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This paper presents a pioneering approach to bolstering network security and privacy by implementing chaotic optical communication with a hybrid optical feedback system (HOFS). The current baseline methods in network security are often less feasible for hybrid feedback systems, including limited robustness, compromised security, and synchronization challenges. Therefore, this paper proposes a hybrid approach to address these shortcomings by integrating the HOFS into chaotic optical communication systems (HOFS-COCS) to overcome the baseline challenges. This paper aims to improve network security while significantly maintaining efficient communication channels. Moreover, We designed two algorithms, one for chaotic maps generation and another for text encryption and decryption, to improve security in the hybrid feedback system. Our findings demonstrate through rigorous experimentation and analysis that the proposed (HOFS-COCS) method significantly improves network security by enabling reliable chaos generation, synchronization, and secure message transmission in chaotic optical communication systems. This research represents a significant advancement towards enhanced secrecy and synchronization in chaotic optical communication systems, promising a paradigm shift in network security protocols. [ABSTRACT FROM AUTHOR]

Self-mixing interference (SMI) has emerged as a powerful non-contact vibration sensing technique, leveraging the inherent coupling between laser emission and external optical feedback. However, conventional SMI systems often face limitations in signal resolution and measurement accuracy, particularly when probing low-amplitude vibrations or low-reflectivity targets. This study proposes a frequency modulation (FM) approach, FM-SMI, to enhance the capabilities of SMI setups. By intentionally modulating the laser frequency of 20 kHz, the FM-SMI technique induces a segmentation of the interference signal, effectively increasing the temporal resolution and facilitating the detection of finer vibration details. Comprehensive experiments involving oscillating speakers and rotating silicon wafers validate the superior performance of the FM-SMI system. Notably, the frequency-modulated signals exhibit stability and robustness, even under low-amplitude vibration conditions or when targeting low-reflectivity surfaces. The enhanced signal quality, coupled with numerical processing techniques, enables precise extraction of vibration characteristics, including amplitude variations and surface topographies. The proposed FM-SMI approach demonstrates its potential as a versatile tool for high-precision, non-contact vibration measurements across diverse applications, such as, non-destructive testing and the characterization of vibration induced by the rotational systems. [ABSTRACT FROM AUTHOR]

Increasingly, artificial intelligent systems look to neuromorphic photonics for its speed and its low loss, high bandwidth interconnects. Silicon photonics has shown promise to enable the creation of large scale neural networks. Here, we propose a monolithic silicon opto-electronic resonator spiking neuron. Existing designs of photonic spiking neurons have difficulty scaling due to their dependence on certain nonlinear effects, materials, and devices. The design discussed here uses optical feedback from the transmission of a continuously pumped microring PN modulator to achieve excitable dynamics. It is cascadable, capable of operating at GHz speeds, and compatible with wavelength-division multiplexing schemes for linear weighting. It is a Class 2 excitable device via a subcritical Hopf bifurcation constructed from devices commonly found in many silicon photonic chip foundries. Silicon photonics can be used to create high-speed large-scale neuromorphic systems for artificial intelligent tasks. Here, the authors discuss the design details and behavior of a resonator spiking neuron that can be fabricated in a commercial silicon photonics foundry process. [ABSTRACT FROM AUTHOR]

Lasing in the bad-cavity regime has promising applications in quantum precision measurement and frequency metrology due to the reduced sensitivity of the laser frequency to cavity-length fluctuations. Thus far, relevant studies have been mainly focused on conventional cavities whose finesse is high enough that the resonance linewidth is sufficiently narrow compared to the cavity's free spectral range, though still in the bad-cavity regime. However, lasing output from the cavity whose finesse is close to the limit of 2 has never been experimentally accessed. Here, we demonstrate an extremely bad-cavity laser, analyze the physical mechanisms limiting cavity finesse, and report on the worst-ever laser cavity with finesse reaching 2.01. The optical cavity has a reflectance close to zero and only provides weak optical feedback. The laser power can be as high as tens of μW and the spectral linewidth reaches a few kHz, over one thousand times narrower than the gain bandwidth. In addition, the measurement of cavity pulling reveals a pulling coefficient of 0.0148, the lowest value ever achieved for a continuous-wave laser. Our findings open up an unprecedentedly innovative perspective for future new ultra-stable lasers, which could possibly trigger future discoveries in optical clocks, cavity QED, continuous-wave superradiant laser, and explorations of quantum many-body physics. [ABSTRACT FROM AUTHOR]

We report the development of an optical feedback linear cavity-enhanced absorption spectroscopy instrument for HO2 detection using a distributed feedback (DFB) diode laser operating at 1506 nm. A direct and accurate method of reflectivity measurement based on the analysis of cavity mode signals was proposed. A differential circuit was used to judge the zero crossing point of the optical feedback cavity mode in the center of the frequency locking region, and shift the laser operating current to the non-resonant region. In this way, a ring-down signal was obtained with a time of 17.9 μs, corresponding to an effective absorption pathlength of 5.37 km. Combining the standing wave condition, the relationship between cavity length and drive voltage of the PZT mounted on the cavity rear mirror is translated into a correlation between the transmitted light wavenumber and the PZT voltage. The spectral resolution was improved from 290 MHz to 97 MHz by precisely tuning the PZT voltage. The achieved detection sensitivity of the system was 7 × 10− 10 cm− 1 with a data acquisition time of 10.6 s. The absorption spectrum of HO2 at 6638.205 cm− 1 was measured at a cell pressure of 50 mbar with a detection limit of 3.24 × 109 molecule/cm3. [ABSTRACT FROM AUTHOR]

