Silicon Photonic Components Research Articles

Low-loss silicon waveguide and an ultrahigh-Q silicon microring resonator in the 2 µm wave band.

Silicon photonic-integrated circuits (PICs) operating in the 2 µm wave band are of great interest for spectroscopic sensing, nonlinear optics, and optical communication applications. However, the performance of silicon PICs in this wave band lags far behind the conventional optical communication band (1310/1550 nm). Here we report the realization of a low-loss waveguide and an ultrahigh-Q microring resonator in the 2 µm wave band on a standard 200 mm silicon photonic platform. The single-mode strip waveguide fabricated on a 220 nm-thick silicon device layer has a record-low propagation loss ∼0.2 dB/cm. Based on the low-loss waveguide, we demonstrate an ultrahigh-Q microring resonator with a measured loaded Q-factor as high as 1.1 × 106 and intrinsic Q-factor of 2 × 106, one order of magnitude higher than prior silicon resonators operating in the same wave band. The extinction ratio of the resonator is higher than 22 dB. These high-performance silicon photonic components pave the way for on-chip sensing applications and nonlinear optics in the 2 µm wave band.

Thermal flux manipulation on the silicon photonic chip to suppress the thermal crosstalk

The integration density of silicon photonic integrated circuit (PIC) is ultimately constrained by various crosstalk mechanisms on the chip. Among them, the most prominent limiting factor is the thermal crosstalk due to the wide use of the thermo-optic effect. High-density silicon PICs strongly demand an advanced structure with better thermal crosstalk suppression ability than the traditional air isolation trench. Inspired by the thermal-metamaterial based on the scattering-cancellation method, we demonstrate a closed heat shield (CHS) structure on a silicon PIC chip, which can manipulate the thermal flux to bypass the temperature-sensitive silicon photonics components. The on-chip CHS structure is a bilayer cylindrical shell fabricated by the standard silicon photonics processing flow. Its outer and inner shell layers are formed by a 6-μm-wide interconnection metal and 4-μm-wide air trench, respectively. Plenty of temperature-sensitive micro-ring resonators inside the CHS are used to probe the temperature profile. The measurement results show that the CHS can reduce the local temperatures by 50%/44%/36% at the locations 29/41/83 μm away from the external heater. In contrast, the conventional air trench of the same dimension reduces the local temperatures by 32%/28%/21% at the same positions. In addition, the response time of the thermal field inside the CHS is around one-half of that in the conventional air trench. Furthermore, the simulation result indicates that if the outer shell of the CHS can contact with the silicon substrate by utilizing the through-silicon-via structure, the thermal crosstalk suppression ability can be improved significantly.

High-speed 4 × 4 silicon photonic plasma dispersive switch, operating at the 2 µm waveband.

The escalating need for expansive data bandwidth, and the resulting capacity constraints of the single mode fiber (SMF) have positioned the 2-μm waveband as a prospective window for emerging applications in optical communication. This has initiated an ecosystem of silicon photonic components in the region driven by CMOS compatibility, low cost, high efficiency and potential for large-scale integration. In this study, we demonstrate a plasma dispersive 4 × 4 photonic switch operating at the 2-μm waveband with the highest switching speed. The demonstrated switch operates across a 45-nm bandwidth, with 10-90% rise and 90-10% fall time of 1.78 ns and 3.02 ns respectively. In a 4 × 4 implementation, crosstalk below -15 dB and power consumption lower than 19.15 mW across all 16 optical paths are indicated. This result brings high-speed optical switching to the portfolio of devices at the promising waveband.

Ultra‐Low Loss SiN Edge Coupler for III‐V/SiN Hybrid Integration

AbstractSilicon photonics is becoming a competitive technology to settle the issues of power and speed in data centers and computing systems. However, the absence of an on‐chip light source restricts the wide and deep application of silicon photonics. Essentially, efficient edge coupling from a Si‐based III‐V laser to silicon photonic components remains the major obstacle for on‐chip integration. Here, two compact edge couplers with ultra‐low coupling loss and eased fabrication for the light transmission from a Si‐based III‐V laser to Si3N4 waveguide are realized. An ultra‐low laser‐to‐chip coupling loss of only 1.175 dB for the butt coupling of a Si‐based InAs QD laser is experimentally demonstrated with high alignment tolerance. The strategies presented here provide a competitive solution for realizing ultra‐efficient integration of Si‐based light sources and silicon photonic components.

