BP-based supervised learning algorithm for multilayer photonic spiking neural network and hardware implementation.

We introduce a supervised learning algorithm for photonic spiking neural network (SNN) based on back propagation. For the supervised learning algorithm, the information is encoded into spike trains with different strength, and the SNN is trained according to different patterns composed of different spike numbers of the output neurons. Furthermore, the classification task is performed numerically and experimentally based on the supervised learning algorithm in the SNN. The SNN is composed of photonic spiking neuron based on vertical-cavity surface-emitting laser which is functionally similar to leaky-integrate and fire neuron. The results prove the demonstration of the algorithm implementation on hardware. To seek ultra-low power consumption and ultra-low delay, it is great significance to design and implement a hardware-friendly learning algorithm of photonic neural networks and realize hardware-algorithm collaborative computing.

Ultrahigh dynamic range and low noise figure programmable integrated microwave photonic filter.

Microwave photonics has adopted a number of important concepts and technologies over the recent pasts, including photonic integration, versatile programmability, and techniques for enhancing key radio frequency performance metrics such as the noise figure and the dynamic range. However, to date, these aspects have not been achieved simultaneously in a single circuit. Here, we report a multi-functional photonic integrated circuit that enables programmable filtering functions with record-high performance. We demonstrate reconfigurable filter functions with record-low noise figure and a RF notch filter with ultra-high dynamic range. We achieve this unique feature using versatile complex spectrum tailoring enabled by an all integrated modulation transformer and a double injection ring resonator as a multi-function optical filtering component. Our work breaks the conventional and fragmented approach of integration, functionality and performance that currently prevents the adoption of integrated MWP systems in real applications.

Fast, closed-loop iterative system-on-chip of deflection efficiency enhancement for a liquid crystal optical phased array.

To implement a liquid crystal optical phased array (LC-OPA) on a practical free-space laser communication terminal, there are two essential parameters: insertion loss and the closed-loop bandwidth required to meet the dynamic linking condition of the acquisition-tracking-pointing sub-system. Real-time hardware platforms and deflection efficiency optimization algorithms have been suggested since the invention of LC-OPA. In this paper, the so-called ZYNQ platform, a field-programmable-gate-array-based heterogeneous system-on-chip (SoC), is utilized to keep real-time response and accelerate data generation, such as beam steering, beamforming, beam enhancement, etc. In addition, a novel, to the best of our knowledge, optimization algorithm is proposed on the concept of dimension reduction of the number of objective variables. After deploying on this heterogeneous SoC platform, numerical simulations and experimental results both verify that, compared to the conventional PC-based system, the integrated SoC platform offers 15.8 times faster iterative speed, a rapid convergence rate, and excellent robustness, yet with less usage of power, physical size, and monetary cost. The efficiency enhancement process costs only a few seconds at any angle, laying the foundation for practical in-line applications.

Spectrum-continuous narrowband filter with an enhanced flat-top passband and sharp roll-off performance.

We demonstrate a spectrum-continuous narrowband filter based on multi-section Mach-Zehnder interferometer (MZI) structures on a silicon nitride platform. Enhanced in-band flatness and sharp out-of-band roll-off features are achieved by cascading MZIs of specific functionalities in a series. This filter is fabricated in a single chip with a total waveguide length exceeding 237 cm. We experimentally demonstrate for the first time, to the best of our knowledge, a four-channel spectrum-continuous filter with a 3 dB bandwidth of 580 MHz, a 1 dB fluctuation bandwidth as high as 356 MHz, and an excellent 20 dB roll-off factor of 1.26 that are obtained simultaneously. Both the optical and electrical experimental results show that the proposed filter can play a key role in integrated microwave photonic front-end systems.

Tilted Nano-Grating Based Ultra-Compact Broadband Polarizing Beam Splitter for Silicon Photonics.

An ultra-compact broadband silicon polarizing beam splitter is proposed based on a tilted nano-grating structure. A light cross coupling can be realized for transverse-magnetic mode, while the transverse-electric light can almost completely output from the through port. The length of the coupling region is only 6.8 µm, while an extinction ratio of 23.76 dB can be realized at a wavelength of 1550 nm. As a proof of concept, the device was fabricated by a commercial silicon photonic foundry. It can realize a 19.84 dB extinction ratio and an 80 nm working bandwidth with an extinction ratio of larger than 10 dB. The presented device also shows a good fabrication tolerance to the structure deviations, which is favorable for its practical applications in silicon photonics.

Analog radio-over-fiber links with a high SFDR for passive distributed antenna system applications using Lagrange multiplier optimization.

An analog radio-over-fiber scheme with a high spurious-free dynamic range (SFDR) is proposed for constructing a passive distributed antenna system (DAS). By developing a Lagrange multiplier constrained optimization model, the best trade-off among RF output power, the polarization incident angle, and the RF power ratio is obtained. Consequently, the third-order intermodulation distortion and second-order harmonic distortion can be suppressed simultaneously simply by varying the polarization incident angle. The simulated and experimental results show that the proposed scheme is effective and feasible. Additionally, this Letter offers valuable insights into the nonlinear optimization, and it may be of great significance in future design and manufacture.

Linearized phase modulated microwave photonic link based on integrated ring resonators.

