In-Phase and Anti-Phase Synchronization in a Laser Frequency Comb.

Coupled clocks are a classic example of a synchronization system leading to periodic collective oscillations. Already in 1665, Christiaan Huygens described this phenomenon as a kind of "sympathy" among oscillators. In this work, we describe the formation of two types of laser frequency combs as a system of oscillators coupled through the beating of the lasing modes. We experimentally show two completely different types of synchronization in a quantum dot laser-in-phase and splay-phase states. Both states can be generated in the same device, just by varying the damping losses of the system. This modifies the coupling among the oscillators. The temporal laser output is characterized using both linear and quadratic autocorrelation techniques. Our results show that both pulses and frequency-modulated states can be generated on demand within the same device. These findings allow us to connect laser frequency combs produced by amplitude-modulated and frequency-modulated lasers and link these to pattern formation in coupled systems such as Josephson-junction arrays.

Passively mode-locked semiconductor quantum dot on silicon laser with 400 Hz RF line width.

Mode-locked InAs/InGaAs quantum dot lasers emitting optical frequency combs centered at 1310 nm are promising sources for high-speed and high-capacity communication applications. We report on the stable optical pulse train generation by a monolithic passively mode-locked edge-emitting two-section quantum dot laser based on a five-stack InAs/InGaAs dots-in-a-well structure directly grown on an on-axis (001) silicon substrate by solid-source molecular beam epitaxy. Optical pulses as short as 1.7 ps at a pulse repetition rate or inter-mode beat frequency of 9.4 GHz are obtained. A minimum pulse-to-pulse timing jitter of 9 fs, corresponding to a repetition rate line width of 400 Hz, is demonstrated. The generated optical frequency combs yield exceptional low amplitude jitter performance and comb widths exceed 5.5 nm at a -3 dB criteria, containing more than 100 comb carriers.

Threshold behavior of optical frequency comb self-generation in an InAs/InGaAs quantum dot laser.

We report on a significant reduction of both the radio-frequency beat note line width at 40.7 GHz and the integrated relative intensity noise of a 1 mm long edge-emitting monolithic Fabry-Perot InAs/InGaAs quantum dot semiconductor laser emitting from the ground state at 1250 nm by injection current control. For increasing injection currents, first an unlocked multi-mode behavior is observed and then, at a certain current above lasing threshold, self-locking of the longitudinal modes due to the internal non-linear effects occurs yielding a beat line width of 20 kHz (-3 dB) in contrast to tens of megahertz for lower injection currents. These results are confirmed by simulations.

Spatial confinement - rapid 2D F- diffusion in micro- and nanocrystalline RbSn2F5.

Diffusion of small ions in materials with confined space for translational dynamics can be quite different to isotropic (3D) diffusion, which is found in the majority of solids. Finding credible indications for 2D diffusion is not as easy as it looks at first glance, especially if only powder samples are available. Here we chose the ternary fluoride RbSn2F5 as a new model system to seek out low-dimensional anion diffusion in a nanocrystalline material. We prepared RbSn2F5via mechanochemically-assisted solid state synthesis and used both ac conductivity spectroscopy and spin-lock NMR relaxation measurements to find evidence that the fluorine ions preferably diffuse between the Rb-rich layers. In both cases the diffusion induced spin-lock NMR rates are only consistent with conductivity data if they are analyzed with the semi-empirical spectral density function for 2D jump diffusion as introduced by P. M. Richards [Solid State Commun., 1978, 25, 1019].

Non-local correlations via chaotic itinerancy in VCSEL with optical feedback.

Similar to edge-emitting lasers, vertical cavity surface emitting lasers (VCSELs) subjected to optical feedback are known for exhibiting erratic fluctuations of their optical power at slow and fast time scales; these are called low-frequency fluctuations (LFF). Here, we demonstrate both experimentally and numerically that the chaotic itinerancy in phase space associated with LFF has a deep connection with the creation of non-local correlations at multiple time scales between the two linear polarization modes. Our result provides a novel framework to interpret the unknown origin of spectral signatures in the optical power of chaotic lasers with optical feedback, which were observed in the past two decades.

Relative intensity noise reduction in a dual-state quantum-dot laser by optical feedback.

