Power Dropouts Research Articles

Effect of Spontaneous Emission Noise and Modulation on Semiconductor Lasers Near Threshold with Optical Feedback

The dynamical behavior of power dropouts in a semiconductor laser with optical feedback, pumped near threshold current, is strongly influenced by quantum noise. This is clearly demonstrated by experiments with modulations on the pumping current or the feedback strength. For the cases without modulation and with only current modulation, the dropouts occur randomly. However the feedback strength modulation locks the dropout events periodically. By numerically modeling these three cases using the Lang–Kobayashi equations with a stochastic term to take into account spontaneous emission noise, it is shown that the observed behavior of the dropouts can be readily reproduced for all three cases. Noise plays a signifcant role in explaining the observed dropout events. A simple explanation of the observed dropout phenomenon is presented, based on the adiabatic motion of the ellipse formed by the steady state solutions of the rate equations due to slow time modulations of the injection current or the feedback strength.

Fast pulsing dynamics of a vertical-cavity surface-emitting laser operating in the low-frequency fluctuation regime

We analyze the dynamics of a vertical-cavity surface-emitting laser with optical feedback operating in the low-frequency fluctuation regime. By focusing on the fast pulsing dynamics, we show that the two linearly polarized modes of the laser exhibit two qualitatively different behaviors: they emit pulses in phase just after a power dropout and they emit pulses out of phase after the recovery process of the output power. As a consequence, two distinct statistical distributions of the fast pulsating total intensity are observed, either monotonically decaying from the noise level or peaked around the mean intensity value. We further show that gain self-saturation of the lasing transition strongly modifies the shape of the intensity distribution.

ENTRAINMENT OF OPTICAL LOW-FREQUENCY FLUCTUATIONS IS ENHANCED BY COUPLING

The control of the low-frequency fluctuations exhibited by two mutually coupled semiconductor lasers is studied experimentally and numerically. We observe that coupling enhances the response of the system to a weak periodic modulation of the injection current of one of the lasers, leading to a highly efficient entrainment of the synchronized low-frequency power dropouts to the external periodic driving. We compare the quality of the entrainment with the one obtained in a single semiconductor laser with optical feedback, showing the beneficial role of coupling in this pursuit. The experimental observations are satisfactorily reproduced by numerical simulations of a set of coupled delay-differential rate equations.

Periodic entrainment of power dropouts in mutually coupled semiconductor lasers

We examine the effect of current modulation in the irregular dropout dynamics exhibited by two mutually coupled semiconductor lasers. Our experimental results show that a weak periodic modulation in the injection current of one of the lasers entrains the power dropouts in a very efficient way. It is also observed that the laser with the highest frequency leads the dynamics independent of which laser is modulated. As a result, the entrainment is anticipative when modulation is applied to the laser with lowest frequency. Numerical simulations of a model based on delay-coupled rate equations successfully reproduce the behavior observed.

Asymmetric and delayed activation of side modes in multimode semiconductor lasers with optical feedback

We study the multimode dynamics of a semiconductor laser with optical feedback operating in the low-frequency fluctuation regime. A multimode extension of the Lang–Kobayashi (LK) model shows, in agreement with experimental observations, that the low-frequency power dropouts exhibited by the main modes are accompanied by sudden, asymmetric, activations of dormant longitudinal side modes. Furthermore, these activations are delayed with respect to the dropouts of the active modes. In order to satisfactorily reproduce both the asymmetric activation of side modes and their delay with respect to the dropouts, the generalized LK model has to include a parabolic gain profile, together with a frequency shift of the gain curve with carrier population.

Synchronization and antisynchronization of chaotic power drop-outs and jump-ups of coupled semiconductor lasers

Experimental observations and numerical simulations of synchronization and antisynchronization of low-frequency power drop-outs and jump-ups of chaotic semiconductor lasers are presented.

Stochastic entrainment of optical power dropouts.

