Silicon Photonics Platform Research Articles

Compact Si-SiN photonic fiber optic gyroscope transceiver for large volume manufacturing

Miniaturized interferometric fiber optic gyroscopes (IFOGs) providing high-precision angular measurement are highly desired in various smart applications. In this work, we present a high-performance Si-SiN photonic FOG transceiver composed of an optical source, polarizer, splitter, and on-chip germanium (Ge) photodetector (PD). The transceiver is assembled in a standard butterfly package with a thermo-electric cooler (TEC). The optical loss (including two edge couplers, as well as one 3 dB splitter) and polarization extinction ratio (PER) are less than 7 dB and greater than 20 dB at room temperature, respectively. Built with the polarization maintaining (PM) fiber coil with 70 mm average diameter and 580 m length, the transceiver-based IFOG exhibits record-low bias stability of 0.022 deg/h at an integration time of 10 s, the angular random walk (ARW) of 0.0012 deg/h, and the bias instability of 0.003 deg/h, to the best of our knowledge. The preliminary reliability test agrees well with the practical requirements. Our work verifies that the on-chip Ge PD is eligible for high-performance FOG applications. Leveraged with the typical CMOS compatible 8-inch (200 mm diameter wafers) silicon photonics platform and decreased fiber splicing points, the presented transceiver provides a promising solution toward a low-loss and miniaturized FOG system with large volume manufacturing capability.

Low dark current and low voltage germanium avalanche photodetector for a silicon photonic link

Germanium (Ge)-silicon (Si)-based avalanche photodetectors (APDs) featured by a high absorption coefficient in the near-infrared band have gained wide applications in laser ranging, free space communication, quantum communication, and so on. However, the Ge APDs fabricated by the complementary metal oxide semiconductor (CMOS) process suffer from a large dark current and limited responsivity, imposing a critical challenge on integrated silicon photonic links. In this work, we propose a p-i-n-i-n type Ge APD consisting of an intrinsic germanium layer functioning as both avalanche and absorption regions and an intrinsic silicon layer for dark current reduction. Consequently, a Ge APD with a low dark current, low bias voltage, and high responsivity can be obtained via a standard silicon photonics platform. In the experimental measurement, the Ge APD is characterized by a high primary responsivity of 1.1 A/W with a low dark current as low as 7.42 nA and a dark current density of 6.1×10−11 A/μm2 at a bias voltage of −2 V. In addition, the avalanche voltage of the Ge APD is −8.4 V and the measured 3 dB bandwidth of the Ge APD can reach 25 GHz. We have also demonstrated the capability of data reception on 32 Gbps non-return-to-zero (NRZ) optical signal, which has potential application for silicon photonic data links.

Sub-volt forward-biased silicon microring modulator at 210 Gb/s.

Low-voltage and efficient optical modulators in the silicon photonic (SiPh) platform are highly desired for realizing high-speed connectivity in chip level interconnects, data center interconnects, and high-performance computing (HPC). With the modulator operating at CMOS compatible voltages, high-voltage modulator drivers are no longer needed, thus reducing driver design complexity and power consumption. We demonstrate a silicon microring modulator (MRM) operating at a driving voltage of 0.8 Vpp. We achieve high modulation efficiency by using a small forward bias of 0.2 V: the forward bias voltage allows the modulator to have an enhanced optical modulation amplitude (OMA) by operating near injection mode, modulating 180 Gb/s (180 Gbaud) non-return-to-zero (NRZ) and 210 Gb/s (105 Gbaud) 4-level pulse amplitude modulation (PAM-4) free of electrical or optical amplification. We also demonstrate the operation at zero bias and achieve up to 200 Gb/s. Error-free operation was observed at 130 Gb/s (NRZ). The absence of an external biasing voltage can further improve energy efficiency and simplifies device integration.

Hybrid III-V/Silicon Quantum Photonic Device Generating Broadband Entangled Photon Pairs

