Integrated line-by-line optical pulse shaper for high-fidelity and rapidly reconfigurable RF-filtering

Broadband microwave signal generation with programmable chirp shapes via low-speed electronics-controlled phase-modulated optical loop

Broadband microwave signals with customized chirp shapes are highly captivating in practical applications. Compared with electronic technology, photonic solutions are superior in bandwidth but suffer from flexible and rapid manipulation of chirp shape or frequency. Here, we demonstrate a concept for generating broadband microwave signals with programmable chirp shapes. Our realization is based on a recirculating phase-modulated optical loop to ultrafast manipulate the laser frequency, which breaks the limitation of the buildup time of the laser from spontaneous emission. Through heterodyne beating the frequency-agile lasers with a continuous-wave laser, microwave signals with ultrafast and programmable chirp shapes are generated. Besides, signal parameters, such as bandwidth, center frequency, and temporal duration, can be reconfigured. In the experiment, highly coherent microwave signals with various customized chirp shapes are generated, where the time resolution for programming the chirp shape is 649 ps. This flexible frequency manipulation characteristic holds promise for many applications, including LiDAR, broadband radar systems, and spectroscopy.

Versatile parallel signal processing with a scalable silicon photonic chip

Silicon photonic signal processors promise a new generation of signal processing hardware with significant advancements in processing bandwidth, low power consumption, and minimal latency. Programmable silicon photonic signal processors, facilitated by tuning elements, can reduce hardware development cycles and costs. However, traditional programmable photonic signal processors based on optical switches face scalability and performance challenges due to control complexity and transmission losses. Here, we propose a scalable parallel signal processor on silicon for versatile applications by interleaving wavelength and temporal optical dimensions. Additionally, it incorporates ultra-low-loss waveguides and low-phase-error optical switch techniques, achieving an overall insertion loss of 10 dB. This design offers low loss, high scalability, and simplified control, enabling advanced functionalities such as accurate microwave reception, narrowband microwave photonic filtering, wide-bandwidth arbitrary waveform generation, and high-speed parallel optical computing without the need for tuning elements calibration. Our programmable parallel signal processor demonstrates advantages in both scale and performance, marking a significant advancement in large-scale, high-performance, multifunctional photonic systems.

Silicon photonic microresonator-based high-resolution line-by-line pulse shaping

Optical pulse shaping stands as a formidable technique in ultrafast optics, radio-frequency photonics, and quantum communications. While existing systems rely on bulk optics or integrated platforms with planar waveguide sections for spatial dispersion, they face limitations in achieving finer (few- or sub-GHz) spectrum control. These methods either demand considerable space or suffer from pronounced phase errors and optical losses when assembled to achieve fine resolution. Addressing these challenges, we present a foundry-fabricated six-channel silicon photonic shaper using microresonator filter banks with inline phase control and high spectral resolution. Leveraging existing comb-based spectroscopic techniques, we devise a system to mitigate thermal crosstalk and enable the versatile use of our on-chip shaper. Our results demonstrate the shaper's ability to phase-compensate six comb lines at tunable channel spacings of 3, 4, and 5 GHz. Specifically, at a 3 GHz channel spacing, we showcase the generation of arbitrary waveforms in the time domain. This scalable design and control scheme holds promise in meeting future demands for high-precision spectral shaping capabilities.

Photonic signal processor based on a Kerr microcomb for real-time video image processing

Signal processing has become central to many fields, from coherent optical telecommunications, where it is used to compensate signal impairments, to video image processing. Image processing is particularly important for observational astronomy, medical diagnosis, autonomous driving, big data and artificial intelligence. For these applications, signal processing traditionally has mainly been performed electronically. However these, as well as new applications, particularly those involving real time video image processing, are creating unprecedented demand for ultrahigh performance, including high bandwidth and reduced energy consumption. Here, we demonstrate a photonic signal processor operating at 17 Terabits/s and use it to process video image signals in real-time. The system processes 400,000 video signals concurrently, performing 34 functions simultaneously that are key to object edge detection, edge enhancement and motion blur. As compared with spatial-light devices used for image processing, our system is not only ultra-high speed but highly reconfigurable and programable, able to perform many different functions without any change to the physical hardware. Our approach is based on an integrated Kerr soliton crystal microcomb, and opens up new avenues for ultrafast robotic vision and machine learning.

