Free-standing microscale photonic lantern spatial mode (De-)multiplexer fabricated using 3D nanoprinting.

Photonic lantern (PL) spatial multiplexers show great promise for a range of applications, such as future high-capacity mode division multiplexing (MDM) optical communication networks and free-space optical communication. They enable efficient conversion between multiple single-mode (SM) sources and a multimode (MM) waveguide of the same dimension. PL multiplexers operate by facilitating adiabatic transitions between the SM arrayed space and the single MM space. However, current fabrication methods are forcing the size of these devices to multi-millimeters, making integration with micro-scale photonic systems quite challenging. The advent of 3D micro and nano printing techniques enables the fabrication of freestanding photonic structures with a high refractive index contrast (photopolymer-air). In this work we present the design, fabrication, and characterization of a 6-mode mixing, 375 µm long PL that enables the conversion between six single-mode inputs and a single six-mode waveguide. The PL was designed using a genetic algorithm based inverse design approach and fabricated directly on a 7-core fiber using a commercial two-photon polymerization-based 3D printer and a photopolymer. Although the waveguides exhibit high index contrast, low insertion loss (-2.6 dB), polarization dependent (-0.2 dB) and mode dependent loss (-4.4 dB) were measured.

Modulation-free laser stabilization technique using integrated cavity-coupled Mach-Zehnder interferometer.

Stable lasers play a significant role in precision optical systems where an electro-optic laser frequency stabilization system, such as the Pound-Drever-Hall technique, measures laser frequency and actively stabilizes it by comparing it to a frequency reference. Despite their excellent performance, there has been a trade-off between complexity, scalability, and noise measurement sensitivity. Here, we propose and experimentally demonstrate a modulation-free laser stabilization method using an integrated cavity-coupled Mach-Zehnder interferometer as a frequency noise discriminator. The proposed architecture maintains the sensitivity of the Pound-Drever-Hall architecture without the need for any modulation. This significantly simplifies the architecture and makes miniaturization into an integrated photonic platform easier. The implemented chip suppresses the frequency noise of a semiconductor laser by 4 orders-of-magnitude using an on-chip silicon microresonator with a quality factor of 2.5 × 106. The implemented passive photonic chip occupies an area of 0.456 mm2 and is integrated on AIM Photonics 100 nm silicon-on-insulator process.

Pulcherriminic acid modulates iron availability and protects against oxidative stress during microbial interactions.

Siderophores are soluble or membrane-embedded molecules that bind the oxidized form of iron, Fe(III), and play roles in iron acquisition by microorganisms. Fe(III)-bound siderophores bind to specific receptors that allow microbes to acquire iron. However, certain soil microbes release a compound (pulcherriminic acid, PA) that, upon binding to Fe(III), forms a precipitate (pulcherrimin) that apparently functions by reducing iron availability rather than contributing to iron acquisition. Here, we use Bacillus subtilis (PA producer) and Pseudomonas protegens as a competition model to show that PA is involved in a peculiar iron-managing system. The presence of the competitor induces PA production, leading to precipitation of Fe(III) as pulcherrimin, which prevents oxidative stress in B. subtilis by restricting the Fenton reaction and deleterious ROS formation. In addition, B. subtilis uses its known siderophore bacillibactin to retrieve Fe(III) from pulcherrimin. Our findings indicate that PA plays multiple roles by modulating iron availability and conferring protection against oxidative stress during inter-species competition.

Redox Reporter - Ligand Competition to Support Signaling in the Cocaine-Binding Electrochemical Aptamer-Based Biosensor.

Electrochemical aptamer-based (E-AB) biosensors have demonstrated capabilities in monitoring molecules directly in undiluted complex matrices and in the body with the hopes of addressing personalized medicine challenges. This sensing platform relies on an electrode-bound, redox-reporter-modified aptamer. The electrochemical signal is thought to originate from the aptamer undergoing a binding-induced conformational change capable of moving the redox reporter closer to the electrode surface. While this is the generally accepted mechanism, it is notable that there is limited evidence demonstrating conformational change or distance-dependent change in electron transfer rates in E-AB sensors. In response, we investigate here the signal transduction of the well-studied cocaine-binding aptamer with different analytical methods and found that this sensor relies on a redox-reporter - ligand competition mechanism rather than a ligand-induced structure formation mechanism. Our results show that the covalently bound redox reporter, methylene blue, binds at or near the ligand binding site on the aptamer resulting in a folded conformation of the cocaine-binding aptamer. Addition of ligand then competes with the redox reporter for binding, altering its electron transfer rate. While we show this for the cocaine-binding aptamer, given the prevalence of methylene blue in E-AB sensors, a similar competition-based may occur in other systems.

