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.

Ridge waveguide couplers with leaky mode resonator-like wavelength responses.

Integrated photonic resonators based on bound states in the continuum (BICs) on the silicon-on-insulator (SOI) platform have the potential for novel, mass-manufacturable resonant devices. While the nature of BIC-based ridge resonators requires the resonators to be extended in the (axial) propagation direction of the resonant mode, the requirement for excitation from the quasi-continuum extends the resonator structures also in the lateral dimensions, resulting in large device footprints. To overcome this footprint requirement, we investigate the translation of BIC-based ridge resonators into a guided mode system with finite lateral dimensions. We draw analogies between the resulting waveguide system and the BIC-based resonators and numerically demonstrate that, analog to the BIC-based resonators, such a waveguide system can exhibit spectrally narrow-band inversion of its transmissive behavior.

Ridge resonators with compact guided mode coupling.

Ridge resonators are a recently introduced integrated photonic circuit element based on bound states in the continuum (BICs) which can produce a single, sharp resonance over a broad wavelength range with high extinction ratio. However, to excite these resonators, a broad beam of laterally unbound slab mode is required, resulting in a large device footprint, which is not attractive for integrated photonic circuits. In this contribution, we propose and numerically validate a guided-mode waveguide structure that can be analogue to the BIC-based ridge resonators. Our simulations show that the proposed guided-mode waveguide structure can produce resonances with similar characteristics, yet with a significantly reduced footprint. Furthermore, we investigate the influence of the resonator's dimensions on the bandwidth of the resonance, demonstrating that resonances with Q-factors from low to very high (> 10000) are feasible. We believe that the reduced footprint and ability to design filters systematically make the guided-mode waveguide resonators an attractive photonic circuit component with particular value for foundry fabricated silicon photonic circuits.

Arbitrary access to optical carriers in silicon photonic mode/wavelength hybrid division multiplexing circuits.

The manipulation of optical modes directly in a multimode waveguide without affecting the transmission of undesired signal carriers is of significance to realize a flexible and simple structured optical network-on-chip. In this Letter, an arbitrary optical mode and wavelength carrier access scheme is proposed based on a series of multimode microring resonators and one multimode bus waveguide with constant width. As a proof-of-concept, a three-mode (de)multiplexing device is designed, fabricated, and experimentally demonstrated. A new, to the best of our knowledge, phase-matching idea is employed to keep the bus waveguide width constant. The mode coupling regions and transmission regions of the microring resonators are designed carefully to selectively couple and transmit different optical modes. The extinction ratio of the microring resonators is larger than 21.0 dB. The mode and wavelength cross-talk for directly (de)multiplexing are less than -12.8 dB and -19.0 dB, respectively. It would be a good candidate for future large-scale multidimensional optical networks.

Ridge resonators: impact of excitation beam and resonator losses.

Photonic resonators based on bound states in the continuum are attractive for sensing and telecommunication applications, as they have the potential to achieve ultra-high Q-factor resonators in a compact footprint. Recently, ridge resonators - leaky mode resonators based on a bound state in the continuum - have been demonstrated on a scalable photonic integrated circuit platform. However, high Q-factor ridge resonators have thus far not been achieved. In this contribution, we investigate the influence of excitation beam width and optical losses on the spectral response of ridge resonators. We show that for practical applications, the space required of the excitation beam is the limiting factor on the highest achievable Q-factor.

Integrated photonic high extinction short and long pass filters based on lateral leakage.

In this contribution we present a new approach to achieve high extinction short and long pass wavelength filters in the integrated photonic platform of lithium niobate on insulator. The filtering of unwanted wavelengths is achieved by employing lateral leakage and is related to the bound state in the continuum phenomenon. We show that it is possible to control the filter edge wavelength by adjusting the waveguide dimensions and that an extinction of hundreds of dB/cm is readily achievable. This enabled us to design a pump wavelength suppression of more than 100 dB in a 3.5 mm long waveguide, which is essential for on-chip integration of quantum-correlated photon pair sources. These findings pave the way to integrate multi wavelength experiments on chip for the next generation of photonic integrated circuits.

