Cost-effective equalization of electro-optic frequency combs in a Sagnac interferometer.

We present a cost-effective electro-optic frequency comb generation and equalization method using a single phase modulator inserted in a Sagnac interferometer layout. The equalization relies on the interference of comb lines generated in both clockwise and counter-clockwise directions. Such a system is capable of providing flat-top combs with flatness values comparable with other approaches proposed in literature, yet offering a simplified synthesis and reduced complexity. The frequency range of operation at hundreds of MHz renders this scheme particularly interesting for some sensing and spectroscopy applications.

Tunable photo-induced second-harmonic generation in a mode-engineered silicon nitride microresonator.

All-optical poling enables reconfigurable and efficient quasi-phase-matching for second-order parametric frequency conversion in silicon nitride integrated photonics. Here, we report broadly tunable milliwatt-level second-harmonic generation in a small free spectral range silicon nitride microresonator, where the pump and its second-harmonic are both always on the fundamental mode. By carefully engineering the light coupling region between the bus and microresonator, we simultaneously achieve critical coupling of the pump as well as efficient extraction of second-harmonic light from the cavity. Thermal tuning of second-harmonic generation is demonstrated with an integrated heater in a frequency grid of 47 GHz over a 10 nm band.

Integrated Backward Second-Harmonic Generation through Optically Induced Quasi-Phase-Matching.

Quasi-phase-matching for efficient backward second-harmonic generation requires sub-µm poling periods, a nontrivial fabrication feat. For the first time, we report integrated first-order quasiphase-matched backward second-harmonic generation enabled by seeded all-optical poling. The self-organized grating inscription circumvents all fabrication challenges. We compare backward and forward processes and explain how grating period influences the conversion efficiency. These results showcase unique properties of the coherent photogalvanic effect and how it can bring new nonlinear functionalities to integrated photonics.

Deep-learning-assisted communication capacity enhancement by non-orthogonal state recognition of structured light.

In light of pending capacity crunch in information era, orbital-angular-momenta-carrying vortex beams are gaining traction thanks to enlarged transmission capability. However, high-order beams are confronted with fundamental limits of nontrivial divergence or distortion, which consequently intensifies research on new optical states like low-order fractional vortex beams. Here, we experimentally demonstrate an alternative mean to increase the capacity by simultaneously utilizing multiple non-orthogonal states of structured light, challenging a prevailing view of using orthogonal states as information carriers. Specifically, six categories of beams are jointly recognized with accuracy of >99% by harnessing an adapted deep neural network, thus providing the targeted wide bandwidth. We then manifest the efficiency by sending/receiving a grayscale image in 256-ary mode encoding and shift keying schemes, respectively. Moreover, the well-trained model is able to realize high fidelity recognition (accuracy >0.8) onto structured beams under unknown turbulence and restricted receiver aperture size. To gain insights of the framework, we further interpret the network by revealing the contributions of intensity signals from different positions. This work holds potential in intelligence-assisted large-capacity and secure communications, meeting ever growing demand of daily information bandwidth.

Temporal Talbot effect of optical dark pulse trains.

The temporal Talbot effect describes the periodic self-imaging of an optical pulse train along dispersive propagation. This is well studied in the context of bright pulse trains, where identical or multiplied pulse trains with uniform bright waveforms can be created. However, the temporal self-imaging has remained unexplored in the dark pulse regime. Here, we disclose such a phenomenon for optical dark pulse trains, and discuss the comparison with their bright pulse counterparts. It is found that the dark pulse train also revives itself at the Talbot length. For higher-order fractional self-imaging, a mixed pattern of bright and dark pulses is observed, as a result of the interference between the Talbot pulses and the background. Such unconventional behaviors are theoretically predicted and experimentally demonstrated by using programmable spectral shaping as well as by optical fiber propagation.

Investigation of temporal Talbot operation in a conventional optical tapped delay line structure.

We propose a novel scheme of temporal Talbot effect achieving optical pulse train repetition-rate multiplication in a conventional tapped delay line structure. While it is generally used for spectral amplitude filtering, we show that such architecture could also be configured for spectral phase-only filtering, as well as for a combination of amplitude and phase filtering regimes. We theoretically derive and numerically simulate the working principle of the concept, followed by a proof-of-principle experimental demonstration using an off-the-shelf Mach-Zehnder delay line interferometer, which corresponds to the simplest version of the proposed structure. We address the efficiency, and potential performance degradation in the presence of power imbalance and delay line length inaccuracy of the architecture, together with applied phase error.

Linearly chirped mid-infrared supercontinuum in all-normal-dispersion chalcogenide photonic crystal fibers.

We demonstrate all-normal dispersion supercontinuum generation in chalcogenide photonic crystal fibers pumped at 2070-2080 nm with a femtosecond fiber laser. At 2.9 kW peak power, the generated supercontinuum has a 3 dB bandwidth of 27.6 THz and -20 dB bandwidth of 75.5 THz. We experimentally investigated the supercontinuum evolution inside our sample fiber at various peak powers and fiber lengths and study the impact of fiber water absorption on the generated supercontinuum spectrum. Multiple tests with fiber length- ranging from 0.34 to 10 cm-provide insight on pulse evolution along fiber length. Our simulations, which utilizes the generalized nonlinear Schrodinger equation model, match perfectly the experiments for all tested pump powers and fiber lengths, and confirm that the output pulse has a linear chirp, allowing linear pulse compression.

