Line shapes of near-infrared DFB and VCSEL diode lasers under the influence of system back reflections

Laser diode line widths and line shapes are experimentally investigated in dependence on the diode current and on back reflections from an optical system. Four distributed-feedback (DFB)-type diode lasers and two vertical-cavity surface-emitting lasers (VCSELs) have been tested within the same optical setup and using the same fitting methods. System back reflection ratios of light reflected back to the laser have been varied between ?1?dB and ?45?dB and were below ?60?dB when all reflections were blocked. The background of this investigation is the evaluation of different laser types with respect to their suitability for sensor applications in which optical back reflections may occur, for example tunable diode-laser spectroscopy (TDLS). While DFB-type lasers showed almost pure Lorentzian line shapes and line widths of a few MHz, the tested VCSELs had a strong Gaussian contribution to the line shape, indicating stronger 1/f noise, which was also observed in the relative intensity noise of these particular lasers. System reflection ratios above ?25?dB had strong effects on the line width in both DFB diode lasers and VCSELs, while some influences have been observed at even lower reflection ratios for DFB diode lasers. As much smaller reflection ratios are typically required in TDLS systems to avoid etalon-like fringes and self-mixing interference effects, we conclude that the influence on the line width is not the most important reason to minimize back reflections in practical TDLS systems or to choose one type of diode laser over the other.     

Synchronization of mutually coupled chaotic lasers in the presence of a shutter

Two mutually coupled chaotic diode lasers exhibit stable isochronal synchronization in the presence of self-feedback. When the mutual communication between the lasers is discontinued by a shutter and the two uncoupled lasers are subject to self-feedback only, the desynchronization time is found to scale as Adtau, where Ad>1 and tau corresponds to the optical distance between the lasers. Prior to synchronization, when the two lasers are uncorrelated and the shutter between them is opened, the synchronization time is found to be much shorter, though still proportional to tau. As a consequence of these results, the synchronization is not significantly altered if the shutter is opened or closed faster than the desynchronization time. Experiments in which the coupling between two chaotic-synchronized diode lasers is modulated with an electro-optic shutter are found to be consistent with the results of numerical simulations.     

Low-frequency dynamics of lasers

An approach to nonlinear dynamics of multimode lasers is developed. It is based on the concept of two systems of eigenoscillations: optical modes and relaxation oscillations. The importance of a correct (not arbitrary) choice of a model is underlined. Characteristic features of two different rate equation models are formulated and compared. A method of selective perturbation on the system is described which makes it possible to study interrelations between optical modes and relaxation oscillations, and to control dynamical behavior of a laser. The possibility of using dynamical regularities for solving both applied and basic problems is illustrated in several examples. (c) 1996 American Institute of Physics.     

Synchronization of chaotic semiconductor laser systems: a vectorial coupling-dependent scenario

We demonstrate the influence of vectorial coupling on the synchronization behavior of complex systems. We study two semiconductor lasers subject to delayed optical feedback which are unidirectionally coherently coupled via their optical fields. Our experimental and numerical results demonstrate a characteristic synchronization scenario in dependence on the relative feedback phase leading cyclically from chaos synchronization to almost uncorrelated states, and back to chaos synchronization. Finally, we reveal the influence of the feedback phase on the dynamics of the solitary delay system.     

Coupling-mediated ghost resonance in mutually injected lasers

We experimentally and numerically study the phenomenon of ghost resonance in coupled nonlinear systems. Two mutually injected semiconductor lasers are externally perturbed in their pump currents by two respective periodic signals of different frequencies f(1) and f(2). For small amplitudes of the external modulations, the two laser intensities display synchronized optical pulses, in the form of dropout events occurring at irregular times. By adjusting the amplitude and frequencies of the driving signals, the system exhibits a ghost resonance in the dropout appearance at a frequency f(r) not present in the distributed inputs.     

Theory of carrier gain dynamics in InGaAs/AlGaAs strained-layer single-quantum-well diode lasers

Calculations of the femtosecond gain dynamics in InGaAs/AlGaAs strained-layer single-quantum-well diode lasers are presented and compared to experiments which use a novel multiple-wavelength pump probe technique. We develop a detailed theoretical model for the gain dynamics in a quantum well laser diode structure to aid in the interpretation of gain dynamics induced by both interband absorption and stimulated emission of photons. In the model, transient gain and differential transmission are computed in a multiband effective mass model including biaxial strain, valence subband mixing, and scattering both within and between subbands. The transient photogeneration of electron-hole pairs by the pump pulse and subsequent relaxation of carriers by both polar optical phonon scattering and carrier-carrier scattering are calculated within a Boltzmann equation framework. A relaxation approximation for the carrier-carrier scattering is made making the coupled Boltzmann equations an effective one dimensional model which are then solved using an adaptive Runge-Kutta technique rather than a more computationally intensive Monte Carlo approach.     