We report a reduction of the relative intensity noise (RIN) in a dual-state emitting quantum-dot laser subject to the state-selective optical feedback on the ground state and excited state. Numerically, we map the evolution of the RIN for variations of the optical feedback phases for both states. We report important differences in the impact of the feedback when applied to the ground or excited state, and observe regimes for which a significant reduction in RIN is achieved. Experimentally, we confirm these results and achieve a 16 dB reduction of the RIN via a careful and independent tuning of the optical feedback phase for each state.

Energy exchange between modes in a multimode two-color quantum dot laser with optical feedback.

We investigate experimentally and theoretically the multimode dynamics of a two-color quantum dot laser subject to time-delayed optical feedback. We unveil energy exchanges between the longitudinal modes of the excited state triggered by variations of the feedback phase, and observe that the modal competition between longitudinal modes appears independently within the ground state and excited state emission. These features are accurately reproduced with a quantum dot laser model extended to take into account multiple modes for both ground and excited states. Finally, we discuss the significant impact of such behavior on feedback-based control of two-color quantum dot lasers.

Two-dimensional tomographic terahertz imaging by homodyne self-mixing.

We realize a compact two-dimensional tomographic terahertz imaging experiment involving only one photoconductive antenna (PCA) simultaneously serving as a transmitter and receiver of the terahertz radiation. A hollow-core Teflon cylinder filled with α-Lactose monohydrate powder is studied at two terahertz frequencies, far away and at a specific absorption line of the powder. This sample is placed between the antenna and a chopper wheel, which serves as back reflector of the terahertz radiation into the PCA. Amplitude and phase information of the continuous-wave (CW) terahertz radiation are extracted from the measured homodyne self-mixing (HSM) signal after interaction with the cylinder. The influence of refraction is studied by modeling the set-up utilizing ZEMAX and is discussed by means of the measured 1D projections. The tomographic reconstruction by using the Simultaneous Algebraic Reconstruction Technique (SART) allows to identify both object geometry and α-Lactose filling.

Picosecond pulse amplification up to a peak power of 42 W by a quantum-dot tapered optical amplifier and a mode-locked laser emitting at 1.26 µm.

We experimentally study the generation and amplification of stable picosecond-short optical pulses by a master oscillator power-amplifier configuration consisting of a monolithic quantum-dot-based gain-guided tapered laser and amplifier emitting at 1.26 µm without pulse compression, external cavity, gain- or Q-switched operation. We report a peak power of 42 W and a figure-of-merit for second-order nonlinear imaging of 38.5 W2 at a repetition rate of 16 GHz and an associated pulse width of 1.37 ps.

Two-state semiconductor laser self-mixing velocimetry exploiting coupled quantum-dot emission-states: experiment, simulation and theory.

We exploit the coupled emission-states of a single-chip semiconductor InAs/GaAs quantum-dot laser emitting simultaneously on ground-state (λ(GS) = 1245 nm) and excited-state (λ(ES) = 1175 nm) to demonstrate coupled-two-state self-mixing velocimetry for a moving diffuse reflector. A 13 Hz-narrow Doppler beat frequency signal at 317 Hz is obtained for a reflector velocity of 3 mm/s, which exemplifies a 66-fold improvement in width as compared to single-wavelength self-mixing velocimetry. Simulation results reveal the physical origin of this signal, the coupling of excited-state and ground-state photons via the carriers, which is unique for quantum-dot lasers and reproduce the experimental results with excellent agreement.

Timing jitter reduction of passively mode-locked semiconductor lasers by self- and external-injection: numerical description and experiments.

In this paper the influence of different feedback (FB) and synchronization schemes on the timing phase noise (TPN) power spectral density (PSD) of a quantum-dot based passively mode-locked laser (MLL) is studied numerically and by experiments. The range of investigated schemes cover hybrid mode-locking, an opto-electrical feedback configuration, an all-optical feedback configuration and optical pulse train injection configuration by means of a master MLL. The mechanism responsible for TPN PSD reduction in the case of FB is identified for the first time for monolithic passively MLL and relies on the effective interaction of the timing of the intra-cavity pulse and the time-delayed FB pulse or FB modulation together with an statistical averaging of the independent timing deviations of both. This mechanism is quantified by means of simulation results obtained by introducing an universal and versatile simple time-domain model.

Performance assessment of the new Xpert® HIV-1 viral load XC assay for quantification of HIV-1 viral loads.