An Efficient Silicon Grating Coupler for a 2 μm Waveband Based on a Polysilicon Overlay

The short-wavelength mid-infrared spectral range of the 2 μm waveband has the advantages of low transmission loss and broad gain bandwidth, making it a promising candidate for the next optical fiber communication window. It is thus highly desired to develop high-performance silicon photonic components in this waveband. Here, an efficient dual-layer grating coupler was designed on a 220 nm thick silicon-on-insulator based on raised polysilicon to address the low directionality issue. For the fiber tilted at an angle of 10°, the grating coupler’s simulated coupling efficiency reaches 80.3% (−0.95 dB) at a wavelength of 2002 nm. The 1 dB bandwidth is 66 nm. The structure is completely compatible with the standard silicon photonic fabrication process, making it suitable for large volume fabrication.

On-chip Y-junction with adaptive power splitting toward ultrabroad bandwidth.

Growing research interests have been directed to the emerging optical communication band at 2-µm wavelengths. The silicon photonic components are highly desired to operate over a broad bandwidth covering both C-band and the emerging 2-µm wave band. However, the dispersions of the silicon waveguides eventually limit the optical bandwidth of the silicon photonic devices. Here, we introduce a topology-optimized Y-junction with a shallow-etched trench and its utility to reverse the detrimental dispersion effect. The shallow trench enables the Y-junction to have an adaptive splitting capability over a broad spectral range. The 0.2-dB bandwidth of the power splitter exceeds 800 nm from 1400 nm to 2200 nm. The device has a compact footprint of 3 µm × 1.64 µm. The device is characterized at the C-band and 2-µm band with a measured excess loss below 0.4 dB for a proof-of-concept demonstration.

Monolithic integration of embedded III-V lasers on SOI

Silicon photonic integration has gained great success in many application fields owing to the excellent optical device properties and complementary metal-oxide semiconductor (CMOS) compatibility. Realizing monolithic integration of III-V lasers and silicon photonic components on single silicon wafer is recognized as a long-standing obstacle for ultra-dense photonic integration, which can provide considerable economical, energy-efficient and foundry-scalable on-chip light sources, that has not been reported yet. Here, we demonstrate embedded InAs/GaAs quantum dot (QD) lasers directly grown on trenched silicon-on-insulator (SOI) substrate, enabling monolithic integration with butt-coupled silicon waveguides. By utilizing the patterned grating structures inside pre-defined SOI trenches and unique epitaxial method via hybrid molecular beam epitaxy (MBE), high-performance embedded InAs QD lasers with monolithically out-coupled silicon waveguide are achieved on such template. By resolving the epitaxy and fabrication challenges in such monolithic integrated architecture, embedded III-V lasers on SOI with continuous-wave lasing up to 85 °C are obtained. The maximum output power of 6.8 mW can be measured from the end tip of the butt-coupled silicon waveguides, with estimated coupling efficiency of approximately -6.7 dB. The results presented here provide a scalable and low-cost epitaxial method for the realization of on-chip light sources directly coupling to the silicon photonic components for future high-density photonic integration.

An Integrated Optical Circuit Architecture for Inverse-Designed Silicon Photonic Components.

In this work, we demonstrate a compact toolkit of inverse-designed, topologically optimized silicon photonic devices that are arranged in a "plug-and-play" fashion to realize many different photonic integrated circuits, both passive and active, each with a small footprint. The silicon-on-insulator 1550-nm toolkit contains a 2 × 2 3-dB splitter/combiner, a 2 × 2 waveguide crossover, and a 2 × 2 all-forward add-drop resonator. The resonator can become a 2 × 2 electro-optical crossbar switch by means of the thermo-optical effect, phase-change cladding, or free-carrier injection. For each of the ten circuits demonstrated in this work, the toolkit of photonic devices enables the compact circuit to achieve low insertion loss and low crosstalk. By adopting the sophisticated inverse-design approach, the design structure, shape, and sizing of each individual device can be made more flexible to better suit the architecture of the greater circuit. For a compact architecture, we present a unified, parallel waveguide circuit framework into which the devices are designed to fit seamlessly, thus enabling low-complexity circuit design.

