STM observation of pit formation and evolution during the epitaxial growth of Si on Si(001) surface

Pit formation and surface morphological evolution in Si（001） homoepitaxy are investigated by using scanning tunneling microscopy. Anti-phase boundary is found to give rise to initial generation of pits bound by bunched DB steps. The terraces break up and are reduced to a critical nucleus size with pit formation. Due to anisotropic kinetics, a downhill bias diffusion current, which is larger along the dimer rows through the centre area of the terrace than through the area close to the edge, leads to the prevalence of pits bound by {101} facets. Subsequent annealing results in a shape transition from {101）-faceted pits to multi-faceted pits.

Pattern recognition in multi-synaptic photonic spiking neural networks based on a DFB-SA chip

Spiking neural networks(SNNs)utilize brain-like spatiotemporal spike encoding for simulating brain functions.Photonic SNN offers an ultrahigh speed and power efficiency platform for implementing high-performance neuromorphic computing.Here,we proposed a multi-synaptic photonic SNN,combining the modified remote supervised learning with delayweight co-training to achieve pattern classification.The impact of multi-synaptic connections and the robustness of the network were investigated through numerical simulations.In addition,the collaborative computing of algorithm and hardware was demonstrated based on a fabricated integrated distributed feedback laser with a saturable absorber(DFB-SA),where 10 different noisy digital patterns were successfully classified.A functional photonic SNN that far exceeds the scale limit of hardware integration was achieved based on time-division multiplexing,demonstrating the capability of hardware-algorithm co-computation.

Performance limit of a multi-frequency probe based coherent optical time domain reflectometry caused by nonlinear effects

The nonlinear effects that limit the performance of the multi-frequency probe（MFP）based coherent optical time domain reflectometry（C-OTDR）are investigated.Based on theoretical analysis and experimental results,compared with conventional C-OTDR,when the probe pulse has power gradient within the pulse width,self-phase modulation（SPM）and cross-phase modulation（XPM）are strengthened in the new C-OTDR scheme.The generation of four-wave mixing（FWM）is dependent on SPM and XPM,and with modulation frequency of phase modulator higher than 40 MHz,the stimulated Brillouin scattering（SBS） threshold can be enhanced by more than 5 dB,which benefits the maximum dynamic range of the MFP C-OTDR.

Experimental demonstration of coherent photonic neural computing based on a Fabry–Perot laser with a saturable absorber

As Moore’s law has reached its limits,it is becoming increasingly difficult for traditional computing architectures to meet the demands of continued growth in computing power.Photonic neural computing has become a promising approach to overcome the von Neuman bottleneck.However,while photonic neural networks are good at linear computing,it is difficult to achieve nonlinear computing.Here,we propose and experimentally demonstrate a coherent photonic spiking neural network consisting of Mach–Zehnder modulators(MZMs)as the synapse and an integrated quantum-well Fabry–Perot laser with a saturable absorber(FP-SA)as the photonic spiking neuron.Both linear computation and nonlinear computation are realized in the experiment.In such a coherent architecture,two presynaptic signals are modulated and weighted with two intensity modulation MZMs through the same optical carrier.The nonlinear neuron-like dynamics including temporal integration,threshold,and refractory period are successfully demonstrated.Besides,the effects of frequency detuning on the nonlinear neuron-like dynamics are also explored,and the frequency detuning condition is revealed.The proposed hardware architecture plays a foundational role in constructing a large-scale coherent photonic spiking neural network.

Development of Fully-Distributed Fiber Sensors Based on Brillouin Scattering

Brillouin scattering based optical fiber sensors(BOFS)have the unique advantages over other sensors such as long distance,fully distributed,and multi-parameter sensing.The progresses on the development of BOFS technology in Nanjing University are reviewed.The key technologies to make BOFS with ultra-long distance,high spatial resolution,and fast measuring speed are discussed and realized.

