Polarization-Insensitive Silicon Photonic Filter With Multimode Waveguide Gratings

A means to calculate the multimodal spectral response of Bragg gratings in general non-circular multimode waveguides is proposed. To illustrate the power of the technique, the spectra of two Bragg temperature sensors are numerically calculated in which coupling between 100 modes is considered. It is shown how the Bragg wavelength in multimode Bragg grating waveguides is affected by the number of modes and energy distribution among them. Good matching of the simulated spectrum of a multimode Bragg grating on a planar inverted rib waveguide to the measured spectrum is seen.

A high-performance monolithic integrated wavelength division multiplexing silicon (Si) photonics receiver chip is fabricated on a silicon-on-insulator platform. The receiver chip has a 25-channel Si nanowire-arrayed waveguide grating, and each channel is integrated with a high-speed waveguide Ge-on-Si photodetector. The central wavelength, optical insertion loss, and cross talk of the array waveguide grating are 1550.6 nm, 5–8 dB, and −12–−15 dB, respectively. The photodetectors show low dark current density of 16.9 mA/cm2 at −1 V and a high responsivity of 0.82 A/W at 1550 nm. High bandwidths of 23 and 29 GHz are achieved at 0 and −1 V, respectively. Each channel can operate at 50 Gbps with low input optical power even under zero bias, which realizes an aggregate data rate of 1.25 Tbps.

Defining elements with reconfigurable input-output characteristics is of importance to achieve flexible circuitry where light can be manipulated and routed using external control signals. We have developed an experimental approach for shaping of the transmission function of multimode silicon photonic waveguides by projecting a pattern of local nonlinear perturbations induced by an ultrafast laser pulse. Making use of the degrees of freedom offered by a spatial light modulator, the technique offers a new approach for studying light transport, for controlling its flow on ultrafast time scale, and for programming functions on a photonic chip.

Mode-division multiplexing (MDM) is an attractive solution for future on-chip networks to enhance the optical transmission capacity with a single laser source. A mode-division reconfigurable optical add/drop multiplexer (ROADM) is one of the key components to construct flexible and complex on-chip optical networks for MDM systems. In this paper, we report on a novel scheme of mode-division ROADM with mode-selective silicon photonic MEMS (micro-electromechanical system) switches. With this ROADM device, data carried by any mode-channels can be rerouted or switched at an MDM network node, i.e., any mode could be added/dropped to/from the multimode bus waveguide flexibly and selectively. Particularly, the design and simulation of adiabatic vertical couplers for three quasi-TE modes (TE0, TE1, and TE2 modes) based on effective index analysis and mode overlap calculation method are reported. The calculated insertion losses are less than 0.08 dB, 0.19 dB, and 0.03 dB for the TE0 mode, TE1 mode, and TE2 mode couplers, respectively, over a wavelength range of 75 nm (1515–1590 nm). The crosstalks are below −20 dB over the bandwidth. The proposed device is promising for future on-chip optical networks with flexible functionality and large-scale integration.

Optical phased arrays (OPAs) have received considerable attention as solid-state beam scanners. However, conventional OPAs that actively control the phase difference between arrays are characterized by excessive power consumption for high-precision beam emission. In this study, we fabricated an OPA comprising Bragg grating and arrayed waveguide grating (AWG). Multi-mode waveguide is used in AWG to reduce the effect of manufacturing error. This device realizes wide and high-resolution two-dimensional beam steering only by sweeping wavelength. FWHM of the emitted beam is 0.534° × 2.27°, and the steering range is 43.9° × 13.5° with 1/64 of the power consumption of conventional OPA.

