Optical soliton crystal micro-comb
Optical frequency combs, composed of discrete and equally spaced frequency lines, are extremely powerful tools for optical frequency metrology. Micro-combs offer the full power of optical frequency combs, but in an integrated form with much smaller footprint [22-24]. They have enabled many breakthroughs through their ability to generate wideband low-noise optical frequency lines for high-resolution optical frequency synthesis , ultrahigh-capacity communications , complex quantum state generation , advanced microwave signal processing , and more.
In this work we use a particular class of microcomb termed soliton crystals. They were so-named because of their crystal-like profile in the angular domain of tightly packed self-localized pulses within micro-ring resonators . They are naturally formed in micro-cavities with appropriate mode crossings, without the need for complex dynamic pumping and stabilization schemes (described by the Lugiato-Lefever equation ). They are characterized by distinctive ‘fingerprint’ optical spectra (Fig. 2f) which arise from spectral interference between the tightly packaged solitons circulating along the ring cavity. This category of soliton micro-comb features deterministic soliton formation originating from the mode crossing-induced background wave and the high intra-cavity power (the mode crossing is measured as in Fig. 2c). This in turn enables simple and reliable initiation via adiabatic pump wavelength sweeping  that can be achieved with manual detuning (the intracavity power during the pump sweeping is shown in Fig. 2d). The key to the ability to adiabatically sweep the pump lies in the fact that the intra-cavity power is over thirty times higher than single-soliton states (DKS), and very close to that of spatiotemporal chaotic states . Thus, the soliton crystal displays much less thermal detuning or instability resulting from the ‘soliton step’ that makes resonant pumping of DKS states more challenging . It is this combination of ease of generation and overall conversion efficiency that makes soliton crystals highly suited for demanding applications such as ONNs.
The coherent soliton crystal microcomb (Fig. 2) was generated by optical parametric oscillation in a single integrated micro-ring resonator (MRR). The MRR (Fig. 2b) was fabricated on a CMOS-compatible doped silica platform [22, 23], featuring a high Q factor of over 1.5 million and a radius of 592 μm, which corresponds to a low free spectral range of ~ 48.9 GHz . The pump laser (Yenista Tunics – 100S-HP) was boosted by an optical amplifier (Pritel PMFA-37) to initiate the parametric oscillation. The soliton crystal microcomb provided over 90 channels over the telecommunications C-band (1540-1570 nm), offering adiabatically generated low-noise frequency comb lines with a small footprint of < 1 mm2 and potentially low power consumption (>100 mW using the technique in ).
Evaluation of the computing performance
Since there are no common standards in the literature for classifying and quantifying the computing speed and processing power of ONNs, we explicitly outline the performance definitions that we use in characterizing our performance. We follow the approach that is widely used to evaluate electronic micro-processors. The computing power of the convolution accelerator—closely related to the operation bandwidth—is denoted as the throughput, which is the number of operations performed within a certain period. Considering that in our system the input data and weight vectors originate from different paths and are interleaved in different dimensions (time, wavelength, and space), we use the temporal sequence at the electrical output port to define the throughput in a more straightforward manner.
At the electrical output port, the output waveform has L+R−1 symbols in total (L and R are the lengths of the input data vector and the kernel weight vector, respectively), among which L−R+1 symbols are the convolution results. Further, each output symbol is the calculated outcome of R multiply-and-accumulate operations or 2R FLOPS, with a symbol duration τ given by that of the input waveform symbols. Thus, considering that L is generally much larger than R in practical convolutional neural networks, the term (L−R+1)/(L+R−1) would not affect the vector computing speed, or throughput, which (in FLOPS) is given by
As such, the computing speed of the vector convolutional accelerator demonstrated here is 2×9×62.9×10 = 11.321 Tera-FLOPS for ten parallel convolutional kernels).
We note that when processing data in the form of vectors, such as audio speech, the effective computing speed of the accelerator would be the same as the vector computing speed 2R/ τ. Yet when processing data in the form of matrices, such as for images, we must account for the overhead on the effective computing speed brought about by the matrix-to-vector flattening process. The overhead is directly related to the width of the convolutional kernels, for example, with 3-by-3 kernels, the effective computing speed would be ~1/3 * 2R/τ, which, however, we note still is in the ultrafast (TeraFLOP) regime due to the high parallelism brought about by the time-wavelength interleaving technique.
For the convolutional accelerator, the output waveform of each kernel (with a length of L−R+1=250,000−9+1=249,992) contains 166×498=82,668 useful symbols that are sampled out to form the feature map, while the rest of the symbols are discarded. As such, the effective matrix convolution speed for the experimentally performed task is slower than the vector computing speed of the convolution accelerator by the overhead factor of 3, and so the net speed then becomes 11.321×82,668/249,991=11.321×33.07% = 3.7437 Tera-FLOPS.