Electrosurgical and ultrasonic devices are used in surgical procedures for hemostatic sealing and bisection of vascular tissues. Previous benchtop studies alternatively demonstrated successful infrared laser sealing and cutting of blood vessels, in a sequential, two-step approach. This study describes a smaller, laparoscopic device compatible design, and simultaneous approach to sealing and bisection of vessels, with potential optical feedback. A 1470-nm infrared diode laser sealed and bisected 40 porcine renal arteries, ex vivo. A reciprocating, side-firing, optical fiber, housed in a transparent square quartz optical chamber (2.7 × 2.7 × 25 mm outer dimensions), delivered laser energy over an 11 mm scan length, with a range of incident powers (41–59 W) and treatment times (5–21 s). Vessel diameters ranged from 2.5 to 4.8 mm. Vessel burst pressure measurements were performed on each cut end (n = 80) with success indicated by pressures exceeding 360 mmHg. All vessel ends were successfully sealed and bisected (80/80). The highest incident power, 59 W, yielded short treatment times of 5–6 s. Peak temperatures on the external chamber surface reached 103 oC. Time to cool down to body temperature measured 37 s. Infrared lasers simultaneously seal and bisect blood vessels, with treatment times comparable to, and temperatures and cooling times lower than reported for conventional devices. Future work will focus on integrating the fiber and chamber into a standard 5-mm-outer-diameter laparoscopic device. Customization of fiber scan length to match vessel size may also reduce laser energy deposition, enabling lower peak temperatures, treatment times, and cooling times. [ABSTRACT FROM AUTHOR]

The propagation of nonlinear waves, such as fires, weather fronts, and disease spread, has drawn attention since the dawn of time. A well-known example of nonlinear wave–fronts–in our daily lives is the domino waves, which propagate equally toward the left or right flank due to their reciprocal coupling. However, there are other situations where front propagation is not fully understood, such as bistable fronts with nonreciprocal coupling. These couplings are characterised by the fact that the energy emitter and receiver are not interchangeable. Here, we study the propagation of nonlinear waves in a bistable optical chain forced by nonreciprocal optical feedback. The spatiotemporal evolution and the front speeds are characterised as a function of the nonreciprocal coupling. We derive an equation to describe the interacting optical elements in a liquid crystal light valve with nonreciprocal optical feedback and compare the experimental results with numerical simulations of the coupled bistable systems. Understanding and harnessing nonlinear wave propagation has been a long-standing research topic in physics. In a controlled optical experiment, the authors realize a coupled bistable optical chain and study nonlinear wave propagation under nonreciprocal optical feedback, revealing unexpected behaviours of nonlinear wave propagation. [ABSTRACT FROM AUTHOR]

In this paper we report an improved method of coherent sensing through the use of a generalized phase-stepping algorithm to extract magnitude and phase information from interferometric fringes acquired by laser feedback interferometry (LFI). Our approach allows for significantly reduced optical sampling and acquisition times whilst also avoiding the need for fitting to complex models of lasers under optical feedback in post-processing. We investigate theoretically the applicability of this method under different levels of optical feedback, different laser parameters, and for different sampling conditions. We furthermore validate its use experimentally for LFI-based sensing using a terahertz (THz)-frequency laser in both far-field and near-field sensing configurations. Finally we demonstrate our approach for two-dimensional nanoscale imaging of the out-of-plane field supported by individual micro-resonators at THz frequencies. Our results show that fully coherent sensing can be achieved reliably with as little as 4 sampling points per imaging pixel, opening up opportunities for fast coherent sensing not only at THz frequencies but across the visible and infra-red spectrum. [ABSTRACT FROM AUTHOR]

The effect of the optical polarized Feedback through feedback strength and delayed time on dynamics, polarization flipping points and chaotic of quantum dot semiconductor lasers is studied using polarization-rotated optical feedback from the transverse electric mode to the transverse magnetic mode. It was found that the change in the behavior of the laser output is little at the small values of the above parameters and perturbed by increasing them, but we got a different behavior that ends in chaos and an excessive increase in capacitance when changing those parameters. [ABSTRACT FROM AUTHOR]

Ever since the mid-1960's, locking the phases of modes enabled the generation of laser pulses of duration limited only by the uncertainty principle, opening the field of ultrafast science. In contrast to conventional lasers, mode spacing in random lasers is ill-defined because optical feedback comes from scattering centres at random positions, making it hard to use mode locking in transform limited pulse generation. Here the generation of sub-nanosecond transform-limited pulses from a mode-locked random fibre laser is reported. Rayleigh backscattering from decimetre-long sections of telecom fibre serves as laser feedback, providing narrow spectral selectivity to the Fourier limit. The laser is adjustable in pulse duration (0.34–20 ns), repetition rate (0.714–1.22 MHz) and can be temperature tuned. The high spectral-efficiency pulses are applied in distributed temperature sensing with 9.0 cm and 3.3 × 10−3 K resolution, exemplifying how the results can drive advances in the fields of spectroscopy, telecommunications, and sensing. Pulses of adjustable duration are generated by a mode-locked random fibre laser that can drive advances in sensing. Rayleigh backscattering from cm-long sections of telecom fibre provides laser feedback and spectral selectivity to the Fourier limit. [ABSTRACT FROM AUTHOR]

Radiative characteristics of microstripe laser diode bar in an external resonator based on an aspherical lens and a flat dielectric mirror were Investigated. The bar had total emitting aperture of 185 μm, formed by 10 stripes with a width of 6 μm separated by mesa-grooves. Operation in the external resonator of the entire emitting aperture was characterized by a multimode generation regime with a peak power of 3 W/6 A. Selection of lateral modal structures and transition to a single-mode regime is possible by limiting the number of stripes involved in optical feedback. Dependences of optical mode reconfiguration were studied by introducing limiting slits into the external cavity. It was shown that limiting the emitting aperture involved in the feedback to 65 μm allows us to demonstrate high-order single-mode operation with far-field divergence for the central lobe of 1°. [ABSTRACT FROM AUTHOR]

The results of the experimental study of the effect of optical feedback (OF) on intensity fluctuations at a selected polarization in a single-mode vertical-cavity surface-emitting laser (VCSEL) are presented. The aim of this study is to look for conditions of enhancement of the level of polarization noise for possible use as a source of randomness for fast random bit generation. It is shown for several VCSEL examples that strong optical feedback can lead to an increase polarization noise, the amplitude and frequency band of which is sufficient for the efficient generation of random bits at a speed of several Gbit/s. This approach makes it possible to implement a fairly compact and structurally simple high-speed random bit generator. [ABSTRACT FROM AUTHOR]