Ordering Eyeglasses Using 3D Head Scan Technology versus Established Online and Storefront Clinic Methods.

SIGNIFICANCE: This study investigates how a new smartphone scanning technology compares with established online and storefront vendors in providing remote measurement and adjustment of prescription eyeglasses. PURPOSE: This study aimed to evaluate a new technology for ordering prescription eyeglasses online. METHODS: Thirty participants with 2.00 to 2.75 D of presbyopia (aged 49 to 74 years) were asked to order eyeglasses with progressive addition lenses from four vendors: one online vendor using a new head scan technology (Topology, San Francisco, CA), two established Web site-based vendors (vendors A and B), and one storefront vendor: Université de Montréal's Vision Clinic (UMVC). The resulting measurements were compared with those of opticians. Participant-reported preferences on visual and physical comfort of eyeglasses were collected after 15-minute trials of eyeglasses from each vendor. RESULTS: Pupillary half-distance measured with Topology matched optician measurements, but online vendors A and B diverged (mean difference, - 0.80 mm [ Z = -2.79; P = .005]). Topology and vendor B segment addition heights were similar to optician measurements, but vendor A diverged (mean, -1.40 mm [ Z = -2.58; P = .01]). The personalization parameter values obtained with Topology were different from optician measurements for pantoscopic angle (-5.30° [ Z = -4.12; P < .001]) and face wrap angle (+1.25° [ Z = -2.94; P = .003]). The UMVC eyeglasses scored best for adjustment (8.71/10 [ Z = -5.53; P < .001]), with Topology coming second (7.23/10). Topology scores were equivalent to UMVC scores for all eight items of patient-reported preferences (nonparametric Friedman analysis of variance, P < .05). CONCLUSIONS: Basic lens centration measurements obtained with Topology compare well with those of opticians, but some aspects of the methodology for measuring personalization parameters could be improved. In comparison with two established online vendors, resulting measurements with Topology are more consistent. Initial wearer satisfaction with Topology eyeglasses was also better.

Acting as a Molecular Tailor: Dye Structural Modifications for Improved Sensitivity toward Lysophosphatidic Acids Sensing.

Lysophosphatidic acids (LPA) are key biomarkers for several physiological processes, the monitoring of which can provide insights into the host's health. Common lab-based techniques for their detection are cumbersome, expensive, and necessitate specialized personnel to operate. LPA-sensitive fluorescent probes have been described, albeit for nonaqueous conditions, which impedes their use in biological matrices. In this paper, we explore in detail the influence of structure on the extent of aggregation-induced fluorescence quenching using specially synthesized styrylpyridinium dyes bearing structural adaptations to bestow them enhanced affinity toward LPA in aqueous media. Spectroscopic investigations supported by time-resolved fluorimetry revealed the contribution of excimer formation to the fluorescence quenching mechanism displayed by the fluorescent probes. Experimental observations of the influence of structure on detection sensitivity were supported by DFT calculations.

Pushing the Limits of Surface-Enhanced Raman Spectroscopy (SERS) with Deep Learning: Identification of Multiple Species with Closely Related Molecular Structures.

Raman spectroscopy is a non-destructive and label-free molecular identification technique capable of producing highly specific spectra with various bands correlated to molecular structure. Moreover, the enhanced detection sensitivity offered by surface-enhanced Raman spectroscopy (SERS) allows analyzing mixtures of related chemical species in a relatively short measurement time. Combining SERS with deep learning algorithms allows in some cases to increase detection and classification capabilities even further. The present study evaluates the potential of applying deep learning algorithms to SERS spectroscopy to differentiate and classify different species of bile acids, a large family of molecules with low Raman cross sections and molecular structures that often differ by a single hydroxyl group. Moreover, the study of these molecules is of interest for the medical community since they have distinct pathological roles and are currently viewed as potential markers of gut microbiome imbalances. A convolutional neural network model was developed and used to classify SERS spectra from five bile acid species. The model succeeded in identifying the five analytes despite very similar molecular structures and was found to be reliable even at low analyte concentrations.

Peta-bit-per-second optical communications system using a standard cladding diameter 15-mode fiber.

Data rates in optical fiber networks have increased exponentially over the past decades and core-networks are expected to operate in the peta-bit-per-second regime by 2030. As current single-mode fiber-based transmission systems are reaching their capacity limits, space-division multiplexing has been investigated as a means to increase the per-fiber capacity. Of all space-division multiplexing fibers proposed to date, multi-mode fibers have the highest spatial channel density, as signals traveling in orthogonal fiber modes share the same fiber-core. By combining a high mode-count multi-mode fiber with wideband wavelength-division multiplexing, we report a peta-bit-per-second class transmission demonstration in multi-mode fibers. This was enabled by combining three key technologies: a wideband optical comb-based transmitter to generate highly spectral efficient 64-quadrature-amplitude modulated signals between 1528 nm and 1610 nm wavelength, a broadband mode-multiplexer, based on multi-plane light conversion, and a 15-mode multi-mode fiber with optimized transmission characteristics for wideband operation.

