Silicon Photonic Systems Research Articles

Fully reconfigurable MEMS-based second-order coupled-resonator optical waveguide (CROW) with ultra-low tuning energy.

Integrated microring resonators are well suited for wavelength-filtering applications in optical signal processing, and cascaded microring resonators allow flexible filter design in coupled-resonator optical waveguide (CROW) configurations. However, the implementation of high-order cascaded microring resonators with high extinction ratios (ERs) remains challenging owing to stringent fabrication requirements and the need for precise resonator tunability. We present a fully integrated on-chip second-order CROW filter using silicon photonic microelectromechanical systems (MEMS) to adjust tunable directional couplers and a phase shifter using nanoscale mechanical out-of-plane waveguide displacement. The filter can be fully reconfigured with regard to both the ER and center wavelength. We experimentally demonstrated an ER exceeding 25 dB and continuous wavelength tuning across the full free spectral range of 0.123 nm for single microring resonator, and showed reconfigurability in second-order CROW by tuning the ER and resonant wavelength. The tuning energy for an individual silicon photonic MEMS phase shifter or tunable coupler is less than 22 pJ with sub-microwatt static power consumption, which is far better than conventional integrated phase shifters based on other physical modulation mechanisms.

Flexible transmission control of mode conversion in a TiO2 cladding silicon subwavelength waveguide by engineering inter-waveguide mode coupling

Mode conversions can be effectively accomplished in coupled mode silicon photonic waveguide systems when specific phase-matching conditions are fulfilled. However, conventional device using such principles is optimized to a certain coupling length, which can only achieve a fixed energy transmission. Moreover, the conventional method to tune the refractive index is insufficient for effective transmission manipulation. In this paper, we systematically analyze the coupling strategy in the silicon-based dual-waveguide coupled-mode system, and propose an effective transmission tuning method by engineering the inter-waveguide mode coupling. We introduce titanium dioxide as cladding material to a subwavelength waveguide-based directional coupler to change the refractive index variable, which is both CMOS compatible and has negative thermo-optical coefficient. By implementing different temperature on waveguide, the dispersion curves of stripe waveguide and subwavelength waveguide can be tailored independently, thus the coupling coefficient can be changed without changing the geometry of waveguide. We demonstrate prototype of TE0-TE1 multi-mode variable optical attenuator. The output can be tuning 0.05%–96.4% of input energy when the temperature rises 0-400 K. We extend the device to TE2 and TE3 with output intensities of 0.04-91.4% and 0.04-88.8% over the temperature rise range of 317 K and 214 K, respectively. Cascading mode converters for different mode-order, we can arbitrarily mix the mode percentage in output waveguide to generate different interference patterns. We demonstrate a reconfigurable optical tweezer using this setup, which is capable of manipulating nanoscale particles. We envision that such methodology can be implemented in flexible multimode optical networks and labs on chip.

Overcoming the Fermi-Level Pinning Effect in the Nanoscale Metal and Silicon Interface.

Silicon-based photodetectors are attractive as low-cost and environmentally friendly optical sensors. Also, their compatibility with complementary metal-oxide-semiconductor (CMOS) technology is advantageous for the development of silicon photonics systems. However, extending optical responsivity of silicon-based photodetectors to the mid-infrared (mid-IR) wavelength range remains challenging. In developing mid-IR infrared Schottky detectors, nanoscale metals are critical. Nonetheless, one key factor is the Fermi-level pinning effect at the metal/silicon interface and the presence of metal-induced gap states (MIGS). Here, we demonstrate the utilization of the passivated surface layer on semiconductor materials as an insulating material in metal-insulator-semiconductor (MIS) contacts to mitigate the Fermi-level pinning effect. The removal of Fermi-level pinning effectively reduces the Schottky barrier height by 12.5% to 16%. The demonstrated devices exhibit a high responsivity of up to 234 μA/W at a wavelength of 2 μm, 48.2 μA/W at 3 μm, and 1.75 μA/W at 6 μm. The corresponding detectivities at 2 and 3 μm are 1.17 × 108 cm Hz1/2 W-1 and 2.41 × 107 cm Hz1/2 W-1, respectively. The expanded sensing wavelength range contributes to the application development of future silicon photonics integration platforms.

