Coupled Laser System Research Articles

Laser welding components for electric vehicles with a high-power blue laser system

Welding copper to itself and other metals is challenging using conventional welding techniques. The process window for welding copper with an infrared (IR) laser, resistance welder, or an ultrasonic welder is very narrow. In the case of the infrared laser, the high reflectivity at these wavelengths makes it difficult to couple the power into the material and control the temperature of the weld puddle. In the case of ultrasonic and resistive welding, the high thermal conductivity of the material and the tendency to create particles cause less than ideal welds. These fundamental problems can be overcome by using a laser with a wavelength that is highly absorbed by these materials. This paper will present recent welding results using a fiber coupled 500-W blue laser system coupled to a welding head to deliver a 215 μm spot size and an average power density of 1.6 MW/cm2. These results will be compared with the authors’ previous results from a free space delivered laser system that was the prototype for the 500-W fiber coupled laser. The fiber coupled laser system performance exceeded the free space performance because of two factors: (1) the welding was able to be performed at normal incidence (90°) to the surface allowing for greater power coupling into the copper and (2) a smaller spot size with a higher power density was used (1.6 MW/cm2 vs 398 kW/cm2). Tests on welding battery components, including stacks of foils, buss bars, hairpins (for motors), and other components, with no porosity and no spatter will be covered. Both copper and mixed metals welding results will be presented. Tests have also been performed with 1 kW of laser power from a processing head with a 400 μm spot size and an average power density of 800 kW/cm2. Both systems have enough power density to initiate the keyhole welding process in copper, stainless steel, and aluminum. The difference in welding speeds for these two systems will be compared in this paper.

Dynamics for encircling an exceptional point in a nonlinear non-Hermitian system.

We study the dynamics near an exceptional point in a nonlinear non-Hermitian system consisting of a pair of resonators. One of the resonators has a linear loss, and the other resonator has a saturable gain. We show that the system dynamics exhibit chiral characteristics. And moreover, unique to the nonlinear system, such dynamics allow one to adiabatically switch between bistable states at the same system parameter. Such bistable switching is potentially interesting in optical memory based on coupled laser systems.

Complex Dynamics Induced by Asymmetry in Coupled Laser Systems

Complex Dynamics Induced by Asymmetry in Coupled Laser Systems

Spectral shaping of lasing in vertically aligned coupled nanowire lasers

We use numerical simulations to study the effects of nanowire geometry on the emission spectra of nanowire-based lasers. The studied nanowire lasers are made from gallium-nitride and the simulations are performed using finite difference time domain method. We show that changes in the diameter of the nanowire lasers also change the effective refractive index of their optical guided modes, which allows control over their Fabry-Perot spectrum. In addition, we show that evanescent coupling of two vertically standing nanowire lasers, having different cross section sizes, leads to a Vernier effect, which results in single mode emission from the small footprint coupled lasers system.

Exploring High Quality Chaotic Signal Generation in a Mutually Delay Coupled Semiconductor Lasers System

High quality chaotic signal generation in a mutually delay coupled semiconductor lasers (MDC-SLs) system is numerically explored by evaluating the time-delay signature (TDS) and complexity of chaotic signals. Autocorrelation function is utilized for quantitatively identifying the TDS of chaotic signal, and Kolmogorov–Sinai entropy and Kaplan–York dimensions are applied to estimate the complexity of chaotic signal. The results show that, under suitable parameters, two sets of chaotic signals with weak TDS and high complexity can be obtained simultaneously. By analyzing the influences of the mutual coupling strength and frequency detuning between the two MDC-SLs on the TDS and complexity of chaotic signals, the optimized parameter regions are specified for simultaneously generating two sets of high quality chaotic signals based on the MDC-SLs system.