Application of the TDFA window in true optical time delay systems.

Recent advances in silicon photonic components operating in the thulium-doped fiber amplifier (TDFA) wavelength regime around 2-µm have shown that these wavelengths hold great promise for on-chip photonic systems. Here we present our work on characterizing a Mach-Zehnder interferometer coupled silicon photonic ring resonator operating in the TDFA window for optical time delay applications. We describe the optical transmission and variable time delay properties of the resonator, including a detailed characterization and comparison of the directional coupler and Mach-Zehnder interferometer base components at both 1930 and 1550 nm wavelengths. The results show tuning of a ring from a 190-ps peak time delay at a resonant extinction ratio of 5.1-dB to a 560-ps peak time delay at an extinction ratio of 11.0-dB, in good agreement with optical models of the device. These results demonstrate significant promise towards the future application of TDFA band devices in optical time delay systems.

Photonic devices for future network designs

AbstractOptical fiber networks, which make up the backbone of the internet, rely on many electrical signal processing devices. Nanoscale silicon photonic network components, such as phase shifters, could boost optical network speed, capacity, and reliability. To design these small but powerful devices, a team at the Swiss Federal Institute of Technology Lausanne uses simulation to optimize both optical and electromechanical performance.

High-Performance Silicon Photonics Using Heterogeneous Integration

The performance of silicon photonic components and integrated circuits has improved dramatically in recent years. As a key enabler, heterogeneous integration not only provides the optical gain which is absent from native Si substrates and enables complete photonic functionalities on chip, but also lays the foundation of versatile integrated photonic device performance engineering. This paper reviews recent progress of high-performance silicon photonics using heterogeneous integration, with emphasis on ultra-low-loss waveguides, single-wavelength lasers, comb lasers, and photonic integrated circuits including optical phased arrays for LiDAR and optical transceivers for datacenter interconnects.

High performance thin-film lithium niobate modulator on a silicon substrate using periodic capacitively loaded traveling-wave electrode

Thin-film lithium niobate (TFLN) based traveling-wave modulators maintain simultaneously excellent performances, including large modulation bandwidth, high extinction ratio, low optical loss, and high modulation efficiency. Nevertheless, there still exists a balance between the driving voltage and modulation bandwidth. Here, we demonstrate an ultra-large bandwidth electro-optic modulator without compromising the driving voltage based on the TFLN platform on a silicon substrate, using a periodic capacitively loaded traveling-wave electrode. In order to compensate the slow-wave effect, an undercut etching technique for the silicon substrate is introduced to decrease the microwave refractive index. Our demonstrated devices represent both low optical and low microwave losses, which leads to a negligible optical insertion loss of 0.2 dB and a large electro-optic bandwidth with a roll-off of 1.4 dB at 67 GHz for a 10 mm-long device. A low half-wave voltage of 2.2 V is also achieved. Data rates up to 112 Gb s−1 with PAM-4 modulation are demonstrated. The compatibility of the proposed modulator to silicon photonics facilitates its integration with matured silicon photonic components using, e.g., hybrid integration technologies.

Photonic crystal lasers: from photonic crystal surface emitting lasers (PCSELs) to hybrid external cavity lasers (HECLs) and topological PhC lasers [Invited

Photonic crystals (PhC) represent an important class of silicon photonics components employed as wavelength selective resonators to act as narrow-band mirrors in integrated lasers due to their small footprint, high surface area, and Q-factor/volume ratio that enables efficient confinement of light, required for improved performances of the laser. These properties of PhCs are key for the potential deployment of PhC based high power, energy efficient and versatile semiconductor lasers for telecom, datacom, optical sensing and biomedical applications. In this paper, we report the main advances on PhC based lasers from photonic crystal surface-emitting lasers (PCSELs) to the new hybrid external cavity laser (HECLs) configurations.

