Fiber Bragg grating (FBG) feedback has initially been investigated as a promising approach to conceal the time-delay signature in optical chaos generation. It has been shown that the laser dynamics vary greatly with respect to the FBG properties, especially to the frequency detuning between the laser emission and the Bragg wavelength. As a result, adjusting the FBG features will lead to significantly different behaviour. Here, we theoretically study the response of FBGs with different lengths but with similar reflectivity: this way, the impulse response is stretched over a longer period of time while its overall shape is maintained. This leads to a broadening of the FBG bandwidth and, thus, to a longer distribution of the feedback over time. In this work, we analyse the effects of the time-distribution variations for long gratings by simply tracking the first Hopf bifurcation and the feedback rate needed to destabilize the laser. The numerical results are generated using a modified version of the well-known Lang and Kobayashi equations. Our results show that the time-distribution of the feedback seem to have little effect in itself on the overall dynamics though it obviously affects the FBG spectra properties. We report stability oscillations of the laser behavior when long, narrow-bandwidth gratings are considered. The influence of the grating length on the specific dynamic details is investigated through the time delay signature (TDS) focusing especially on the implication of the stability oscillations on the TDS. We report that although variation of the TDS for long grating are observed the better TDS suppression is achieved with relatively short gratings.
Laser designed to emit at multiple and controllable modes, or multi-wavelength lasers, have the potential to become key building blocks in future compact THz or mm-wave transceivers. Combined with optical injection, these lasers can function as low-noise THz sources or even enable all-optical THz signal processing. Among the various multi-wavelength laser concepts, DBR-based lasers stand out because of their simplicity principle to control and switch the output wavelength of the laser. The extra wavelengths also add new degrees of freedom and interesting new features for laser dynamics. Yet, the coupling between the different laser modes has not been carefully considered so far. Here, we experimentally and numerically analyze the effect of nonlinear mode coupling and interactions in a dual-wavelength laser under optical injection. We focus particularly on the evolution of the locking bandwidth for different gain coefficients between the injected and non-injected modes. In addition, we report a wavelength shift of the non-injected mode which follows the evolution of the detuning in the other mode. Our work brings a new important insight into the mode competition taking place in multi-wavelength lasers, pushing them forward towards novel applications.
Multi-wavelength lasers – i.e. devices that can emit simultaneously on different wavelengths – represent an interesting opportunity for terahertz and telecom applications. In this context, implementing such devices with a generic process appears to be a key requirement to fully benefit from the development of these new platforms for Photonic Integrated Circuits (PICs). However, the genericity of the process comes at the cost of certain limitations in terms of design or performances. Through InP Multi-Project Wafer (MPW) runs, we have developed compact dual-wavelength laser (DWL) designs by taking advantage of the properties of Distributed Bragg Reflectors (DBRs). We use two detuned DBRs as narrow-bandwidth frequency-selective mirrors on one side, and, on the other side, we close the laser cavity with either a broad bandwidth multi-mode interference reflector (MIR) or a third DBR with a broader bandwidth than the first two. While each design successfully shows dual-wavelength emission, their emission properties significantly differ from one to another. In this contribution, we characterize the different laser designs and focus on the DBRs themselves. In particular, we study their characteristics when a current is applied or the temperature is changed. Next, we investigate and analyse their impact on the emission properties of the dual-wavelength lasers. Thus, we provide further insight on the relevance and potential of the proposed DWLs and highlight key points for the tailoring of these devices for a given application.
In this conference proceeding, we present a novel technique to control the emission of dual-wavelength lasers. Using a well designed external cavity, we demonstrate that tuning the optical feedback phase allows to efficiently tailor the output power balance between the two wavelengths emitted by the laser. With this technique, a complete switch between one and the other mode can also be achieved and we report a suppression ratio up to 40 dB. Due to its simplicity, the structure can be monolithically integrated easily with the laser itself and offers a precise control of the dual-wavelength emission using a single control parameter.
We investigate the behaviour of a multimode two-color quantum dot laser subject to optical feedback. In particular, we focus on the effects of a variation of the external cavity length at the micrometer scale on the laser emission characteristics and especially on its optical spectrum. For each mode, we observe oscillations of the output power with different spectral amplitudes. No clustering or mode grouping effect is observed. Theoretically, we demonstrate a good agreement with a multimode two-color quantum dot laser model.
In this contribution, we experimentally report recurrent switching between ground and excited state emission in a quantum dot laser controlled by optical feedback. We demonstrate that changing the phase of the optical feedback can efficiently induce switching between the two emission processes of the laser. Experimentally, by using an external mirror placed on a piezo-actuator, we were able to achieve incomplete switching between ground and excited state emission, i.e. without complete extinction of the modes. The switching takes place for variations of the external cavity length at the wavelength scale, i.e. around 1.2 um. Theoretically, we successfully link this switching behaviour with the evolution of the modal gain difference between the two modes induced by the variations of the optical feedback phase.
