ABSTRACT
We have performed experimental and numerical studies enabling clear insight into the physical mechanisms underlying the super-mode noise mitigation in harmonically mode-locked (HML) fiber lasers using the resonant continuous wave (CW) injection. New experiments have refined the requirements to the positions inside the laser spectrum assigned to the injected CW component, a Kelly sideband, and the transparency peaks of the birefringent fiber filter. In particular, we have proved experimentally that the noise mitigation effect is dominating with the CW injected to the long-wavelength side of laser spectrum. Injection to the opposite side destroys the HML operation regime. Our numerical simulations confirm these specific features. To get the result, we have simulated phase-locking between the CW and a single soliton. Then, the developed model has been applied to the laser cavity operating multiple pulses in the presence of the gain depletion and recovery mechanism responsible for harmonic pulse arrangement. We clearly demonstrate how the CW injection accelerates or slows down the HML process enabling the generation of additional inter-pulse forces.
ABSTRACT
We develop a theoretical framework to analyze the mechanism of refractive index changes (RIC) in double-clad Yb³âº doped optical fibers under resonant core or clad pumping, and with signal amplification. The model describes and compares thermal and electronic contributions to the phase shifts induced on the amplified signal at 1064 nm and the probe signal at 1550 nm, i.e. located inside and outside of the fiber amplification band, respectively. The ratio between the thermal and electronic phase shifts is evaluated as a function of the pump pulse duration, the gain saturation, the amplified beam power and for a variety of fiber parameters.
ABSTRACT
In this paper, we completely study the wavelength dependency of differential group delay (DGD) in uniform fiber Bragg gratings (FBG) exhibiting birefringence. An analytical expression of DGD is established. We analyze the impact of grating parameters (physical length, index modulation and apodization profile) on the wavelength dependency of DGD. Experimental results complete the paper. A very good agreement between theory and experience is reported.
ABSTRACT
We analyze theoretically the dynamics of a semiconductor laser subject to optical feedback, on the basis of the well-known Lang-Kobayashi equations. Previous investigations on this laser system suggest that a small linewidth enhancement factor (alpha factor) stabilizes the laser dynamics. By contrast, we unveil here optical feedback induced instabilities which are present for a small value of alpha but which disappear when alpha increases above alpha approximately 1. By combining numerical simulations and modern continuation methods for delay-differential equations, we unveil cascades of subcritical and supercritical Hopf bifurcations on the first external-cavity mode (ECM). We unveil for the first time, to our knowledge, the occurrence of subcritical Hopf bifurcation points for intermediate values of the EC length, i.e. close to the boundary between the short and the long EC regimes. They lead to severe laser instabilities such as large intensity and possibly chaotic pulsations. Moreover, these Hopf bifurcation cascades for small values of alpha are shown to be responsible for different bifurcation scenarios leading to restabilization of the first ECM and to ECM bistability.
ABSTRACT
Vertical-cavity surface-emitting lasers subjected to weak polarization-insensitive optical feedback are studied experimentally and theoretically. We find that the feedback induces random anticorrelated hopping between the two orthogonal linearly polarized modes. This polarization mode hopping is accompanied by rapid anticorrelated oscillations in the linearly polarized intensities at the external-cavity frequency. The study of a simple stochastic delay differential equation suggests that these oscillations generated by the delay are typical of any hopping phenomenon between states.
ABSTRACT
A current-modulated vertical-cavity surface-emitting laser is shown to exhibit interesting nonlinear dynamics in its two orthogonal linearly polarized (LP), fundamental transverse modes. The intensities of the two LP modes may exhibit in-phase time-periodic dynamics or chaotic regimes with combination of in-phase and antiphase dynamics at two different time scales. Chaotic dynamics are found in a large range of laser and modulation parameters.
ABSTRACT
We investigate numerically the low-frequency fluctuation regime in a laser diode subject to optical feedback. We demonstrate that a saddle-node ghost can induce this regime.
