Dynamics of coupled self-pulsating semiconductor lasers

We introduce and analyze a model for the dynamics of two coupled self-pulsating semiconductor lasers. We investigate the role of the complex coupling coefficient in the static and dynamic properties of the device. We find conditions for the emergence of coherent laser pulses, in which the two lasers display synchronous coherent self-pulsations (self-pulsating super modes). Nonlinear dynamics and two different routes to chaos are also individuated and discussed.     

Modeling self-pulsating DFB lasers with an integrated phase tuningsection

A theoretical model of a self-pulsating three-section DFB laser with an integrated phase tuning section is established. It is based on traveling wave equations and the standard carrier rate equations. Parameters of an existing device are used for applying the model. Key conditions and characteristics of self-pulsations (SPs) are modeled and compared with experimental results. The important role of phase tuning for turning on the SP is pointed out. The dependence of the SP regime on the detuning between the Bragg wavelengths in the laser and reflector is determined and the essential role of phase-readjustment is identified. Frequency tuning via the laser currents, as well as the pulse shape at various frequencies, is investigated. This allows us to identify the mechanism for frequency tuning. The model turns out to be a good tool to improve our knowledge of the self-pulsation effect and to design optimized devices     

混合光栅型的三段式自脉动DFB激光器

制作了1.55μm InGaAsP-InP三段式混合光栅型DFB激光器.观察到了20GHz左右的自脉动信号.讨论了自脉动的产生机制,并且对调相区所起的作用进行了研究.     

High-frequency pulsations in DFB lasers with amplified feedback

We describe the basic ideas behind the concept of distributed feedback (DFB) lasers with short optical feedback for the generation of high-frequency self-pulsations and show the theoretical background describing realized devices. It is predicted by theory that the self-pulsation frequency increases with increasing feedback strength. To provide evidence for this, we propose a novel device design which employs an amplifier section in the integrated feedback cavity of a DFB laser. We present results from numerical simulations and experiments. It has been shown experimentally that a continuous tuning of the self-pulsation frequency from 12 to 45 GHz can be adjusted via the control of the feedback strength. The numerical simulations, which are in good accordance with experimental investigations, give an explanation for a self-stabilizing effect of the self-pulsations due to the additional carrier dynamic in the integrated feedback cavity.     

Widely frequency-tunable amplified feedback lasers for 10-GHz optical pulsation

Monolithic amplified feedback semiconductor lasers are demonstrated as a new solution to 10-GHz optical pulsation, where self-pulsations are generated according to the concept of a single-mode laser with shortly delayed optical feedback. They consist of a loss-coupled distributed feedback section operating as a single-mode laser and an integrated feedback cavity including a phase control, an amplifier, and a transparency section. The pulsation frequency is continuously tunable in the range of 7-11.5 GHz with an extinction ratio above 6.5 dB, which indicates that precise control of a cavity length is not needed.     

Self-Pulsating Amplified Feedback Laser Based on a Loss-Coupled DFB Laser

Monolithic self-pulsating semiconductor lasers called amplified feedback lasers (AFLs) can generate high-frequency self-pulsations according to the concept of a single-mode laser with shortly delayed optical feedback, which consist of a distributed-feedback (DFB) laser, a phase control, and an amplifier section. Since mode degeneracy of the DFB section, which should operate as a single-mode laser, affects the self-pulsation, single-mode characteristics of the DFB section are critical for the self-pulsation. The effect of a complex coupling in the DFB section on the self-pulsation is numerically analyzed to reveal that the complex coupling provides a wide operation range for the self-pulsation. Also, self-pulsating AFLs based on a loss-coupled DFB laser are experimentally demonstrated to verify the self-pulsation characteristics and the capability for all-optical clock recovery.     