Emulation of deep-ultraviolet lithography using rapid-prototyping, electron-beam lithography for silicon photonics design.

We demonstrate a method to emulate the optical performance of silicon photonic devices fabricated using advanced deep-ultraviolet lithography (DUV) processes on a rapid-prototyping electron-beam lithography process. The method is enabled by a computational lithography predictive model generated by processing SEM image data of the DUV lithography process. We experimentally demonstrate the emulation method's accuracy on integrated silicon Bragg grating waveguides and grating-based, add-drop filter devices, two devices that are particularly susceptible to DUV lithography effects. The emulation method allows silicon photonic device and system designers to experimentally observe the effects of DUV lithography on device performance in a low-cost, rapid-prototyping, electron-beam lithography process to enable a first-time-right design flow.

Petabit-Scale Silicon Photonic Interconnects With Integrated Kerr Frequency Combs

Silicon photonics holds significant promise in revolutionizing optical interconnects in data centers and high performance computers to enable scaling into the Pb/s package escape bandwidth regime while consuming orders of magnitude less energy per bit than current solutions. In this work, we review recent progress in silicon photonic interconnects leveraging chip-scale Kerr frequency comb sources and provide a comprehensive overview of massively scalable silicon photonic systems capable of capitalizing on the large number of wavelengths provided by such combs. We first consider the high-level architectural constraints and then proceed to detail the corresponding fundamental device designs supported by both simulated and experimental results. Furthermore, the majority of experimentally measured devices were fabricated in a commercial 300 mm foundry, showing a clear path to volume manufacturing. Finally, we present various system-level experiments which illustrate successful proof-of-principle operation, including flip-chip integration with a co-designed CMOS application-specific integrated circuit (ASIC) to realize a complete Kerr comb-driven electronic-photonic engine. These results provide a viable and appealing path towards future co-packaged silicon photonic interconnects with aggregate per-fiber bandwidth above 1 Tb/s, energy consumption below 1 pJ/bit, and areal bandwidth density greater than 5 Tb/s/mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Silicon photonic microelectromechanical systems add-drop ring resonator in a foundry process

Photonic add-drop filters are crucial components for the implementation of wavelength division multiplexing (WDM) in fiber-optic communication systems. The recent progress in photonic integration has shown the potential to integrate photonic add-drop filters alongside high-performance photonic building blocks on a chip to construct compact and complex photonic-integrated circuits for WDM. Typically, implementations are based on micro-ring resonators with integrated heaters or free carrier dispersion-based modulators to adjust the filter wavelength. However, heaters suffer from high power consumption, and free carriers result in optical absorption losses, limiting the scalability toward very-large-scale circuits. We demonstrate the design, simulation, fabrication, and experimental characterization of a compact add-drop filter based on a vertically movable, MEMS-actuated ring resonator. The MEMS-actuated add-drop filter is implemented in IMEC’s iSiPP50G silicon photonics platform and realized using a short post-processing flow to safely release the suspended MEMS structures in a wafer-level compatible process. The filter exhibits a through port linewidth of ∼1 nm (124.37 GHz) at 1557.1 nm, and it retains a port extinction of 20 dB and a port isolation of >50 dB under 27 V of actuation voltage. The combination of low-power consumption and a compact footprint demonstrates the suitability for very-large-scale integration in photonic circuits.