Photonic lantern tip/tilt detector for adaptive optics systems.

In this work, we demonstrate a four-core multicore fiber photonic lantern tip/tilt wavefront sensor. To diagnose the low-order Zernike aberrations, we exploit the ability of the photonic lantern to encode the characteristics of a complex incoming beam at the multimode facet of the sensor to intensity distributions at the multicore fiber output. Here, we provide a comprehensive numerical analysis capable of predicting the performance of fabricated devices and experimentally demonstrate the concept. Two receiver architectures are implemented to discern tip/tilt information by (i) imaging the four-core fiber facet on a 2D detector and (ii) direct power measurement of the single mode outputs using a multicore fiber multiplexer and photodetectors. For both receiver schemes, an angular detection window of ∼0.4∘ at 1064 nm can be achieved. Our results are expected to further facilitate the development of intensity-based fiber wavefront sensors for adaptive optics systems.

Experimental characterization of group and phase delays induced by bending and twisting in multi-core fibers.

The local variations of group and phase propagation delays induced by bending and twisting a coupled core three-core fiber are experimentally characterized, for the first time, to the best of our knowledge, along the fiber length, with millimeter-scale spatial resolution. The measurements are performed by means of spectral correlation analysis on the fiber's Rayleigh backscattered signal, enabling for a distributed measurement of the perturbation effects along the fiber length. A mathematical model validating the experimental results is also reported.

Demonstration of terabit coherent on-chip optical interconnects employing mode-division multiplexing.

We experimentally demonstrate a net capacity per wavelength of 1.23 Tb/s with 30 GBaud 16-ary quadrature amplitude modulation (16-QAM) mode-division multiplexing (MDM) signals over a single silicon-on-insulator (SOI) multimode waveguide for optical interconnects employing $11 \times 11$ multiple-in-multiple-out (MIMO) digital signal processing. In order to simplify the receiver architecture for coherent optical interconnects, we further propose and evaluate an on-chip self-homodyne coherent detection (SHCD) scheme. In the experiment, 30 Gbaud quadrature phase shift keying (QPSK) signals carried by 10 waveguide modes are successfully recovered with bit error rates (BERs) below 7% forward error correction (FEC) threshold using the pilot tone delivered by ${{\rm TE}_0}$ mode as a local oscillator. Around 10% penalty on error vector magnitude (EVM) is observed due to modal cross talk compared to homodyne detection.

Machine Learning Enables Selection of Epistatic Enzyme Mutants for Stability Against Unfolding and Detrimental Aggregation.

Machine learning (ML) has pervaded most areas of protein engineering, including stability and stereoselectivity. Using limonene epoxide hydrolase as the model enzyme and innov'SAR as the ML platform, comprising a digital signal process, we achieved high protein robustness that can resist unfolding with concomitant detrimental aggregation. Fourier transform (FT) allows us to take into account the order of the protein sequence and the nonlinear interactions between positions, and thus to grasp epistatic phenomena. The innov'SAR approach is interpolative, extrapolative and makes outside-the-box, predictions not found in other state-of-the-art ML or deep learning approaches. Equally significant is the finding that our approach to ML in the present context, flanked by advanced molecular dynamics simulations, uncovers the connection between epistatic mutational interactions and protein robustness.

Time reversed optical waves by arbitrary vector spatiotemporal field generation.

Lossless linear wave propagation is symmetric in time, a principle which can be used to create time reversed waves. Such waves are special "pre-scattered" spatiotemporal fields, which propagate through a complex medium as if observing a scattering process in reverse, entering the medium as a complicated spatiotemporal field and arriving after propagation as a desired target field, such as a spatiotemporal focus. Time reversed waves have previously been demonstrated for relatively low frequency phenomena such as acoustics, water waves and microwaves. Many attempts have been made to extend these techniques into optics. However, the much higher frequencies of optics make for very different requirements. A fully time reversed wave is a volumetric field with arbitrary amplitude, phase and polarisation at every point in space and time. The creation of such fields has not previously been possible in optics. We demonstrate time reversed optical waves with a device capable of independently controlling all of light's classical degrees of freedom simultaneously. Such a class of ultrafast wavefront shaper is capable of generating a sequence of arbitrary 2D spatial/polarisation wavefronts at a bandwidth limited rate of 4.4 THz. This ability to manipulate the full field of an optical beam could be used to control both linear and nonlinear optical phenomena.