We show that the natural pulsed behavior, in the form of sudden power dropouts, exhibited by semiconductor lasers subject to optical feedback can be entrained by the joint action of external noise and weak periodic driving. These power dropouts, which in the absence of forcing do not occur periodically, acquire the periodicity of the harmonic driving for an optimal amount of external noise, in what constitutes a form of stochastic resonance. This phenomenon is analyzed by means of a generalized Lang-Kobayashi model with external nonwhite noise in the modulated pump current, in terms of both the temporal correlation and the amplitude of the noise.

Dynamics of modal power distribution in a multimode semiconductor laser with optical feedback

The dynamics of power distribution between longitudinal modes of a multimode semiconductor laser subjected to external optical feedback is experimentally analyzed in the low-frequency fluctuation regime. Power dropouts in the total light intensity are invariably accompanied by sudden activations of several longitudinal modes. These activations are seen not to be simultaneous to the dropouts, but to occur after them. The phenomenon is statistically analysed in a systematic way, and the corresponding delay is estimated.

Low-frequency fluctuations in external cavity semiconductor lasers: understanding based on a simple dynamical model

We investigate the dynamical origin of low-frequency fluctuations (LFFs) in external cavity semiconductor lasers by utilizing a simplified, three-dimensional model derived from the infinite-dimensional Lang-Kobayashi (LK) equations. The simplified model preserves the dynamical properties of the external-cavity modes (ECMs) and antimodes which play a fundamental role in the generation of LFFs. This model yields a clear picture of the dynamical origin of the LFFs. Two distinct regions are observed in parameter space, one with the presence of the maximum-gain mode (MGM), and another without it. In particular, we show that, in the absence of noise, LFFs are a consequence of the dynamical interactions among different ECMs and antimodes. When a small amount of noise is present, LFFs result from an intermittent switching of trajectories among different coexisting attractors in the phase space. The presence of double peaks in the distribution of power dropout times, which has been observed recently in experiments, is explained, and a scaling relation is obtained between the average switching time and the noise strength.

Low-frequency fluctuations in a semiconductor laser with phase conjugate feedback

We present numerical results obtained for a semiconductor laser with phase conjugate optical feedback. We observe both low and high frequency dynamics similar to that observed with conventional optical feedback. Here also the low frequency fluctuations correspond to power drop-outs while the high frequency fluctuations being associated with fast pulses in the power. Due to the absence of conventional external cavity modes in phase conjugate feedback, current models do not predict this behaviour.

Synchronization of power dropouts in coupled semiconductor lasers with external feedback

Synchronization of power dropouts in coupled semiconductor lasers with external feedback

Recovery process of low-frequency fluctuations in laser diodes with external optical feedback.

The recovery process of low-frequency fluctuations is experimentally investigated in a semiconductor laser with external optical feedback. Monotonic stepwise increase of the light output intensity is observed for various external cavity lengths. The time interval of each step corresponds to the round-trip time of the external cavity. Transitions between steps and dynamics within each step are analyzed. Experimental results are discussed with reference to the scenario of chaotic mode itinerancy. Statistical properties of time intervals between power dropout events are also discussed.

Semiconductor laser with optical feedback: from excitable to deterministic low-frequency fluctuations.

Semiconductor lasers with optical feedback present a regime in which power dropouts are observed. Although this regime has been extensively studied, there is no agreement about whether the dropouts are deterministically or stochastically generated. In this paper we will study the statistics of time intervals between dropouts assuming noise-driven simple excitable models. We explain the appearance of characteristic times in the first return maps.

Numerical statistics of power dropouts based on the Lang-Kobayashi model.

The statistics of power dropouts in semiconductor lasers subjected to delayed optical feedback have been numerically investigated using the Lang-Kobayashi model. The data from the numerical simulations have then been used to calculate the probability distribution functions, mean values, and return maps of the time that elapses between dropouts. In addition, the transition from the "low frequency fluctuation" to the "coherence collapse" regime has also been investigated. The numerical simulations compare well with both experimental results, obtained from multilongitudinal mode lasers, and analytical results obtained from other theoretical models. Evidence of "excitability" within the Lang-Kobayashi model is also reported.

