Refractory times for excitable dual-state quantum dot laser neurons.

Excitable photonic systems show promise for ultrafast analog computation, several orders of magnitude faster than biological neurons. Optically injected quantum dot lasers display several excitable mechanisms with dual-state quantum lasers recently emerging as true all-or-none excitable artificial neurons. For use in applications, deterministic triggering is necessary and this has previously been demonstrated in the literature. In this work we analyze the crucially important refractory time for this dual-state system, which defines the minimum time between distinct pulses in any train.

Inhibitory and excitatory integration with a quantum dot laser neuron.

Neuromorphic computing has garnered a lot of attention in recent years. Excitable photonic systems in particular demonstrate great potential for ultrafast, controllable spike processing. Optically injected quantum dot lasers display several distinct excitable regimes. We demonstrate here that optically injected dual-state quantum dot lasers can display the classic leaky integrate-and-fire mechanism where the integration of several sub-threshold perturbations can yield an effective supra-threshold perturbation. Intriguingly, a contrasting integrate-and-inhibit mechanism is demonstrated in this work where the integration of two supra-threshold perturbations yields an effective sub-threshold perturbation similar to the pre-pulse inhibition mechanism of biological neurons. This is the first such mechanism in neuromorphic photonics to the best of our knowledge.

Asymmetric excitable phase triggering in an optically injected semiconductor laser.

One of the defining characteristics of excitability is the existence of an excitable threshold: the minimum perturbation amplitude necessary to produce an excitable response. We analyze an optically injected dual state quantum dot laser, previously shown to display a dual state stochastic excitable dynamic. We show that deterministic triggering of this dynamic can be achieved via optical phase perturbations. Further, we demonstrate that there are in fact two asymmetric excitable thresholds in this system corresponding to the two possible directions of optical phase perturbations. For fast enough perturbations, an excitable interval arises, and there is a limit to the perturbation amplitude, above which excitations no longer arise, a phenomenon heretofore unobserved in studies of excitability.

Anxiety and threat-related attentional biases in adolescents with fragile X syndrome.

BACKGROUND: Fragile X syndrome (FXS) is a single-gene disorder highly associated with anxiety; however, measuring anxiety symptoms in FXS and other neurogenetic syndromes is challenged by common limitations in language, self-awareness and cognitive skills required for many traditional assessment tasks. Prior studies have documented group-level differences in threat-related attentional biases, assessed via eye tracking, in FXS and non-FXS groups. The present study built on this work to test whether attentional biases correspond to clinical features of anxiety among adolescents and young adults with FXS. METHODS: Participants included 21 male adolescents with FXS ages 15-20 years who completed an adapted eye-tracking task that measured attentional bias towards fearful faces of varied emotional intensity. RESULTS: Among participants without anxiety disorders, attentional bias towards fear increased across age, similar to non-FXS paediatric anxiety samples. In contrast, participants with anxiety disorders exhibited greater stability in fear-related attentional biases across age. Across analyses, subtle fear stimuli were more sensitive to within-group anxiety variability than full-intensity stimuli. CONCLUSIONS: Our results provide novel evidence that although threat-related attentional biases may correspond with anxiety outcomes in FXS, these associations are complex and vary across developmental and task factors. Future studies are needed to characterise these associations in more robust longitudinal samples, informing whether and how eye-tracking tasks might be optimised to reliably predict and track anxiety in FXS.

Excitable interplay between lasing quantum dot states.

The optically injected semiconductor laser system has proven to be an excellent source of experimental nonlinear dynamics, particularly regarding the generation of excitable pulses. Typically for low-injection strengths, these pulses are the result of a small above-threshold perturbation of a stable steady state, the underlying physics is well described by the Adler phase equation, and each laser intensity pulse is accompanied by a 2π phase rotation. In this article, we show how, with a dual-state quantum dot laser, a variation of type I excitability is possible that cannot be described by the Adler model. The laser is operated so that emission is from the excited state only. The ground state can be activated and phase locked to the master laser via optical injection while the excited state is completely suppressed. Close to the phase-locking boundary, a region of ground-state emission dropouts correlated to excited-state pulses can be observed. We show that the phase of the ground state undergoes bounded rotations due to interactions with the excited state. We analyze the system both experimentally and numerically and find excellent agreement. Particular attention is devoted to the bifurcation conditions needed for an excitable pulse as well as its time evolution.

Experimental electric field visualisation of multi-mode dynamics in a short cavity swept laser designed for OCT applications.

An experimental study into the modal dynamics of a short cavity, fast frequency-swept laser is presented. This commercially available external cavity swept source is designed for use in optical coherence tomography (OCT) applications and displays a number of dynamic lasing regimes during the course of the wavelength sweep. Interferometric full electric field reconstruction is employed, allowing for measurement of the laser operation in a time-resolved, single-shot manner. Recovery of both the phase and intensity of the laser output across the entire sweep enables direct visualization of the laser instantaneous optical spectrum. The electric field reconstruction technique reveals the presence of multi-mode dynamics, including coherent mode-locked pulses. During the main part of the imaging sweep, the laser is found to operate in a second harmonic sliding frequency mode-locking regime. Examination of the modal evolution of this coherent regime reveals evidence of previously unobserved frequency switching dynamics.