Microcomb-driven silicon photonic systems

Microcombs have sparked a surge of applications over the past decade, ranging from optical communications to metrology1–4. Despite their diverse deployment, most microcomb-based systems rely on a large amount of bulky elements and equipment to fulfil their desired functions, which is complicated, expensive and power consuming. By contrast, foundry-based silicon photonics (SiPh) has had remarkable success in providing versatile functionality in a scalable and low-cost manner5–7, but its available chip-based light sources lack the capacity for parallelization, which limits the scope of SiPh applications. Here we combine these two technologies by using a power-efficient and operationally simple aluminium-gallium-arsenide-on-insulator microcomb source to drive complementary metal–oxide–semiconductor SiPh engines. We present two important chip-scale photonic systems for optical data transmission and microwave photonics, respectively. A microcomb-based integrated photonic data link is demonstrated, based on a pulse-amplitude four-level modulation scheme with a two-terabit-per-second aggregate rate, and a highly reconfigurable microwave photonic filter with a high level of integration is constructed using a time-stretch approach. Such synergy of a microcomb and SiPh integrated components is an essential step towards the next generation of fully integrated photonic systems.

Broadband Nonvolatile Electrically Controlled Programmable Units in Silicon Photonics

Programmable photonic integrated circuits (PICs) have recently gained significant interest due to their potential in creating next-generation technologies ranging from artificial neural networks and microwave photonics to quantum information processing. The fundamental building block of such programmable PICs is a tunable 2 x 2 switch, traditionally controlled by the thermo-optic or free-carrier dispersion. Yet, these implementations are power-hungry, volatile, and have a large footprint (typically > 100 um). Therefore, a truly 'set-and-forget' type 2 x 2 switch with zero static power consumption is highly desirable for large-scale PICs. Here, we report a broadband nonvolatile electrically programmable 2 x 2 silicon photonic switch based on the phase-change material Ge2Sb2Te5. The directional coupler switch exhibits a compact coupling length (64 um), small insertion loss (<2 dB), and minimal crosstalk (<-8 dB) across the entire telecommunication C-band while maintaining a record-high endurance of over 2,800 switching cycles. This demonstrated switch constitutes a critical component for realizing future generic programmable silicon photonic systems.

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.

Single-Event Transients in a Commercially Available, Integrated Germanium Photodiode for Silicon Photonic Systems

Ion-induced SETs were captured from a germanium photodiode (PD) in an integrated silicon photonics technology platform. Statistics from 400 ion-strike events were extracted for various optical powers and LETs incident on the PD. The results show that the transient peak decreases with increasing optical power. TCAD simulation results indicate that photo-generated carriers decrease the built-in electric field of the PD, thereby decreasing the transient peak. The photo-generated carriers also increase the probability of Auger recombination, which further reduces the transient peak under illuminated conditions, but this effect is small and does not contribute significantly to the transient peak reduction. Finally, a short case study with an ON–OFF keying modulation scheme is presented to show how these transients have the potential to corrupt bits in realistic communication systems. Furthermore, it is also shown that hardening techniques can be utilized to mitigate bit errors in communication systems that use integrated Ge PDs.

Plasmon-assisted polarization-sensitive photodetection with tunable polarity for integrated silicon photonic communication systems

To establish high-bandwidth chip-to-chip interconnects in optoelectronic integrated circuits, requires high-performance photon emitters and signal receiving components. Regarding the photodetector, fast device concepts like Schottky junction devices, large carrier mobility materials and shrinking the channel length will enable higher operation speed. However, integrating photodetectors in highly scaled ICs technologies is challenging due to the efficiency-speed trade-off. Here, we report a scalable and CMOS-compatible approach for an ultra-scaled germanium (Ge) based photodetector with tunable polarity. The photodetector is composed of a Ge Schottky barrier field effect transistor with monolithic aluminum (Al) source/drain contacts, offering plasmon assisted and polarization-resolved photodetection. The ultra-scaled Ge photodetector with a channel length of only 200 nm shows high responsivity of about R = 424 A W−1 and a maximum polarization sensitivity ratio of TM/TE = 11.

High-power Si-Ge photodiode assisted by doping regulation.

High-power silicon-based photodiodes are key components in many silicon photonics systems, such as microwave photonics systems, an optical interconnection system with multi-level modulation formats, etc. Usually, the saturation power of the silicon-germanium (Si-Ge) photodiode is limited by the space-charge screening (SCS) effect and the feasibility of the fabrication process. Here, we propose a high saturation power Si-Ge photodiode assisted by doping regulation. Through alleviating the SCS effect of the photodiode, we successfully demonstrate an 85.7% improvement on the saturation power and a 57% improvement on the -1 dB compression photocurrent. The proposed high-power Si-Ge photodiode requires no specific fabrication process and will promote the low-cost integrated silicon photonics systems for more applications.