Double-stadium Si-MZI racetrack microring resonator circuits: way to generate optical digital patterns

Silicon (Si) photonic components, namely, grating couplers and three-port splitters and couplers, have been designed and combined to form double-stadium Si-Mach–Zehnder interferometer (MZI) racetrack microring resonator circuits for efficient control over resonance in a certain wavelength range and free-spectral range (FSR). It was found that variable microring resonator arms and the values of free-space distance (between the arms) at 200 nm and 5 µm can yield two and six OFF-phase zones, respectively, within the wavelength range of 1.5–1.6 µm. Furthermore, it was observed that the smaller arms downsize the FSR, whereas the larger arms upsize its value within the transmission spectrum. The usefulness of these structures can be conceptualized in the area of optical digital pattern code generation because suitably controlling the MZI arm lengths would create certain optical digital patterns in the frequency domain of operation. Apart from these, the usefulness of racetrack optical delay lines remains in optical sensing or biomedical sensing as well.

Temperature Dependence of the Steering Angles of a Silicon Photonic Optical Phased Array

Silicon photonic optical phased arrays leveraging from the well established CMOS fabrication process have become promising candidates to realize miniaturized beam steering systems. While different implementations of optical phased arrays have been successfully demonstrated, no investigations on the influence of thermal distortions on the response of these systems have been performed so far. Due to the high thermo-optic coefficient of silicon, the refractive index and thus the functionality of silicon photonic components depend strongly on the temperature. In highly integrated systems, consisting of several photonic components, in which laser sources and control electronics are directly integrated on top of the photonic IC, heat dissipated by active components leads to temperature gradients and offsets, affecting the functionality of the photonic system. Optical phased arrays consisting of an array of phase sensitive components, are particularly sensitive towards temperature. Therefore, precise investigations of the temperature dependence of the OPA functionality are required. This article aims to expand the understanding of the influence of thermal distortions on the steering angles of optical phased arrays. Thereto, analytical expressions describing the temperature dependence of the steering angles are provided and experimentally validated. The expressions are provided in a general form so that they can be applied to any OPA system regardless of its implementation.

Novel adiabatic coupler for III-V nano-ridge laser grown on a Si photonics platform.

While III-V lasers epitaxially grown on silicon have been demonstrated, an efficient approach for coupling them with a silicon photonics platform is still missing. In this paper, we present a novel design of an adiabatic coupler for interfacing nanometer-scale III-V lasers grown on SOI with other silicon photonics components. The starting point is a directional coupler, which achieves 100% coupling efficiency from the III-V lasing mode to the Si waveguide TE-like ground mode. To improve the robustness and manufacturability of the coupler, a linear-tapered adiabatic coupler is designed, which is less sensitive to variations and still reaches a coupling efficiency of around 98%. Nevertheless, it has a relatively large footprint and exhibits some undesired residual coupling to TM-like modes. To improve this, a more advanced adiabatic coupler whose geometry is varied along its propagation length is designed and manages to reach ∼100% coupling and decoupling within a length of 200 μm. The proposed couplers are designed for the particular case of III-V nano-ridge lasers monolithically grown using aspect-ratio-trapping (ART) together with nano-ridge engineering (NRE) but are believed to be compatible with other epitaxial III-V/Si integration platforms recently proposed. In this way, the presented coupler is expected to pave the way to integrating III-V lasers monolithically grown on SOI wafers with other photonics components, one step closer towards a fully functional silicon photonics platform.

Hierarchical Design and Optimization of Silicon Photonics

Silicon photonics is a rapidly maturing platform for optical communication and sensing. As systems leveraging silicon photonics have grown in size and complexity, so too has the demand for high performance silicon photonics components. In order to meet these demands, we propose a hierarchical approach to design and optimization of silicon photonics components. Our approach applies simple physical analysis to choose an effective starting geometry for a two-step gradient-based shape optimization. This optimization employs carefully chosen geometrical constraints in order to consistently produce robust, high performance devices which satisfy practical fabrication constraints of deep UV lithography. In order to demonstrate the versatility of method, we optimize a 3 dB coupler which achieves better than 0.04 dB excess loss over the O-band, a four port 3-dB coupler which achieves better than 0.41 dB excess loss and near 50:50 splitting over the O-band, and a fabrication-tolerant waveguide crossing which achieves better than 0.075 dB insertion loss over the O-band even when subject to $\pm \text{10}$ % silicon thickness variations. These results pave the way for high efficiency silicon photonic component libraries.