New wide single-mode strip and grating loaded (SGL) channel waveguides made of silicon nitride on the oxide buffer layer of a planar silicon-on-insulator waveguide are studied. The central 10-lm-wide strip produces a multi-mode channel waveguide and diffraction gratings with a period 0.6 lm built on the structure edges produce mode-dependent additional losses due to radiation to the surrounding medium. The optical properties of these waveguides are discussed using the results of a three-dimensional numerical simulation by the FDTD and BPM methods. It is shown that a wide SGL waveguide is quasi-single-mode one because ithas a small propagation loss (∼ 0.3 dB cm-1) for the fundamental mode and a high (up to -20 dB cm-1) loss forthe higher order modes. The new SGL waveguides are CMOS compatible and can become basic for fabricating new photonic elements, including tunable optical filters and multi-plexers based on the multireflector technology.

We are delighted to introduce a Special Issue on Optical Interconnects for the IET Optoelectronics magazine. This issue presents the current status as well as some of the outstanding progress that has been witnessed in the rapidly evolving area of optical interconnects. Today, data centres (DC) and high-performance computers are struggling with limited interconnect capacity and increased power consumption while trying to meet overwhelming bandwidth demands. As photonics technologies have been rapidly maturing, optical interconnects have come into the spotlight as the means to cope with the emerging bandwidth, energy, and capacity needs. As such, a wealth of photonic interconnect solutions across the DC hierarchy has unfolded, covering from electro-optic boards to transceiver development, on-chip and on-board switches and interconnects as well as novel noninvasive monitoring methods for light controlling for compensating thermal crosstalk, fabrication tolerances and unstable environmental conditions. At the same time, the increased penetration of optical interconnects in short-reach communications, such as board and chip levels, has triggered new challenges in photonics technologies calling for new innovations with respect to integration techniques and packaging methods. This Special Issue of IET includes integrated circuits, assembly techniques, systems-level architectures, and controlling methods related to short reach ‘chip-to-chip’, ‘in-the-box’, and ‘box-to-box’ communications. Specific areas covered by this special issue include: Silicon photonic transceiver technologies and flat DC topologies; High-speed electro-optic hosting boards and fast, energy-efficient electronics; DC switches and co-packaged optic (CPO) technologies; and Noninvasive controlling techniques for complex photonic integrated circuits. The four papers in this special section promote high-performance optical interconnect architectures and key technologies to tackle the rising bandwidth demands of DCs. They cover (a) co-packaged DC optics, (b) silicon technology transceiver modules and high-speed hosting boards for multisocket server board (MSB) interconnects and (c) automated controlling architecture for complex photonic systems; Items (b) and (c) draw from impressive achievements accomplished through the recently completed European H2020 Project ICT-STREAMS. This report written by C. Mickenberg tries to shed light on motivating factors behind co-packaged optics (CPO), the trade-offs involved and their potential impact on data centre network architectures. The article reviews the evolution of DC switch application-specific integrated circuit (ASICs) and DC optics, provides a future road map and finally discusses the potential impact of CPO on the network architecture. This topic is covered by two articles. The first, written by M. Morallis-Pegios, proposes using silicon photonics to advance interconnect throughput, socket connectivity and energy efficiency in MSB layouts by enabling a flat-topology wavelength division multiplexing (WDM) point-to-point optical MSB interconnect scheme. WDM silicon photonic transceivers coassembled with their electronic circuits achieving a 50-Gb/s line rate and 400-Gb/s aggregate data transmission are demonstrated. The article also proposes silicon photonic Arrayed Waveguide Grating Router (AWGRs) that can offer collision-less time-of-flight connectivity for up to 16 nodes. The second article covering that area is written by T. Lamprecht and reports on the development of electro-optic hosting boards for on-board transceiver subassemblies which is required to eventually form the Tb/s multi-socket onboard communication. The article also discusses the electrical interface as well as photonic board requirements, including the manufacturing technique and the assembly concept that was followed to deliver terabyte per second multichip optical communication. This article, written by F. Zanetto, introduces a novel dithering technique for real-time control of multiple cascaded photonic integrated devices, proposing a calibration-free strategy to support the development and real-life diffusion of increasingly complex photonic systems. The article elaborates on the implementation of a calibration-free automated control loop exploiting integral controllers to close a real-time feedback and stabilise the optimum working point of the employed photonic devices. The effectiveness of the proposed architectural approach is experimentally validated with the error-free routing and transmission of a 30-Gbit/s optical signal traversing a cascade of micro-ring resonator filters. All of the articles selected for this Special Issue present a wide variety of key topics and significant results addressing optical interconnects in high-performance data communication systems, including new progress on architectures, systems and packaging techniques as well as novel optimised controlling methods. As guest editors, we would like to thank all of the authors for their high-quality submissions that significantly contribute to the optical interconnect field. We would also like to thank the reviewers for their valuable and constructive technical comments that greatly enhance the Special Issue, because their feedback and comments ensure the high quality of the IET Optoelectronics Journal. We hope that this IET Special Issue will provide a useful reference of the state-of-the-art in optical interconnects to readers, and we anticipate such technological perspectives will stimulate new advancements and technological outcomes that will in turn enrich the growing field of optical interconnects. This work was supported by the E.C. through the H2020-ICT-STREAMS research project (688172).

