Challenges and solutions for high-speed integrated silicon photonics

In this paper, we present novel architectures for non-volatile directed logic circuits, that demonstrate low insertion losses, high extinction ratios, as well as multi-bit and broadband operations. The non-volatile switching operations are performed using a non-volatile phase change material Ge2Sb2Se4Te1 (GSST). The phase of the GSST is controlled by electro-thermal switching (from amorphous to crystalline and vice versa) using graphene micro-heaters. We demonstrate all the fundamental logic operations (such as OR, AND, XOR, NOR, NAND, and XNOR), adder, comparator, and parity checker with multi-bit operation capability. Moreover, we demonstrate directed logic architectures that can perform multiple logic operations (which includes providing inverted outputs) using the same logic circuit. The designs of the logic circuits show low insertion losses (< 1 dB) and high extinction ratios ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$&gt;$ </tex-math></inline-formula> 10 dB) with broadband operations ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\sim $ </tex-math></inline-formula> 100 nm) operating in the telecommunication C-band. To the best of our knowledge, this is the first demonstration of the broadband operation for photonic directed logic circuits. For cascading multiple logic operations, the proposed architectures are ideal in terms of logic delay, extinction ratios and the ability to drive the number of other circuits. This study provides a practical technique to design future high-speed nanoscale non-volatile photonic integrated circuits for logic computing applications.

Silicon photonics (SiPh) has emerged as a promising technology for photonic integrated circuits (PICs). One of the basic components in SiPh is the directional coupler (DC), which plays an important role in signal monitoring with the requirement of low wavelength dependence and low loss. This paper proposes a broadband and low-loss DC designed for signal power tapping on the 3 μm silicon-on-insulator (SOI) waveguide platform. By utilizing the advantages of multi-micron waveguides and replacing one of the straight waveguides with an optimized arc-shaped waveguide in the coupling region, the proposed DC enhances spectral stability and improves transmission with negligible loss. Experimental evidence indicates that the proposed DC showcases a minimal variation in the tapping ratio. From 1470 nm to 1630 nm, the largest deviation away from the tapping ratio at 1550 nm is 1.433%. Additionally, the device exhibits a low excess loss of −0.27 dB. These results suggest that the proposed device is well-suited to reliable signal power tapping and monitoring, particularly within PICs.

Silicon photonics has attracted extensive attention in recent years as a promising solution for next generation high-speed, low energy consumption, and low cost data transmission systems. Although a few experiments indicated board-level and long haul communication capability, major and near-future application of silicon photonics is commonly seen as Ethernet at 100Gb/s and beyond, such as interconnects in data centers, where O-Band (near 1310 nm wavelength) has been standardized for its low fiber dispersion. However, almost all silicon photonics devices demonstrated up to date operate at C-Band (1530 nm to 1560 nm), the fiber loss and erbium amplification window, probably due to the wider availability of lasers and testing apparatus at this wavelength. Typical C-Band devices cannot operate at O-Band, thus the whole device library needs to be redesigned and recalibrated for O-Band applications. In this paper, we present an ultra compact, low loss, and low crosstalk waveguide crossing operating at O-Band. It is designed using the finite difference time domain method coupled with a particle swarm optimization. Device footprint is only 6 μm × 6 μm. The measured insertion loss is 0.19±0.02 dB across an 8-inch wafer. Cross talk is lower than -35 dB. We also report a second waveguide crossing with a 9 μm × 9 μm footprint with 0.017±0.005 dB insertion loss. Finally we summarize the performance of our overall O-Band device library, including low-loss waveguides, high-speed modulators, and photodetectors.