Detuning Boundaries of Linear and Nonlinear Dynamics in a System of Coupled Lasers

The dynamics in a system of integrated self- and mutually-coupled lasers are analysed. For the first time, we present an overall comparison of the experimental and modelled dynamics of this system as functions of the frequency detuning and coupling magnitude parameters. First, the nontrivial parameters affecting the dynamics, namely the group index and group index dispersion, the relaxation oscillation frequencies of the two lasers, and the coupling magnitude, are calibrated. A transition in dynamics depending on detuning is then demonstrated, for a range of coupling magnitudes, in both the experimental measurements and by using a travelling-wave model of the system. The dynamics are analysed rigorously using the correlation dimension of the time series to identify regions of linear and nonlinear interaction, and distinct dynamic boundaries separating these regions are then calculated. In addition, we show, using results from the travelling-wave model, the profound effect of the coupling phase on the dynamics.

Inverse Vernier effect in coupled lasers

In this report we study the Vernier effect in coupled laser systems consisting of two cavities. We show that depending on the nature of their coupling, not only can the "supermodes" formed at the overlapping resonances of the coupled cavities have the lowest thresholds and lase first as previously suggested, leading to a manifestation of the typical Vernier effect now in an active system; these supermodes can also have increased thresholds and are hence suppressed, which can be viewed as an inverse Vernier effect. We attribute this effect to detuning-dependent Q-spoiling, and it can lead to an increased free spectrum range and possibly single-mode lasing, which may explain the experimental findings of several previous work. We illustrate this effect using two coupled micro-ring cavities and a micro-ring cavity coupled to a slab cavity, and we discuss its relation to the existence of exceptional points in coupled lasers.

Phase trapping and slipping in a forced hydrodynamically self-excited jet

AbstractIn a recent study on a coupled laser system, Thévenin et al. (Phys. Rev. Lett., vol. 107, 2011, 104101) reported the first experimental evidence of phase trapping, a partially synchronous state characterized by frequency locking without phase locking. To determine whether this state can arise in a hydrodynamic system, we reanalyse the data from our recent experiment on a periodically forced self-excited low-density jet (J. Fluid Mech., vol. 726, 2013, pp. 624–655). We find that this jet exhibits the full range of phase dynamics predicted by model oscillators with weak nonlinearity. These dynamics include (i) phase trapping between phase drifting and phase locking when the jet is forced far from its natural frequency and (ii) phase slipping during phase drifting when it is forced close to its natural frequency. This raises the possibility that similar phase dynamics can be found in other similarly self-excited flows. It also strengthens the validity of using low-dimensional nonlinear dynamical systems based on a universal amplitude equation to model such flows, many of which are of industrial importance.

Control of dynamical instability in semiconductor quantum nanostructures diode lasers: Role of phase-amplitude coupling

We numerically investigate the complex nonlinear dynamics for two independently coupled laser systems consisting of (i) mutually delay-coupled edge emitting diode lasers and (ii) injection-locked quantum nano-structures lasers. A comparative study in dependence on the dynamical role of alpha parameter, which determine the phase-amplitude coupling of the optical field, in both the cases is probed. The variation of alpha lead to conspicuous changes in the dynamics of both the systems, which are characterized and investigated as a function of optical injection strength for the fixed coupled-cavity delay time. Our analysis is based on the observation that the cross-correlation and bifurcation measures unveil the signature of enhancement of amplitude-death islands in which the coupled lasers mutually stay in stable phase-locked states. In addition, we provide a qualitative understanding of the physical mechanisms underlying the observed dynamical behavior and its dependence on alpha. The amplitude death and the existence of multiple amplitude death islands could be implemented for applications including diode lasers stabilization.

Optical spectrum behaviour of a coupled laser system under chaotic synchronization conditions

Synchronization characteristics of two bidirectionally coupled semiconductor lasers, one operating in a chaotic regime with low-frequency fluctuations and the other with free laser beam emission, were experimentally investigated. The chaotic synchronization regimes and optical spectral behavior of the coupled system were analyzed with respect to the optical spectra emitted initially by the two uncoupled lasers operated under the same feedback conditions. It was observed that the number of synchronization regimes that can be obtained and their stability depend on the similarity of spectral structures of the uncoupled lasers emissions. The dominant active laser modes of the coupled system emission coincide with the laser modes of the one or both uncoupled laser emissions, depending on the operating synchronization regime. We have associated changes in the optical spectrum of the coupled system with the synchronization regimes. The repartition of power between the active modes of a coupled system allows identification of the synchronization regime.