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 1×8 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 1µm-thick top silicon layer, which was composed of 5×8 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 1µm 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 1×4 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).

The compatibility of silicon photonics with existing CMOS fabrication processes enables the possibility of large scale manufacturing of integrated photonics for communication applications above 100Gbit/s. However, the design of silicon photonics devices that are able to fulfil the high-performance requirements of broad optical bandwidth ( 50nm), low loss (2dB), high-speed (25Gbit/s), low driving voltage (≥ –5V), large fabrication tolerance (±20nm) and high reproducibility across the wafer is a unique practical challenge. In this work, we present a library of passive and active integrated silicon photonic devices that meet the stringent design requirements for communication applications. The design, modelling and experimental results are presented for low loss edge couplers with insertion loss < 2dB, low-loss waveguides and bends, high-performance polarization beam splitter with extinction ratio above 25 dB, broadband directional couplers with optical bandwidth above 50nm, and arrayed waveguide gratings (AWGs) with low insertion loss < 1dB. Last but not least, high-speed silicon modulators (25Gbit/s) with phase efficiency VπL ≤ 2V.cm at DC reverse bias of ≥ –5V and low propagation loss of ≤ ∼1dB/mm are demonstrated.

Silicon photonics has emerged as a critical enabling technology for a diverse range of applications, from high-speed data communication and computing to advanced sensing and quantum information processing. This paper provides a comprehensive review of recent progress in the foundational passive devices that underpin this technological revolution. We survey the state of the art in fundamental building blocks, including strip, rib, and silicon nitride waveguides, with a focus on achieving ultra-low propagation loss. The review details essential components for light coupling and splitting, such as grating couplers, edge couplers, multimode interference couplers, and directional couplers, citing their typical performance metrics. Key wavelength filtering and routing components, including high-Q ring resonators, Mach–Zehnder interferometers, and arrayed waveguide gratings, are analyzed. Furthermore, we provide a comparative overview of the capabilities of major photonic foundries operating on a multi-project wafer model. The paper concludes by discussing persistent challenges in packaging and polarization management, and explores future trends driven by co-packaged optics, inverse design methodologies, and the expansion of silicon photonics into new application domains.

We propose a novel approach to demonstrate simultaneous multi-wavelength locking during temperature changes in a silicon photonic polarization insensitive microring-based wavelength division multiplexing (WDM) receiver. The DC component of a single monitoring photodetector at the through port of the microring filter array is exploited as a feedback signal with no additional power consumption. This feedback signal is used in control circuitry to properly tune the microring filters using ohmic heating, thus creating a feedback loop for thermal adaptation. We describe the necessary information, specifically each microring filter's room temperature resonant wavelength and tunability, which can be used to calibrate and achieve proper wavelength configurability and locking. In addition, we describe a simple control algorithm based on an adaptive gradient method often used in machine learning, allowing the receiver to endlessly demultiplex at different temperatures. We successfully achieve thermal adaptation over a temperature range >37°C and demultiplex a 4 × 25 Gb/s on-off-keying signal of 150 GHz channel spacing, all while the polarization is scrambling.