In this paper, we demonstrated several AWG-based silicon photonics devices fabricated with deep UV lithography on SOI platform, including cascaded AWG, AWG-based receiver with fiber-to-waveguide converters, AWG-based OADM and folded AWG with ring-reflectors. In a thin top silicon layer, the silicon nanowire AWG has a performance of relative high crosstalk. In order to reduce the crosstalk, we designed and fabricated a 4-channel cascaded AWG. Compared to the normal single silicon photonics arrayed waveguide grating with a crosstalk of ~ -12.5 dB, the crosstalk of more than 20 dB has been dramatically improved in this cascaded AWG. The AWG-based OADM device is composed of two 1&times;8 AWGs, MZI thermo-optic switch array and crossing array, by which 8 signals can be loaded or uploaded. The channel spacing is around 3.2 nm at the wavelength of 1550nm and the crosstalk is less than -20 dB. The dynamic extinction is around 15 dB by thermal effect and the power consumption for a single channel is less than 12.3 mW. The 8-channel receiver was designed and fabricated on a SOI wafer with 1&micro;m-thick top silicon layer, which was composed of 5&times;8 AWG and 8 Ge-based butt-coupled photodetector array. The AWG device was designed on a thick SOI wafer for a good crosstalk performance. However, The silicon waveguide with a height of 1&micro;m has a coupling issue with a cleaved standard optical fiber. In order to reduce the coupling loss with the cleaved fiber, a new cantilevered fiber-to-waveguide converter was designed to integrate at the output/input waveguide of this receiver. The measured coupling loss of this converter is around 2.4dB/facet by using a cleaved single mode fiber. The 3dB-bandwidth of this butt-coupled photodetector is more than 30 GHz at a reverse bias of -1V. The transmission capability of this receiver is more than 400Gbps. Compared to other silicon photonics devices, the AWG size is not small. A 1&times;4 folded silicon photonics AWG was designed and fabricated on a SOI wafer with 340nm-thick top silicon layer. This folded AWG was integrated with ring-mirrors at the end of arrayed waveguides. The optical signals are launched into the input waveguide and coupled into the arrayed waveguides through the input slab waveguides. At the end of arrayed waveguides, the signals are reflected by the ring-mirrors. The reflected signals go back to the arrayed waveguides and the input slab waveguide in sequence, and then they are separated into the four output waveguides. The measured transmission loss of this folded AWG is about 3.5 dB and the crosstalk is around 20 dB. The measured channel spacing is 6.7 nm. This size of the demonstrated arrayed waveguide grating is only half of the normal arrayed waveguide grating. Acknowledgment： This work was supported by the National Natural Science Foundation of China (No. 61674072, No. 61565011 and No. 51304097).

New arrayed waveguide gratings (AWGs) are proposed with asymmetry slab waveguides for combined optical performance of low insertion loss and wide passband. The predicted performance is demonstrated using 16 channel, 100 GHz channel spacing AWGs.

Optoelectronic integration technologies based on silicon-on-insulator (SOI) can bring revolutionary change to on-chip arrayed waveguide grating (AWG) demodulation systems. In this study, we present several ultrasmall 1 x N AWGs for an SOI-based AWG demodulation integration microsystem of different scales. The core sizes of the fabricated AWGs are smaller than 400 x 600 μm 2 . Experimental results match the simulation results, indicating that AWGs have a good transmission spectrum of low crosstalk below -20 dB and low insertion loss below -6.5 dB. The fabricated AWGs can be perfectly applied to improve the integration level and performance of the SOI-based AWG demodulation integration microsystem.

The programmable photonic integrated circuit (PIC) is an enabling technology behind optical interconnects and quantum information processing. Conventionally, the programmability of PICs is driven by the thermo-optic effect, free carrier dispersion, or mechanical tuning. These effects afford either high speed or a large extinction ratio, but all require constant power or bias to maintain the states, which is undesirable for programmability with infrequent switching. Recent progress in programmable PICs based on nonvolatile phase-change materials (PCMs) offers an attractive solution to a truly "set-and-forget" switch that requires zero static energy. Here, we report an essential building block of large-scale programmable PICs─a racetrack resonator with independent control of coupling and phase. We changed the resonance extinction ratio (ER) without perturbing the resonance wavelength, leveraging a programmable unit based on a directional coupler and a low-loss PCM Sb2Se3. The unit is only 33-μm-long and has an operating bandwidth over 50 nm, a low insertion loss (∼0.36 dB), high ER (∼15 dB), and excellent fabrication yield of over 1000 cycles endurance across nine switches. The work is a crucial step toward future large-scale energy-efficient programmable PICs.

