Self-emergence of robust solitons in a microcavity.

In many disciplines, states that emerge in open systems far from equilibrium are determined by a few global parameters1,2. These states can often mimic thermodynamic equilibrium, a classic example being the oscillation threshold of a laser3 that resembles a phase transition in condensed matter. However, many classes of states cannot form spontaneously in dissipative systems, and this is the case for cavity solitons2 that generally need to be induced by external perturbations, as in the case of optical memories4,5. In the past decade, these highly localized states have enabled important advancements in microresonator-based optical frequency combs6,7. However, the very advantages that make cavity solitons attractive for memories-their inability to form spontaneously from noise-have created fundamental challenges. As sources, microcombs require spontaneous and reliable initiation into a desired state that is intrinsically robust8-20. Here we show that the slow non-linearities of a free-running microresonator-filtered fibre laser21 can transform temporal cavity solitons into the system's dominant attractor. This phenomenon leads to reliable self-starting oscillation of microcavity solitons that are naturally robust to perturbations, recovering spontaneously even after complete disruption. These emerge repeatably and controllably into a large region of the global system parameter space in which specific states, highly stable over long timeframes, can be achieved.

11 TOPS photonic convolutional accelerator for optical neural networks.

Convolutional neural networks, inspired by biological visual cortex systems, are a powerful category of artificial neural networks that can extract the hierarchical features of raw data to provide greatly reduced parametric complexity and to enhance the accuracy of prediction. They are of great interest for machine learning tasks such as computer vision, speech recognition, playing board games and medical diagnosis1-7. Optical neural networks offer the promise of dramatically accelerating computing speed using the broad optical bandwidths available. Here we demonstrate a universal optical vector convolutional accelerator operating at more than ten TOPS (trillions (1012) of operations per second, or tera-ops per second), generating convolutions of images with 250,000 pixels-sufficiently large for facial image recognition. We use the same hardware to sequentially form an optical convolutional neural network with ten output neurons, achieving successful recognition of handwritten digit images at 88 per cent accuracy. Our results are based on simultaneously interleaving temporal, wavelength and spatial dimensions enabled by an integrated microcomb source. This approach is scalable and trainable to much more complex networks for demanding applications such as autonomous vehicles and real-time video recognition.

Dual-polarization RF channelizer based on microcombs.

We report a dual-polarization radio frequency (RF) channelizer based on microcombs. Two high-Q micro-ring resonators (MRRs) with slightly different free spectral ranges (FSRs) are used: one MRR is pumped to yield soliton crystal microcombs ("active"), and the other MRR is used as a "passive" periodic optical filter supporting dual-polarization operation to slice the RF spectrum. With the tailored mismatch between the FSRs of the active and passive MRRs, wideband RF spectra can be channelized into multiple segments featuring digital-compatible bandwidths via the Vernier effect. Due to the use of dual-polarization states, the number of channelized spectral segments, and thus the RF instantaneous bandwidth (with a certain spectral resolution), can be doubled. In our experiments, we used 20 microcomb lines with ∼ 49 GHz FSR to achieve 20 channels for each polarization, with high RF spectra slicing resolutions at 144 MHz (TE) and 163 MHz (TM), respectively; achieving an instantaneous RF operation bandwidth of 3.1 GHz (TE) and 2.2 GHz (TM). Our approach paves the path towards monolithically integrated photonic RF receivers (the key components - active and passive MRRs are all fabricated on the same platform) with reduced complexity, size, and unprecedented performance, which is important for wide RF applications with digital-compatible signal detection.

Low-loss fiber-to-chip edge coupler for silicon nitride integrated circuits.

Silicon nitride (SiN) integrated optical waveguides have found a wide range of applications due to their low loss, broad wavelength transmission band and high nonlinearity. However, the large mode mismatch between the single-mode fiber and the SiN waveguide creates a challenge of fiber coupling to these waveguides. Here, we propose a coupling approach between fiber and SiN waveguides by utilizing the high-index doped silica glass (HDSG) waveguide as the intermediary to smooth out the mode transition. We achieved fiber-to-SiN waveguide coupling efficiency of lower than 0.8 dB/facet across the full C and L bands with high fabrication and alignment tolerances.

Investigating the thermal robustness of soliton crystal microcombs.