Signal processing is key to communications and video image processing for astronomy, medical diagnosis, autonomous driving, big data and AI. Menxi Tan and colleagues report a photonic processor operating at 17Tb/s for ultrafast robotic vision and machine learning.

Comparison of Microcomb-Based Radio-Frequency Photonic Transversal Signal Processors Implemented with Discrete Components Versus Integrated Chips

RF photonic transversal signal processors, which combine reconfigurable electrical digital signal processing and high-bandwidth photonic processing, provide a powerful solution for achieving adaptive high-speed information processing. Recent progress in optical microcomb technology provides compelling multi-wavelength sources with a compact footprint, yielding a variety of microcomb-based RF photonic transversal signal processors with either discrete or integrated components. Although they operate based on the same principle, the processors in these two forms exhibit distinct performances. This paper presents a comparative investigation of their performances. First, we compare the performances of state-of-the-art processors, focusing on the processing accuracy. Next, we analyze various factors that contribute to the performance differences, including the tap number and imperfect response of experimental components. Finally, we discuss the potential for future improvement. These results provide a comprehensive comparison of microcomb-based RF photonic transversal signal processors implemented using discrete and integrated components and provide insights for their future development.

High-resolution wide-band optical frequency comb control using stimulated Brillouin scattering

We introduce a technique to manipulate an optical frequency comb on a line-by-line basis using stimulated Brillouin scattering (SBS). The narrow-linewidth SBS process has been used to address individual lines in optical frequency combs, but previous demonstrations required a dedicated laser to modulate each comb tooth, prohibiting complete comb control. Here, we use a pair of frequency shifting fiber optic loops to generate both an optical frequency comb and a train of frequency-locked pulses that can be used to manipulate the comb via SBS. This approach enables control of the entire frequency comb using a single seed laser without active frequency locking. To demonstrate the versatility of this technique, we generate and manipulate a comb consisting of 50 lines with 200 MHz spacing. By using polarization pulling assisted SBS, we achieve a modulation depth of 30 dB. This represents a scalable approach to control large numbers of comb teeth with high resolution using standard fiber-optic components.

Optical frequency comb and picosecond pulse generation based on a directly modulated microcavity laser

Optical frequency comb (OFC) and picosecond pulse generation are demonstrated experimentally based on a directly modulated AlGaInAs/InP square microcavity laser. With the merit of a high electro-optics modulation response of the microcavity laser, power-efficient OFCs with good flatness are produced. Ten 8-GHz-spaced optical tones with power fluctuation less than 3 dB are obtained based on the laser modulated by a sinusoidal signal. Moreover, the comb line number is enhanced to 20 by eliminating the nonlinear dynamics through optical injection locking. Owing to the high coherence of the OFC originating from the directly modulated microcavity laser, a 6.8 ps transform-limited pulse is obtained through dispersion compensation. The optical pulse is further compressed to 1.3 ps through the self-phase modulation effect in high nonlinear fiber.

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 diagnosis^1-7. 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.

Reconfigurable radiofrequency filters based on versatile soliton microcombs

The rapidly maturing integrated Kerr microcombs show significant potential for microwave photonics. Yet, state-of-the-art microcomb-based radiofrequency filters have required programmable pulse shapers, which inevitably increase the system cost, footprint, and complexity. Here, by leveraging the smooth spectral envelope of single solitons, we demonstrate microcomb-based radiofrequency filters free from any additional pulse shaping. More importantly, we achieve all-optical reconfiguration of the radiofrequency filters by exploiting the intrinsically rich soliton configurations. Specifically, we harness the perfect soliton crystals to multiply the comb spacing thereby dividing the filter passband frequencies. Also, the versatile spectral interference patterns of two solitons enable wide reconfigurability of filter passband frequencies, according to their relative azimuthal angles within the round-trip. The proposed schemes demand neither an interferometric setup nor another pulse shaper for filter reconfiguration, providing a simplified synthesis of widely reconfigurable microcomb-based radiofrequency filters.