Novel Measures for Thermal Management of Silicon Photonic Optical Phased Arrays

Silicon photonics has become a promising technology for fabrication of compact, highly integrated systems. When dealing with phase sensitive silicon photonic components, however, temperature control and in particular, reduction of temperature gradients across phase sensitive components becomes crucial. Solid-state optical beam steering using optical phased arrays (OPAs), is an exemplary system, which could be miniaturized by using silicon photonics and where temperature control is of utmost importance. In order to reduce the system cost and size as well as to increase the steering velocity, it is desired to replace mechanical beam steering systems by solid-state steering systems. Due to their compatibility with CMOS fabrication processes, silicon photonics based OPAs are promising candidates to become the next generation beam steering solution. However, due to the large thermo-optic coefficient of silicon, these phased arrays are extremely sensitive to temperature. Thermal management of silicon-photonic based OPAs requires not only heat dissipation of integrated active components, but also reduction of temperature gradients across the phased array. Here a novel concept for thermal management of silicon photonic OPAs is proposed and analyzed using finite-element simulations. The proposed concepts show a formidable efficiency in improving the heat dissipation as well as in reducing temperature gradients across the array.

Progress of radiation effects of silicon photonics devices

Silicon photonics is a fundamental technology, which has great potential applications in optical interconnection for telecom, datacom, and high performance computers, as well as in bio-photonics. Currently considered are the photonics integrated circuits that are able to work in harsh environments such as high energy equipment and future space systems including satellites, space stations and spacecraft. The understanding of the radiation effects of the photonics devices is critical for fabricating radiation hardened photonic integrate chips and maintaining the performance of the devices and the systems. In this paper, the recent progress of the radiation effects of silicon photonic components is summarized. The effects of the high energy particles that possibly degrade the performance of the device are explained, and the response of the passive and active device under radiation are reviewed comprehensively, including waveguides, ring resonators, modulators, detectors, lasers and optical fibers and so on. For passive devices, radiation-induced effects include accelerated-oxidation of the structures, radiation-generated lattice defects, and amorphous densification or compaction in the optical materials. The effective refractive index of the passive device may change consequently, leading the working frequency to shift, the optical confinement to decrease, and the optical power to leak, which accounts for the extra loss or other performance degradation behaviors. For photodetectors and lasers, radiation-induced displacement damage will be dominant. The induced point defects localized in the silicon layer bring about deep level in the forbidden band, acting as generation-recombination centers or trap centers of tunneling effect, which will compensate for either donor or acceptor levels, degrading the response of these optoelectronic device significantly. The plasma dispersion effect is the mainstream approach to building the silicon electro-optic modulators, which will suffer ionization damage in the high energy particle environment, because the interface-trapped hole caused by ionizing radiation reduces the carrier concentration in the depletion region and even induces the pinch-off of the p-doped side of the modulator, which may result in device failure. To improve the radiation hardness of the silicon photonic device, the passivation of the surface, optimization of the waveguide shape, and the choice of appropriate thickness of the buried oxide layer are possible solutions, and more effective approaches are still to be developed.

Silicon Photonics in Optical Coherent Systems

Optical coherent systems, which employ the interference of a received optical signal with a local laser to achieve high signal-to-electrical-noise ratio and capture the magnitude, phase, and polarization of the signal, have grown dramatically in the last decade due to improvements in digital signal processors. The optics of both the transmitter and receiver have grown in complexity, and its deployment has benefitted greatly from the integration offered by silicon photonics. Today’s coherent systems range from 1.2-Tb/s fiber-optic communication transceivers to optical coherence tomography medical instruments. This paper will give an overview of optical coherent systems, present the state-of-the-art in silicon photonic components shipping today, and discuss where silicon photonics is going in this field.