A two-dimensional (2D) solid-state random laser emitting in the visible is demonstrated, in which optical feedback is provided by a controlled disordered arrangement of air-holes in a dye-doped polymer film. We find an optimal scatterer density for which threshold is minimum and scattering is the strongest. We show that the laser emission can be red-shifted by either decreasing scatterer density or increasing pump area. We show that spatial coherence is easily controlled by varying pump area. Such a 2D random laser provides with a compact on-chip tunable laser source and a unique platform to explore non-Hermitian photonics in the visible. [ABSTRACT FROM AUTHOR]

Diagnosing the root causes of cloud feedback in climate models and reasons for inter-model disagreement is a necessary first step in understanding their wide variation in climate sensitivities. Here we bring together two analysis techniques that illuminate complementary aspects of cloud feedback. The first quantifies feedbacks from changes in cloud amount, altitude, and optical depth, while the second separates feedbacks due to cloud property changes within specific cloud regimes from those due to regime occurrence frequency changes. We find that in the global mean, shortwave cloud feedback averaged across ten models comes solely from a positive within-regime cloud amount feedback countered slightly by a negative within-regime optical depth feedback. These within-regime feedbacks are highly uniform: In nearly all regimes, locations, and models, cloud amount decreases and cloud albedo increases with warming. In contrast, global-mean across-regime components vary widely across models but are very small on average. This component, however, is dominant in setting the geographic structure of the shortwave cloud feedback: Thicker, more extensive cloud types increase at the expense of thinner, less extensive cloud types in the extratropics, and vice versa at low latitudes. The prominent negative extratropical optical depth feedback has contributions from both within- and across-regime components, suggesting that thermodynamic processes affecting cloud properties as well as dynamical processes that favor thicker cloud regimes are important. The feedback breakdown presented herein may provide additional targets for observational constraints by isolating cloud property feedbacks within specific regimes without the obfuscating effects of changing dynamics that may differ across timescales. [ABSTRACT FROM AUTHOR]

Excitable cells can be stimulated or inhibited by optogenetics. Since optogenetic actuation regimes are often static, neurons and circuits can quickly adapt, allowing perturbation, but not true control. Hence, we established an optogenetic voltage-clamp (OVC). The voltage-indicator QuasAr2 provides information for fast, closed-loop optical feedback to the bidirectional optogenetic actuator BiPOLES. Voltage-dependent fluorescence is held within tight margins, thus clamping the cell to distinct potentials. We established the OVC in muscles and neurons of Caenorhabditis elegans, and transferred it to rat hippocampal neurons in slice culture. Fluorescence signals were calibrated to electrically measured potentials, and wavelengths to currents, enabling to determine optical I/V-relationships. The OVC reports on homeostatically altered cellular physiology in mutants and on Ca2+-channel properties, and can dynamically clamp spiking in C. elegans. Combining non-invasive imaging with control capabilities of electrophysiology, the OVC facilitates high-throughput, contact-less electrophysiology in individual cells and paves the way for true optogenetic control in behaving animals. Optogenetic actuation regimes are often static, which allows perturbation, but not true control of neuronal activity. Here, the authors describe an all-optical method for bidirectional steering of membrane potential, in closed loop, in C. elegans muscles and neurons, and rat hippocampal slice culture. The 'optogenetic voltage clamp' uses two microbial rhodopsin actuators and the rhodopsin voltage indicator QuasAr. [ABSTRACT FROM AUTHOR]

The experimental results were compared with the different perovskite quantum dot–doped materials, which explicitly include losses, gain, lasing, and threshold. The random lasers emerge multiple emission peaks at different central wavelengths, due to the recurrent optical gain feedback and the localized field resonance effect in vicinity of particles. By the operation of the applied different devices, the perovskite quantum dots can be better materials for generating photons and can create stronger light scattering for transition from incoherent lasing to coherent lasing. Contrary to conventional random lasers, we found that the nature of the random lasing is dictated by the perovskite quantum dots, and the quantum dots with a high refractive index can be advantageous for random lasing in mediums. The experiments also reveal a regime including the fundamental aspects of surface plasmon resonance light scattering for facile active lasing mode switching. These results will guide future design of different quantum dot random lasers with controlled lasing wavelength output. [ABSTRACT FROM AUTHOR]

The diversity of observed nonlinear dynamics in laser diodes subjected to optical feedback shows promise as an excellent candidate for chaos-based commercial applications. Thus, works in the last decade have primarily focused on system performances, geometric configurations, and balancing their trade-offs. We demonstrate an optical feedback system operating on phase-conjugate feedback exhibiting state-of-the-art chaos bandwidth values reaching ≈ 30 GHz. We report numerous high-frequency, spatiotemporally complex, chaotic dynamics undocumented in the past four decades. We highlight the underlying physics involving a three-tier temporal interaction mechanism between laser relaxation oscillations, phase-conjugate feedback induced external cavity modes, and chaotic bursts repeating each delay time in the extended cavity. We show supporting real-time high-definition system outputs captured by modern large bandwidth oscilloscopes. The presented work shows to our knowledge, the highest bandwidth and complexity entropy to-date in an optical chaos from a single laser, thereby proving the unnecessary need for further complexity using cascading lasers. The chaotic behavior due to the non-linearity present in a time-delayed feedback system has potential applications for secure optical communication and encryption. Here, a laser diode with phase-conjugate feedback is reported with state-of-the-art broadband, spatiotemporally complex, and high-entropy chaos. [ABSTRACT FROM AUTHOR]

Optical frequency combs in the terahertz frequency range are long-awaited frequency standards for spectroscopy of molecules and high-speed wireless communications. However, a terahertz frequency comb based on a low-cost, energy-efficient, and room-temperature-operating device remains unavailable especially in the frequency range of 0.1 to 3 THz. In this paper, we show that the resonant-tunneling-diode (RTD) oscillator can be passively mode-locked by optical feedback and generate a terahertz frequency comb. The standard deviation of the spacing between the comb lines, i.e., the repetition frequency, is reduced to less than 420 mHz by applying external bias modulation. A simulation model successfully reproduces the mode-locking behavior by including the nonlinear capacitance of RTD and multiple optical feedback. Since the mode-locked RTD oscillator is a simple semiconductor device that operates at room temperature and covers the frequency range of 0.1 to 2 THz (potentially up to 3 THz), it can be used as a frequency standard for future terahertz sensing and wireless communications. The authors report passive mode-locking and frequency-comb generation induced by optical feedback in the resonant-tunneling-diode terahertz oscillator, a semiconductor-based oscillator covering the frequency range of 0.1 to 2 THz. [ABSTRACT FROM AUTHOR]

Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 1015 Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si3N4 photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required. Stable and tunable integrated lasers are fundamental building blocks for applications from spectroscopy to imaging and communication. Here the authors present a narrow linewidth hybrid photonic integrated laser with low frequency noise and fast linear wavelength tuning. They then provide an efficient FMCW LIDAR demonstration. [ABSTRACT FROM AUTHOR]

We demonstrate numerical analysis of the dynamics and noise of semiconductor laser (SL) with optical feedback (OFB) and nonlinear gain (NLG). The simulations are based on an improved time-delay rate equations model that takes into account the multiple round-trips of the lasing field in the external cavity. The temporal trajectory of the photon number, bifurcation diagrams, and relative intensity noise (RIN) is used to perform this analysis. The results show that when NLG included the Hopf bifurcation point moved towards the direction of increasing OFB strength. Period doubling, or sub-harmonics operation, is the route to chaos of SL depending on whether NLG is included or not. In the periodic oscillation (PO) and route to chaos regimes, including the NLG causes a significant frequency shift relative to the frequency in the case without NLG. Under strong OFB, the inclusion of NLG changes chaotic dynamics to continuous wave operation or PO depending on the OFB strength and characterized by RIN near to or higher than the RIN of the solitary laser. At high-frequency regime, the RIN is characterized by the compound cavity frequency or the external cavity frequency depending on the NLG whether included or not. [ABSTRACT FROM AUTHOR]

The current research work presents an evidence on overall synchronization of loosely bound strength in chaotic systems along with a new coupled design based on dual quantum dot light-emitting diodes (QDLEDs) in order to generate n-scroll attractors. To characterize these phenomena, the researchers used a theoretical approach on the basis of time series and phase space maps, i.e., attractors. In case of coupled QDLED attractors, the phases are generally locked during synchronous regime while the amplitudes are correlated. With the proposed construction scheme, both frequency detuning and coupling strength of two systems can be tuned independently. Further, chaotic attractors with even or else odd count of scrolls can also be easily generated. The study also demonstrated distinct attractors with n scrolls obtained using coupled design. [ABSTRACT FROM AUTHOR]

Active particles can self-propel by exploiting locally available energy resources. When powered by light, these resources can be distributed with high resolution allowing spatio-temporal modulation of motility. Here we show that the random walks of light-driven bacteria are rectified when they swim in a structured light field that is obtained by a simple geometric transformation of a previous system snapshot. The obtained currents achieve an optimal value that we establish by general theoretical arguments. This optical feedback is used to gather and confine bacteria in high-density and high-activity regions that can be dynamically relocated and reconfigured. Moving away from the boundaries of these optically confined states, the density decays to zero in a few tens of micrometers, exhibiting steep exponential tails that suppress cell escape and ensure long-term stability. Our method is general and scalable, providing a versatile tool to produce localized and tunable active baths for microengineering applications and systematic studies of non-equilibrium phenomena in active systems. Light can be used to precisely modulate the speed of active particles in space and time. Here, the authors rectify and confine bacteria using an optical feedback loop that couples bacteria topast configurations. [ABSTRACT FROM AUTHOR]

Decision making using photonic technologies has been intensively researched for solving the multi-armed bandit problem, which is fundamental to reinforcement learning. However, these technologies are yet to be extended to large-scale multi-armed bandit problems. In this study, we conduct a numerical investigation of decision making to solve large-scale multi-armed bandit problems by controlling the biases of chaotic temporal waveforms generated in semiconductor lasers with optical feedback. We generate chaotic temporal waveforms using the semiconductor lasers, and each waveform is assigned to a slot machine (or choice) in the multi-armed bandit problem. The biases in the amplitudes of the chaotic waveforms are adjusted based on rewards using the tug-of-war method. Subsequently, the slot machine that yields the maximum-amplitude chaotic temporal waveform with bias is selected. The scaling properties of the correct decision-making process are examined by increasing the number of slot machines to 1024, and the scaling exponent of the power-law distribution is 0.97. We demonstrate that the proposed method outperforms existing software algorithms in terms of the scaling exponent. This result paves the way for photonic decision making in large-scale multi-armed bandit problems using photonic accelerators. [ABSTRACT FROM AUTHOR]

We numerically study the behavior of a semiconductor ring laser subject to bidirectional delayed optical feedback, when the isolated laser is in the quasi-unidirectional regime. The optical feedback, provided by two external reflectors located in front of the ring output waveguides, can modify the laser regime produced by the cross-saturation between the clockwise and the counter-clockwise mode. Thus, the system exhibits new different regimes, most of which are asymmetric and bidirectional, with alternating dominant mode. Two of these regimes are of special interest in view of applications, because the laser switching period, between the clockwise and the counter-clockwise mode, is linearly related to the time of flight from the laser to one or both reflectors. In these operating conditions, the laser is thus suitable to implement a telemeter. A convenient electrical output signal is obtained by a photodiode located behind one (partially reflecting) fixed mirror, or by measuring the voltage drop across the laser junction. Simulations are performed by mathematical models based on rate-equations, assuming typical literature parameters for a 1 mW ring laser. [ABSTRACT FROM AUTHOR]