For microcomb-based radiofrequency filters pulse shapers are required, which increase the system cost, footprint, and complexity. Here, the authors bypass this need by exploiting versatile soliton states inherent in microresonator and achieve reconfigurable radiofrequency filters.

Programmable broadband optical field spectral shaping with megahertz resolution using a simple frequency shifting loop

Controlling the temporal and spectral properties of light is crucial for many applications. Current state-of-the-art techniques for shaping the time- and/or frequency-domain field of an optical waveform are based on amplitude and phase linear spectral filtering of a broadband laser pulse, e.g., using a programmable pulse shaper. A well-known fundamental constraint of these techniques is that they can be hardly scaled to offer a frequency resolution better than a few GHz. Here, we report an approach for user-defined optical field spectral shaping using a simple scheme based on a frequency shifting optical loop. The proposed scheme uses a single monochromatic (CW) laser, standard fiber-optics components and low-frequency electronics. This technique enables efficient synthesis of hundreds of optical spectral components, controlled both in phase and in amplitude, with a reconfigurable spectral resolution from a few MHz to several tens of MHz. The technique is applied to direct generation of arbitrary radio-frequency waveforms with time durations exceeding 100 ns and a detection-limited frequency bandwidth above 25 GHz.

Full spectral and temporal control of light has a multitude of applications but is often limited in frequency resolution. The authors implement a scheme using a frequency shifting optical loop for optical field spectral shaping with a high degree of control and megahertz resolution

Microwave photonic multiband filter with independently tunable passband spectral properties

Multiband RF filters with independently controllable passbands are an essential component in dynamic multiband RF communications. Unfortunately, even a fixed multiband RF filter without the capability to adjust the passband properties individually is very difficult to achieve using either RF electronics or microwave photonic technologies. In microwave photonic approaches, the critical limitation is the close relationship between passbands-the tuning of one passband leads to a change in another, hindering the ability to independently control each passband. In this Letter, a programmable microwave photonic multiband filter with full control of amplitude, frequency, bandwidth, group delay slope, and the spectral shape of each passband has been experimentally demonstrated. A multiband filter design algorithm has also been developed that considers each RF passband as an individual, then uses inverse Fourier transform and filter design rule to determine the corresponding optical parameters and combines a series of shaped cosine functions to achieve the desired RF properties.

Space-time focusing in a highly multimode fiber via optical pulse shaping

Fields propagating through a highly scattering material will be distorted in both space (intensity speckles) and time (spectral and temporal speckles), inhibiting tasks such as imaging and communication in both the optical and radio frequency regions. In optics, research thus far has demonstrated spatial focusing, image transmission, and short pulse delivery through bulk scattering materials and multimode fibers by taking advantage of spatial wavefront-shaping techniques. Here, we exploit spectral phase shaping for reference-free characterization of spectral and temporal speckle, and space-time focusing of broadband ultrafast pulses distorted by modal dispersion in a multimode fiber. We show that temporal speckle fields at different multimode fiber output locations are uncorrelated and demonstrate the ability to focus a short pulse at a specific output spatial location, while keeping the field at other output locations noise-like, offering opportunities to expand multimode fiber imaging and communication capacity.

Rapidly reconfigurable high-fidelity optical arbitrary waveform generation in heterogeneous photonic integrated circuits

This paper demonstrates rapidly reconfigurable, high-fidelity optical arbitrary waveform generation (OAWG) in a heterogeneous photonic integrated circuit (PIC). The heterogeneous PIC combines advantages of high-speed indium phosphide (InP) modulators and low-loss, high-contrast silicon nitride (Si₃N₄) arrayed waveguide gratings (AWGs) so that high-fidelity optical waveform syntheses with rapid waveform updates are possible. The generated optical waveforms spanned a 160 GHz spectral bandwidth starting from an optical frequency comb consisting of eight comb lines separated by 20 GHz channel spacing. The Error Vector Magnitude (EVM) values of the generated waveforms were approximately 16.4%. The OAWG module can rapidly and arbitrarily reconfigure waveforms upon every pulse arriving at 2 ns repetition time. The result of this work indicates the feasibility of truly dynamic optical arbitrary waveform generation where the reconfiguration rate or the modulator bandwidth must exceed the channel spacing of the AWG and the optical frequency comb.