Roadmapping the next generation of silicon photonics.

Silicon photonics has developed into a mainstream technology driven by advances in optical communications. The current generation has led to a proliferation of integrated photonic devices from thousands to millions-mainly in the form of communication transceivers for data centers. Products in many exciting applications, such as sensing and computing, are around the corner. What will it take to increase the proliferation of silicon photonics from millions to billions of units shipped? What will the next generation of silicon photonics look like? What are the common threads in the integration and fabrication bottlenecks that silicon photonic applications face, and which emerging technologies can solve them? This perspective article is an attempt to answer such questions. We chart the generational trends in silicon photonics technology, drawing parallels from the generational definitions of CMOS technology. We identify the crucial challenges that must be solved to make giant strides in CMOS-foundry-compatible devices, circuits, integration, and packaging. We identify challenges critical to the next generation of systems and applications-in communication, signal processing, and sensing. By identifying and summarizing such challenges and opportunities, we aim to stimulate further research on devices, circuits, and systems for the silicon photonics ecosystem.

Correction: Integrated silicon photonic MEMS.

[This corrects the article DOI: 10.1038/s41378-023-00498-z.].

Correction: Integrated 4-terminal single-contact nanoelectromechanical relays implemented in a silicon-on-insulator foundry process.

Correction for 'Integrated 4-terminal single-contact nanoelectromechanical relays implemented in a silicon-on-insulator foundry process' by Yingying Li et al., Nanoscale, 2023, https://doi.org/10.1039/d3nr03429a.

Integrated 4-terminal single-contact nanoelectromechanical relays implemented in a silicon-on-insulator foundry process.

Integrated nanoelectromechanical (NEM) relays can be used instead of transistors to implement ultra-low power logic circuits, due to their abrupt turn off characteristics and zero off-state leakage. Further, realizing circuits with 4-terminal (4-T) NEM relays enables significant reduction in circuit device count compared to conventional transistor circuits. For practical 4-T NEM circuits, however, the relays need to be miniaturized and integrated with high-density back-end-of-line (BEOL) interconnects, which is challenging and has not been realized to date. Here, we present electrostatically actuated silicon 4-T NEM relays that are integrated with multi-layer BEOL metal interconnects, implemented using a commercial silicon-on-insulator (SOI) foundry process. We demonstrate 4-T switching and the use of body-biasing to reduce pull-in voltage of a relay with a 300 nm airgap, from 15.8 V to 7.8 V, consistent with predictions of the finite-element model. Our 4-T NEM relay technology enables new possibilities for realizing NEM-based circuits for applications demanding harsh environment computation and zero standby power, in industries such as automotive, Internet-of-Things, and aerospace.

Wideband parametric baseband macromodeling of linear and passive photonic circuits via complex vector fitting.

A novel wideband parametric baseband macromodeling technique for passive photonic devices and circuits is presented. It allows to efficiently estimate the baseband scattering representations of a linear, passive photonic system as a function of a set of design variables, such as geometrical layout or substrate features. The proposed technique relies on the interpolation of macromodels computed via a complex vector fitting (CVF) algorithm, by adopting a methodology based on amplitude and frequency scaling that preserves, by construction, the physical properties of the system, such as causality, stability and passivity. For a specified combination of the design parameters, a rational CVF model is derived that can be simulated by a wide range of ordinary differential equation (ODE) solvers or circuit simulators. Additionally, time-domain simulations using the computed model can be performed at arbitrary optical carrier frequencies, thus allowing for the simulation of multi-wavelength systems. Two application examples are presented to demonstrate the flexibility and advantages of the proposed method.

Integrated silicon photonic MEMS.