The past decade has witnessed major advances in the development and system-level applications of photonic integrated microcombs, that are coherent, broadband optical frequency combs with repetition rates in the millimeter-wave to terahertz domain. Most of these advances are based on harnessing of dissipative Kerr solitons (DKS) in microresonators with anomalous group velocity dispersion (GVD). However, microcombs can also be generated with normal GVD using localized structures that are referred to as dark pulses, switching waves or platicons. Compared with DKS microcombs that require specific designs and fabrication techniques for dispersion engineering, platicon microcombs can be readily built using CMOS-compatible platforms such as thin-film (i.e., thickness below 300 nm) silicon nitride with normal GVD. Here, we use laser self-injection locking to demonstrate a fully integrated platicon microcomb operating at a microwave K-band repetition rate. A distributed feedback (DFB) laser edge-coupled to a Si3N4 chip is self-injection-locked to a high-Q (> 107) microresonator with high confinement waveguides, and directly excites platicons without sophisticated active control. We demonstrate multi-platicon states and switching, perform optical feedback phase study and characterize the phase noise of the K-band platicon repetition rate and the pump laser. Laser self-injection-locked platicons could facilitate the wide adoption of microcombs as a building block in photonic integrated circuits via commercial foundry service. 'Here the authors provide the demonstration of platicon comb generation in an integrated photonic chip using laser self-injection locking, They take advantage of platicons generation in normal GVD resonators, which significantly relaxes the material and geometry design restrictions [ABSTRACT FROM AUTHOR]

Optical feedback is an efficient way to narrow and stabilise semi-conductor lasers. As a step forward for ultra-stable, yet highly tunable sources, we developed a new prototype of a three-mirror V-shaped optical cavity (VCOF). It is made of a precisely machined Zerodur spacer, that holds 3 optical contacted mirrors forming a high finesse V-shaped cavity. This arrangement allows the resonant light to be sent back to the seeding laser, triggering a drastic narrowing of the emission linewidth well below the cavity mode width without the need for high-bandwidth active electronic lock of the laser on the cavity. Low expansion material and precise temperature control of the reference cavity leads to a source with Hz level frequency drift and 70 Hz-level emission linewidth at 215 THz. We discuss the benefits of this new prototype compared to its predecessor for high-sensitivity cavity ring down spectroscopy (CRDS). To serve this purpose, the frequency stabilised laser was characterised on short, mid and long time scales, using a high finesse etalon, a self referenced optical frequency comb and a long term saturated CRDS absorption Lamb dip measurement on water. [ABSTRACT FROM AUTHOR]

Optical feedback is a powerful technique to stabilise and narrow semi-conductor lasers. As a step forward for field deployable, ultra-stable yet tunable sources, we analyse and model the opto-mechanical design of a V-shaped cavity optical feedback (VCOF) reference cavity. We estimate the relative contributions of sources of external disturbance on the optical resonance frequency of the cavity, and ultimately define the minimal thermal and mechanical shielding requirements to face field conditions. We attest of the robustness of the developed system, and show that external sources of disturbance are only secondary contributors to the overall stability of the cavity. The suite of analytical tools developed in the process paves the way for lighter and more compact cavity designs, more adapted to field deployment. [ABSTRACT FROM AUTHOR]

The presented research explores the sensing capability of the Self-Mixing Optical Feedback Interferometry (SM-OFI). The study evaluates the impact of different trade-off parameters like injection current, incident angle, target velocity, ambient temperature, the numerical aperture of collimating lens, etc. All the modes of SM signal acquisition are compared and investigate the influence of the acquisition scheme on the quality of the signal. The incident angle, wavelength, and external cavity length are also being trade-offs to increase the SNR and decrease of speckle effect. The study confirms that SNR increased linearly (R-square = 0.97) corresponds to numerical aperture. It was also analyzed that the external cavity length has a direct impact on the Doppler spectrum. The main aim of the study is to lay down the experimental narrative and to provide a simple mathematical relationship. [ABSTRACT FROM AUTHOR]

Purpose: Transscleral controlled cyclophotocoagulation (COCO) is a transscleral 810-nm diode laser cyclophotocoagulation that automatically adjusts the applied laser energy utilizing an optical feedback loop. The present study investigates the influence of pseudoexfoliation (PEX) on the efficacy of COCO in a Caucasian study population. Methods: Retrospective data from 130 consecutive eyes were analyzed during a 2-year follow-up. Baseline characteristics, intraocular pressure (IOP), number of IOP-lowering medications, visual field, best-corrected visual acuity (BCVA), and secondary surgical interventions (SSI) were analyzed. The primary endpoint was IOP reduction at M24 compared to baseline, and the secondary endpoints were IOP course, reduction of IOP-lowering medications, surgical success, and IOP-lowering SSIs stratified by PEX and baseline IOP. Results: IOP reductions of −35, −39, −25, −25, −23, −34, and −36% could be achieved from baseline to D1, W1, M1, M3, M6, M12, and M24 (all p < 0.001), respectively, while there was a significant overall reduction over time (p < 0.001) in the number of topical IOP-lowering medications postoperatively. The proportion of eyes requiring additional systemic IOP-lowering medication reduced from 31 to 0% at M24 (p = 0.025). Eyes without PEX and IOP < 30 mmHg at baseline had the lowest risk for IOP-lowering SSIs (p < 0.03). BCVA dropped at M12 (0.25 [95% CI: 0.12–0.38]), and the drop persisted during the following 12 months. Conclusion: The present study demonstrates a midterm IOP-lowering effect after COCO while reducing the burden for topical and systemic IOP-lowering medications. Patients without PEX and IOP < 30 mmHg have a lower risk of SSI. The procedure per se cannot be excluded as causative for the decreased postoperative BCVA. Further prospective investigations are suggested. [ABSTRACT FROM AUTHOR]

We study a coherently driven semiconductor laser cavity containing a single quantum dot(QD) (as gain medium) with optical feedback under Markovian approximation. We have obtained coupled operator equations for the model Hamiltonian using standard input-output formalism of cavity-QED and have found that these equations do not have any finite steady state solutions. We have also used an exact numerical framework based on Matlab platform qotoolbox, to compute the temporal dynamics of the mean excitation number of laser cavity mode under high feedback coupling regime. We have further studied the photon correlations of both the cavity mode as well as external feedback mode to feedback identify the laser parameters and coupling strength that give the nonclassical sub-Poissonian photon statistics. This work is useful for coherent control of photon statistics and photon correlations in the semiconductor laser with optical feedback. [ABSTRACT FROM AUTHOR]