Laser-noise-induced correlations and anti-correlations in electromagnetically induced transparency

High degrees of intensity correlation between two independent lasers were observed after propagation through a rubidium vapor cell in which they generate Electromagnetically Induced Transparency (EIT). As the optical field intensities are increased, the correlation changes sign (becoming anti-correlation). The experiment was performed in a room temperature rubidium cell, using two diode lasers tuned to the ⁸⁵Rb D₂ line (λ= 780 nm). The cross-correlation spectral function for the pump and probe fields is numerically obtained by modeling the temporal dynamics of both field phases as diffusing processes. We explored the dependence of the atomic response on the atom-field Rabi frequencies, optical detuning and Doppler width. The results show that resonant phase-noise to amplitude-noise conversion is at the origin of the observed signal and the change in sign for the correlation coefficient can be explained as a consequence of the competition between EIT and Raman resonance processes.     

Time delay induced partial death patterns with conjugate coupling in relay oscillators

We investigate the dynamics of delay-coupled relay oscillators with conjugate (or dissimilar) coupling and find the partial death with the phase-flip transition. This phenomenon is quite general and occurs for the limit cycle as well as chaotic relay oscillators. In the regime of partial death, parts of the system oscillate with large amplitude, while other element stays at rest. Using the Stuart-Landau and Rössler oscillators, we demonstrate that partial amplitude death is a robust dynamical state in coupled oscillators. We also studied the mismatch delay and find different types of dynamical pattern with partial death.     

Nonreciprocal switching thresholds in coupled nonlinear microcavities

A concept for the design of nonlinear optical diodes is proposed that uses the multistability of coupled nonlinear microcavities and the dependence of switching thresholds on the direction of incidence. A typical example of such a diode can be created by combining two mirror-symmetric microcavities where modes of the opposite parity dominate. It is shown that a strong nonreciprocal behavior can be achieved together with a negligible insertion loss. To describe the dynamical properties of such systems, a model based on the coupled-mode theory is developed, and a possible implementation in the form of multilayered structures is considered.     

Bistable polarization switching in mutually coupled vertical-cavity surface-emitting lasers

We theoretically investigate the polarization-resolved dynamics of two vertical-cavity surface-emitting semiconductor lasers that are mutually coupled through coherent optical injection. We find a sequence of bistable polarization switchings that can be induced by changing either the coupling strength or the optical propagation phase. The successive polarization switchings are correlated with the creation of new compoundcavity modes when these parameters are continuously varied.     

Resurgence of oscillation in coupled oscillators under delayed cyclic interaction

This paper investigates the emergence of amplitude death and revival of oscillations from the suppression states in a system of coupled dynamical units interacting through delayed cyclic mode. In order to resurrect the oscillation from amplitude death state, we introduce asymmetry and feedback parameter in the cyclic coupling forms as a result of which the death region shrinks due to higher asymmetry and lower feedback parameter values for coupled oscillatory systems. Some analytical conditions are derived for amplitude death and revival of oscillations in two coupled limit cycle oscillators and corresponding numerical simulations confirm the obtained theoretical results. We also report that the death state and revival of oscillations from quenched state are possible in the network of identical coupled oscillators. The proposed mechanism has also been examined using chaotic Lorenz oscillator.     

Diode laser based light sources for biomedical applications

Diode lasers are by far the most efficient lasers currently available. With the ever‐continuing improvement in diode laser technology, this type of laser has become increasingly attractive for a wide range of biomedical applications. Compared to the characteristics of competing laser systems, diode lasers simultaneously offer tunability, high‐power emission and compact size at fairly low cost. Therefore, diode lasers are increasingly preferred in important applications, such as photocoagulation, optical coherence tomography, diffuse optical imaging, fluorescence lifetime imaging, and terahertz imaging. This review provides an overview of the latest development of diode laser technology and systems and their use within selected biomedical applications. 670 nm external cavity diode laser for Raman spectroscopy built on a 13 × 4 mm² microbench (Copyright FBH/Schurian.com ).     

Synchronization and time shifts of dynamical patterns for mutually delay-coupled fiber ring lasers

A pair of coupled erbium doped fiber ring lasers is used to explore the dynamics of coupled spatiotemporal systems. The lasers are mutually coupled with a coupling delay less than the cavity round-trip time. We study synchronization between the two lasers in the experiment and in a delay differential equation model of the system. Because the lasers are internally perturbed by spontaneous emission, we include a noise source in the model to obtain stochastic realizations of the deterministic equations. Both amplitude synchronization and phase synchronization are considered. We use the Hilbert transform to define the phase variable and compute phase synchronization. We find that synchronization increases with coupling strength in the experiment and the model. When the time series from two lasers are time shifted in either direction by the delay time, approximately equal synchronization is frequently observed, so that a clear leader and follower cannot be identified. We define an algorithm to determine which laser leads the other when the synchronization is sufficiently different with one direction of time shift, and statistics of switches in leader and follower are studied. The frequency of switching between leader and follower increases with coupling strength, as might be expected since the lasers mutually influence each other more effectively with stronger coupling.     

