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.

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.

Optimising microring resonator based optical frequency comb distillation for optical communications systems.

Microring resonators (MRR) can be used as devices for filtering out broadband noise on optical frequency combs, in cases where significant amplification of a generated comb is required. While comb distillation has been demonstrated experimentally for optical communication systems, approaches to optimise device and sub-system parameters have not been explored. Here, we investigate how the performance of comb distillation through micro-ring filtering depends on device parameters. We also explore device parameter dependent performance when the comb and MRR are misaligned in line spacing. For the device platform we investigate, we find that the required optical signal-to-noise ratio (OSNR) of a comb line can be reduced by 16 dB, independent of modulation format, using a MRR with a resonance bandwidth of 100 MHz and coupling loss of 3 dB.

Optimizing DC restoration in Kramers-Kronig optical single-sideband receivers.

Kramers-Kronig optical single-sideband receivers remove the signal-signal beat interference (SSBI) that occurs when detecting a signal that has electrical signals mapped onto its optical field at the transmitter; such signals support electronic dispersion compensation without the need for a coherent receiver. To use the full range of the analog-to-digital converter's (ADC) range, it is best to a.c.-couple the photocurrent, to remove its DC content; however, the DC must be restored digitally before the KK algorithm is applied. Recent publications have concentrated on perfectly determining the restored DC's required level from the signal, with a view this is optimal for lowering error rates. In this paper, we investigate signal-signal beat interference (SSBI) cancellation in a single photodiode receiver using Kramers-Kronig receiver algorithm, with large variations in optical carrier-to-signal power ratio (CSPR) and DC offset level. Through simulations and experiments, we find a strategy to optimize the signal quality without the need of an extensive search for the DC offset value. We also find that a theoretically perfect determination of the original DC level does not provide best signal quality especially for low CSPRs; in order to achieve maximum cancellation of signal-signal beat interference, the level of the restored DC has an optimum value that depends on the optical CSPR. We define a digital CSPR, which is the value of the CSPR in the digital domain after DC restoration. Our measurements show that we simply need to bias the signal upwards and make the minimum signal above zero by 0.1% of the r.m.s. signal amplitude when the optical CSPR is low. For higher values of optical CSPR, the optimal digital CSPR is about 2-dB lower than the optical CSPR, and the optimal DC offset can be calculated from this digital CSPR. We find that the boundary between our low optical CSPR region and high optical CSPR region depends on the noise level in the system.

Effective linewidth reduction in self-homodyne coherent reception by stimulated Brillouin scattering-based optical carrier recovery.

In this paper, we demonstrate a self-homodyne coherent system with a significantly narrowed effective linewidth using optical carrier recovery based on stimulated Brillouin scattering (SBS), employing only coarse path length matching. The effective linewidth of the SBS-based receiver system is reduced from 75 kHz to less than 2 kHz, which is estimated by Lorentzian fitting of power spectra, and confirmed by simulation results of the tolerance window length for phase noise compensation (PNC) with different linewidth. Both experimental and numerical studies on the tracking requirements on PNC algorithms confirm effective linewidth reduction to this level, and show a 32x relaxation of the phase recovery tracking window length. This highlights the potential to significantly reduce the computational complexity of PNC even in coarsely optimized SBS-based self-homodyne coherent systems, providing an alternative to using demanding ultra-low linewidth lasers.

Wide-range optical carrier recovery via broadened Brillouin filters.

Stimulated Brillouin scattering has great potential for wide-wavelength-range optical carrier recovery, as it can act as a parametrically defined narrowband gain filter. However, due to the dispersion of the Brillouin frequency shift, prior demonstrations have been limited in wavelength range. Here, we demonstrate that frequency modulating the pump light for a gain filter based on stimulated Brillouin scattering enables optical carrier recovery for a broad range of input wavelengths. We demonstrate highly selective (<150M H z bandwidth) amplification for optical carriers over an 18 nm wide wavelength range in the optical communications C-band, an ∼6× improvement over using an unmodulated pump. Measurements of the noise properties of these spectrally broadened gain filters, in both amplitude and phase, indicate the noise performance and SNR are maintained over a wide wavelength range. Our technique provides a potential solution for highly selective, wavelength agnostic optical carrier recovery.

Optical sampling to enhance Nyquist-shaped signal detection under limited receiver bandwidth.

Insufficient receiver bandwidth destroys the orthogonality of Nyquist-shaped pulses, generating inter-symbol interference (ISI). We propose using an optical pre-sampler to alleviate the requirement on the receiver bandwidth through pulse re-shaping. Experiments and simulations using an optically shaped 40-Gbaud Nyquist-shaped on-off-keying signal (N-OOK) show receiver sensitivity improvements of 4- and 7.1-dB under 18- and 11-GHz receiver electrical bandwidths, respectively.