The demand for integrated photonic chips combining the generation and manipulation of quantum states of light is steadily increasing, driven by the need for compact and scalable platforms for quantum information technologies. While photonic circuits with diverse functionalities are being developed in different single material platforms, it has become crucial to realize hybrid photonic circuits that harness the advantages of multiple materials while mitigating their respective weaknesses, resulting in enhanced capabilities. Here, we demonstrate a hybrid III-V/silicon quantum photonic device combining the strong second-order nonlinearity and direct band gap of the III-V semiconductor platform with the high maturity and CMOS compatibility of the silicon photonic platform. Our device embeds the spontaneous parametric down-conversion (SPDC) of photon pairs into an AlGaAs source and their vertical routing to an adhesively bonded silicon-on-insulator circuitry, within an evanescent coupling scheme managing both polarization states. This enables the on-chip generation of broadband (>40 nm) telecom photons by type-0 and type-2 SPDC from the hybrid device, at room temperature and with internal pair generation rates exceeding 105s−1 for both types, while the pump beam is strongly rejected. Two-photon interference with 92% visibility (and up to 99% upon 5-nm spectral filtering) proves the high energy-time entanglement quality of the produced quantum state, thereby enabling a wide range of quantum information applications on chip, within a hybrid architecture compliant with electrical pumping and merging the assets of two mature and highly complementary platforms in view of out-of-the-lab deployment of quantum technologies. Published by the American Physical Society 2024

Integrated Pockels Modulators on Silicon Photonics Platform

AbstractElectro‐optic (EO) modulators are essential components in various fields, including optical communication, free‐space communication, microwave photonics, sensing, and light detection and ranging. The EO modulation enables the fast conversion of electric signals into optical signals, facilitating the precise manipulation of light. With advancements in fabrication processing techniques, next‐generation integrated EO modulators have demonstrated substantial improvements in modulation efficiency, bandwidth, and footprint. Here, the latest research progress in integrated EO modulation, focusing on the principle of the Pockels effect, key modulation metrics, novel EO thin‐film material platforms, and innovative device architectures is overviewed. Finally, it is evaluated different schemes and provide perspectives on future trends in developing integrated EO modulators, highlighting both the advantages and challenges of integrated EO modulation, including waveguide and electrode engineering, integrated methods, and other applications for large‐scale photonic integrated circuits.

A novel bistable optical phase shifter using non-volatile MEMS actuator for application in photonic integrated circuits

Recently, micro-electromechanical systems (MEMS) actuators have been employed to perform the tuning mechanisms in photonic integrated circuits (PICs). Photonic components with MEMS-based tuning mechanisms offer several advantages, including small dimensions, low power consumption, fast tuning time, and high compatibility with silicon photonic platforms. These features enable the integration of photonic components on a single chip, which is demanded for complex applications such as artificial intelligence applications including machine learning, neural networks, and neuromorphic computing. Phase shifters are one of the most important components in PICs. This paper introduces a novel bistable optical phase shifter that utilizes a MEMS-based tuning mechanism to maintain its last state without consuming electrical power. The proposed device boasts impressive functional characteristics including a phase shift of 2.32π, a tuning time of 1.3μs, an optical loss of 0.03 dB, and compact dimensions of 35 μm by 35 μm while operating at a voltage of 18V. Given these remarkable features, the proposed device is an appealing candidate for integration in complex PICs such as photonics-based AI systems.

Unidirectional and highly dispersive grating emitters for silicon photonic beam steerers.

Grating emitters are the key component used in silicon photonic beam steerers to create light emission and allow dispersion-based steering. Conventional grating emitters are inefficient due to bidirectional emission, and their angular dispersion is only on the order of 0.1°/nm. In this paper, we develop unidirectional grating emitters by choosing a top cladding material with a refraction index higher than that of the bottom cladding. The refractive index difference allows the phase-matching condition to be satisfied only for the upward diffraction. We demonstrate experimentally grating emitters with a directionality of 83% and an angular dispersion of 0.79°/nm. These grating emitters have a simple device structure compatible with the standard silicon photonic platform.

Multilayer Reconfigurable 3D Photonics Integrated Circuits Based on Deposition Method

AbstractPhotonics integrated circuits (PICs) can overcome the bottlenecks in communication capacity. 3D PIC technology is an attractive method for increasing density effectively and combining different materials on one chip for functional integration. In this study, a low‐cost and reconfigurable 3D integrated silicon photonics (SiPhs) platform is proposed fabricated by depositing polycrystalline silicon (poly‐Si) on crystalline silicon (c‐Si), which is patterned through deep ultraviolet (DUV) stepper‐based manufacturing process. The high mobility of poly‐Si ensures high‐speed and power‐efficient modulation. Based on the proposed 3D SiPhs platform, an 8 × 8 optical switch whose units are separated in different layers is fabricated and packaged. The switch shows average insertion losses of −13.98 and −15.86 dB, while working in “all‐cross” and “all‐bar” states, respectively. The crosstalk is lower than −19 and −11 dB for “all‐cross” and “all‐bar” states. Additionally, an overlayer switch located on the 1st and 3rd layers is proposed and experimentally demonstrated, which offers overlapping capabilities that are more compact compared with switches in a single‐layer platform. This validates the possibility of the large‐density integration scalability of the platform.