For the deep CNN the convolutional accelerator front end layer has a vector computing speed of 2×25×11.9×3 = 1.785 Tera-FLOPS while the matrix convolution speed for 5x5 kernels is 1.785×6×26/(900−25+1) = 317.9 Giga-FLOPS. For the fully connected layer of the deep CNN, according to Eq. (4), the output waveform of each neuron would have a length of 2R−1, while the useful (relevant output) symbol would be the one locating at R+1, which is also the result of 2R operations. As such, the computing speed of the fully connected layer would be 2R / (τ*(2R−1)) per neuron. With R =72 during the experiment and ten neurons simultaneous operating, the effective computing speed of the matrix multiplication would be 2R / (τ*(2R−1)) × 10 = 2×72 / (84ps* (2×72−1)) = 119.83 Giga-FLOPS.
In addition, the intensity resolution (i.e., the bit-resolution for digital systems) for analog ONNs is mainly limited by the signal-to-noise ratio (SNR). To achieve 8-bit resolution, the SNR of the system needs to reach over 20∙log10(28) = 48 dB. This is within the capability of our accelerator and so our system speed in Terabits/s is simply our speed in FLOPs times 8 – ie., not reduced by our OSNR.
To achieve the designed kernel weights, the generated microcomb was shaped in power using liquid crystal on silicon based spectral shapers (Finisar WaveShaper 4000S). We used two WaveShapers in the experiments - the first was used to flatten the microcomb spectrum while the precise comb power shaping required to imprint the kernel weights was performed by the second, located just before the photo-detection. A feedback loop was employed to improve the accuracy of comb shaping, where the error signal was generated by first measuring the impulse response of the system with a Gaussian pulse input and comparing it with the ideal channel weights. (Figure S6 and S7 show the shaped impulse response for the convolutional layer and the fully connected layer of the CNN).
The electrical input data was temporally encoded by an arbitrary waveform generator (Keysight M8195A) and then multicast onto the wavelength channels via a 40 GHz intensity modulator (iXblue). For the 500×500 image processing, we used sample points at a rate of 62.9 Giga samples/s to form the input symbols. We then employed a 2.2 km length of dispersive fibre that profiided a progressive delay of 15.9 ps/channel, precisely matched to the input baud rate. For the convolutional layer of the CNN, we used 5 sample points at 59.421642 Giga Samples/s to form each single symbol of the input waveform, which also matched with the progressive time delay (84 ps) of the 13km dispersive fibre (the generated electronic waveforms for 50 images are shown as Fig. S8 and S9, which served as the electrical input signal for the convolutional and fully connected layers, respectively).
For the convolutional accelerator in both experiments - the 500×500 image processing experiment and the convolutional layer of the CNN - the second Waveshaper simultaneously shaped and de-multiplexed the wavelength channels into separate spatial ports according to the configuration of the convolutional kernels. As for the fully connected layer, the second Waveshaper simultaneously performed the shaping and power splitting (instead of de-multiplexing) for the ten output neurons. Here, we note that the de-multiplexed or power-split spatial ports were sequentially detected and measured. However, these two functions could readily be achieved in parallel with a commercially available 20-port optical spectral shaper (WaveShaper 16000S, Finisar) and multiple photodetectors.
The negative channel weights were achieved using two methods. For the 500×500 image processing experiment and the convolutional layer of the CNN, the wavelength channels of each kernel were separated into two spatial outputs by the WaveShaper according to the signs of the kernel weights, and then detected by a balanced photodetector (Finisar XPDV2020). Conversely, for the fully connected layer the weights were encoded in the symbols of the input electrical waveform during the electrical digital processing stage. Incidentally, we demonstrate the possibility using of different methods to impart negative weights, both of which work in the experiments.
Finally, the electrical output waveform was sampled and digitized by a high-speed oscilloscope (Keysight DSOZ504A, 80 Giga Symbols/s) to extract the final convolved output.
In the CNN, the extracted outputs of the convolution accelerator were further processed digitally, including rescaling to exclude the loss of the photonic link via a reference bit, and then mapped onto a certain range using a nonlinear tanh function. The pooling layer’s functions were also implemented digitally, following the algorithm introduced in the network model.
The residual discrepancy or inaccuracy in our work for both the recognition and convolving functions, as compared to the numerical calculations, was due to the deterioration of the input waveform caused by intrinsic limitations in the performance of the electrical arbitrary waveform generator. Addressing this would readily lead to a higher degree of accuracy (i.e., closer agreement with the numerical calculations).