Stability of Optically Injected Two‐State Quantum‐Dot Lasers

Simultaneous two‐state lasing is a unique property of semiconductor quantum‐dot (QD) lasers. This not only changes steady‐state characteristics of the laser device but also its dynamic response to perturbations. In this paper we investigate the dynamic stability of QD lasers in an external optical injection setup. Compared to conventional single‐state laser devices, we find a strong suppression of dynamical instabilities in two‐state lasers. Furthermore, depending on the frequency and intensity of the injected light, pronounced areas of bistability between both lasing frequencies appear, which can be employed for fast optical switching in all‐optical photonic computing applications. These results emphasize the suitability of QD semiconductor lasers in future integrated optoelectronic systems where a high level of stability is required.     

Dual-lag synchronization between coupled chaotic lasers due to path-delay interference

We study experimentally the synchronization dynamics of two semiconductor lasers coupled unidirectionally via two different delayed paths. The emitter laser operates in a chaotic regime characterized by low-frequency fluctuations due to optical feedback and induces a synchronized dynamical activity in the receiver laser, which operates in the continuous-wave regime when uncoupled. Different delays in the two coupling paths lead to the coexistence of two time lags in the synchronized dynamics of the oscillators. This dual-lag synchronization degrades the average synchronization quality of the system of coupled lasers and hinders the transmission of information between them. Numerical simulation results agree with the experimental observations, and allow us to explore this phenomenon in a wide parameter range, and quantify the degree of signal transmission degradation caused by this chaotic path-delay interference.     

Absorptive and dispersive bistability in semiconductor injection lasers

Recent progress in the study of both absorptive and dispersive bistability in semiconductor injection lasers is reported. Inhomogeneously excited semiconductor lasers as an absorptive case, and laser diode optical amplifiers and optical injection locking systems of laser diodes as dispersive cases, are described. Applications of bistable semiconductor lasers, such as optical memories, optical regenerative amplifiers and all-optical switching, are also discussed.     

Crowd synchrony and quorum sensing in delay-coupled lasers

Crowd synchrony and quorum sensing arise when a large number of dynamical elements communicate with each other via a common information pool. Previous evidence has shown that this type of coupling leads to synchronization, when coupling is instantaneous and the number of coupled elements is large enough. Here we consider a situation in which the transmission of information between the system components and the coupling pool is not instantaneous. To that end, we model a system of semiconductor lasers optically coupled to a central laser with a delay. Our results show that, even though the lasers are nonidentical due to their distinct optical frequencies, zero-lag synchronization arises. By changing a system parameter, we can switch between two different types of synchronization transition. The dependence of the transition with respect to the delay-coupling parameters is studied.     

Resonance amplifier model describing diode lasers coupled to short external resonators

A model of a regenerative resonance amplifier has been used to describe Fabry-Perot GaAs diode lasers coupled to short external resonators. The optical gain and the equivalent input of the resonance amplifier are controlled by means of appropriate rate equations. Homogeneous line-broadening and proportionality between gain and electron density have been assumed in this approach. The model was applied to a diode laser coupled to an external plane mirror placed at a variable distance from one diode mirror. A numerical evaluation of the coupling coefficient between laser and external cavity enables to predict the measured power output and mode selective properties of the coupled system. A similar analysis was done for diode lasers coupled to external hemispherical resonators. The model calculation confirms the previously demonstrated mode selective properties of the hemispherical configuration which permitted stable single mode operation.     

基于外光注入互耦合垂直腔面发射激光器的混沌随机特性研究

通过在互耦合垂直腔面发射激光器(VCSELs)系统中增加外光注入, 建立了一种基于偏振可调光反馈VCSEL驱动互耦合VCSELs混沌系统模型, 分析了增加外光驱动对互耦合激光器随机特性的影响. 以不可预测度作为随机特性的评价指标, 采用信息论中的排列熵作为相应量化工具, 对系统输出混沌信号的不可预测性进行定量分析.数值研究了光强度、时延、偏振旋转角度以及驱动激光器与耦合激光器间的频率失谐对输出信号随机特性的影响.结果表明: 外光注入能够增大互耦合VCSELs输出混沌信号的排列熵, 即外光注入能够有效提高耦合系统的随机特性; 驱动激光器可调偏振片偏转角度调节到45° 附近, 注入强度适中, 满足耦合强度大于驱动激光器自反馈强度条件, 系统输出信号的排列熵较大; 在耦合时延与驱动激光器反馈时延不相等的同时, 增加驱动激光器与耦合激光器频率失谐, 外光注入互耦合VCSELs的随机特性能够得到进一步提高.