All-optical OFDM demultiplexing with optical partial Fourier transform and coherent sampling.

We propose a novel scheme with a "time-lens"-based partial optical Fourier transform (OFT) and coherent sampling for high-speed complex orthogonal frequency-division multiplexing (OFDM) signal detection. Compared with all-optical OFDM demultiplexing with a matched optical filter, our proposed method replaces specialized optical filters with commercially available equipment, which relaxes stringent manufacturing and operational requirements. Our simulation shows that even with a partial OFT, theoretically, close to inter-channel interference-free performance is possible. In addition, we performed a proof-of-concept experiment of 16×10 Gbaud quadrature phase-shift keying (QPSK) all-optical OFDM detection, with all the bit error rates far below the 7% hard-overhead forward error correction limit.

Single-photodiode per polarization receiver with signal-signal beat interference suppression through heterodyne detection.

We show that a simplified, single-photodiode per polarization heterodyne receiver is able to directly suppress signal-signal beat interference (SSBI), without the need for cancellation in the digital domain. We characterize performance degradation due to SSBI, and show that a strong LO in the receiver can mitigate SSBI. Transmission of 400 Gb/s-class signals is shown over single fiber spans of up to 160 km, and over field-deployed metropolitan area fiber. These results indicate that a single photodiode can be used to receive complex optical signals in high speed fiber systems without the need for SSBI cancellation in the digital domain.

Inter-channel nonlinear phase noise compensation using optical injection locking.

We propose optical injection locking (OIL) to enable compensation of the inter-channel nonlinear phase noise, which is dominated by cross-phase modulation (XPM). In this paper, injection locking is used to create a local oscillator for a homodyne receiver from a residual carrier. The locking is fast enough to follow XPM-phase distortion, but slow enough to reject the signal bands, which are spaced slightly away from the pilot. The homodyne receiver thus partially cancels XPM, as it is common to the signals and the pilot. An experimental 7-channel WDM system gives 1-dB (0.7-dB) improvement in the peak Q of the center channel, for QPSK (16-QAM) modulated OFDM subcarriers, and increased the transmission reach by 320 km. The optimum performance was achieved at an injection ratio of -45 dB, with the injected power as low as -24.5 dBm.

Simple optoelectronic frequency-offset estimator for coherent optical OFDM.

We propose a carrier frequency-offset estimator for optical OFDM systems using off-the-shelf optical components and simple digital processing as a replacement for the purely digital signal processing using the cyclostationarity property of optical OFDM signals with cyclic prefix. Simulations show the system accuracy of <4% estimate error within the range [-1250 + 1250] MHz offsets for a single polarization 28-Gbaud OFDM signal with 15% cyclic prefix. The effects of the system parameters on the performance are investigated.

Polarization independent optical injection locking for carrier recovery in optical communication systems.

An optical injection locking (IL) system that is independent of the incoming signal's polarization is demonstrated for carrier recovery in coherent optical communication systems. A sub-system that enables polarization independence is discussed and experimentally verified. The system is tested over a 20-km test field link using a broad-linewidth laser (40 MHz), and shows the suppression of phase noise when using the carrier recovered by injection locking as the local oscillator.

Hardware-efficient signal generation of layered/enhanced ACO-OFDM for short-haul fiber-optic links.

Layered/enhanced ACO-OFDM is a promising candidate for intensity modulation and direct-detection based short-haul fiber-optic links due to its both power and spectral efficiency. In this paper, we firstly demonstrate a hardware-efficient real-time 9.375 Gb/s QPSK-encoded layered/enhanced asymmetrical clipped optical OFDM (L/E-ACO-OFDM) transmitter using a Virtex-6 FPGA. This L/E-ACO-OFDM signal is successfully transmitted over 20-km uncompensated standard single-mode fiber (S-SMF) using a directly modulated laser. Several methods are explored to reduce the FPGA's logic resource utilization by taking advantage of the L/E-ACO-OFDM's signal characteristics. We show that the logic resource occupation of L/E-ACO-OFDM transmitter is almost the same as that of DC-biased OFDM transmitter when they achieve the same spectral efficiency, proving its great potential to be used in a real-time short-haul optical transmission link.

Cyclic spectra for wavelength-routed optical networks.

We propose occupying the guard bands in closely spaced WDM systems with redundant signal spectral components to increase tolerance to frequency misalignment and channel shaping from multiplexing elements. By cyclically repeating the spectrum of a modulated signal, we show improved tolerance to impairments due to add/drop multiplexing with a commercial wavelength selective switch in systems using 5%-20% guard bands on a 50 GHz DWDM grid.