An on-chip linearization method for phase modulated microwave photonic link based on integrated ring resonators is proposed. By properly tailoring the phase and amplitude of optical carrier band and second-order sidebands, the third-order intermodulation distortion (IMD3) components can be suppressed. Theoretical analysis are taken and a proof-of-concept experiment is carried out. Experimental results demonstrate that IMD3 is suppressed by 21.7 dB. When the noise of the link is properly optimized, an SFDR of 112.7 dB·Hz2/3 can be achieved. This opens the possibility of integrating linearization into a functional photonic integrated circuit.

Method of fiber transfer delay measurement based on phase quantization and delay synthesis.

A novel method, to the best of our knowledge, of fiber transfer delay (FTD) measurement based on phase quantization and delay synthesis is proposed and demonstrated. By detecting the differential phase shifts of a set of frequency-multiplied RF signals transmission through the fiber link with and without the FTD under the test, the ${2}\pi $2π phase ambiguity problem can be solved. To avoid the phase quantization error near the digital quantization boundary, a self-check and error-correction method is proposed so as to greatly improve the reliability of measurement. In the experiment, the measurement repeatability around 0.018 ps within a period of 80 s is achieved for a back-to-back fiber link, and a test resolution of 0.03 ps is proved with a motorized tunable delay line. The system is available for measurement of a large FTD range up to 100 µs with no dead zone.

Photonic generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading.

A novel (to the best of our knowledge) photonic approach for the generation of background-free frequency-doubled phase-coded microwave pulses with immunity to periodic power fading is proposed. By switching between the carrier suppressed double sideband (CS-DSB) modulation with reverse quadrature phase difference and carrier suppressed single sideband (CS-SSB) modulation, background-free phase-coded microwave pulses with frequency doubling of the local oscillator signal can be obtained. Thanks to the CS-DSB/CS-SSB modulation, the generated microwave pulses can be directly transmitted via single-mode fiber (SMF) without the influence of the periodic power fading. Since no filter is utilized, the proposed scheme owns a wide frequency tunable range. An experiment is conducted to verify the proposed approach. Two 1 Gbit/s phase-coded microwave pulses with center frequencies of 6 and 12 GHz are generated and successfully transmitted through a 25 km SMF. The pulse compression ratio and main-to-sidelobe ratio after the transmissions are measured as 13 and 8.84 dB, respectively.

Tunable OEO-based photonic RF receiver with image frequency rejection.

A tunable optoelectronic oscillator (OEO)-based photonic radio frequency (RF) heterodyne receiver with the capability of image frequency rejection is proposed and experimentally demonstrated. The OEO incorporating a dual-electrode Mach-Zehnder modulator (DEMZM) operates as a high quality local oscillator (LO) and a broadband mixer simultaneously, with one electrode port of the DEMZM used for the feedback of the generated LO signal and the other port for receiving external RF signals. Two channels of immediate frequency (IF) signals can be achieved in optical domain by beating the +1-order sideband of LO with the +1-order sideband of input RFs and beating the -1-order sideband of LO with the -1-order sideband of input RFs correspondingly. Through setting the bias angle of the DEMZM at π/4 or 5π/4 and quadrature combining of the two channels of the IF signal, the combined IF signal down-converted from the target RF signal gets enhanced due to the in-phase sum of two channels, while the combined IF signal down-converted from the image frequency can be rejected due to out-of-phase sum of two channels. In the experiment, the input RF signals with operating frequency range from 8.85 to 14.85 GHz are successfully received with the image rejection ratio reaching 42.6 dB, and the spurious free dynamic range of the system is better than 94.8 dB Hz2/3.

Trajectory variations of optical Bloch oscillations for Airy beams in transversely and longitudinally modulated photonic lattices.

We consider four types of modulated photonic lattices and numerically study the optical Bloch oscillations of Airy beams in these periodic structures. Our results demonstrate that in transversely modulated photonic lattices, the oscillation period increases and the amplitude reduces as the separation variation index increases. However, the increasing of the width variation index could lead to a consistent reduction of oscillation period and amplitude. On the other hand, in three kinds of longitudinally bent photonic lattices, the optical Bloch oscillation trajectories of Airy beams follow the waveguide shape perfectly within the whole propagation length. For another three kinds of refractive index longitudinally varied lattices, the optical Bloch oscillation period and amplitude are limited with the increasing of refractive index, contributing to the presentation of dramatically different Airy propagation trajectories. Our study proposes a new view to manipulate the Airy trajectory in periodic structures.

Nonlinear evolution of Airy-like beams generated by modulated waveguide arrays.

We numerically study the formation of modulated waveguide generated Airy-like beams and their subsequent evolution in homogeneous medium. The results show that the Airy-like beams could be generated from narrow Gaussian beams propagating in one-dimensional transverse separation modulated unbent, cosine bent, or logarithm bent waveguide arrays, respectively. The waveguide-generated Airy-like beams maintain their characteristics when propagating without nonlinearity or under the self-defocusing nonlinearity in homogeneous medium, while the beams are distorted under the self-focusing nonlinearity. The deformation depends on the waveguide bending and the outgoing angles of the Airy-like beams. Our results provide a new way to generate and manipulate the Airy-like beam.