This paper presents novel designs and simulation results of electro-optic modulators in integrated silicon photonics platforms. The electro-optic modulators described in this paper are based on the modulation of the photonic bandgap in a silicon photonic crystal slab waveguide, consisting of a phase change material (germanium selenide) sandwiched in between silicon layers. Three-dimensional finite difference time domain (FDTD) modeling is employed to simulate two configurations of electro-optic modulators based on mode-gap shifting in photonic crystal slab waveguides—one that is designed for operation with the transverse electric (TE) polarization of light and the other for the transverse magnetic (TM) polarization. When the electric field is applied across the germanium selenide layer surrounded by doped silicon layers, there is a change of phase of the germanium selenide layer. A large refractive index contrast between the two phases of the germanium selenide layer—i.e., between the amorphous phase and the crystalline phase—on application of the electric field is responsible for shifting of the mode gap in the dispersion characteristics of the PhC slab waveguide. Our results show excellent performance of the electro-optic modulators in terms of the high On/Off extinction ratio ( &gt; 37 d B in the TE configuration and &gt; 28 d B in the TM configuration), broadband switching capability ( ∼ 100 n m ), low insertion loss in the On state ( &lt; 2 d B ), high range of tunability (by changing the lattice constant and radius of air holes), and low operating voltage ( &lt; 4 V ). The proposed devices can be used as electro-optic modulators in future nanoscale photonic integrated circuits.

This paper proposes a cascade-connected arrayed-waveguide grating (AWG) as a solution to the problem of crosstalk accumulation in a large-scale AWG multiplexer/demultiplexer (MUX/DEMUX) and demonstrates a 64-channel cascaded AWG module with a very low background crosstalk of less than -80 dB and a total crosstalk of about -34 dB. In this paper, the authors densely integrate 64 additional compactly designed crosstalk-suppressing AWGs whose bandwidths were carefully optimized and directly attach them to a conventional 64-channel AWG. Consequently, in addition to a very low crosstalk, a low insertion loss and a compact size without passband shape distortion are achieved with this module. Based on the performance of the cascaded AWG module, it is then estimated that it is possible to realize a 1000-channel AWG MUX/DEMUX that is free from the problem of crosstalk accumulation.

We have developed an NxN cyclic-frequency router with improved performance by employing two types of modified configuration; a uniform-loss and cyclic-frequency (ULCF) arrayed-waveguide grating (AWG) and an interconnected multiple AWG. We have demonstrated a compact 50-GHz-spacing 64x64 ULCF-AWG router with low and uniform insertion losses of 5.4-6.8 dB and frequency deviations from the grid of less than plusmn8 GHz. We have also demonstrated a 100-GHz-spacing 8x8 interconnected multiple-AWG router with a practical configuration, very low and uniform insertion losses of 2.3-3.4 dB, and frequency deviations from the grid of less than plusmn6 GHz. We discuss the suitable or realizable scale N of the two types of routers by comparison with a conventional AWG router in terms of optical and dimensional performance and productivity.

Nowadays, the development of modern optical systems relies on optical device size minimization and operating power reduction. Optical modulator based on silicon on insulator (SOI) platform is a key element in different optical systems. Therefore, the optical modulator with compact size and low insertion loss could improve the optical system efficiency. In this work, a novel compact optical modulator based on hybrid plasmonic/silicon layers is introduced. The full vectorial finite element method (FV-FEM) is used to numerically analyze the proposed design. Vanadium dioxide (VO2) is also utilized as a cap layer to control the modulation process. The insertion loss (IL) and extinction ratio (ER) of the suggested modulator are equal to 2.1 dB/µm and 28 dB/µm, respectively, at the operating wavelength 1.55 µm. Consequently, high figure-of-merit (FoM) =ER/IL = 13.5 is achieved with an optical bandwidth (ER > 3 dB) greater than 1 µm, which is large in comparison to pervious designs.