Soliton crystals are a novel form of microcomb, with relatively high conversion efficiency, good thermal robustness, and simple initiation among the methods to generate them. Soliton crystals can be easily generated in microring resonators with an appropriate mode-crossing. However, fabrication defects can significantly affect the mode-crossing placement and strength in devices. To enable soliton crystal states to be harnessed for a broader range of microcomb applications, we need a better understanding of the link between mode-crossing properties and the desired soliton crystal properties. Here, we investigate how to generate the same soliton crystal state in two different microrings, how changes in microring temperature change the mode-crossing properties, and how mode-crossing properties affect the generation of our desired soliton crystal state. We find that temperature affects the mode-crossing position in these rings but without major changes in the mode-crossing strength. We find that our wanted state can be generated over a device temperature range of 25 ∘C, with different mode-crossing properties, and is insensitive to the precise mode-crossing position between resonances.

On-chip generation of high-dimensional entangled quantum states and their coherent control.

Optical quantum states based on entangled photons are essential for solving questions in fundamental physics and are at the heart of quantum information science. Specifically, the realization of high-dimensional states (D-level quantum systems, that is, qudits, with D > 2) and their control are necessary for fundamental investigations of quantum mechanics, for increasing the sensitivity of quantum imaging schemes, for improving the robustness and key rate of quantum communication protocols, for enabling a richer variety of quantum simulations, and for achieving more efficient and error-tolerant quantum computation. Integrated photonics has recently become a leading platform for the compact, cost-efficient, and stable generation and processing of non-classical optical states. However, so far, integrated entangled quantum sources have been limited to qubits (D = 2). Here we demonstrate on-chip generation of entangled qudit states, where the photons are created in a coherent superposition of multiple high-purity frequency modes. In particular, we confirm the realization of a quantum system with at least one hundred dimensions, formed by two entangled qudits with D = 10. Furthermore, using state-of-the-art, yet off-the-shelf telecommunications components, we introduce a coherent manipulation platform with which to control frequency-entangled states, capable of performing deterministic high-dimensional gate operations. We validate this platform by measuring Bell inequality violations and performing quantum state tomography. Our work enables the generation and processing of high-dimensional quantum states in a single spatial mode.

Thermo-Optic Response and Optical Bistablility of Integrated High-Index Doped Silica Ring Resonators.

The engineering of thermo-optic effects has found broad applications in integrated photonic devices, facilitating efficient light manipulation to achieve various functionalities. Here, we perform both an experimental characterization and a theoretical analysis of these effects in integrated microring resonators made from high-index doped silica, which have had many applications in integrated photonics and nonlinear optics. By fitting the experimental results with theory, we obtain fundamental parameters that characterize their thermo-optic performance, including the thermo-optic coefficient, the efficiency of the optically induced thermo-optic process, and the thermal conductivity. The characteristics of these parameters are compared to those of other materials commonly used for integrated photonic platforms, such as silicon, silicon nitride, and silica. These results offer a comprehensive insight into the thermo-optic properties of doped silica-based devices. Understanding these properties is essential for efficiently controlling and engineering them in many practical applications.

High parametric efficiency in laser cavity-soliton microcombs.

Laser cavity-soliton microcombs are robust optical pulsed sources, usually implemented with a microresonator-filtered fibre laser. In such a configuration, a nonlinear microcavity converts the narrowband pulse resulting from bandwidth-limited amplification to a background-free broadband microcomb. Here, we theoretically and experimentally study the soliton conversion efficiency between the narrowband input pulse and the two outputs of a four-port integrated microcavity, namely the 'Drop' and 'Through' ports. We simultaneously measure on-chip, single-soliton conversion efficiencies of 45% and 25% for the two broadband comb outputs at the 'Drop' and 'Through' ports of a 48.9 GHz free-spectral range micro-ring resonator, obtaining a total conversion efficiency of 72%.

Second-harmonic generation in a high-index doped silica micro-ring resonator.

We report the first, to the best of our knowledge, observation of second-harmonic generation (SHG) in a high-index doped silica micro-ring resonator, due to the symmetry-breaking-induced χ(2) at the core and cladding interface of the waveguide. The generated SH power is shown to have quadratic dependence on the in-cavity power of the fundamental pump at around 1550 nm. The pumping wavelength sweep method is adopted to fulfill the phase-matching condition for maximum conversion efficiency of SHG. This work offers a new approach to generate a visible source for the visible-light integrated optical platform from infrared-visible light conversion.

All-optical RF spectrum analyzer with a 5 THz bandwidth based on CMOS-compatible high-index doped silica waveguides.