All-optical organic photochemical integrated nanophotonic memory: low-loss, continuously tunable, non-volatile

Controlling changes in the optical properties of photonic devices allows photonic integrated circuits (PICs) to perform useful functions, leading to a large breadth of applications in communications, computing, and sensing. Many mechanisms to change optical properties exist, but few allow doing so in a reversible, non-volatile manner. Without such mechanisms, power inefficiencies and use of external memory are inevitable. In this work, we propose and experimentally demonstrate reversible, non-volatile phase actuation of a silicon nitride PIC with thermally stable photochromic organic molecules vapor-deposited within a slot waveguide structure. The use of a high-core-index platform allows the photochemical phase actuation of a planar-resonator-based photonic memory unit, which enables positive and negative signal weighting and permits integrated spectroscopic analysis. We show properties of this all-optical memory for a silicon photonics platform, including low loss in the optical C-band, first-order photokinetics of the photoconversion, bidirectional scalable switching, and continuous tuning. Such features are critical for memories in analog applications such as quantum, microwave, and neuromorphic photonics, where bipolar weights, low loss, and precision are paramount. More generally, this work suggests that back-end-of-line-compatible vapor deposition of organic molecules into silicon photonic circuits is promising to introduce non-silicon-native functionality.

Plasmonic Radiation from Spin-Momentum Locking.

Chiral light emission plays a key role in sensing, tomography, quantum communication, among others. Whereas, achieving highly pure, tunable chirality emission across a broad spectrum currently presents significant challenges. Free-electron radiation emerges as a promising solution to surpass these barriers, especially in hard-to-reach regimes. Here, chiral free-electron radiation is presented by exploiting the spin-momentum locking (SML) property of spoof surface plasmons (SSPs). When the phase velocity of free electrons matches that of the SSPs, the SSPs can be excited. By implementing wavenumber compensation through perturbations, the confined SSPs are transformed into free-space free-electron radiation. Owing to the law of angular momentum conservation, this process converts the transverse spin angular momentum of SSPs into the longitudinal spin angular momentum of free-electron radiation during the process, producing pure, tunable, and chiral free-electron radiation across a broad spectrum. This method achieves an optimal degree of circular polarization approaching -1. The innovative methodology can be adapted to SML-enabled guided states or silicon photonics platforms, offering new avenues for achieving chiral emission.

What can be integrated on the silicon photonics platform and how?

We review the integration techniques for incorporating various materials into silicon-based devices. We discuss on-chip light sources with gain materials, linear electro-optic modulators using electro-optic materials, low-power piezoelectric tuning devices with piezoelectric materials, highly absorbing materials for on-chip photodetectors, and ultra-low-loss optical waveguides. Methodologies for integrating these materials with silicon are reviewed, alongside the technical challenges and evolving trends in silicon hybrid and heterogeneously integrated devices. In addition, potential research directions are proposed. With the advancement of integration processes for thin-film materials, significant breakthroughs are anticipated, leading to the realization of optoelectronic monolithic integration featuring on-chip lasers.

Scalable free-space photonic antennas in foundry SOI silicon photonic platforms.

We present a flexible, scalable, and low-noise design scheme for coupling free-space light into a silicon-on-insulator (SOI) electronic-photonic integrated circuit. The proposed scheme utilizes arrays of grating couplers with compact, inverse-designed power combining networks to couple a distributed optical collection area to a single output waveguide, forming a photonic antenna. Fabrication density compliance is maintained regardless of the antenna size, and the collection area can be scaled while maintaining a fixed noise floor. Using experimental grating array antennas fabricated in the GF45CLO platform, we demonstrate up to a 6.7× increase in the signal-to-noise ratio (SNR) of a lens-less monolithic free-space photonic receiver using a 4×4 grating array.

Innovative Integration of Dual Quantum Cascade Lasers on Silicon Photonics Platform.

