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.

Correlated twin-photon generation in a silicon nitride loaded thin film PPLN waveguide.

Photon-pair sources based on thin film lithium niobate on insulator technology have a great potential for integrated optical quantum information processing. We report on such a source of correlated twin-photon pairs generated by spontaneous parametric down conversion in a silicon nitride (SiN) rib loaded thin film periodically poled lithium niobate (LN) waveguide. The generated correlated photon pairs have a wavelength centred at 1560 nm compatible with present telecom infrastructure, a large bandwidth (21 THz) and a brightness of ∼2.5 × 105 pairs/s/mW/GHz. Using the Hanbury Brown and Twiss effect, we have also shown heralded single photon emission, achieving an autocorrelation g H(2)(0)≃0.04.

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.

Adaptive dispersion compensation using a photonic integrated circuit finite impulse response filter.

Optical equalization can be used for chromatic dispersion compensation in optical communication systems to improve the system performance; however, optical signal processing (OSP) is generally specifically designed for transmission channels, that is non-adaptive to dynamic transmission distortions compared with digital signal processing (DSP). In this contribution, we demonstrate optical equalization using a photonic integrated circuit (PIC) filter for chromatic dispersion compensation, with static and adaptive techniques: (a) the static optical equalizer is calibrated based on the known fiber dispersion and length, by using the fractional delay reference method; (b) the adaptive optical equalizer is updated iteratively to compensate transmission impairments based on a least-mean squares (LMS) algorithm. Experimental results show that both the static optical equalizer and the adaptive optical LMS equalizer can give an 18-dB Q-factor for a 14-Gbd QPSK signal transmitting over 30 km. To highlight the capability of the optical equalizers, we use simulations to show the improvement in dispersion compensating characteristics by implementing additional taps.

Minimum complexity integrated photonic architecture for delay-based reservoir computing.

Reservoir computing is an analog bio-inspired computation scheme for efficiently processing time-dependent signals, the photonic implementations of which promise a combination of massive parallel information processing, low power consumption, and high-speed operation. However, most of these implementations, especially for the case of time-delay reservoir computing, require extensive multi-dimensional parameter optimization to find the optimal combination of parameters for a given task. We propose a novel, largely passive integrated photonic TDRC scheme based on an asymmetric Mach-Zehnder interferometer in a self-feedback configuration, where the nonlinearity is provided by the photodetector, and with only one tunable parameter in the form of a phase shifting element that, as a result of our configuration, allows also to tune the feedback strength, consequently tuning the memory capacity in a lossless manner. Through numerical simulations, we show that the proposed scheme achieves good performance -when compared to other integrated photonic architectures- on the temporal bitwise XOR task and various time series prediction tasks, while greatly reducing hardware and operational complexity.

Integrated lithium niobate polarization beam splitter based on a photonic-crystal-assisted multimode interference coupler.

Lithium niobate on insulator (LNOI) is a promising platform for high-speed photonic integrated circuits (PICs) that are used for communication systems due to the excellent electro-optic properties of lithium niobate (LN). In such circuits, the high-speed electro-optical modulators and switches need to be integrated with passive circuit components that are used for routing the optical signals. Polarization beam splitters (PBSs) are one of the fundamental passive circuit components for high-speed PICs that can be used to (de)multiplex two orthogonal polarization optical modes, enabling on-chip polarization division multiplexing (PDM) systems, which are suitable for enhancing the data capacity of PICs. In this Letter, we design and experimentally demonstrate a high-performance PBS constructed by a photonic crystal (PC)-assisted multimode interference (MMI) coupler. The measured polarization extinction ratio (ER) of the fabricated device is 15 dB in the wavelength range from 1525 to 1565 nm, which makes them suitable for the high-speed and large data capacity PICs required for future communication systems.

Integrated lithium niobate optical mode (de)interleaver based on an asymmetric Y-junction.

Lithium niobate on insulator (LNOI) platforms promise unique advantages in realizing high-speed, large-capacity, and large-scale photonic integrated circuits (PICs) by leveraging lithium niobate's attractive material properties, which include electro-optic and nonlinear optic properties, low material loss, and a wide transparency window. Optical mode interleavers can increase the functionality of future PICs in LNOI by enabling optical mode division multiplexing (MDM) systems, allowing variable mode assignment while maintaining high channel utilization and capacity. In this Letter, we experimentally demonstrate an optical mode interleaver based on an asymmetric Y-junction on the LNOI platform, which exhibits an insertion loss of below 0.46 dB and modal cross talk of below -13.0 dB over a wavelength range of 1500-1600 nm. The demonstrated mode interleaver will be an attractive circuit component in future high-speed and large-capacity PICs due to its simple structure, scalability, and capacity for efficient and flexible mode manipulation on the LNOI platform.

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.

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.

Low loss CMOS-compatible silicon nitride photonics utilizing reactive sputtered thin films.