ABSTRACT
We study numerically the synchronization of external-cavity semiconductor lasers in a master-slave configuration, based on a Lang-Kobayashi-type model. Depending on the feedback and coupling strengths, the slave laser synchronizes with the injected optical field or with the injected field but lags in time. We show that these two types of synchronization present different robustness with respect to the noise, frequency detuning, and current modulation of the master laser.
ABSTRACT
Experiments have yielded polarization self-modulation in vertical-cavity surface-emitting lasers (VCSELs) subject to a pi/2 polarization-rotating optical feedback. The phenomenon has been simulated numerically, but its bifurcation has never been explained. We show that polarization self-modulation results from a Hopf bifurcation mechanism that can be analyzed in terms of the laser feedback parameters. Our analysis predicts other bifurcations for low values of the feedback rate, which explain why more-complex time-dependent outputs have been observed as alternatives to polarization self-modulation.
ABSTRACT
We study numerically two distant unidirectionally coupled single-mode semiconductor lasers subject to coherent optical feedback. We show that two fundamentally different types of chaotic synchronization can occur depending on the strengths of the coupling and of the feedback of the receiver laser.
ABSTRACT
It is shown that the adiabatic following of the dark, nonabsorbing state improves significantly the electromagnetically induced transparency performance and slows down the group velocity of the probe pulse. This concept can be used for fast selective gating of one pulse out of a pulse train.
ABSTRACT
We propose a secure communication scheme based on anticipating synchronization of two chaotic laser diodes, one subject to incoherent optical feedback and the other to incoherent optical injection. This scheme does not require fine tuning of the optical frequencies of both lasers as is the case for other schemes based on chaotic laser diodes subject to coherent optical feedback and injection. Our secure communication scheme is therefore attractive for experimental investigation.
ABSTRACT
We propose a technique for measuring both pulse width and amplitude jitter noises of high-repetition-rate optical pulse trains and the cross correlation between these noises as well. The technique is based on time-domain amplitude demodulation of three harmonic components of the detected pulse train. We applied this technique to characterize noises of a gigahertz optical pulse train generated by an actively mode-locked Er-doped fiber laser. Correlation between pulse width jitter and pulse amplitude jitter was observed at low frequencies in this laser. Unlike relaxation oscillation noise, low-frequency noise is free from pulse energy jitter. Owing to its ability to measure pulse width jitter in addition to amplitude and phase jitters, this technique is of great interest for characterizing noises of a wide variety of optical pulse train sources.
ABSTRACT
We demonstrate experimentally all-optical stabilization of a single-mode laser diode subject to external optical feedback operating in the low-frequency fluctuations (LFF) regime, by the technique of applying a second delayed optical feedback. We interpret our results as suppression of LFF through destruction of the antimodes responsible for the LFF crises and stabilization of the laser through creation of new maximum gain modes, in agreement with recent theoretical predictions.
ABSTRACT
We demonstrate for the first time to our knowledge, experimentally and theoretically, that the pulse-to-pulse amplitude fluctuations that occur in pulse trains generated by actively mode-locked Er-doped fiber lasers in a repetition-rate-doubling rational-harmonic mode-locking regime are completely eliminated when the modulation frequency is properly tuned. Irregularity of the pulse position in the train was found to be the only drawback of this regime. One could reduce the irregularity to a value acceptable for applications by increasing the bandwidth of the optical filter installed in the laser cavity.
ABSTRACT
Stabilization of an actively mode-locked fiber laser in the frequency-doubling rational-harmonic mode-locking regime is demonstrated experimentally for the first time to the authors' knowledge. The stabilization is achieved by a method based on minimization of the average optical power at the second output of a dual-output Mach-Zehnder modulator used as a mode locker. This method produces long-term stable operation of the laser with ~35-dB suppression of the pulse-to-pulse amplitude fluctuation caused by rational-harmonic frequency doubling.
ABSTRACT
We demonstrate numerically that low-frequency fluctuations (LFF's) observed in a laser diode subjected to a first optical feedback with a short delay are suppressed by means of an adequate second optical feedback. The general idea of this technique is based on the observation that second feedback can suppress the antimodes that are responsible for the crises in the LFF regime. Furthermore, we observe that the second optical feedback can steer an unstable laser that is biased near threshold into a stable regime.
