device. In this regard, this paper presents the design methodology of a proposed Si/SiGe integrated Bragg grating (IBG) filter that includes the influence of stress effect during filter design. Moreover, it also reports the variation in the optical performance of the device due to stress effect. The methodology utilizes both analytical and numerical approaches. IBG filters realized over SOI platform are designed using the proposed method for C-band of operation and having different 3-dB bandwidths. Refractive index variation along the length of the proposed filter has been achieved by introducing SiGe alloy filled grooves, which are corrugated in silicon strip waveguide. The efficacy of the methodology has been demonstrated for varying waveguide width and three different Ge fractions, i.e., 0.10, 0.15 and 0.20 in SiGe. The stress at Si-SiGe interface due to the lattice and thermal mismatch has been incorporated in a modified transfer matrix method (TMM), which is in its unmodified form can only be used under no-stress condition. The proposed design methodology has been tested for designing three IBG filters having 10 nm, 5 nm and 1.6 nm bandwidths along with other suitable user-defined parameters; such as maximum insertion loss (IL), minimum extinction ratio (ER), Ge fraction in SiGe alloy, and center wavelength. It has been found that our proposed methodology, which is based on modified TMM, can find possible combinations of width, length, SiGe groove depth of the IBG filters that meet user-defined specifications even in presence of stress. This paper also compares the results generated by the proposed modified TMM with the results from commercially available photonic device design tool. It has been found that the proposed method can result IBG filters having estimated optical performances with an accuracy of < 10%, <8%, and <0.01% for ER, bandwidth, and resonance wavelength, respectively.

Numerical design and optimization of a high compact, broadband optical three-mode selective converter by manipulating ITO-based controllable phase shifters integrated on silicon-on-insulator waveguides

Recently, indium tin oxide has been very attractive for thermo-optic (TO)-controlled silicon photonic devices, due to its high transparency, strong TO phase dependency, high conductivity, and CMOS compatibility. These properties enable the reduction of the gap between the silicon core and the heater, leading to a considerably low electric power consumption and high switching speed. We present an investigation of a numerical simulation to design and optimize a compact ITO microheater for tuning the TO phase shift in a proof-of-concept of a three-mode converter (TMC) with broadband and ultralow loss properties. We show that the TMC can operate the 3-dB bandwidth up to 100 nm. In addition, the designed device can attain relatively large fabrication tolerances for width and height of ±50 and ±5 nm, respectively. In addition, the proposed mode converter consumes a total power of less than 90 mW and obtains a fast switching time of less than 8 μs. Moreover, the device can be integrated into an estimated compact footprint of 8 μm × 2160 μm. Such excellent performances make ITO a strong candidate for low-loss TO phase shifters and open up an alternative method for realizing ultrafast and high-speed mode division multiplexing systems and large-scale applications.

An on-chip integrated microfiber–silicon–graphene hybrid structure photodetector

Silicon photonic devices have great potential for photocommunication, and silicon-based photodetectors have attracted wide attention. Here, we report an on-chip integrated microfiber–silicon–graphene hybrid structure photodetector that can operate in the visible and near-infrared ranges. The detector has a responsivity of ∼136 mA W−1 at 808 nm and a rise time of ∼1.1 μs. At a reverse bias of 5 V, we achieved a responsivity of ∼1350 mA W−1. Our device provides an option for on-chip integration.

An analog electronic emulator of non-linear dynamics in optical microring resonators

The microring resonator is a ubiquitous building block of optical integrated circuits. Owing to its unique non-linear properties, it appears well-suited as a node in the realization of photonic networks, such as spiking neural networks. However, experimental investigation requires complex setups, incurring considerable device fabrication times and costs. Inherent physical constraints limit the ranges of the parameters determining the timescales and strengths of the non-linear behaviors. Moreover, on-the-fly changes to these parameters and node couplings are not easily realizable. On the other hand, numerical simulations are computationally heavy, particularly for large networks, and do not entirely replace measurement on a physical apparatus. To mitigate this problem, we introduce and realize an analog electronic emulator that implements dynamics, attempting to reproduce the self-pulsing phenomenon in an optical microresonator. It can be readily constructed with off-the-shelf components, and is well-suited for building complex networks. The circuit is explicitly based on a mathematical model of the microresonator, subject to some approximations, parameter adjustments and rescalings. Initial results comparing experimental data from the emulator and a representative silicon photonic device plus numerical simulations suggest a satisfactory level of agreement in the temporal dynamics and response to parametric sweeps when scaled for the different response times. This work exemplifies the relevance of explicitly establishing correspondences between physical systems having widely different features but governed by similar dynamics.

Silicon photonic beam steering module with backside coupling elements toward dense heterogeneous integration with drive electronics

Solid state beam steering devices are key elements in low cost, robust, and three-dimensional imaging systems. Here, we present a silicon photonic beam steering device based upon an 8 × 8 grating coupler focal plane array approach fed by a thermo-optic Mach–Zehnder switching tree. In this device, transmission of light from the grating couplers is made through the backside of the chip using topside mirrors allowing for both high-efficiency out-coupling and direct flip-chip integration of drive electronics, providing a path to scale to denser focal plane arrays with large numbers of points in the future. A −13.8 dB fiber-to-fiber transmission was achieved in our preliminary test around 1523 nm for the beam steering chip.