BACKGROUND: HIV-1 RNA quantification is a key component of treatment monitoring. OBJECTIVES: To assess the performance of a redesigned HIV-1 RNA quantitative assay that uses a dual-target approach: Xpert® HIV-1 Viral Load (VL) XC. STUDY DESIGN: Fresh and frozen samples (N = 533) from HIV-1 positive patients tested with Abbott HIV-1 assays (Alinity m and RealTime [m2000]) were retested using the new Xpert XC assay. Three samples with known underquantification using the previous single-target Xpert assay were retested. RESULTS: The Xpert XC assay yielded valid results in 98.5% (N = 528/536) of cases and showed high sensitivity in 80 fresh samples that had undetectable VLs or ≤1.7 log copies/mL with Alinity m. Linear regression and Bland-Altman analyses showed high concordance with the Abbott tests for quantified samples over a wide VL range (1.6-6.9 copies/mL), including non-B subtypes (mean difference=-0.1±0.23 log copies/mL). Mutations associated with integrase resistance did not impact the results. Very good linearity and reproducibility was shown for the tested subtypes B, CRF06_cpx, and CRF02_AG. Xpert XC VLs in samples that were previously underquantified using the original single-target Xpert assay were similar to those detected by the Abbott assays (±0.11 log copies/mL). CONCLUSIONS: The Xpert XC assay showed excellent correlation with the Abbott assays for all tested HIV-1 subtypes. Sensitivity, linearity and accuracy were high in the therapeutically relevant VL range. With a time to result of only 90 min, this on-demand decentralized assay is a safe, reliable and fast option for VL monitoring in HIV-1-infected patients.

Clinical evaluation of the automated Abbott RealTime SARS-CoV-2, Alinity m SARS-CoV-2, and Alinity m Resp-4-Plex assays.

BACKGROUND: Detection of SARS-CoV-2 infections relies on the use of sensitive, accurate and high throughput RT-PCR assays. OBJECTIVES: We assessed the analytical performance of the Abbott RealTime SARS-CoV-2 (RT-SARS), Alinity m SARS-CoV-2 (AlinSARS) assays and compared the clinical performance of the RT-SARS, AlinSARS, and Alinity m Resp-4-Plex (Alin4Plex) assays to the Seegene Allplex assay (Allplex) and an inhouse test (Inhouse). RESULTS: We found 100 % positive percent agreement (PPA) and 100 % negative percent agreement (NPA) comparing RT-SARS and Allplex. RT-SARS, AlinSARS and Inhouse showed 100 % NPA and 100 % PPA across all assays, except for the RdRp target of Inhouse (PPA = 84 %). Similarly, Alin4Plex and Allplex showed high agreement with specimens containing either SARS-CoV-2, influenza A, influenza B, or RSV. Detection rates of 100 % for SARS-CoV-2 at 50 copies/mL, high precision, and no cross-reactivity with non-SARS-CoV-2 respiratory pathogens were observed for RT-SARS and AlinSARS. AlinSARS detected SARS-CoV-2 in spiked throat washes and in specimens infected with SARS-CoV-2 Alpha or Beta variants. CONCLUSIONS: The newly developed RT-SARS, AlinSARS, and Alin4Plex assays proved to be useful for detecting SARS-CoV-2 RNA in clinical samples.

3D-printable portable open-source platform for low-cost lens-less holographic cellular imaging.

Digital holographic microscopy is an emerging, potentially low-cost alternative to conventional light microscopy for micro-object imaging on earth, underwater and in space. Immediate access to micron-scale objects however requires a well-balanced system design and sophisticated reconstruction algorithms, that are commercially available, however not accessible cost-efficiently. Here, we present an open-source implementation of a lens-less digital inline holographic microscope platform, based on off-the-shelf optical, electronic and mechanical components, costing less than $190. It employs a Blu-Ray semiconductor-laser-pickup or a light-emitting-diode, a pinhole, a 3D-printed housing consisting of 3 parts and a single-board portable computer and camera with an open-source implementation of the Fresnel-Kirchhoff routine. We demonstrate 1.55 µm spatial resolution by laser-pickup and 3.91 µm by the light-emitting-diode source. The housing and mechanical components are 3D printed. Both printer and reconstruction software source codes are open. The light-weight microscope allows to image label-free micro-spheres of 6.5 µm diameter, human red-blood-cells of about 8 µm diameter as well as fast-growing plant Nicotiana-tabacum-BY-2 suspension cells with 50 µm sizes. The imaging capability is validated by imaging-contrast quantification involving a standardized test target. The presented 3D-printable portable open-source platform represents a fully-open design, low-cost modular and versatile imaging-solution for use in high- and low-resource areas of the world.