We report an all-optical radio-frequency (RF) spectrum analyzer with a bandwidth greater than 5 THz, based on a 50 cm long spiral waveguide in a CMOS-compatible high-index doped silica platform. By carefully mapping out the dispersion profile of the waveguides for different thicknesses, we identify the optimal design to achieve near-zero dispersion in the C-band. To demonstrate the capability of the RF spectrum analyzer, we measure the optical output of a femtosecond fiber laser with an ultrafast optical RF spectrum in the terahertz regime.

2D Layered Graphene Oxide Films Integrated with Micro-Ring Resonators for Enhanced Nonlinear Optics.

Layered 2D graphene oxide (GO) films are integrated with micro-ring resonators (MRRs) to experimentally demonstrate enhanced nonlinear optics. Both uniformly coated (1-5 layers) and patterned (10-50 layers) GO films are integrated on complementary-metal-oxide-semiconductor (CMOS)-compatible doped silica MRRs using a large-area, transfer-free, layer-by-layer GO coating method with precise control of the film thickness. The patterned devices further employ photolithography and lift-off processes to enable precise control of the film placement and coating length. Four-wave-mixing (FWM) measurements for different pump powers and resonant wavelengths show a significant improvement in efficiency of ≈7.6 dB for a uniformly coated device with 1 GO layer and ≈10.3 dB for a patterned device with 50 GO layers. The measurements agree well with theory, with the enhancement in FWM efficiency resulting from the high Kerr nonlinearity and low loss of the GO films combined with the strong light-matter interaction within the MRRs. The dependence of GO's third-order nonlinearity on layer number and pump power is also extracted from the FWM measurements, revealing interesting physical insights about the evolution of the GO films from 2D monolayers to quasi bulk-like behavior. These results confirm the high nonlinear optical performance of integrated photonic resonators incorporated with 2D layered GO films.

Third-harmonic generation in CMOS-compatible highly doped silica micro-ring resonator.

We present the first demonstration of visible emission from highly doped silica glass micro-ring resonators (MRRs) through a third-harmonic generation (THG) nonlinear process. We obtain green light conversion efficiency of 2.7×10-5 W-2 in a MRR with loaded Q-factor of 1.4×106 pumped in the telecom band. A thermal nonlinear model is developed to account for the in-cavity power dependence of the resonance detuning. Using the extracted thermal nonlinear coefficients, the measured TH resonance shift is calibrated by subtracting the thermal nonlinear-induced phase mismatch to obtain the theoretical threefold wavelength relationship along with the measured cubic power relationship.

Highly reconfigurable hybrid laser based on an integrated nonlinear waveguide.

The ability of laser systems to emit different adjustable temporal pulse profiles and patterns is desirable for a broad range of applications. While passive mode-locking techniques have been widely employed for the realization of ultrafast laser pulses with mainly Gaussian or hyperbolic secant temporal profiles, the generation of versatile pulse shapes in a controllable way and from a single laser system remains a challenge. Here we show that a nonlinear amplifying loop mirror (NALM) laser with a bandwidth-limiting filter (in a nearly dispersion-free arrangement) and a short integrated nonlinear waveguide enables the realization and distinct control of multiple mode-locked pulsing regimes (e.g., Gaussian pulses, square waves, fast sinusoidal-like oscillations) with repetition rates that are variable from the fundamental (7.63 MHz) through its 205th harmonic (1.56 GHz). These dynamics are described by a newly developed and compact theoretical model, which well agrees with our experimental results. It attributes the control of emission regimes to the change of the NALM response function that is achieved by the adjustable interplay between the NALM amplification and the nonlinearity. In contrast to previous square wave emissions, we experimentally observed that an Ikeda instability was responsible for square wave generation. The presented approach enables laser systems that can be universally applied to various applications, e.g., spectroscopy, ultrafast signal processing and generation of non-classical light states.

Advanced RF and microwave functions based on an integrated optical frequency comb source.

We demonstrate advanced transversal radio frequency (RF) and microwave functions based on a Kerr optical comb source generated by an integrated micro-ring resonator. We achieve extremely high performance for an optical true time delay aimed at tunable phased array antenna applications, as well as reconfigurable microwave photonic filters. Our results agree well with theory. We show that our true time delay would yield a phased array antenna with features that include high angular resolution and a wide range of beam steering angles, while the microwave photonic filters feature high Q factors, wideband tunability, and highly reconfigurable filtering shapes. These results show that our approach is a competitive solution to implementing reconfigurable, high performance and potentially low cost RF and microwave signal processing functions for applications including radar and communication systems.

Robust soliton crystals in a thermally controlled microresonator.