Square wave excitability in quantum dot lasers under optical injection.

Quantum dot lasers display many unique dynamic phenomena when optically injected. Bistability has been predicted in a region of high injection strength. Experimentally, we show that a square wave phenomenon, rather than a phase-locked bistability, is observed in this region. The squares can manifest as a periodic train but also as noise-driven Type II excitable events. We interpret the appearance of the square waves as a thermally induced breaking of the bistability. Indeed, we find experimentally that over the duration of a square, the relative detuning between the master and the slave evolves deterministically. A relatively simple, physically motivated, rate equation model is presented and displays excellent agreement with the experiment.

Delay induced high order locking effects in semiconductor lasers.

Multiple time scales appear in many nonlinear dynamical systems. Semiconductor lasers, in particular, provide a fertile testing ground for multiple time scale dynamics. For solitary semiconductor lasers, the two fundamental time scales are the cavity repetition rate and the relaxation oscillation frequency which is a characteristic of the field-matter interaction in the cavity. Typically, these two time scales are of very different orders, and mutual resonances do not occur. Optical feedback endows the system with a third time scale: the external cavity repetition rate. This is typically much longer than the device cavity repetition rate and suggests the possibility of resonances with the relaxation oscillations. We show that for lasers with highly damped relaxation oscillations, such resonances can be obtained and lead to spontaneous mode-locking. Two different laser types--a quantum dot based device and a quantum well based device-are analysed experimentally yielding qualitatively identical dynamics. A rate equation model is also employed showing an excellent agreement with the experimental results.

Direct experimental measurement of single-mode and mode-hopping dynamics in frequency swept lasers.

A time-resolved study is presented of the single-mode and mode-switching dynamics observed in swept source vertical cavity surfing emitting lasers and swept wavelength short external cavity lasers. A self-delayed interferometric technique is used to experimentally measure the phase and intensity of these frequency swept lasers, allowing direct examination of the modal dynamics. Visualisation of the instantaneous optical spectrum reveals mode-hop free single mode lasing in the case of the vertical cavity laser, with a tuning rate of 6.3 GHz/ns. More complex mode-switching behaviour occurs in the external cavity laser, with the mode-hopping dynamics found to be dominated by the deterministic movement of the spectral filter. Evidence of transient multi-mode operation and mode-pulling is also presented.

Effectiveness of interferon-free therapy for the treatment of HCV-patients with compensated cirrhosis treated through the Irish early access program.

BACKGROUND: We investigated the real-world effectiveness of interferon-free regimens for the treatment of patients with compensated cirrhosis infected with hepatitis C virus (HCV). METHOD: Using the Irish national HCV treatment registry, the effectiveness and safety of interferon-free regimens for HCV-infected patients treated between April 2015 and August 2016, was determined. RESULTS: A SVR12 was achieved in 86% of subjects treated with sofosbuvir/ledipasvir ± ribavirin (SOF/LDV±RBV), 93% treated with paritaprevir, ombitasvir and ritonavir combined with dasabuvir ± ribavirin (3D±RBV) and 89% treated with sofosbuvir/daclatasvir ± ribavirin (SOF/DCV±RBV). The discontinuation rate was 5% and the on-treatment mortality rate was 1%. CONCLUSION: The availability of interferon-free regimens represents a significant breakthrough for the treatment of HCV infection. Treatments options, with high SVR12 rates, are now available for patients with compensated cirrhosis who were unsuitable for treatment with interferon-based regimens. Data obtained from studies conducted in real world practice provide robust information fundamental for input into future economic evaluations for agents used for the treatment of HCV infection.

Injection-induced, tunable all-optical gating in a two-state quantum dot laser.

We demonstrate a tunable all-optical gating phenomenon in a single-section quantum dot laser. The free-running operation of the device is emission from the excited state. Optical injection into the ground state of the material can induce a switch to emission from the ground state with complete suppression of the excited state. If the master laser is detuned from the ground-state emitting frequency, a periodic train of ground-state dropouts can be obtained. These dropouts act as gates for excited-state pulsations: during the dropout, the gate is opened and gain is made available for the excited state, and the gate is closed again when the dropout ends. Numerical simulations using a rate equation model are in excellent agreement with experimental results.

Optically induced hysteresis in a two-state quantum dot laser.

Quantum dot lasers can lase from the ground state only, simultaneously from both the ground and first excited states and from the excited state only. We examine the influence of optical injection at frequencies close to the ground state when the free-running operation of the device is excited state lasing only. We demonstrate the existence of an injection-induced bistability between ground state dominated emission and excited state dominated emission and the consequent hysteresis loop in the lasing output. Experimental and numerical investigations are in excellent agreement. Inhomogeneous broadening is found to be the underlying physical mechanism driving the phenomenon.