A distributed optical fiber sensing system for synchronous vibration and loss measurement

We propose a fully distributed fusion system combining phase-sensitive optical time-domain reflectometry(Φ-OTDR) and OTDR for synchronous vibration and loss measurement by setting an ingenious frequency sweep rate(FSR) of the optical source. The relationships between FSR, probe pulse width and repeat period are given to balance the amplitude fluctuation of OTDR traces, the dead zone probability and the measurable frequency range of vibration events. In the experiment, we achieve synchronous vibration and loss measurement with FSR of 40 MHz/s, the proble pulse width of 100 ns and repeat rate of 0.4 ms. The fluctuation of OTDR trace is less than 0.45 dB when the signalto-noise ratio(SNR) is over 12 dB for a captured vibration event located at 9.1 km. The proposed method can be used for not only detection but also early warning of damage events in optical communication networks.

Demodulation of the multi-peak fiber Bragg grating sensor based on partial wavelength scan

Until now, a high-efficiency demodulation method for fiber Bragg grating(FBG) sensors has been a challenge.In this Letter, by employing multi-peak FBGs, an FBG sensor with a partial wavelength scan is proposed and initially demonstrated. By demodulating a near-symmetrical multi-peak FBG and an asymmetrical multi-peak FBG in the strain experiment, sensor sensitivities of 1.02 pm/με and 1.01 pm/με are measured for the interrogation system, respectively. The average demodulation deviations for the two sensors are 1.81% and 0.4%,respectively. The proposed method is expected to realize high-efficiency and low-cost FBG interrogators.

Numerical simulation and experiment of optothermal response of biological tissue irradiated by continuous xenon lamp

A finite element method computation model for analyzing optothermal interaction of polychromatic light and biology tissue is proposed and proven by experiment. A continuous xenon lamp is employed as an example. First, the spectral energy distribution of the xenon lamp is measured and found to be equivalent to a series of quasi-chromatic light with different central wavelengths, different energies, and certain bandwidth. Next, according to the reported thermal and optical parameters of porcine skin and porcine liver, the temporal temperature distributions of these tissues irradiated by each quasi-chromatic light are simulated. Then, the thermal effect is superimposed to obtain the whole optothermal temporal temperature distribution. Moreover, the optothermal response experiments of fresh porcine skin and porcine liver tissues irradiated by continuous xenon lamp are carried out. The results of the simulation and experiment are analyzed and compared, and are found to be commendably matched.

Photonic integrated spiking neuron chip based on a self-pulsating DFB laser with a saturable absorber

We proposed and experimentally demonstrated a simple and novel photonic spiking neuron based on a distributed feedback(DFB)laser chip with an intracavity saturable absorber(SA).The DFB laser with an intracavity SA(DFBSA)contains a gain region and an SA region.The gain region is designed and fabricated by the asymmetric equivalentπ-phase shift based on the reconstruction-equivalent-chirp technique.Under properly injected current in the gain region and reversely biased voltage in the SA region,periodic self-pulsation was experimentally observed due to the Q-switching effect.The self-pulsation frequency increases with the increase of the bias current and is within the range of several gigahertz.When the bias current is below the self-pulsation threshold,neuronlike spiking responses appear when external optical stimulus pulses are injected.Experimental results show that the spike threshold,temporal integration,and refractory period can all be observed in the fabricated DFB-SA chip.To numerically verify the experimental findings,a time-dependent coupled-wave equation model was developed,which described the physics processes inside the gain and SA regions.The numerical results agree well with the experimental measurements.We further experimentally demonstrated that the weighted sum output can readily be encoded into the self-pulsation frequency of the DFB-SA neuron.We also benchmarked the handwritten digit classification task with a simple single-layer fully connected neural network.By using the experimentally measured dependence of the self-pulsation frequency on the bias current in the gain region as an activation function,we can achieve a recognition accuracy of 92.2%,which bridges the gap between the continuous valued artificial neural networks and spike-based neuromorphic networks.To the best of our knowledge,this is the first experimental demonstration of a photonic integrated spiking neuron based on a DFB-SA,which shows great potential to realizing large-scale multiwavelength photonic spiking neural network chips.