Reducing power dissipation in electro-optic modulators is a key step for widespread application of silicon photonics to optical communication. In this work, we design Mach–Zehnder modulators in the silicon-on-insulator platform, which make use of slow light in a waveguide grating and of a reverse-biased p-n junction with interleaved contacts along the waveguide axis. After optimizing the junction parameters, we discuss the full simulation of the modulator in order to find a proper trade-off among various figures of merit, such as modulation efficiency, insertion loss, cutoff frequency, optical modulation amplitude, and dissipated energy per bit. Comparison with conventional structures (with lateral p-n junction and/or in rib waveguides without slow light) highlights the importance of combining slow light with the interleaved p-n junction, thanks to the increased overlap between the travelling optical wave and the depletion regions. As a surprising result, the modulator performance is improved over an optical bandwidth that is much wider than the slow-light bandwidth.

In this paper a novel microwave photonics beamformer device concept, for single side band (SSB) 40 GHz modulated signals, is presented. The proposed device comprises tunable lasers, flat-top arrayed waveguide gratings (AWG), a Mach Zender Modulator (MZM), an all-pass ring resonator and photodetectors. The device can be produced as a photonic integrated circuit. The signals from the lasers (one for each beamformer radiant element) are multiplexed by the first AWG, modulated, and passed through the all-pass ring resonator. The AWG channel spacing and the ring resonator Free Spectral Range (FSR) are both set to be equal to 100 GHz. The signal is demultiplexed by a second AWG and finally photodetected. By tuning each laser within its corresponding AWG passband, the phase difference between the optical carrier and the 40 GHz microwave modulated signal for each beamformer element can be controlled. The difference is determined by the phase response of the all-pass ring resonator. A critical part of the design is the alignment between the resonances of the ring resonator and both AWGs, but this can be alleviated by using a single AWG in fold-back configuration. The power provided to each beamformer element is different due to the intrinsic non-uniform losses of the AWGs and the ring resonators, but this can also be solved either by properly setting the lasers power, or by means of additional optical amplifiers. The presented analysis is independent of the integration technology. In Silicon photonics, the AWGs and ring resonator can be produced, while the (hybrid) integration of lasers, modulator, photodetectors (and eventually amplifiers) is a challenge. The device can be monolithically integrated on semi-insulating InP technology.

Integrated Mach-Zehnder modulators are key components in silicon photonic devices, which rely on a reverse-biased p-n junction to modulate the optical signal via a change of the waveguide refractive index. Reducing their energy consumption is a crucial issue towards applications of silicon photonics to optical communication. In this presentation, we describe the design of slow light structures consisting of silicon grating waveguides, which have an increased group index close to the photonic band edge. Also, we discuss how the design of p-n junctions affects matching between the optical field mode and the depletion region, in order to improve the modulation efficiency and to reduce the energy dissipation per bit of the modulator.

Silicon photonics is a fast growing technology for high bandwidth optical communications. Bandwidth can be strongly enhanced with the use of multimode systems, but integrated photonics still has to operate in the single-mode regime. Indeed, most multi-mode photonic devices such as waveguide bends cannot preserve the integrity of the modes. Here we present an alternative inverse-design method to design an ultra-compact multimode waveguide bend. The convergence and performance of the algorithm are studied, along with a transform method that allows to design devices made of two materials only. The optimal device is studied with full electromagnetic simulations to compare its behavior with a more traditional circular waveguide bend. A broadband transmission with very low intermodal coupling is achieved. This illustrates the potential of the objective-first inverse-design method to design ultra-compact broadband photonic devices.