For the first time, we demonstrate the hybrid integration of dual distributed feedback (DFB) quantum cascade lasers (QCLs) on a silicon photonics platform using an innovative 3D self-aligned flip-chip assembly process. The QCL waveguide geometry was predesigned with alignment fiducials, enabling a sub-micron accuracy during assembly. Laser oscillation was observed at the designed wavelength of 7.2 μm, with a threshold current of 170 mA at room temperature under pulsed mode operation. The optical output power after an on-chip beam combiner reached sub-milliwatt levels under stable continuous wave operation at 15 °C. The specific packaging design miniaturized the entire light source by a factor of 100 compared with traditional free-space dual lasers module. Divergence values of 2.88 mrad along the horizontal axis and 1.84 mrad along the vertical axis were measured after packaging. Promisingly, adhering to i-line lithography and reducing the reliance on high-end flip-chip tools significantly lowers the cost per chip. This approach opens new avenues for QCL integration on silicon photonic chips, with significant implications for portable mid-infrared spectroscopy devices.

Observation of Topological Transition in Floquet Non-Hermitian Skin Effects in Silicon Photonics.

Non-Hermitian physics has greatly enriched our understanding of nonequilibrium phenomena and uncovered novel effects such as the non-Hermitian skin effect (NHSE) that has profoundly revolutionized the field. NHSE has been predicted in systems with nonreciprocal couplings which, however, are challenging to realize in experiments. Without nonreciprocal couplings, the NHSE can also emerge in systems with coexisting gauge fields and loss or gain (e.g., in Floquet non-Hermitian systems). However, such Floquet NHSE remains largely unexplored in experiments. Here, we realize the Floquet NHSEs in periodically modulated optical waveguides integrated on a silicon photonic platform. By engineering the artificial gauge fields induced by the periodical modulation, we observe various Floquet NHSE phases and unveil their rich topological transitions. Remarkably, we discover the transitions between the unipolar NHSE phases and an unconventional bipolar NHSE phase, which is accompanied by the directional reversal of the NHSEs. The underlying physics is revealed by the band winding in complex quasienergy space which undergoes a topology change from isolated loops with the same winding to linked loops with opposite windings. Our work unfolds a new route toward Floquet NHSEs originating from the interplay between gauge fields and dissipation effects, and thus offers fundamentally new ways for steering light and other waves.

High-order Autler–Townes splitting in electrically tunable photonic molecules

Whispering gallery mode optical microresonators represent a promising avenue for realizing optical analogs of coherent light–atom interactions, circumventing experimental complexities. All-optical analogs of Autler–Townes splitting have been widely demonstrated, harnessing coupled optical microresonators, also known as photonic molecules, wherein the strong coupling between resonant fields enables energy level splitting. Here, we report the characterizations of Autler–Townes splitting in waveguide-coupled microring dimers featuring mismatched sizes. By exploiting backscattering-induced coupling via Rayleigh and Mie scatterers in individual rings, high-order Autler–Townes splitting has been realized, yielding supermode hybridization in a multi-level system. Upon resonance detuning using an integrated phase shifter, intra-cavity coupling-induced splitting becomes almost indistinguishable at the zero-detuning point where the strong inter-cavity coupling counteracts the imbalance of backscattering strengths in individual rings. Through demonstrations on the maturing silicon photonics platform, our findings establish a framework of electrically tunable photonic molecules for coupling-mediated Autler–Townes splitting, offering promising prospects for on-chip signal generation and processing across classical and quantum regimes.

Micro-transfer printed high-speed InP-based electro-absorption modulator on silicon-on-insulator

A high-speed InP-based electro-absorption modulator (EAM) on 220 nm silicon-on-insulator (SOI) is designed, fabricated, and measured. The III–V device is heterogeneously integrated to the SOI using transfer printing, with direct bonding. The printing accuracy of the device was within ±0.5 μm. This design evanescently couples light between the III–V waveguide and the SOI via a taper region in the InP ridge for high transmission. This method is a flexible and robust method of transferring an InP EAM to SOI, where multiple device variations have been transferred. At 1550 nm, the printed EAM has a measured electrical bandwidth of up to 40 GHz, an extinction ratio (ER) of 30 dB from 0 to −6 V, and an insertion loss of 6.5 dB, which reduces with longer wavelengths. An ER of 25 dB is obtained over a spectral bandwidth of 30 nm with biasing to −8 V. Open-eye diagrams were measured up to 50 Gbps in a back-to-back measurement. This device is suitable for applications in high-speed communications and sensing, leveraging the added advantage of III–V absorption modulation on a silicon photonics platform.