We model and investigate the dynamics and modulation performance of semiconductor laser integrated with two short external cavities facing the front and back facets with the aim to enhance the modulation bandwidth of the laser for use in high-speed photonics. The coupled cavities provide double optical feedback (DOFB) to the laser cavity through the partially reflecting facets of the laser cavity. The study is based on modifying the rate equations of the laser to include multiple reflections of laser radiations in the external cavities. Therefore, it accounts for the regime of strong OFB that causes bandwidth enhancement. We introduce correspondence between the laser stability under DOFB and the modulation response characteristics. Also, we allocate the ranges of the DOFB that induce photon–photon resonance (PPR) effect as the main contributor to the bandwidth enhancement. We show that the intensity modulation (IM) response can be tailored by varying the reflectivity of the external mirrors when the external cavities are too short to stabilise the laser output. Modulation bandwidth better than 55 GHz is predicted under strong double OFB when the external cavities are as short as 2 mm. Stronger DOFB is found to enhance the PPR effect and induce resonant modulation over a narrow frequency range around frequencies reaching 45 GHz. [ABSTRACT FROM AUTHOR]

We report on the possibility of applying optical feedback (OFB) from a short external cavity to change the multimode hopping oscillation in long-wavelength semiconductor laser to single-mode oscillation. We characterize the influence of OFB on the modal oscillations and the associated noise spectrum of the multimode laser. The study is based on a modified time-delay multimode rate-equation model of the laser that includes mechanisms of spectral gain suppression along with OFB induced due to multiple reflections by an external reflector. The study is applied to 1.55 μm-InGaAsP laser that exhibits multimode hopping in its solitary version and supports wide bandwidth. The noise is evaluated in terms of the relative intensity noise. We show that strong OFB changes impact of the asymmetric gain suppression on the mode structure of the laser so that it changes the side mode suppression ratio (SMSR) and induces jumping of the strongest modes to longer or shorter wavelengths in random-like fashion. The change of SMSR of the laser occurs mainly when strong OFB induces multiperiodic and chaos oscillations. We specify the ranges of OFB over which SMO is induced. The low-frequency noise is lowest when the laser maintains CW operation or exhibits period-1 oscillations, while it is maximized when the laser dynamics are chaotic. [ABSTRACT FROM AUTHOR]

We describe a novel chaotic macropulse green laser designed employing the pulse modulation of a 520 nm laser diode with optical feedback. We analyze the characteristics of these chaotic macropulses, in view of effects of the feedback intensity, injection current, modulation voltage, duty cycle, and modulation frequency on the pulses, including their full width at half maximum (FWHM) values and peak side-lobe levels (PSLs). We determine an optimum feedback intensity of 11.7% based on the maximum peak optical power and the scope of the amplitude jitter of the chaotic macropulse. The optimum injection currents are approximately between 60 and 70 mA, and the modulation voltages between 2.0 and 3.0 V are limited by the radio-frequency (RF) power. We found that chaos also occurred at the bottom of the macropulse when the duty cycle was less than approximately 30%. The modulation frequencies were loaded from 4 to 20 MHz; this range was restricted by the performance of the RF input port on the laser diode mount. The range resolution of the chaotic macropulses is approximately 3 cm in seawater; it is achieved with lower power consumption than a continuous wave chaotic laser. [ABSTRACT FROM AUTHOR]

In this work, via autocorrelation function (ACF) and permutation entropy (PE) methods, we numerically investigate the time-delay signature (TDS) characteristics of the chaotic signal output from an optoelectronic oscillator (OEO) after introducing an extra optical feedback loop. The results demonstrate that, for such a chaotic system, both the optoelectronic feedback with a delay time of T1 and the optical feedback with a delay time of T2 contribute to the TDS of generated chaos. The TDS of the chaotic signal should be evaluated within a large time window including T1 and T2 by the strongest peak in the ACF curve of the chaotic signal, and the strongest peak may locate at near T1 or T2. Through mapping the evolution of the TDS in the parameter space of the optical feedback strength and time, certain optimized parameter regions for achieving a chaotic signal with a relatively weak TDS can be determined. [ABSTRACT FROM AUTHOR]

We present a chaotic lidar for underwater ranging and three-dimension (3D) imaging applications. The lidar transmitter based on a laser diode with free space optical feedback is designed to generate 520 nm wavelength wideband chaotic signal. This transmitter is used with a correlation-based lidar receiver to perform a ranging demonstration in clean water with a measured attenuation coefficient of 0.06 m−1 at 520 nm. The experimental results prove that 1.2 cm mean absolute error (MAE) and 3.4 cm range resolution can be achieved and kept in a range of 140 cm at least. Moreover, employing a two-axis mirror for laser scanning, the possibility of the designed chaotic lidar system for 3D imaging is also verified. [ABSTRACT FROM AUTHOR]

A 2D carrier transport model to be used in studying the spatial current dynamics in laser thyristors is presented. The model takes into account such features as optical feedback, impact ionization, and drift velocity saturation in strong electric fields. It is shown that there is current localization during laser-thyristor switch-on. A relationship is demonstrated between the distribution nonuniformity of the control current and its amplitude and position of the initial switch-on region. The time of laser-thyristor switch-on is 13 ns at a feed voltage of 26V, with a time of switch-on spreading over the entire 200-μm stripe width of ~65 ns. These parameters remain invariable irrespective of the switch-on spatial dynamics. [ABSTRACT FROM AUTHOR]

Feedback control mechanisms are ubiquitous in science and technology, and play an essential role in regulating physical, biological and engineering systems. The standard second law of thermodynamics does not hold in the presence of measurement and feedback. Most studies so far have extended the second law for discrete, Markovian feedback protocols; however, non-Markovian feedback is omnipresent in processes where the control signal is applied with a non-negligible delay. Here, we experimentally investigate the thermodynamics of continuous, time-delayed feedback control using the motion of an optically levitated, underdamped microparticle. We test the validity of a generalized second law which bounds the energy extracted from the system, and study the breakdown of feedback cooling for very large time delays. The second law of thermodynamics cannot be straightforwardly applied to systems with non-Markovian feedback control, and previous extensions to cover that regime have not been tested experimentally. Here, the authors bridge this gap using an optically levitated microsphere in a feedback-cooling system. [ABSTRACT FROM AUTHOR]