Low temperature deposition of low loss silicon nitride (SiN) thin-films is very attractive as it opens opportunities for realization of multi-layer photonic chips and hybrid integration of optical waveguides with temperature sensitive platforms such as processed CMOS silicon electronics or lithium niobate on insulator. So far, the most common low-temperature deposition technique for SiN is plasma enhanced chemical vapor deposition (PECVD), however such SiN thin-films can suffer from significant losses at C-band wavelengths due to unwanted hydrogen bonds. In this contribution we present a back end of line (< 400°C), low loss SiN platform based on reactive sputtering for telecommunication applications. Waveguide losses of 0.8 dB/cm at 1550 nm and as low as 0.6 dB/cm at 1580 nm have been achieved for moderate confined waveguides which appear to be limited by patterning rather than material. These findings show that reactive sputtered SiN thin-films can have lower optical losses compared to PECVD SiN thin-films, and thus show promise for future hybrid integration platforms for applications such as high Q resonators, optical filters and delay lines for optical signal processing.

Improved second harmonic performance in periodically poled LNOI waveguides through engineering of lateral leakage.

In this contribution, we investigate the impact of lateral leakage for linear and nonlinear optical waveguides in lithium niobate on an insulator (LNOI). Silicon nitride (SiN) loaded and direct patterned lithium niobate cross-sections are investigated. We show that lateral leakage can take place for the TE mode in LNOI ridge waveguides (X-cut lithium niobate), due to the birefringence of the material. This work gives guidelines for designing waveguides in LNOI that do not suffer from the lateral leakage effect. By applying these design considerations, we avoided the lateral leakage effect at the second harmonic wavelength of a nonlinear optical waveguide in LNOI and demonstrate a peak second harmonic generation conversion efficiency of ~1160% W-1cm-2.

Optical frequency comb based system for photonic refractive index sensor interrogation.

In this contribution, we demonstrate how an optical frequency comb can be used to enhance the functionality of an integrated photonic biosensor platform. We show that if an optical frequency comb is used to sample the spectral response of a Mach-Zehnder interferometer and if the line spacing is arranged to sample the periodic response at 120° intervals, then it is possible to combine these samples into a single measurement of the interferometer phase. This phase measurement approach is accurate, independent of the bias of the interferometer and robust against intensity fluctuations that are common to each of the comb lines. We demonstrate this approach with a simple silicon photonic interferometric refractive index sensor and show that the benefits of our approach can be obtained without degrading the lower limit of detection of 3.70×10-7 RIU.

Low loss (Al)GaAs on an insulator waveguide platform.

In this Letter, we demonstrate a low loss gallium arsenide and aluminum gallium arsenide on an insulator platform by heterogenous integration. The resonators on this platform exhibit record high quality factors up to 1.5×106, corresponding to a propagation loss ∼0.4 dB/cm. For the first time, to the best of our knowledge, the loss of integrated III-V semiconductor on insulator waveguides becomes comparable with that of the silicon-on-insulator waveguides. This Letter should have a significant impact on photonic integrated circuits (PICs) and become an essential building block for the evolving nonlinear PICs and integrated quantum photonic systems in the future.

Quasi-phase matching via femtosecond laser-induced domain inversion in lithium niobate waveguides.

We demonstrate an all-optical fabrication method of quasi-phase matching structures in lithium niobate (LiNbO3) waveguides using a tightly focused femtosecond near-infrared laser beam (wavelength of 800 nm). In contrast to other all-optical schemes that utilize a periodic lowering of the nonlinear coefficient χ(2) by material modification, here the illumination of femtosecond pulses directly reverses the sign of χ(2) through the process of ferroelectric domain inversion. The resulting quasi-phase matching structures, therefore, lead to more efficient nonlinear interactions. As an experimental demonstration, we fabricate a structure with the period of 2.74 µm to frequency double 815 nm light. A maximum conversion efficiency of 17.45% is obtained for a 10 mm long waveguide.

Experimental demonstration of bidirectional light transfer in adiabatic waveguide structures.

We propose and demonstrate a novel type of optical integrated structure consisting of three adiabatically coupled waveguides arranged in an N-shaped geometry. Unlike conventional adiabatic three-waveguide couplers mimicking the stimulated Raman adiabatic passage process which utilize solely the counter-intuitive coupling and, thus, operate only in one direction, our structure achieves complete bidirectional light transfer between two waveguides through the counter-intuitive and intuitive coupling in either direction over a wide wavelength range. Moreover, the light transfer through the intuitive coupling is more efficient and robust than through the counter-intuitive coupling.

Lithium niobate photonics: Unlocking the electromagnetic spectrum.

Lithium niobate (LN), first synthesized 70 years ago, has been widely used in diverse applications ranging from communications to quantum optics. These high-volume commercial applications have provided the economic means to establish a mature manufacturing and processing industry for high-quality LN crystals and wafers. Breakthrough science demonstrations to commercial products have been achieved owing to the ability of LN to generate and manipulate electromagnetic waves across a broad spectrum, from microwave to ultraviolet frequencies. Here, we provide a high-level Review of the history of LN as an optical material, its different photonic platforms, engineering concepts, spectral coverage, and essential applications before providing an outlook for the future of LN.