Consistency and complexity in coupled semiconductor lasers with time-delayed optical feedback

Consistency of response in a system driven repeatedly by a complex signal has been observed in many nonlinear dynamical systems. We investigate the consistency of unidirectionally coupled semiconductor lasers with optical feedback and measure the complexity of the entire laser system by using the Lyapunov spectrum. The complexity strongly depends on the degree of consistency. It is found that the complexity of the coupled laser system can be classified into three regions. When the system shows consistency, the complexity of the entire laser system corresponds to that of the solitary drive laser. In the inconsistency region, the complexity of the entire laser system corresponds to the sum of the complexity of the uncoupled drive and response lasers. The complexity increases more than the sum of the two solitary lasers near the boundary of the consistency region, where new dynamical fluctuations appear due to the optical carrier interaction between the two lasers.

Dynamics of two laterally coupled semiconductor lasers: Strong- and weak-coupling theory

The stability and nonlinear dynamics of two semiconductor lasers coupled side to side via evanescent waves are investigated by using three different models. In the composite-cavity model, the coupling between the lasers is accurately taken into account by calculating electric field profiles (composite-cavity modes) of the whole coupled-laser system. A bifurcation analysis of the composite-cavity model uncovers how different types of dynamics, including stationary phase-locking, periodic, quasiperiodic, and chaotic intensity oscillations, are organized. In the individual-laser model, the coupling between individual lasers is introduced phenomenologically with ad hoc coupling terms. Comparison with the composite-cavity model reveals drastic differences in the dynamics. To identify the causes of these differences, we derive a coupled-laser model with coupling terms which are consistent with the solution of the wave equation and the relevant boundary conditions. This coupled-laser model reproduces the dynamics of the composite-cavity model under weak-coupling conditions.

Continuous wavelet transform in the analysis of burst synchronization in a coupled laser system

The transition to synchronization of a pair of coupled chaotic CO2 lasers is investigated numerically in a model system. This system displays episodes of bursting of different predominant frequencies. Due to the multiple time scales present in this system, we use a complex continuous wavelet transform to perform the synchronization analysis. Thus it enables us to resolve the time of occurrence as well as the frequency of an event in a given time series up to an intrinsic uncertainty. Furthermore, due to the complex nature of that wavelet transform, it yields a direct estimate of the system's phase. We show that, as the coupling strength of the laser system is increased, the mutual coherency increases differently for different frequencies. Additionally we test our method with experimental data.

BOUNDS FOR THE SET CONTAINING ALL COMPACT INVARIANT SETS OF THE LINEARLY COUPLED LASER SYSTEM

In this paper we study the localization problem of all compact invariant sets of the five-dimensional coupled laser system by applying the first-order extremum conditions. Our main results consist in finding a number of localization sets formed by frusta, a circular cylinder, two parabolic cylinders and some other quadratic surfaces. Parameters of these localization sets are computed explicitly.

Laser ignition of flammable mixtures via a solid core optical fiber

To date no commercial fiber coupled laser systems have reached the irradiance and pulse energy required for flammable mixtures ignition. In this work we report preliminary results on the ignition of two-phase mixtures promoted by a laser delivering pulses through optical fiber. Experiments undertaken on free beam path configurations have allowed identification of the optical parameters required for laser ignition. The fiber coupled system used is based on a Q-switched nanosecond laser operating at 1064 nm. The fiber input angle and the focal length have been identified as the most important parameters. We demonstrated the possibility of delivering nanosecond pulses of 30 mJ focused onto a spot of 200 μm through a solid core optical fiber, and to promote ignition of n-heptane/air and JP4/air mixtures.