We demonstrated a multipass-assisted dual-comb gas sensor for the simultaneous and sensitive detection of C2H2 and NH3 near 1.5 μm using a single free-running fiber laser. Two pulse trains generated from the shared cavity in the passively mode-locked fiber laser ensure the mutual coherence and common-mode noise rejection. One of the comb pulse trains passed through an open-path multipass cell with an effective path length of 4 m. A synchronized acquisition technique was adopted for more efficient interferogram acquisition compared with the conventional continuous acquisition method. At an acquisition time of 1.5 s, we obtained a signal-to-noise ratio of more than 600 by 1500 averages of the acquired spectrum of 0.1% C2H2. Pressure broadening of C2H2 and NH3 in the frequency range of 6503–6530 cm−1 between 0.1 and 1 bar were captured by the dual-comb sensor. Finally, the free-running fiber laser-based dual-comb sensor successfully analyzed the C2H2/NH3 mixtures, enabling real-time gas sensing without using complex optical and electronic feedback systems. [ABSTRACT FROM AUTHOR]

The peak–valley point serves as the intersection for the derivative of both the self-mixing interference signal and its anti-cosine transform. In this study, a peak–valley point detection algorithm based on the phase unwrapping method for self-mixing interference was proposed, The proposed method had the advantage of identifying the peak–valley point without setting a threshold. In addition, the relative error of the algorithm did not exceed 1% of the simulation signal, when the optical feedback factor is within the range from 0.1 to 4. Further, the experimental results were found to be in accordance with the theoretical analysis, with a relative error of 4.9%. [ABSTRACT FROM AUTHOR]

We demonstrate that the bandwidth of a terahertz (THz) quasi time-domain spectroscopy (QTDS) system can be increased by controlled optical feedback into the multimode laser diode. The THz spectral broadening correlates well with the broadening of the optical spectrum. Our results show that the phase of the feedback does not affect the optical or THz bandwidth in general, although certain combinations of feedback power and phase can create additional high-frequency components in the THz spectrum, which are not present otherwise. For the optimal combination of feedback power and phase, the bandwidth of the THz QTDS system is increased from 1.7 to 2.5 THz. [ABSTRACT FROM AUTHOR]

Remote laser welding systems can usually focus a laser beam to diameters of 0.1 mm. Therefore, high quality clamping and precise teaching is needed in order to achieve appropriate process tolerance for a sound weld. This brings reservation in the field of small series and user-customized manufacturing, where product individualization requires flexible and adaptive systems as workpiece geometry is not exact due to manufacturing tolerances and thermal deformations during welding. The preparation for welding is therefore often time-consuming. To solve this, we have developed an innovative system, which enables in-line adaptive 3D seam tracking. The system consists of an industrial robot (Yaskawa MC2000), a scanning head (HighYag RLSK; working area 200 mm × 300 mm × 200 mm) with optical triangulation feedback and a fiber laser (IPG, YRL-400-AC; 400 W). A feed-forward loop was used to achieve positioning accuracy of under 0.05 mm during on-the-fly welding. Experimental results show that between welding speeds of 25 and 150 cm/min, average tracking deviations are 0.043 mm and 0.276 mm in y and z directions, respectively. Moreover, teaching times for a specified seam can be shortened for more than 10 times due to the fact that only rough seam teaching is required. The proposed system configuration could be adapted to other classical welding processes. [ABSTRACT FROM AUTHOR]

In this paper we propose a new architecture for enhancing the performance of a transimpedance amplifier used for low-currents, and in particular, that used in biosensing. It is usually the first block in biomedical acquisition systems for converting a current in the nanoampere and picoampere range into a proportional voltage, with an amplitude suitable for further processing. There exist two main amplifier topologies for achieving this, current-mode and shunt-feedback mode. This paper introduces a shunt-feedback amplifier that embodies current-mode operation and thereby offers the advantages of both existing schemes. A conventional shunt-feedback amplifier has a number of stages and requires compensation components to achieve stability of the feedback loop. The exemplary circuit described is inherently stable because a high gain is effectively achieved in one stage that has a dominant pole controlling the frequency response. Exhibiting complementary symmetry, the configuration has an input port that is very close to earth potential. This enables the configuration to handle bidirectional input signals such are as met with in electrochemical ampero-metric biosensors. For the 0.35 µm process adopted and ± 3.3 V rail supplies, the power dissipation is 330 µW. With a transimpedance gain of 120 dBΩ the incremental input and output resistances are less than 2 Ω and the − 3 dB bandwidth for non-optical input currents is 8.2 MHz. The input referred noise current is 3.5 pA/√Hz. [ABSTRACT FROM AUTHOR]

We designed a compact and efficient high-power laser that generates chaotic green light by perturbing a 520 nm laser diode with optical feedback. This achieved a continuous output power of 30 mW with a bandwidth of more than 1 GHz and an electrical-to-green conversion efficiency of 6.25%. All possible chaotic states were demonstrated for different diode-driving currents and feedback intensities. The FWHM and the peak side-lobe level (PSL) are affected by the injection current and feedback intensity. The FWHM increases with increasing feedback intensity, and the PSL increases with both injection current and feedback intensity. The compact chaotic laser could be used in underwater detection to suppress backscattering because of its intrinsic intensity modulation at high frequency. We finally found the optimum range of chaotic states for underwater detection, for which the ratio of input current to threshold current is 1.5 : 2.1. [ABSTRACT FROM AUTHOR]