Ultra-Short Pulse Generation in a Three Section Tapered Passively Mode-Locked Quantum-Dot Semiconductor Laser.

We experimentally and theoretically investigate the pulsed emission dynamics of a three section tapered semiconductor quantum dot laser. The laser output is characterized in terms of peak power, pulse width, timing jitter and amplitude stability and a range of outstanding pulse performance is found. A cascade of dynamic operating regimes is identified and comprehensively investigated. We propose a microscopically motivated traveling-wave model, which optimizes the computation time and naturally allows insights into the internal carrier dynamics. The model excellently reproduces the measured results and is further used to study the pulse-generation mechanism as well as the influence of the geometric design on the pulsed emission. We identify a pulse shortening mechanism responsible for the device performance, that is unique to the device geometry and configuration. The results may serve as future guidelines for the design of monolithic high-power passively mode-locked quantum dot semiconductor lasers.

Mismatch in cation size causes rapid anion dynamics in solid electrolytes: the role of the Arrhenius pre-factor.

Crystalline ion conductors exhibiting fast ion dynamics are of utmost importance for the development of, e.g., sensors or rechargeable batteries. In some layer-structured or nanostructured compounds fluorine ions participate in remarkably fast self-diffusion processes. As has been shown earlier, F ion dynamics in nanocrystalline, defect-rich BaF2 is much higher than that in the coarse-grained counterpart BaF2. The thermally metastable fluoride (Ba,Ca)F2, which can be prepared by joint high-energy ball milling of the binary fluorides, exhibits even better ion transport properties. While long-range ion dynamics has been studied recently, less information is known about local ion hopping processes to which 19F nuclear magnetic resonance (NMR) spin-lattice relaxation is sensitive. The present paper aims at understanding ion dynamics in metastable, nanocrystalline (Ba,Ca)F2 by correlating short-range ion hopping with long-range transport properties. Variable-temperature NMR line shapes clearly indicate fast and slow F spin reservoirs. Surprisingly, from an atomic-scale point of view increased ion dynamics at intermediate values of composition is reflected by increased absolute spin-lattice relaxation rates rather than by a distinct minimum in activation energy. Hence, the pre-factor of the underlying Arrhenius relation, which is determined by the number of mobile spins, the attempt frequency and entropy effects, is identified as the parameter that directly enhances short-range ion dynamics in metastable (Ba,Ca)F2. Concerted ion migration could also play an important role to explain the anomalies seen in NMR spin-lattice relaxation.

Dynamics of a passively mode-locked semiconductor laser subject to dual-cavity optical feedback.

We study the influence of dual-cavity optical feedback on the emission dynamics and timing stability of a passively mode-locked semiconductor laser using a delay differential equation model and verify the timing stability results by an initial experiment. By bifurcation analysis in dependence of the feedback delay times and feedback strength bistability, quasiperiodic and chaotic dynamics, as well as fundamental mode-locking are investigated, yielding a comprehensive overview on the nonlinear emission dynamics arising due to dual-cavity optical feedback. Optimum self-locking ranges for improving the timing stability by dual-cavity optical feedback are identified. A timing jitter reduction and an increase of the repetition rate tuning range of up to a factor of three, compared with single-cavity feedback, are predicted for the parameter ranges investigated. Improved timing stability on short and long timescales is predicted for dual-cavity feedback through the suppression of noise-induced fluctuations. Based on the numerical predictions, experimentally, a maximum timing jitter reduction up to a factor of 180 is found, accompanied by a side-band reduction by a factor of 58 dB, when both feedback cavities are resonant.

Treatment of Diffuse Pulmonary Hemorrhage with Factor VIIa.

Blunt thoracic trauma resulting in lung contusion with severe diffuse pulmonary hemorrhage and massive hemoptysis is rare and has a poor prognosis. Treatment options are limited. We report a case of the successful use of recombinant activated factor VII (NovoSeven™) in the treatment of life-threatening diffuse pulmonary hemorrhage secondary to an isolated blunt force thoracic injury without relevant traumatic coagulopathy.