Longitudinal mode competition in chirped grating distributedfeedback lasers

Gain competition often inhibits the simultaneous lasing of multiple longitudinal modes in homogeneously broadened laser systems. A stability analysis is developed to demonstrate that the two lowest order degenerate longitudinal modes in an index-coupled distributed feedback (DFB) laser will lase simultaneously when the index grating is chirped asymmetrically along the axis of the device. This chirped grating structure is shown to decrease the gain competition by reducing the spatial overlap between the degenerate modes. Stable mode beating between the two lowest order lasing modes results, and this beating produces high-frequency output self-pulsations which can be used for millimeter-wave and soliton pulse train generation. An exact closed-form expression for the output intensity of an antisymmetrically chirped index-coupled DFB laser, as a function of the unsaturated gain, is also derived. The expression is valid for arbitrary levels of gain saturation     

Electrically switchable self-pulsations in integratable multisection DFB-lasers

An integratable version of a self-pulsating AR-coated multisection DFB-laser with a phase-tuning section integrated between two DFB sections is presented. It is shown that the phase current can act as an electrical switch for turning ON and OFF the self-pulsations. In addition, the frequency stability can be improved by properly adjusting the phase current. The frequency of the self-pulsations is electrically tunable over more than one octave continuously.     

Self-pulsating semiconductor lasers: theory and experiment

We report detailed measurements of the pump-current dependency of the self-pulsating frequency of semiconductor CD lasers. A distinct kink in this dependence is found and explained using a rate-equation model. The kink denotes a transition between a region where the self-pulsations are weakly sustained relaxation oscillations and a region where Q-switching takes place. Simulations show that spontaneous emission noise plays a crucial role in the cross-over     

Temporal dynamics and facet coating requirements of monolithic MOPAsemiconductor lasers

Dynamics of temporal instabilities in tapered broad-area InGaAs SQW semiconductor master oscillator/power amplifier lasers are experimentally investigated. Multigigahertz self-pulsations are evident in the optical and temporal spectra along with coupled-cavity modes due to the finite output-facet reflectivity. From a previously developed time-dependent coupled-wave model, it is shown that by adjusting the grating strength in the distributed Bragg reflector (DBR) section of the master oscillator the antireflection facet coating requirements can be relaxed. Increased stable operation at high powers is then possible     

Mode competition and mode locking in compound cavity semiconductorlasers

This letter concerns the mode-competition effect for compound-cavity lasers. Considering the mode properties of such a system we show, that the side cavity mode having the same threshold gain as the lasing one, may correspond to either stable or unstable modes of laser operation. In the last case the mode competition leads to locking of the lasing and side modes yielding high-frequency self-pulsations in some range of the system parameters. This effect can be used for the design of self-pulsating SLs generating optical pulses in the microwave region without an intracavity saturable absorber     

Impact of gain dispersion on the spatio-temporal dynamics ofmultisection lasers

We present a refined model for multi-section lasers, introducing an additional equation for material polarization in the well-known traveling wave model. We investigate the polarization-induced changes in the spectral properties of the optical waveguide. Finally, we show the relevance of this model for a more realistic simulation of the complicated dynamical behavior of multi-section distributed feedback (DFB) lasers, such as fast self-pulsations, multi-stability, and hysteresis effects due to mode competition     

A novel structure of DFB laser for the generation of 40 GHz self-pulsation

1.55·μm InGaAsP-InP two-section DFB lasers with varied ridge width, both gain-coupled and index-coupled, have been fabricated. Self-pulsations with frequencies around 40 GHz are observed. The related mechanism and the tunability of generated self-pulsations is studied.     

Dispersive self-Q-switching in self-pulsating DFB lasers

Self-pulsations reproducibly achieved in newly developed lasers with two distributed feedback sections and with an additional phase tuning section are investigated. The existence of the dispersive self-Q-switching mechanism for generating the high-frequency self-pulsations is verified experimentally for the first time. This effect is clearly distinguished from other possible self-pulsation mechanisms by detecting the single-mode type of the self-pulsation and the operation of one section near the transparency current density using it as a reflector with dispersive feedback. The operating conditions for generating this self-pulsation type are analyzed. It is revealed that the required critical detuning of the Bragg wavelengths of the two DFB sections is achieved by a combination of electronic wavelength tuning and current-induced heating. The previous reproducibility problems of self-pulsations in two-section DFB lasers operated at, in principle, suited current conditions are discussed, and the essential role of an electrical phase-control section for achieving reproducible device properties is pointed out. Furthermore, it is demonstrated that phase tuning can be used for extending the self-pulsation regime and for optimizing the frequency stability of the self-pulsation. Improved performance of the devices applied as optical clocks thus can be expected     