The secondary feedback self-mixing interference (2FSMI) technology used to measure the micro-vibrations of objects is more accurate than the self-mixing interference (SMI) technique, and current modulation is a commonly used modulation method. Therefore, current modulation and 2FSMI technology are combined to measure micro-vibrations with different amplitudes within a wavelength. First, the interference signal is current-modulated by a sawtooth wave. Second, the collected signal is preprocessed. Third, the preprocessed signal is subject to fast Fourier transform spectrum analysis. Finally, the micro-displacement of the vibrating object is reconstructed. Experimental results show that the maximum error of this method in reconstructing the micro-displacement of a vibrating object is less than 13 nm, which is approximately half that of the SMI technique. Moreover, this method can reconstruct not only sinusoidal vibrations but also arbitrary waveform vibrations. Therefore, using this method to measure object vibration can yield high measurement accuracy without increasing the experimental device. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the possibility of using optical injection to efficiently suppress the time-delay (TD) signatures of chaotic signals in a large experimentally accessible parameter range of semiconductor ring lasers (SRLs).We also study how this optical injection can improve the signal bandwidths. The injection signal is obtained from a master SRL with either optical self- or cross-feedback. For optical selffeedback configurations, it is found that the suppression of TD signatures is similar to what has been found in injected Fabry-Perot semiconductor lasers, i.e., narrow range of parameters with respect to the detuning and injection strengths. For cross-feedback configurations, however, the TD signatures can be suppressed in a wide range of parameters; meanwhile, the bandwidths are significantly improved for the same range of parameters. This is particularly useful for the security in chaos-based communications and also for generating random bits with improved properties. [ABSTRACT FROM AUTHOR]

This paper presents a laser-based system for a local transformation of the magnetic properties in austenitic stainless steel. The process is applied to linear actuators with an integrated, in-axis position measurement where durability and high precision are needed. The developed system consists of a 400-W, single-mode fiber laser, the focusing optics, and a three-axis actuation system. An important feature is the optical feedback for the adaptive, high-speed (40 kHz) laser power regulation. The experimental results show that the optical feedback effectively compensates for the external disturbances, like an increased roughness and a gradual heat accumulation. The regulation decreases the dispersion of the magnetic signal's response by more than 100% and shortens the transient time at the beginning of the inscription process from 100 to 15 ms. [ABSTRACT FROM AUTHOR]

We present a numerical analysis that focuses, for the first time to our knowledge, on the feedback-induced dynamics in a semiconductor passively mode-locked laser with sub-wavelength resolution. Our results and the corresponding theoretical explanations elucidate several aspects of the laser dynamics under self-injection including inherent properties of mode-locked lasers such as pulse intensity noise and timing jitter. We show that the dynamics of the laser exhibit a periodicity in the wavelength scale apparent only on integer multiples of the laser cavity and decays in the time scale of the pulse duration following the coherence of the mode-locked laser. The corresponding phenomena are dominant for external cavities that are shorter than the laser cavity and superimposed on the previously reported dynamics of the semiconductor mode-locked lasers for longer external delays. Since these dynamics are triggered by low feedback levels, our study could be useful for the optimization of the laser operation in cases where ultra-short external cavity lengths are involved (integrated designs, power collection with fiber tapers, etc.). [ABSTRACT FROM AUTHOR]

A passive way of eliminating optical feedback between a pump laser and an extracavity two-pass pumped Raman laser in a compact arrangement was demonstrated. Feedback was suppressed by arranging the circuit elements in a configuration where the pump-laser 100% mirror and the Raman-laser output mirror formed an unstable cavity characterized by a substantially higher threshold for generating the fundamental radiation as compared with the stable pump-laser cavity. The method was demonstrated using an eye-safe KGd(WO) Raman laser pumped by a pulsed KGd(WO):Nd laser with the working transition $$ {}^{{}^4}F_{3/2}\to {}^4I_{13/2} $$ as an example. The Raman laser emitted pulses of energy ~11.5 mJ at an electrical pump energy of ~10 J, corresponding to a conversion efficiency of ~31%. The Stokes pulse width (~17 ns) was comparable to the pump pulse width (~20 ns) owing to the extracavity location of the Raman laser. [ABSTRACT FROM AUTHOR]

A simple external optical feedback system has been applied to a distributed feedback quantum cascade laser (DFB QCL) for cavity ring-down spectroscopy (CRDS) and a clear effect of feedback was observed. A long external feedback path length of up to 4m can decrease the QCL linewidth to around 50kHz, which is of the order of the transmission linewidth of our high finesse ring-down cavity. The power spectral density of the transmission signal from high finesse cavity reveals that the noise at frequencies above 20kHz is reduced dramatically. [ABSTRACT FROM AUTHOR]

Purpose: To investigate structural changes in the retina by histologic evaluation and in vivo spectral domain optical coherence tomography (SD-OCT) following selective retina therapy (SRT) controlled by optical feedback techniques (OFT). Methods: SRT was applied to 12 eyes of Dutch Belted rabbits. Retinal changes were assessed based on fundus photography, fluorescein angiography (FAG), SD-OCT, light microscopy, transmission electron microscopy (TEM), and scanning electron microscopy (SEM) at each of the following time points: 1 h, and 1, 3, 7, 14 and 28 days after SRT. BrdU (5'-bromo-2'-deoxy-uridine) incorporation assay was also conducted to evaluate potential proliferation of RPE cells. Results: SRT lesions at1 h after SRT were ophthalmoscopically invisible. FAG showed leakage in areas corresponding to SRT lesions, and hyperfluorescence disappeared after 7 days. SD-OCT showed that decreased reflectivity corresponding to RPE damage was restored to normal over time in SRT lesions. Histologic analysis revealed that the damage in SRT lesions was primarily limited to the retinal pigment epithelium (RPE) and the outer segments of the photoreceptors. SEM and TEM showed RPE cell migration by day 3 after SRT, and restoration of the RPE monolayer with microvilli by 1 week after SRT. At 14 and 28 days, ultrastructures of the RPE, including the microvilli and tight junctions, were completely restored. The outer segments of the photoreceptors also recovered without sequelae. Interdigitation between the RPE and photoreceptors was observed. BrdU incorporation assay revealed proliferation of RPE on day 3 after SRT, and peak proliferation was observed on day 7 after SRT. Conclusion: Based on multimodal imaging and histologic assessment, our findings demonstrate that SRT with OFT could selectively target the RPE without damaging the neurosensory retina. Therefore, the use of SRT with OFT opens the door to the possibility of clinical trials of well-defined invisible and nondestructive retina therapy, especially for macular disease. [ABSTRACT FROM AUTHOR]