Emergence of resonant mode-locking via delayed feedback in quantum dot semiconductor lasers.

With conventional semiconductor lasers undergoing external optical feedback, a chaotic output is typically observed even for moderate levels of the feedback strength. In this paper we examine single mode quantum dot lasers under strong optical feedback conditions and show that an entirely new dynamical regime is found consisting of spontaneous mode-locking via a resonance between the relaxation oscillation frequency and the external cavity repetition rate. Experimental observations are supported by detailed numerical simulations of rate equations appropriate for this laser type. The phenomenon constitutes an entirely new mode-locking mechanism in semiconductor lasers.

Optical ultrafast random number generation at 1 Tb/s using a turbulent semiconductor ring cavity laser.

A simple method of high-speed random bit generation is presented that utilizes the turbulent output of a fiber ring cavity semiconductor laser. Random bits are generated by multi-bit sampling of the chaotic optical waveform passed through a simple post-processing procedure, leading to generation rates up to and potentially exceeding 1 Tb/s. The resulting random bit streams are tested statistically using a software package designed to test random number generators, the NIST statistical test suite. The bit streams pass each of these test sets, indicating their suitability for use in random number generation applications. This novel technique allows the generation of random bits from less complex experimental conditions than previously reported, while improving upon recent previous studies in terms of bit rate and quality of bits.

Single shot, time-resolved measurement of the coherence properties of OCT swept source lasers.

A novel, time-resolved interferometric technique is presented that allows the reconstruction of the complex electric field output of a swept source laser in a single-shot measurement. The power of the technique is demonstrated by examining a short cavity swept source designed for optical coherence tomography (OCT) applications with a spectral width of over 100 nm. The novel analysis allows a time-resolved real-time characterization of the roll-off, optical spectrum, linewidth, and coherence properties of a dynamic, rapidly swept laser source.

Dissipative Phase Solitons in Semiconductor Lasers.

We experimentally demonstrate the existence of nondispersive solitary waves associated with a 2π phase rotation in a strongly multimode ring semiconductor laser with coherent forcing. Similarly to Bloch domain walls, such structures host a chiral charge. The numerical simulations based on a set of effective Maxwell-Bloch equations support the experimental evidence that only one sign of chiral charge is stable, which strongly affects the motion of the phase solitons. Furthermore, the reduction of the model to a modified Ginzburg-Landau equation with forcing demonstrates the generality of these phenomena and exposes the impact of the lack of parity symmetry in propagative optical systems.

Experimental classification of dynamical regimes in optically injected lasers.

We present a reliable and fast technique to experimentally categorise the dynamical state of optically injected two mode and single mode lasers. Based on the experimentally obtained time-traces locked, unlocked and chaotic states are distinguished for varying injection strength and detuning. For the two mode laser, the resulting experimental stability diagram provides a map of the various single mode and two mode regimes and the transitions between them. This stability diagram is in strong agreement with the theoretical predictions from low-dimensional dynamical models for two mode lasers. We also apply our method to the single mode laser and retain the close agreement between theory and experiment.

Dynamics of a short cavity swept source OCT laser.

We investigate the behaviour of a short cavity swept source laser with an intra cavity swept filter both experimentally and theoretically. We characterise the behaviour of the device with real-time intensity measurements using a fast digital oscilloscope, showing several distinct regimes, most notably regions of mode-hopping, frequency sliding mode-locking and chaos. A delay differential equation model is proposed that shows close agreement with the experimental results. The model is also used to determine important quantities such as the minimum and maximum sweep speeds for the mode-locking regime. It is also shown that by varying the filter width the maximum sweep speed can be increased but at a cost of increasing the instantaneous linewidth. The consequent impacts on optical coherence tomography applications are analysed.

Incoherent optical triggering of excitable pulses in an injection-locked semiconductor laser.

We experimentally study the response of an injection-locked quantum dot semiconductor laser in the excitable regime to perturbations from an external, incoherent laser. We show that excitable pulses may be triggered both for perturbation wavelengths close to that of the quantum dot device and wavelengths detuned even by a few tens of nanometers.

On-chip optical phase locking of single growth monolithically integrated Slotted Fabry Perot lasers.

This work investigates the optical phase locking performance of Slotted Fabry Perot (SFP) lasers and develops an integrated variable phase locked system on chip for the first time to our knowledge using these lasers. Stable phase locking is demonstrated between two SFP lasers coupled on chip via a variable gain waveguide section. The two lasers are biased differently, one just above the threshold current of the device with the other at three times this value. The coupling between the lasers can be controlled using the variable gain section which can act as a variable optical attenuator or amplifier depending on bias. Using this, the width of the stable phase locking region on chip is shown to be variable.