Wideband tunable REC-DFB laser array using thin-film heaters on the submount

A wideband wavelength-tunable 4×5 distributed feedback(DFB)semiconductor laser array based on the reconstructionequivalent-chirp(REC)technique using a simple tuning scheme is demonstrated.It consists of 20 DFB lasers with 4×5matrix interleaving distributions,two-level cascaded Y-branch optical combiners,and one active semiconductor opticalamplifier(SOA),all in-series integrated on one chip.Unlike the traditional thermal-electric cooler(TEC)-based wavelength-tuning scheme,the tunable 4×5 REC-DFB laser array achieves a faster and broader continuous wavelength-tuningrange using TaN thin-film heaters integrated on the AlN submount.By changing the injection current of the TaN resistorfrom 0 to 190 mA,the proposed tunable laser achieves a wavelength-tuning range of∼2.5 nm per channel and a total tuningof over 50 nm.This study opens up new avenues for realizing cost-effective and wide-tuning-range semiconductor lasers.

Spiking information processing in a single photonic spiking neuron chip with double integrated electronic dendrites

Dendrites,branches of neurons that transmit signals between synapses and soma,play a vital role in spiking information processing,such as nonlinear integration of excitatory and inhibitory stimuli.However,the investigation of nonlinear integration of dendrites in photonic neurons and the fabrication of photonic neurons including dendritic nonlinear integration in photonic spiking neural networks(SNNs)remain open problems.Here,we fabricate and integrate two dendrites and one soma in a single Fabry–Perot laser with an embedded saturable absorber(FP-SA)neuron to achieve nonlinear integration of excitatory and inhibitory stimuli.Note that the two intrinsic electrodes of the gain section and saturable absorber(SA)section in the FP-SA neuron are defined as two dendrites for two ports of stimuli reception,with one electronic dendrite receiving excitatory stimulus and the other receiving inhibitory stimulus.The stimuli received by two electronic dendrites are integrated non-linearly in a single FP-SA neuron,which generates spikes for photonic SNNs.The properties of frequency encoding and spatiotemporal encoding are investigated experimentally in a single FP-SA neuron with two electronic dendrites.For SNNs equipped with FP-SA neurons,the range of weights between presynaptic neurons and postsynaptic neurons is varied from negative to positive values by biasing the gain and SA sections of FP-SA neurons.Compared with SNN with all-positive weights realized by only biasing the gain section of photonic neurons,the recognition accuracy of Iris flower data is improved numerically in SNN consisting of FP-SA neurons.The results show great potential for multi-functional integrated photonic SNN chips.

Broadband Brillouin slow light with multiple-longitudinal-mode,tunable pump

We propose and demonstrate broadband Brillouin slow light using a multiple-longitudinal-mode tunable fiber laser as Brillouin pump. A tunable broadband Brillouin pump with a tuning range from 1 520 to 1 555 nm is generated using a fiber ring laser with a semiconductor optical amplifier （SOA） as its gain medium. The pump spectrum consists of a large number of longitudinal modes separated by 6 MHz. The 3-dB bandwidth is about 11.5 GHz, and its fluctuation is less than 100 MHz within the tuning range. An 8-Gb/s data signal can be delayed by up to 83.0 ps （bit error rate 〈 10^- 9） at 17-dBm pump power.

Tunable high-Q tapered silica microcylinder filter

We propose and demonstrate a tunable optical filter based on a tapered silica microcylinder, whose diam- eter is gradually varied along its longitudinal axis. The tapered microcylinder comprises a slowly varying transition zone fabricated through the fiber drawing technique. Given the tapered diameter of the micro- cylinder, the resonant wavelengths can be tuned by displacing the tapered microcylinder. This tunable filter exhibits a high Q-factor （-106） and a large tuning range （〉1 free spectral range （FSR））.