Leakage mechanisms of sub-pA InGaAs/GaAs nano-ridge waveguide photodetectors monolithically integrated on a 300-mm Si wafer

We report on a comprehensive temperature dependent dark current study of high-quality InGaAs/GaAs multi quantum well waveguide photodetectors monolithically integrated on silicon. They are integrated through metalorganic vapor-phase selective-area epitaxial growth in a 300 mm CMOS pilot line. Defects resulting from the metamorphic growth of III-V devices on Si make these devices susceptible to different leakage mechanisms at higher operating temperatures. For the high-temperature operation of complex photonics-electronics integrations, understanding the leakage mechanisms of the devices has critical significance. This will help to optimize designs promptly and ensure the reliability and longevity of such devices under extreme operating conditions. The photodetector devices exhibit dark currents below 1 pA, at room temperature and −1 V bias voltage, limited by the noise floor of the measurement setup. To resolve the different leakage mechanisms contributing to the dark current, the devices were measured at elevated temperatures and the results were cross-validated with device simulations. The devices exhibited very low dark currents, with a median below 0.1 nA at 195 °C, suggesting very high-quality material growth. Through device models, leakage mechanisms related to Shockley-Read-Hall (SRH) recombination at bulk volume defects are found to be the main factor contributing to the dark current. The surface SRH recombination is found to be limited, yet affecting the forward bias dark current due to the shortening of the diffusion paths of the majority carriers. Also, the device model shows that the actual dark currents at room temperature can be as low as 0.01 pA, more than 1-order lower than the measured levels. This study emphasizes the high quality of the III-V nano-ridge waveguide devices grown on Si, which can potentially expand the capabilities of silicon photonics platforms further.

Self-holding magneto-optical switch integrated on silicon photonic platforms.

We demonstrated what we believe to be a novel self-holding waveguide switch integrated on silicon photonic platforms utilizing the magneto-optical (MO) effect with electrical switching operation. In this study, we designed thin-film magnet arrays and electrodes positioned beside the waveguide. The switching states were changed by flipping the magnetization of thin-film magnets with an applied current of 700 mA, and the switching states were successfully maintained with zero current. The fabricated MO switch has an extinction ratio (ER) of 16.5 dB and an insertion loss (IL) of 7.3 dB at a wavelength of 1546.2 nm, with a phase shift of π/2. This study shows the potential of self-holding silicon photonic switches, enabling large-scale photonic integrated circuits with extremely low energy consumption.

Multilayer stacked crystalline silicon switch with nanosecond-order switching time.

To realize compact and denser photonic integrated circuits, three-dimensional integration has been widely accepted and researched. In this article, we demonstrate the operation of a 3D integrated silicon photonic platform fabricated through wafer bonding. Benefiting from the wafer bonding process, the material of all layers is c-Si, which ensures that the mobility is high enough to achieve a nanosecond response via the p-i-n diode shifter. Optical components, including multimode interferences (MMIs), waveguide crossing, and Mach-Zehnder interferometer (MZI)-based switch, are fabricated in different layers and exhibit great performance. The interlayer coupler and crossing achieve a 0.98 dB coupling loss and <-43.58 dB cross talk, while the crossing fabricated in the same layer shows <-36.00 dB cross talk. A nanosecond-order switch response is measured in different layers.

A peak enhancement of frequency response of waveguide integrated silicon-based germanium avalanche photodetector

Avalanche photodetectors (APDs) featuring an avalanche multiplication region are vital for reaching high sensitivity and responsivity in optical transceivers. Waveguide-coupled Ge-on-Si separate absorption, charge, and multiplication (SACM) APDs are popular due to their straightforward fabrication process, low optical propagation loss, and high detection sensitivity in optical communications. This paper introduces a lateral SACM Ge-on-Si APD on a silicon-on-insulator (SOI) wafer, featuring a 10 μm-long, 0.5 μm-wide Ge layer at 1310 nm on a standard 8-inch silicon photonics platform. The dark current measures approximately 38.6 μA at −21 V, indicating a breakdown voltage greater than −21 V for the device. The APDs exhibit a unit-gain responsivity of 0.5 A/W at −10 V. At −15 V, their responsivity reaches 2.98 and 2.91 A/W with input powers of −10 and −25 dBm, respectively. The device's 3-dB bandwidth is 15 GHz with an input power of −15 dBm and a gain is 11.68. Experimental results show a peak in impedance at high bias voltages, attributed to inductor and capacitor (LC) circuit resonance, enhancing frequency response. Furthermore, 20 Gbps eye diagrams at −21 V and −9 dBm input power reveal signal to noise ratio (SNRs) of 5.30. This lateral SACM APD, compatible with the stand complementary metal oxide semiconductor (CMOS) process, shows that utilizing the peaking effect at low optical power increases bandwidth.