Design of an on-chip electrically driven, position-adapted, fully integrated erbium-based waveguide amplifier for silicon photonics

Erbium-doped waveguide amplifiers are important for silicon photonic large-scale integration. They boost the optical signal to compensate for the on-chip loss of the whole system, such as insertion, transmission, and coupling loss. Herein, a GaAs-on-silicon-pumped erbium-based waveguide amplifier is proposed. The optical amplifier is based on waveguides evanescently coupled with an erbium-doped thin-film gain medium. The erbium-doped gain region is locally pumped by a hybrid GaAs-SiN/SiO vertical-cavity-emitting pump laser. It can be selectively fabricated on the silicon photonic chip that has good position flexibility when integrated with other devices. The amplifier provides high-quality electrically driven amplification and realizes the full integration of amplifiers with the silicon photonic system without any external pump light source. The modeling analyses show that the proposed amplifier design has a maximum saturated gain of 42.5 dB/cm with a modulation bandwidth of ∼42 GHz. This high-gain, large-bandwidth device fully utilizes the advantages of erbium-doped materials and silicon-based III-V semiconductors, while overcoming their issues, and opens up a new pathway for on-chip amplification.

CORNERSTONE’s Silicon Photonics Rapid Prototyping Platforms: Current Status and Future Outlook

The field of silicon photonics has experienced widespread adoption in the datacoms industry over the past decade, with a plethora of other applications emerging more recently such as light detection and ranging (LIDAR), sensing, quantum photonics, programmable photonics and artificial intelligence. As a result of this, many commercial complementary metal oxide semiconductor (CMOS) foundries have developed open access silicon photonics process lines, enabling the mass production of silicon photonics systems. On the other side of the spectrum, several research labs, typically within universities, have opened up their facilities for small scale prototyping, commonly exploiting e-beam lithography for wafer patterning. Within this ecosystem, there remains a challenge for early stage researchers to progress their novel and innovate designs from the research lab to the commercial foundries because of the lack of compatibility of the processing technologies (e-beam lithography is not an industry tool). The CORNERSTONE rapid-prototyping capability bridges this gap between research and industry by providing a rapid prototyping fabrication line based on deep-UV lithography to enable seamless scaling up of production volumes, whilst also retaining the ability for device level innovation, crucial for researchers, by offering flexibility in its process flows. This review article presents a summary of the current CORNERSTONE capabilities and an outlook for the future.

Optical gain sensitivity of BGaAs/GaP quantum wells to admixtures of group III and V atoms

21st-century studies in the field of epitaxy brought observable progress in a field of heterogeneous integration of III-V materials onto silicon photonic systems. BGaAs/GaP quantum wells (QWs) are a new material system, which can be grown on GaP/Si templates and thereby can be integrated with a Si platform. This work presents calculations of the material optical gain spectra for the QWs modified by an incorporation of group III and V elements, in both the QWs region and its barriers. The gain spectra are calculated based on the 8-band k · p model and Fermi’s Golden Rule. Incorporations of indium and phosphorus are analyzed for the thin film of the QWs, and boron, aluminum, indium, and arsenic for its barriers. Since an energy of gain peak in the BGaAs/GaP QWs can be tuned within a range of 690-730 nm and the positive optical gain is presented for the QWs with these modifications, this system is promising for further development of red-light lasers integrated with Si.