Silicon photonics has emerged as a mature technology that is expected to play a key role in critical emerging applications, including very high data rate optical communications, distance sensing for autonomous vehicles, photonic-accelerated computing, and quantum information processing. The success of silicon photonics has been enabled by the unique combination of performance, high yield, and high-volume capacity that can only be achieved by standardizing manufacturing technology. Today, standardized silicon photonics technology platforms implemented by foundries provide access to optimized library components, including low-loss optical routing, fast modulation, continuous tuning, high-speed germanium photodiodes, and high-efficiency optical and electrical interfaces. However, silicon's relatively weak electro-optic effects result in modulators with a significant footprint and thermo-optic tuning devices that require high power consumption, which are substantial impediments for very large-scale integration in silicon photonics. Microelectromechanical systems (MEMS) technology can enhance silicon photonics with building blocks that are compact, low-loss, broadband, fast and require very low power consumption. Here, we introduce a silicon photonic MEMS platform consisting of high-performance nano-opto-electromechanical devices fully integrated alongside standard silicon photonics foundry components, with wafer-level sealing for long-term reliability, flip-chip bonding to redistribution interposers, and fibre-array attachment for high port count optical and electrical interfacing. Our experimental demonstration of fundamental silicon photonic MEMS circuit elements, including power couplers, phase shifters and wavelength-division multiplexing devices using standardized technology lifts previous impediments to enable scaling to very large photonic integrated circuits for applications in telecommunications, neuromorphic computing, sensing, programmable photonics, and quantum computing.

Vacuum-sealed silicon photonic MEMS tunable ring resonator with an independent control over coupling and phase.

Ring resonators are a vital element for filters, optical delay lines, or sensors in silicon photonics. However, reconfigurable ring resonators with low-power consumption are not available in foundries today. We demonstrate an add-drop ring resonator with the independent tuning of round-trip phase and coupling using low-power microelectromechanical (MEMS) actuation. At a wavelength of 1540 nm and for a maximum voltage of 40 V, the phase shifters provide a resonance wavelength tuning of 0.15 nm, while the tunable couplers can tune the optical resonance extinction ratio at the through port from 0 to 30 dB. The optical resonance displays a passive quality factor of 29 000, which can be increased to almost 50 000 with actuation. The MEMS rings are individually vacuum-sealed on wafer scale, enabling reliable and long-term protection from the environment. We cycled the mechanical actuators for more than 4 × 109 cycles at 100 kHz, and did not observe degradation in their response curves. On mechanical resonance, we demonstrate a modulation increase of up to 15 dB, with a voltage bias of 4 V and a peak drive amplitude as low as 20 mV.

Micro-transfer printing InP C-band SOAs on advanced silicon photonics platform for lossless MZI switch fabrics and high-speed integrated transmitters.

We present an approach for the heterogeneous integration of InP semiconductor optical amplifiers (SOAs) and lasers on an advanced silicon photonics (SiPh) platform by using micro-transfer-printing (µTP). After the introduction of the µTP concept, the focus of this paper shifts to the demonstration of two C-band III-V/Si photonic integrated circuits (PICs) that are important in data-communication networks: an optical switch and a high-speed optical transmitter. First, a C-band lossless and high-speed Si Mach-Zehnder interferometer (MZI) switch is demonstrated by co-integrating a set of InP SOAs with the Si MZI switch. The micro-transfer-printed SOAs provide 10 dB small-signal gain around 1560 nm with a 3 dB bandwidth of 30 nm. Secondly, an integrated transmitter combining an on-chip widely tunable laser and a doped-Si Mach-Zehnder modulator (MZM) is demonstrated. The laser has a continuous tuning range over 40 nm and the transmitter is capable of 40 Gbps non-return-to-zero (NRZ) back-to-back transmission at wavelengths ranging from 1539 to 1573 nm. These demonstrations pave the way for the realization of complex and fully integrated photonic systems-on-chip with integrated III-V-on-Si components, and this technique is transferable to other material films and devices that can be released from their native substrate.

Programmable wavelength filter with double ring loaded MZI.

We propose a novel filter circuit that incorporates a double ring resonator with a balanced Mach-Zehnder interferometer (MZI). The circuit has a response equivalent to a conventional ring loaded MZI filter, but with added flexibility in terms of configurability. The second-order filter can also be cascaded to realize higher-order filters. The circuit incorporates a two-stage input and output coupler to further reduce the effect of dispersion. A combination of local and global optimization strategies to program the filter, using tailored objective functions, have been tested in simulation and experiments. To our best knowledge, this is the first time a global optimization strategy is directly used in ARMA filter synthesis and simulation without any additional requirement. We further extend the optimization strategy into experiments and demonstrated its use in practical case for programmable filter circuits.