Talbot effect on orbital angular momentum beams: azimuthal intensity repetition-rate multiplication.

We propose and experimentally demonstrate the azimuthal Talbot effect on orbital angular momentum (OAM) beams. By applying predetermined phases to a number of OAM beams carrying different topological charges, the intensity petal is self-imaged in the azimuthal angle, with arbitrary azimuthal repetition-rate multiplication. The close analogy between temporal and azimuthal Talbot self-imaging is studied. In addition, the effect of amplitude apodization of the OAM spectrum on the resulting intensity pattern, and the azimuthal Talbot effect on Laguerre-Gaussian beams of the same radial indices, are experimentally investigated. All of our experimental images are in excellent agreement with simulation results.

Fiber fuse in chalcogenide photonic crystal fibers.

We observe fiber fuse in tapered GeAsSe photonic crystal fibers (PCF) at around 7 MW/cm2 of intra-core intensity. Vertically cleaved facets from the un-tapered regions and the tapered regions were imaged. The images show shallow voids of different shapes confined to the fiber core. After longitudinally polishing a segment of the PCF, we imaged the PCF internal structure's top view, revealing the fuse voids' geometries and periodicity. In addition, fiber fuse was terminated in one PCF sample by a fast laser shutdown, hence saving a small segment from catastrophic damage. Four-wave-mixing was performed on this transmissive segment to estimate the dispersion. The results yielded an evident hole-pitch ratio change after fiber fuse. To our knowledge, this is the first report of fiber fuse on non-silica glass fibers and the first study of its aftermath on this un-destroyed segment of PCFs.

Large-scale photonic computing with nonlinear disordered media.

Neural networks find widespread use in scientific and technological applications, yet their implementations in conventional computers have encountered bottlenecks due to ever-expanding computational needs. Photonic computing is a promising neuromorphic platform with potential advantages of massive parallelism, ultralow latency and reduced energy consumption but mostly for computing linear operations. Here we demonstrate a large-scale, high-performance nonlinear photonic neural system based on a disordered polycrystalline slab composed of lithium niobate nanocrystals. Mediated by random quasi-phase-matching and multiple scattering, linear and nonlinear optical speckle features are generated as the interplay between the simultaneous linear random scattering and the second-harmonic generation, defining a complex neural network in which the second-order nonlinearity acts as internal nonlinear activation functions. Benchmarked against linear random projection, such nonlinear mapping embedded with rich physical computational operations shows improved performance across a large collection of machine learning tasks in image classification, regression and graph classification. Demonstrating up to 27,648 input and 3,500 nonlinear output nodes, the combination of optical nonlinearity and random scattering serves as a scalable computing engine for diverse applications.

Bright and dark Talbot pulse trains on a chip.

Temporal Talbot effect, the intriguing phenomenon of the self-imaging of optical pulse trains, is extensively investigated using macroscopic components. However, the ability to manipulate pulse trains, either bright or dark, through the Talbot effect on integrated photonic chips to replace bulky instruments has rarely been reported. Here, we design and experimentally demonstrate a proof-of-principle integrated silicon nitride device capable of imprinting the Talbot phase relation onto in-phase optical combs and generating the two-fold self-images at the output. We show that the GHz-repetition-rate bright and dark pulse trains can be doubled without affecting their spectra as a key feature of the temporal Talbot effect. The designed chip can be electrically tuned to switch between pass-through and repetition-rate-multiplication outputs and is compatible with other related frequencies. The results of this work lay the foundations for the large-scale system-on-chip photonic integration of Talbot-based pulse multipliers, enabling the on-chip flexible up-scaling of pulse trains' repetition rate without altering their amplitude spectra.

Photo-induced cascaded harmonic and comb generation in silicon nitride microresonators.

Silicon nitride (Si3N4) is an ever-maturing integrated platform for nonlinear optics but mostly considered for third-order [χ(3)] nonlinear interactions. Recently, second-order [χ(2)] nonlinearity was introduced into Si3N4 via the photogalvanic effect, resulting in the inscription of quasi-phase-matched χ(2) gratings. However, the full potential of the photogalvanic effect in microresonators remains largely unexplored for cascaded effects. Here, we report combined χ(2) and χ(3) nonlinear effects in a normal dispersion Si3N4 microresonator. We demonstrate that the photo-induced χ(2) grating also provides phase-matching for the sum-frequency generation process, enabling the initiation and successive switching of primary combs. In addition, the doubly resonant pump and second-harmonic fields allow for effective third-harmonic generation, where a secondary optically written χ(2) grating is identified. Last, we reach a broadband microcomb state evolved from the sum-frequency-coupled primary comb. These results expand the scope of cascaded effects in microresonators.

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.