PurposeThis paper aims to design a radio frequency micro-electro-mechanical system (RF MEMS)-based phase shifter using chamfered coplanar waveguide (CPW) transmission line (t-line) with open-circuit interdigital metal–air–metal (ID MAM) capacitors.Design/methodology/approachThe proposed phase shifter achieves maximum differential phase shift with low loss at Ku band. The phase shifter is built with one switchable fixed-fixed beam (MEMS switch) on chamfered CPW t-line in series with two planar open-circuit ID MAM capacitors. An equivalent circuit model for the proposed phase shifter is derived, and its parameters are extracted using an electromagnetic (EM) solver.FindingsThe MEMS switch is actuated using an electrostatic method with the calculated residual stress of 44.26 MPa. The fabricated phase shifter exhibits low insertion loss, close to 0.14 dB at 17 GHz, with the maximum phase shift of 15.06°. The return loss is greater than 23 dB between 12 and 18 GHz.Originality/valueThis phase shifter presents a promising solution for low loss applications in the Ku band with a maximum phase shift. As the maximum phase shift of 15.06° is achieved for a unit cell with low insertion loss, the phase shifter is found to be feasible for modern electronically tunable phased arrays used for satellite communication and radar systems.

Recent advancements in mobile communication have escalated the demand for faster data rates, requiring higher carrier frequencies and compact, high-performance, and low-cost radio frequency (RF) filters. Micro-acoustic resonators offer significant advantages in mobile phone filtering due to their low loss and compact size. Addressing the need for low-cost filter solutions for higher frequencies, this study presents a silicon-substrate-based surface acoustic wave (SAW) technology platform to enable high-performance resonators and filters for 6G X-band (7–12 GHz) centimeter-wave (cmWave) wireless communications. Based on a silicon (Si) substrate, we propose a novel design scheme to excite shear vertical surface acoustic waves (SV-SAW) on a 128°Y LiNbO3/SiO2/poly-Si/Si layer stack and realize high-frequency resonators above 6 GHz with high-performance: electromechanical coupling coefficient (k2) of 7.6% ~ 8.9% and high-quality factor (Q) ranging from 193–679. The synthesized filters based on those high-performance resonators show low insertion loss (1.47 to 2.20 dB) and 3-dB bandwidth from 308 to 373 MHz. Especially, the demonstrated filter with a center frequency (fc) at 8.63 GHz exhibits a low insertion loss of only 1.5 dB, which is the best when compared to all other LiNbO3 acoustic filters in this frequency range, 3-dB bandwidth of 373 MHz, and decent out-of-band rejection across the entire 1–15 GHz range. These results mark a significant step forward for the microwave acoustics field and pave the way for enabling solidly-mounted, low-cost, and miniature-size SAW filters for emerging 6G cmWave wireless communications.

In this paper, we present the design of low-loss and polarization-insensitive semiconductor hollow waveguides using a highly reflective GaAs/AlAs mirrors. We experimentally demonstrate a low-loss slab hollow waveguide with a variable air core, exhibiting a low propagation loss of below 0.2 dB/cm.

We discuss recent progress and challenges in realizing Bragg-grating devices on the submicron silicon-on-insulator platform for next-generation optical communications applications, such as on-chip optical interconnects and signal processing. In particular, we focus on grating-assisted, wavelength-selective couplers, known as contra-directional couplers (contra-DCs). In contrast to conventional two-port Bragg gratings operating in the reection mode, contra-DCs are four-port devices with very weak backreections and, therefore, can be easily integrated with other photonic components on a chip. In order to provide a reliable on-chip wavelength-division multiplexing (WDM) solution for high-speed optical interconnects, we have developed high-performance add-drop filters and, furthermore, wavelength multiplexers/demultiplexers with combined advantages of at-top responses, low insertion loss (&lt; 1 dB), and low crosstalk (&lt; -23 dB). These WDM devices are ultra-compact and highly tolerant to temperature uctuations (up to &plusmn;50 &deg;C), showing great potential for large-scale integration and low-power consumption. We further discuss a novel four-port Bragg photonic resonator for high-speed, low-power optical switching. Using a coupler-chirped design with uniform Bragg gratings, we have recently achieved an on-chip, continuously tunable photonic delay line with low insertion loss. These system-orientated devices indicate great potential for large-scale integration of Bragg-grating-defined functions using CMOS-compatible silicon photonics technology.