Self-pulsations in vertical-cavity surface emitting lasers

The dynamic characteristics of surface emitting lasers with a saturable absorbing layer are modelled and shown to exhibit self-sustained pulsations up to 10 GHz. A set of coupled rate equations is used to investigate the regions of self-pulsations, and show that there is an optimum position for the absorbing layer within the cavity structure to produce the widest range of pulsation frequencies     

Optical feedback on self-pulsating semiconductor lasers

Weak optical-feedback effects on the statistical properties of self-pulsations in narrow-stripe semiconductor lasers are analyzed using Lang-Kobayashi-type equations. The self-pulsation features are compared with the characteristics of excited relaxation oscillations. We determine the operating regime in which the randomizing effect of spontaneous-emission noise destroys pulse coherence. In this regime, only phase-insensitive effects of optical feedback are possible, and optimum jitter reduction is achieved with delay times of the order of an integer-odd multiple of the free-running pulsation period. In the high-pump operating regime, interpulse coherence is retained and the optical-feedback phase is shown to be instrumental for pulse-jitter control. Our results show that for cavity lengths up to 10 cm, variations on the order of half an optical wavelength induce jitter variations of one order of magnitude     

Observation of bistability in GaAs quantum-well vertical-cavitysurface-emitting lasers

Two different types of bistability in proton-implanted GaAs quantum-well (QW) vertical-cavity surface-emitting lasers (VCSELs) have been observed. The first type of bistability has a small hysteresis width (~50 μA) in the light versus current and voltage versus current characteristics. Light-induced large negative differential resistance, random fluctuations, and self-pulsations are observed at the switching point. The emission patterns show that the bistability occurs at a spatially localized area under the output facet that covers only a small fraction of the N1S-μm-diameter aperture. The bistability stems from spatially localized saturable absorption. The second type of bistability has a large hysteresis width (~1 μA) in the L-1 characteristics and is observed well above the threshold current. In this case, no observable bistable loop exists in the voltage versus current characteristics, and the bistability is associated with transverse mode-hopping     

紫外激光与半导体相互作用研究进展综述

紫外激光与半导体相互作用是当今国内外研究的热点。综述了紫外激光与半导体相互作用在光电子产业、激光加工、激光表面改性等方面的应用。介绍了紫外激光与半导体相互作用的基本原理,总结了紫外激光烧蚀半导体的理论模型,包括热传导模型、载流子耦合扩散模型、光化学模型、表面热蒸发模型、双温模型、表面充电模型等。总结了关于损伤形貌、烧蚀阈值和紫外激光损伤半导体机理的实验研究。提出了紫外激光与半导体相互作用可能的研究和新的应用方向。     

Distributed-feedback grating used as a lateral-mode selector in phase-locked antiguided arrays

We show here, for the first time, that a distributed-feedback (DFB) grating can act as a lateral-mode discriminator if located below the active region of a phase-locked antiguided array. Spatial-mode selection from such a lower-DFB (LDFB) grating in a resonant antiguided structure (ROW-LDFB) relies on the fact that the optical field distribution below the active region is strongly array-mode dependent. In particular, it is shown that at and near resonance a ROW-LDFB structure strongly favors resonant-mode oscillation, while suppressing oscillation of high-order modes. ROW-LDFB devices thus accomplish both spatial- and frequency-mode selection in a single structure. Furthermore, for effective intermodal discrimination, there is no need for interelement loss or Talbot-type filters, thus eliminating all potential sources of self-pulsations.     

激光与聚合物的相互作用及其应用

激光与聚合物相互作用研究是当今国内外研究的热点.综述了激光与聚合物相互作用在激光加工、液晶光学器件激光束控制、微激光等离子体推进器等方面的应用.介绍了激光与聚合物相互作用的基本原理,总结了激光烧蚀聚合物的理论模型,包括光化学模型、光热模型、稳态模型、羽饰屏蔽模型以及光热—光化学模型.总结了关于烧蚀率、烧蚀阈值和聚合物激光烧蚀分解机理的实验研究.提出了激光与聚合物相互作用可能的研究和新的应用方向.