Synchronization on fast and slow dynamics in drive-response systems

Two types of synchronization, Achronal synchronization and Isochronous synchronization, are investigated numerically when unidirectionally coupled laser systems are considered both on fast and slow dynamics by studying the correlation function. Although the synchronization behaviors are found to be separated on relatively fast dynamics of chaotic fluctuations while blended together on slow dynamics of dropout events, their main features revealed are in good agreement, which most probably suggests that the observation based on detections on slow dynamics is of importance when it is the only choice as detection on fast dynamics is usually extremely hard to be done.

Generalized models as a universal approach to the analysis of nonlinear dynamical systems

We present a universal approach to the investigation of the dynamics in generalized models. In these models the processes that are taken into account are not restricted to specific functional forms. Therefore a single generalized models can describe a class of systems which share a similar structure. Despite this generality, the proposed approach allows us to study the dynamical properties of generalized models efficiently in the framework of local bifurcation theory. The approach is based on a normalization procedure that is used to identify natural parameters of the system. The Jacobian in a steady state is then derived as a function of these parameters. The analytical computation of local bifurcations using computer algebra reveals conditions for the local asymptotic stability of steady states and provides certain insights on the global dynamics of the system. The proposed approach yields a close connection between modelling and nonlinear dynamics. We illustrate the investigation of generalized models by considering examples from three different disciplines of science: a socioeconomic model of dynastic cycles in china, a model for a coupled laser system and a general ecological food web.

Preference of Chaotic Synchronization in a Coupled LaserSystem**The project supported by the Natural Science Foundation ofJiangsu Province of China under Grant No. BK2001138

In a coupled laser system, the dynamics of the receiving laser isinvestigated when two separate transmitting lasers are injectedinto the receiving laser with different coupling strengths. It isshown that the phenomenon of preference of chaotic synchronizationappears under appropriate coupling conditions. The receiving laserwill entrain the pulses of either one or both transmitting laserswhen the coupling is strong while it keeps its own dynamics whenthe coupling is weak.

COLOR MAPS OF THE KAPLAN–YORKE DIMENSION IN OPTICALLY DRIVEN LASERS: MAXIMIZING THE DIMENSION AND ALMOST-HAMILTONIAN CHAOS

In this paper we investigate the geometrical complexity of chaotic attractors in optically injected lasers by calculating the Kaplan–Yorke dimension (D KY ) together with the largest Lyapunov exponent (LLE). For the first time to the best of our knowledge, a color map is constructed indicating regions in the parameter space with high and low dimensionality. It is shown that as the injection power K of the master to the slave laser is increased to high values, the dimension of the attractor decreases for all ranges of the frequency detuning ω. The relation between the trace of the Jacobian matrix and the Kaplan–Yorke dimension shows that the optically injected laser is a system capable to span the dimension anywhere between 2 and 3. A qualitative analysis is finally made in order to identify the material parameters of the slave device for this coupled laser system for a result in the maximized possible dimension, leading to the need for a high electron lifetime and a high linewidth enhancement factor that occur at low temperature operation for a solitary slave semiconductor laser.

Bifurcations and interacting modes in coupled lasers: A strong-coupling theory

The paper presents a theoretical study of synchronization between two coupled lasers. A theory valid for arbitrary coupling between lasers is used. Its key feature is that the laser field is decomposed in terms of the composite-cavity modes reflecting the spatial field dependence over the entire coupled-laser system. The ensuing multimode equations are reduced to class-B, and further to class-A equations which resemble competing species equations. Bifurcation analysis, supported by insight provided by analytical solutions, is used to investigate influences of pump, carrier decay rate, polarization decay rate, and coupling mirror losses on synchronization between lasers. Population pulsation is found to be an essential mode competition mechanism responsible for bistability in the synchronized solutions. Finally, we discovered that the mechanism leading to laser synchronization changes from strong composite-cavity mode competition in class-A regime to frequency locking of composite-cavity modes in class-B regime.