Wave power dropouts associated with radial field intervals in high speed solar wind

Examination of 770 days of heliospheric magnetic field data recorded within polar solar wind by the Ulysses spacecraft revealed a number of occasions when the wave power decreased by more than an order of magnitude for several hours. These power dropouts were found to be associated with intervals when the magnetic field was closely aligned with the radial direction. A case by case study showed that there was no obvious signature in bulk plasma parameters. A statistical study of 24 intervals when the field was close to radial for at least 6 hours showed that the wave power in those intervals was significantly lower than that outside. There were no significant power changes when the field lay close to either the Parker spiral or an arbitrary direction. Although they are likely to be of solar origin, the exact source for these events is not clear.

Semiconductor laser dynamics subject to strong optical feedback

An experimental investigation of a semiconductor laser subject to moderate to strong optical feedback shows that sporadic power discontinuities occur everywhere along the boundary separating feedback regimes IV and V. These low-frequency fluctuations (LFF's) do not occur just near the solitary-laser threshold as previously thought. Furthermore, although the LFF's appear as power dropouts near threshold, they manifest as power jump-ups well above threshold. Theoretical modeling shows reasonable agreement with experiments.

Power partition fluctuations in two-mode-degenerate distributed-feedback lasers

Measurements are presented along with a theoretical analysis of power partition fluctuations in a distributed feedback (DFB) laser operating in continuous wave with the two DFB degenerate modes as the only significant longitudinal modes. The power ratio of the two modes was varied from 1.1 to 2500:1 by electrical control using a two-electrode DFB laser. The laser was observed to exhibit (1) transient power dropouts and (2) two-mode relaxation oscillation. The dropout depths were less than 30% of the main-mode power in all cases, except when the laser was subjected to external optical feedback. The relaxation oscillations of the two-mode power are anticorrelated, which is different from the relaxation oscillation of the total power due to photon-carrier interaction. The calculated result based on a Fokker-Planck analysis agrees with the experiment, except for the two-mode relaxation oscillation, which was not considered in the theory.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Analyses of mode partition and mode hopping in semiconductor lasers

Mode power fluctuations in semiconductors laser due to mode partition and mode hopping are discussed. The power dropout probability P/sub e/ in the mode partition was measured for a wide range, 1.6*10/sup -6/ >

Power dropout statistics of nearly-single-longitudinal-mode semiconductor lasers

An experimental study of power dropouts in the main mode of a nearly-single-longitudinal-mode GaAlAs semiconductor laser is presented. Knowledge of the statistics of main-mode power dropout interarrival times, depths, and widths is necessary to estimate the performance of a communication system using such laser diodes. We found that at a typical laser operating bias ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I/I_{th} = 2.1</tex> ), the mean dropout width was 1.8 ns, dropout depths were exponentially distributed, and dropouts falling below 90 percent of the average main-mode power occurred at a rate of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.5 \times 10^{3}</tex> s <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> . Also, the rate of dropouts falling below 90 percent of the average main-mode power was found to increase substantially near mode hops, sometimes by several orders of magnitude.

Mode power partition events in nearly single-frequency lasers

Frequent power dropouts are observed in the single dominant line of a number of dc-biased 1.3-μm injection lasers, each of which exhibits a nearly single-longitudinal-mode CW spectrum. During the dropout events, some of which essentially extinguish the dominant line for 1 to 8 ns, total laser output power is observed to remain constant: the power lost from the dominant mode appears in a side mode of very low average power. These events are capable of causing bit errors in a communications system. It was found that the frequency of events greater than a given threshold value falls exponentially with an increasing ratio of main-to-side-mode powers and that this ratio must exceed 100:1 in order to avoid a significant contribution to system error rates when the laser is operated continuously and an external modulator is used. These observations are in agreement with recently published calculations. The requirements on mode power ratio for the case of directly modulated lasers are much more severe. We present a calculation of the optical power penalty resulting from these mode power fluctuations.