Silicon photonic microelectromechanical phase shifters for scalable programmable photonics.

Programmable photonic integrated circuits are emerging as an attractive platform for applications such as quantum information processing and artificial neural networks. However, current programmable circuits are limited in scalability by the lack of low-power and low-loss phase shifters in commercial foundries. Here, we demonstrate a compact phase shifter with low-power photonic microelectromechanical system (MEMS) actuation on a silicon photonics foundry platform (IMEC's iSiPP50G). The device attains (2.9π±π) phase shift at 1550 nm, with an insertion loss of (0.33-0.10+0.15)dB, a Vπ of (10.7-1.4+2.2)V, and an Lπ of (17.2-4.3+8.8)µm. We also measured an actuation bandwidth f-3dB of 1.03 MHz in air. We believe that our demonstration of a low-loss and low-power photonic MEMS phase shifter implemented in silicon photonics foundry compatible technology lifts a main roadblock toward the scale-up of programmable photonic integrated circuits.

Disk-loaded silicon micro-ring resonator for high-Q resonance.

By adding two disks to a standard silicon micro-ring resonator, a very high-quality factor (Q) asymmetric resonance with Q values as high as 7.773 × 105 and slope rates in excess of 880 dB/nm can be achieved. A circuit model has been proposed for this device based on which an analysis has been carried out that can predict the effect of reflections in the coupling components. Depending on the coupling coefficient between the disks and the micro-ring resonator (MRR), it is possible to use this design in three regimes, with different spectral features. Moreover, it is shown that the disks introduce a discontinuity in the transmission spectrum and the relative positioning of the disks in the ring provides a new degree of freedom in the design step. The proposed device features a high extinction ratio (ER) around 1550 nm and could be fabricated in any standard silicon photonics technology without requiring any extra materials or processing steps. The proposed resonator has a high sensitivity of ΔλRes (nm)/Δn > 299 nm/RIU, which makes it suitable for sensing applications and efficient modulators.

Programmable photonic circuits.

The growing maturity of integrated photonic technology makes it possible to build increasingly large and complex photonic circuits on the surface of a chip. Today, most of these circuits are designed for a specific application, but the increase in complexity has introduced a generation of photonic circuits that can be programmed using software for a wide variety of functions through a mesh of on-chip waveguides, tunable beam couplers and optical phase shifters. Here we discuss the state of this emerging technology, including recent developments in photonic building blocks and circuit architectures, as well as electronic control and programming strategies. We cover possible applications in linear matrix operations, quantum information processing and microwave photonics, and examine how these generic chips can accelerate the development of future photonic circuits by providing a higher-level platform for prototyping novel optical functionalities without the need for custom chip fabrication.

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.

Correlation between pattern density and linewidth variation in silicon photonics waveguides.

We describe the correlation between the measured width of silicon waveguides fabricated with 193 nm lithography and the local pattern density of the mask layout. In the fabrication process, pattern density can affect the composition of the plasma in a dry etching process or the abrasion rate in a planarization step. Using an optical test circuit to extract waveguide width and thickness, we sampled 5841 sites over a fabricated wafer. Using this detailed sampling, we could establish the correlation between the linewidth and average pattern density around the test circuit, as a function of the radius of influence. We find that the intra-die systematic width variation correlates most with the pattern density within a radius of 200 µm, with a correlation coefficient of 0.57. No correlation between pattern density and the intra-die systematic thickness variation is observed. These findings can be used to predict photonic circuit yield or to optimize the circuit layout to minimize the effect of local pattern density.

Tolerant, broadband tunable 2 × 2 coupler circuit.

We propose a circuit design for a broadband tunable 2 × 2 waveguide coupler, consisting of a two-stage Mach-Zehnder interferometer with electro-optic phase shifters in each stage. We demonstrate that such design can be configured as a tunable coupler with arbitrary coupling ratio and with a uniform response over 50-nm spectral range around 1550 nm. The design is also tolerant to fabrication variations that affect the coupling ratios of the directional couplers.