High-Speed Measurement-Device-Independent Quantum Key Distribution with Integrated Silicon Photonics

Measurement-device-independent quantum key distribution (MDI-QKD) removes all detector side channels and enables secure QKD with an untrusted relay. It is suitable for building a star-type quantum access network, where the complicated and expensive measurement devices are placed in the central untrusted relay and each user requires only a low-cost transmitter, such as an integrated photonic chip. Here, we experimentally demonstrate a 1.25 GHz silicon photonic chip-based MDI-QKD system using polarization encoding. The photonic chip transmitters integrate the necessary encoding components for a standard QKD source. We implement random modulations of polarization states and decoy intensities, and demonstrate a finite-key secret rate of 31 bps over 36 dB channel loss (or 180 km standard fiber). This key rate is higher than state-of-the-art MDI-QKD experiments. The results show that silicon photonic chip-based MDI-QKD, benefiting from miniaturization, low-cost manufacture and compatibility with CMOS microelectronics, is a promising solution for future quantum secure networks.

Physics and applications of quantum dot lasers for silicon photonics

Abstract Photonic integrated circuits (PICs) have enabled numerous high performance, energy efficient, and compact technologies for optical communications, sensing, and metrology. One of the biggest challenges in scaling PICs comes from the parasitic reflections that feed light back into the laser source. These reflections increase noise and may cause laser destabilization. To avoid parasitic reflections, expensive and bulky optical isolators have been placed between the laser and the rest of the PIC leading to large increases in device footprint for on-chip integration schemes and significant increases in packaging complexity and cost for lasers co-packaged with passive PICs. This review article reports new findings on epitaxial quantum dot lasers on silicon and studies both theoretically and experimentally the connection between the material properties and the ultra-low reflection sensitivity that is achieved. Our results show that such quantum dot lasers on silicon exhibit much lower linewidth enhancement factors than any quantum well lasers. Together with the large damping factor, we show that the quantum dot gain medium is fundamentally dependent on dot uniformity, but through careful optimization, even epitaxial lasers on silicon can operate without an optical isolator, which is of paramount importance for the future high-speed silicon photonic systems.

Dynamic trapping and releasing photonics beyond delay-bandwidth limit in cascaded photonic crystal nanocavities

Controlling the flow of light on-chip is of great importance for quantum computing and optical signal processing. In this paper, we present a theoretical study to reveal the underlying physics of how to effectively trap, store and release a signal pulse, and eventually break the delay-bandwidth limit, based on controllable EIT-like effect in dynamically tuned standing-wave cascaded nanocavities. Using this mechanism, we design a compact silicon photonic crystal system with long storing time and a delay-bandwidth product over 460, which is about two orders of magnitude greater than the reported results obtained by other methods based on static resonator system, and the trapped signal pulse can be released on demand.

Compact broadband suspended silicon photonic directional coupler.

Directional couplers are extensively used in photonic integrated circuits as basic components for efficient on-chip photonic signal routing. Conventionally, directional couplers are fully encapsulated in the technology's waveguide cladding material. In this Letter, we demonstrate a compact broadband directional coupler, fully suspended in air and exhibiting efficient power coupling in the cross state. The coupler is designed and built based on IMEC's iSiPP50G standard platform, and hydrofluoric (HF) vapor-etching-based post-processing allows to release the freestanding component. A low insertion loss of 0.5 dB at λ=1560nm and a 1 dB bandwidth of 35 nm at λ=1550nm have been confirmed experimentally. With a small footprint of 20µm×30µm and high mechanical stability, this directional coupler can serve as a basic building block for large-scale silicon photonic microelectromechanical systems (MEMS) circuits.

Towards silicon photonic neural networks for artificial intelligence

Brain-inspired photonic neural networks for artificial intelligence have attracted renewed interest. For many computational tasks, such as image recognition, speech processing and deep learning, photonic neural networks have the potential to increase the computing speed and energy efficiency on the orders of magnitude compared with digital electronics. Silicon Photonics, which combines the advantages of electronics and photonics, brings hope for the large-scale photonic neural network integration. This paper walks through the basic concept of artificial neural networks and focuses on the key devices which construct the silicon photonic neuromorphic systems. We review some recent important progress in silicon photonic neural networks, which include multilayer artificial neural networks and brain-like neuromorphic systems, for artificial intelligence. A prototype of silicon photonic artificial intelligence processor for ultra-fast neural network computing is also proposed. We hope this paper gives a detailed overview and a deeper understanding of this emerging field.