All-optical digital-to-analog converter based on cross-phase modulation with temporal integration.

We propose and experimentally demonstrate an all-optical digital-to-analog converter based on cross-phase modulation with temporal integration. The scheme is robust for driving signal noise due to the low-pass filtering feature of the temporal integrator. The proof-of-concept experiment demonstrates the generation of pulse-amplitude modulation (PAM) sequences up to eight levels. The performance of random PAM 2 and PAM 4 signals with different optical signal-to-noise ratios of the binary driving signal is also investigated. The scheme is scalable for high-speed operation with an appropriate dispersion profile of the nonlinear medium.

Cycle-slip-less low-complexity phase recovery algorithm for coherent optical receivers.

We propose and experimentally validate a blind phase recovery algorithm based on tracking low-frequency components of the phase noise, which we call "filtered carrier-phase estimation (F-CPE)." Tracking only the low-frequency components allows F-CPE to reduce the computational complexity by using a frequency-domain equalizer and to simplify the partitioning of a 16 quadrature amplitude modulation (16QAM) constellation. Further, this approach eliminates cycle slips by suppressing the impact of amplified spontaneous emission on phase noise estimation. The experimental results demonstrate cycle-slip-free operation for 15 and 32 GBd 16QAM signals. Additionally, the proposed method showed similar or better sensitivity compared with the blind-phase-search algorithm, near standard forward error correction thresholds of modern wavelength division multiplexing systems.

Unscented Kalman filters for polarization state tracking and phase noise mitigation.

Simultaneous polarization and phase noise tracking and compensation is proposed based on an unscented Kalman filter (UKF). We experimentally demonstrate the tracking under noise-loading and after 800-km single-mode fiber transmission with 20-Gbaud QPSK and 16-QAM signals. These experiments show that the proposed UKF outperforms both conventional blind tracing algorithms and a previously proposed extended Kalman filter, at the cost of higher complexity. Additionally, we propose and test modified Kalman filter algorithms to reduce computational complexity.

Folded orthogonal frequency division multiplexing.

We propose and demonstrate a new sub-carrier multiplexing scheme, utilizing orthogonal, periodic-sinc-shaped sub-carrier spectra. This 'folded' OFDM allows for multi-carrier bands to be generated with the precise, rectangular frequency definition of Nyquist WDM. We show that this scheme can be implemented with 10 GHz sub-bands, showing a 0.5-dB implementation penalty and successful transmission over 4160-km. We further investigate 40-GHz bands in an add/drop multiplexing scenario on a 50-GHz WDM grid, and show that folded OFDM can provided advantages over conventional OFDM in bandwidth-limited systems.

Banded all-optical OFDM super-channels with low-bandwidth receivers.

We propose a banded all-optical orthogonal frequency division multiplexing (AO-OFDM) transmission system based on synthesising a number of truncated sinc-shaped subcarriers for each sub-band. This approach enables sub-band by sub-band reception and therefore each receiver's electrical bandwidth can be significantly reduced compared with a conventional AO-OFDM system. As a proof-of-concept experiment, we synthesise 6 × 10-Gbaud subcarriers in both conventional and banded AO-OFDM systems. With a limited receiver electrical bandwidth, the experimental banded AO-OFDM system shows 2-dB optical signal to noise ratio (OSNR) benefit over conventional AO-OFDM at the 7%-overhead forward error correction (FEC) threshold. After transmission over 800-km of single-mode fiber, ≈3-dB improvement in Q-factor can be achieved at the optimal launch power at a cost of increasing the spectral width by 14%.

Sub-GHz-resolution C-band Nyquist-filtering interleaver on a high-index-contrast photonic integrated circuit.

Modern optical communications rely on high-resolution, high-bandwidth filtering to maximize the data-carrying capacity of fiber-optic networks. Such filtering typically requires high-speed, power-hungry digital processes in the electrical domain. Passive optical filters currently provide high bandwidths with low power consumption, but at the expense of resolution. Here, we present a passive filter chip that functions as an optical Nyquist-filtering interleaver featuring sub-GHz resolution and a near-rectangular passband with 8% roll-off. This performance is highly promising for high-spectral-efficiency Nyquist wavelength division multiplexed (N-WDM) optical super-channels. The chip provides a simple two-ring-resonator-assisted Mach-Zehnder interferometer, which has a sub-cm2 footprint owing to the high-index-contrast Si3N4/SiO2 waveguide, while manifests low wavelength-dependency enabling C-band (> 4 THz) coverage with more than 160 effective free spectral ranges of 25 GHz. This device is anticipated to be a critical building block for spectrally-efficient, chip-scale transceivers and ROADMs for N-WDM super-channels in next-generation optical communication networks.