Multiport swept-wavelength interferometer with laser phase noise mitigation employing a broadband ultra-weak FBG array.

Optical vector network analyzers (OVNAs) based on swept-wavelength interferometry are applied widely in optical metrology and sensing to measure the complex transfer functions of optical components, devices, and fibers. Phase noise from laser sweep nonlinearities degrades the measurement quality as the distance increases and limits the usage of the OVNA in characterizing systems with long impulse responses as required in space-division multiplexing links with a high mode count or in the presence of large modal differential group delay (DGD). In this Letter, we use a densely distributed broadband ultra-weak fiber Bragg grating array to directly measure the distortion due to phase noise at a 5-m increment up to 400 m and use this measured data to directly eliminate the distortion. We experimentally extend the measurement range of the swept-wavelength OVNA over 400 m and successfully characterize a 2-km six-mode multimode fiber link with an accumulated impulse response as wide as 20 ns.

Carrier-less phase retrieval receiver leveraging digital upsampling.

Phase retrieval (PR) receivers can reconstruct the full electrical field of the signal using only intensity measurements without any optical carrier. In this Letter, we investigate the requirement of digital upsampling and receiver bandwidth of the PR receiver based on alternative projection employing a dispersive element. An iteration scheme averaging the interleaved upsampled symbols to maintain two samples per symbol for the estimated complex-valued signal is proposed and experimentally demonstrated with fast algorithm convergence. The PR uses a modified Gerchberg-Saxton algorithm. Experimentally, we measure Nyquist-shaped 30-GBaud quadrature phase shift keying signals after 55-km single-mode fiber transmission using only 110 and 250 iterations to reach, respectively, the 20% and 7% forward-error correction threshold levels.

Thinking outside the shell: novel sensors designed from plasmon-enhanced fluorescent concentric nanoparticles.

The alteration of photophysical properties of fluorophores in the vicinity of a metallic nanostructure, a phenomenon termed plasmon- or metal-enhanced fluorescence (MEF), has been investigated extensively and used in a variety of proof-of-concept demonstrations over the years. A particularly active area of development in this regard has been the design of nanostructures where fluorophore and metallic core are held in a stable geometry that imparts improved luminosity and photostability to a plethora of organic fluorophores. This minireview presents an overview of MEF-based concentric core-shell sensors developed in the past few years. These architectures expand the range of applications of nanoparticles (NPs) beyond the uses possible with fluorescent molecules. Design aspects that are being described include the influence of the nanocomposite structure on MEF, notably the dependence of fluorescence intensity and lifetime on the distance to the plasmonic core. The chemical composition of nanocomposites as a design feature is also discussed, taking as an example the use of non-noble plasmonic metals such as indium as core materials to enhance multiple fluorophores throughout the UV-Vis range and tune the sensitivity of halide-sensing fluorophores operating on the principle of collisional quenching. Finally, the paper describes how various solid substrates can be functionalized with MEF-based nanosensors to bestow them with intense and photostable pH-sensitive properties for use in fields such as medical therapy and diagnostics, dentistry, biochemistry and microfluidics.

Phase retrieval with fast convergence employing parallel alternative projections and phase reset for coherent communications.

Phase-retrieval (PR) receivers can reconstruct complex-valued signals using only direct detection without the use of any optical carriers. We propose and demonstrate two PR receiver solutions with faster and better convergence. First, we demonstrate a PR receiver based on parallel alternative projections that are produced by propagating the signal through an array of dispersive elements of increasing length followed by direct detection. Fast convergence and high retrieved phase accuracy are achieved using a modified Gerchberg-Saxton (GS) algorithm that uses each projection as an intensity constraint. Second, we achieve similar performances employing an enhanced single projection GS algorithm with selective phase reset using symbol-wise GS errors. We experimentally reconstruct a 30 Gbaud QPSK signal after 55 km single-mode fiber transmission using the proposed solutions with a reduced number of iterations.

Temporal and spectral coding over amplified spontaneous emission for secure optical coherent communications.

We demonstrate secure optical coherent communications employing low-coherence matched detection based on the randomness of amplified spontaneous emission (ASE) noise. Two-level physical-layer optical encryption is achieved through temporal and spectral coding over a broadband ASE source. An ASE-carried signal and unmodulated carrier are polarization multiplexed, transmitted over a same single-mode fiber (SMF), and separated with the aid of polarization tracking before having matched detection at the receiving side. The impact of chromatic dispersion on the low-coherence matched detection system is analyzed and experimentally investigated. We experimentally realize optically coded 20 Gbaud QPSK and 8-PSK signals transmission over a 43 km SMF span with a maximum line rate of 60 Gbits/s.