Silicon ring resonators with a free spectral range robust to fabrication variations.

We propose a design method for silicon ring resonators (RRs) with a free spectral range (FSR) insensitive to fabrication variations. Two waveguide-core widths are used in the RR, with opposite signs of the group-index derivative with respect to the width. This results in cancellation of the width-dependent FSR changes. The systematic deviation of the realized width from the design width is determined and is used for calibrating the calculated relation of group index versus width. This enables a more accurate FSR value and well-aimed robust performance. We present two robust design examples. Experimental results match well with the predictions. For the deliberately introduced ±10 nm core-width change, the FSR variation of the robust designs is only about 30% of the value measured from the RR with a single core width. This design method can be used to improve the performance of photonic integrated circuits using multiple RRs. As the FSR of a RR is not easily tunable, the robust design is beneficial to applications where an accurate FSR is required, such as in microwave photonics.

Pure phase modulation based on a silicon plasma dispersion modulator.

Plasma dispersion modulators (PDMs), such as carrier injection modulators and carrier depletion modulators, are widely used for high speed phase modulation in silicon photonic circuits, but they suffer from spurious intensity modulation. This can be a problem in coherent communication systems that make use of complex multi-level quadrature modulation formats, as well as analog applications such as microwave photonics. In this article, a method to achieve pure phase modulation using PDMs is proposed based on a configurable modulator circuit. The configurable modulator is implemented as a Mach-Zehnder interferometer with a PDM and tunable couplers (TCs). The spurious intensity modulation of the phase modulated lightwave can be compensated by tuning the coupling ratios of the TCs and the phase delay between the two arms of the MZI. Simulation results show that for a depletion modulator, the 1.26 dB spurious intensity modulation can be suppressed down to 0.023 dB within a phase range of 0.4π, and for injection modulator, the 1.27 dB spurious intensity modulation can be suppressed down to 0.07 dB within a phase range of 0.76π.

Tunable electromagnetically induced transparency in integrated silicon photonics circuit.

We comprehensively simulate and experimentally demonstrate a novel approach to generate tunable electromagnetically induced transparency (EIT) in a fully integrated silicon photonics circuit. It can also generate tunable fast and slow light. The circuit is a single ring resonator with two integrated tunable reflectors inside, which form an embedded Fabry-Perot (FP) cavity inside the ring cavity. The mode of the FP cavity can be controlled by tuning the reflections using integrated thermo-optic tuners. Under correct tuning conditions, the interaction of the FP mode and the ring resonance mode will generate a Fano resonance and an EIT response. The extinction ratio and bandwidth of the EIT can be tuned by controlling the reflectors. Measured group delay proves that both fast light and slow light can be generated under different tuning conditions. A maximum group delay of 1100 ps is observed because of EIT. Pulse advance around 1200 ps is also demonstrated.

Digitally controlled multiplexed silicon photonics phase shifter using heaters with integrated diodes.

We present a silicon side heater with integrated diode to provide multiplexed control of different elements in a photonic circuit based on the polarity of the driving signal. The diode introduces an asymmetric electrical response where the heater is only active under forward bias. This can be used to address multiple heaters through the same electrical electrical contacts. We demonstrate push-pull operation on a Mach-Zehnder interferometer with heaters in both arms, as well as time-multiplexed operation of multiple heaters by modulating the driving signal. We extend this work by demonstrating how pulse width modulation (PWM) and duobinary-PWM can be used to improve the linearity of the response of the phase shifters.

Experimental demonstration of a single silicon ring resonator with an ultra-wide FSR and tuning range.

We present an experimental realization of a pseudo single-mode silicon ring resonator with an ultra-wide free spectral range and tuning range. The device is a single microring resonator with a tunable reflector integrated inside. The reflector is designed to have zero reflection for only one resonance of the ring, while all other resonances will suffer strong reflection. Given that the reflection inside a ring resonator leads to resonance splitting and degradation of the extinction ratio (ER), we obtain a ring resonator where only a single resonance has a large ER, while all others have a very low ER. The large ER resonance can be continuously tuned using metal heaters to achieve a broad tuning range over 55 nm with 16 mW of power injected into the phase shifter.