We demonstrate robust soliton crystals generation with a fixed frequency pump laser through a thermoelectric-cooler-based thermal-tuning approach in a butterfly-packaged complementary-metal-oxide-semiconductor-compatible microresonator. Varieties of soliton crystal states, exhibiting "palm-like" optical spectra that result from the strong interactions between the dense soliton ensembles and reflect their temporal distribution directly, are experimentally observed by sweeping one cavity resonance across the pump frequency from the blue-detuned side by reducing the operating temperature of the resonator. Benefitting from the tiny intra-cavity energy change, repeatable interconversion between the chaotic modulation instability and stable soliton crystal states can be successfully achieved via simple tuning of the temperature or pump power, showing the easy accessibility and excellent stability of such soliton crystals. This work could facilitate microresonator-based optical frequency combs towards a portable, adjustable, and low-cost system while avoiding the requirements of delicate frequency-sweeping pump techniques.

Practical system for the generation of pulsed quantum frequency combs.

The on-chip generation of large and complex optical quantum states will enable low-cost and accessible advances for quantum technologies, such as secure communications and quantum computation. Integrated frequency combs are on-chip light sources with a broad spectrum of evenly-spaced frequency modes, commonly generated by four-wave mixing in optically-excited nonlinear micro-cavities, whose recent use for quantum state generation has provided a solution for scalable and multi-mode quantum light sources. Pulsed quantum frequency combs are of particular interest, since they allow the generation of single-frequency-mode photons, required for scaling state complexity towards, e.g., multi-photon states, and for quantum information applications. However, generation schemes for such pulsed combs have, to date, relied on micro-cavity excitation via lasers external to the sources, being neither versatile nor power-efficient, and impractical for scalable realizations of quantum technologies. Here, we introduce an actively-modulated, nested-cavity configuration that exploits the resonance pass-band characteristic of the micro-cavity to enable a mode-locked and energy-efficient excitation. We demonstrate that the scheme allows the generation of high-purity photons at large coincidence-to-accidental ratios (CAR). Furthermore, by increasing the repetition rate of the excitation field via harmonic mode-locking (i.e. driving the cavity modulation at harmonics of the fundamental repetition rate), we managed to increase the pair production rates (i.e. source efficiency), while maintaining a high CAR and photon purity. Our approach represents a significant step towards the realization of fully on-chip, stable, and versatile sources of pulsed quantum frequency combs, crucial for the development of accessible quantum technologies.

Multichannel phase-sensitive amplification in a low-loss CMOS-compatible spiral waveguide.

We investigate single-channel and multichannel phase-sensitive amplification (PSA) in a highly nonlinear, CMOS-compatible spiral waveguide with ultralow linear and negligible nonlinear losses. We achieve a net gain of 10.4 dB and an extinction ratio of 24.6 dB for single-channel operation, as well as a 5 dB gain and a 15 dB extinction ratio spanning over a bandwidth of 24 nm for multiple-channel operation. In addition, we derive a simple analytic solution that enables calculating the maximum phase-sensitive gain in any Kerr medium featuring linear and nonlinear losses. These results not only give a clear guideline for designing PSA-based amplifiers but also show that it is possible to implement both optical regeneration and amplification in a single on-chip device.

Integrated frequency comb source based Hilbert transformer for wideband microwave photonic phase analysis.

We demonstrate a photonic RF Hilbert transformer for broadband microwave in-phase and quadrature-phase generation based on an integrated frequency optical comb, generated using a nonlinear microring resonator based on a CMOS compatible, high-index contrast, doped-silica glass platform. The high quality and large frequency spacing of the comb enables filters with up to 20 taps, allowing us to demonstrate a quadrature filter with more than a 5-octave (3 dB) bandwidth and an almost uniform phase response.

CMOS compatible integrated all-optical radio frequency spectrum analyzer.

We report an integrated all-optical radio frequency spectrum analyzer based on a ~4 cm long doped silica glass waveguide, with a bandwidth greater than 2.5 THz. We use this device to characterize the intensity power spectrum of ultrahigh repetition rate mode-locked lasers at repetition rates up to 400 GHz, and observe dynamic noise related behavior not observable with other techniques.

Integrated frequency comb source of heralded single photons.

We report an integrated photon pair source based on a CMOS-compatible microring resonator that generates multiple, simultaneous, and independent photon pairs at different wavelengths in a frequency comb compatible with fiber communication wavelength division multiplexing channels (200 GHz channel separation) and with a linewidth that is compatible with quantum memories (110 MHz). It operates in a self-locked pump configuration, avoiding the need for active stabilization, making it extremely robust even at very low power levels.