High-speed electro-optic modulator based on silicon nitride loaded lithium niobate on an insulator platform.

Electro-optic (EO) modulators, which convert signals from the electrical to optical domain plays a key role in modern optical communication systems. Lithium niobate on insulator (LNOI) technology has emerged as a competitive solution to realize high-performance integrated EO modulators. In this Letter, we design and experimentally demonstrate a Mach-Zehnder interferometer-based modulator on a silicon nitride loaded LNOI platform, which not only takes full advantage of the excellent EO effect of LiNbO3, but also avoids the direct etching of LiNbO3 thin film. The measured half-wave voltage length product of the fabricated modulator is 2.24 V·cm, and the extinction ratio is ∼20dB. Moreover, the 3 dB EO bandwidth is ∼30GHz, while the modulated data rate for on-off key signals can reach up to 80 Gbps.

On-chip switchable and reconfigurable optical mode exchange device using cascaded three-waveguide-coupling switches.

Data exchange between different data channels can offer more flexible and advanced functions for many optical networks. In this paper, we propose a switchable and reconfigurable data exchange device for arbitrary two optical mode channels based on three-waveguide-coupling (TWC) switches in mode-division multiplexing (MDM) networks. The working principle of the TWC switches is numerically analyzed using the coupled supermode theory. As a proof of concept, switchable data exchange between arbitrary two mode channels among the first three-order quasi-transverse electric modes is experimentally demonstrated successfully. The insertion losses of the device are less than 5.6 dB, including the coupling loss of the multiplexer and demultiplexer, while the mode crosstalk is less than -13.0 dB for all functions. The proposed device is expected to offer more flexibility to on-chip MDM networks due to its low loss, low crosstalk and good scalability.

On-chip biochemical sensor using wide Gaussian beams in silicon waveguide-integrated plasmonic crystal.

An on-chip biochemical sensor based on two-dimensional waveguide-integrated plasmonic crystal formed by a nanogap tile (NGT) array is realized. By using on-chip optical lenses, an ultra-wide collimated Gaussian beam is launched, coupled with surface plasmonic crystals and collected with relatively low additional insertion loss, allowing a large sensing area. The optical field enhancement and stop-band shift of the NGT device for biochemical sensing are numerically and experimentally demonstrated with sensitivity reaching up to ${\sim}{260}\;{\rm nm/RIU}$∼260nm/RIU. Our sensor is demonstrated with monolayer thiol molecules illustrating that it can be functionalized with this class of molecule which is commonly used with bulk surface plasmon resonance sensors.

Microwave engineering filter synthesis technique for coupled ridge resonator filters.

Wavelength filters are among the most important building blocks required for integrated optical circuits. However, existing filter building blocks provide only basic functionality with limited options for controlling the filter function unless sophisticated filter design techniques are employed. Conversely, in the microwave regime, elegant and powerful filter synthesis techniques exist which use coupled resonators. As waveguide ring resonators have emerged, researchers in the optical domain have sought to translate these techniques but the multi-wavelength spacing required to couple optical ring resonator structures severely limits the types of filters that can be realized. In this paper we show how recently reported ridge resonance structures can be arranged as coupled resonators with very close spacing and thus can be harnessed to achieved many of the filter functionalities available in the field of microwave engineering. Our filter is comprised of multiple parallel ridges on a common silicon slab, with each resonator exhibiting a resonant frequency and quality factor which can be controlled through engineering the geometry of the ridge. It is thus possible to choose appropriate combinations of ridge geometries to satisfy the conditions required by filter synthesis prototype. We demonstrate through rigorous simulation how our approach can be used to achieve high order optical bandpass filters at 1.55 µm center wavelength with Butterworth or Chebychev responses and analyse the impact of non-ideal behaviours on filter performance.