We investigate theoretically the synchronization properties of the polarization chaos dynamics generated by a free-running vertical-cavity surface-emitting laser (VCSEL). Here, we focus on a one-way master-slave configuration - or unidirectional coupling - with two chaotic VCSELs. The spin-flip model is used to model the two devices and derived to account for the coupling between them. We demonstrate that the chaotic dynamics generated by the two lasers can indeed synchronize in the proposed configuration. The synchronization appears to be of high quality as we obtain a high-level of similarity between the emission characteristics of the master and slave laser dynamics.
We discuss the impact of asymmetries and noise on the nonlinear dynamics of vertical-cavity surface-emitting lasers (VCSEL). We focus in particular on the effects of these features on the chaotic dynamics that can be generated by a free-running VCSEL due to the intrinsic competition between polarization modes taking place in these devices. Experimentally, we observe significant asymmetries especially in the statistics of the chaotic dynamics. We show that these behaviour can be explained theoretically by a combined effect of the system asymmetries and the noise. This work therefore brings new light on the interplay between deterministic and stochastic processes taking place in VCSELs.
We investigate the spectrally-resolved relative intensity noise (RIN) of a dual state emitting quantum-dot (QD) laser in dependence on the laser biasing conditions. We study the RIN under free-running conditions as well as under external optical feedback (OFB). We ï¬nd an improvement in RIN of the free-running laser when ground-state (GS) and excited-state (ES) emit simultaneously as compared to a single-state emission. Furthermore, we ï¬nd an improvement in RIN under external OFB.
Laser diodes typically behave like damped oscillators: they are generally expected to only show damped relaxation oscillations toward a stable fixed point. In vertical-cavity surface-emitting lasers (VCSELs), the picture appears to be quite different as polarization dynamics can be experimentally observed including bifurcations to self- pulsation and even chaos. Physically, the circular geometry of VCSELs makes the polarization selection very weak and, thus, the additional degree of freedom can enable complex dynamical behavior in the laser diode. Here we report on a new dynamical behavior in a free-running VCSEL: we observe a bistability between two limit cycles oscillating around two distinct elliptical polarization states whose main axes are symmetrical with respect to the polarization at threshold. Although the existence of two symmetric elliptical polarizations and the associated limit cycles are predicted by the San Miguel, Feng and Moloney (SFM) model, the hysteresis cycle observed experimentally highlights the importance of asymmetry in the dynamics from the elliptically polarized states. We demonstrate that this behavior can be accurately reproduced in theory within the SFM framework when taking into account a small misalignment between the phase and amplitude anisotropies of the laser cavity. Our results bring new light into VCSEL polarization dynamics and provide a very good qualitative agreement with the bifurcation scenario predicted by the SFM model.
During the last five years, optical chaos-based random bit generators (RBGs) attracted a lot of attention and demonstrated impressive performances with bit rates up to hundreds of Gbps. However all the suggested schemes use external injection schemes (optical injection or feedback) to turn the lasers into chaos, hence strongly increasing setup complexity. On the other hand, we reported that a laser diode can generate a chaotic output without the need for external perturbation or forcing, hence unveiling a highly simplified way to generate an optical chaos at high frequency. However the low dimension and limited number of positive Lyapunov exponent casted doubts about its direct use for chaos-based applications. Here we make a proof-of-concept demonstration for a Random Bit Generator based on polarization chaos. We therefore suggest a highly simplified RBG scheme using only a free-running laser and small-bandwidth acquisition electronics and demonstrate convincing performances with bit rates up to 100 Gbps without unusual or complex post-processing methods. We link these performances to the double-scroll structure of the chaotic attractor rather than the bandwidth of the dynamics, hence bringing new light on the importance of chaos topology for chaos-based applications. In addition our scheme exhibit a strong potential as it enables a low-cost and/or integrated in parallel on-chip scheme.
Experiments on quantum well and recently quantum dot VCSELs have shown that the increase of injection
current may lead to a transition from a linearly polarized light emission at threshold to a region of nonlinear
dynamics (self-pulsing) accompanying polarization switching between either two orthogonal linearly polarized
states or non-orthogonal elliptically polarized states. The dynamics occurs on a nanosecond time-scale and
relates either to the birefringence induced frequency splitting or to the relaxation oscillation frequency.
In this contribution, we bring new light into the bifurcation mechanisms explaining the occurrence of deterministic
self-pulsing accompanying polarization switching. We demonstrate theoretically that depending on
the laser parameters, different polarization switching scenarios may be observed with self-pulsing dynamics at
a dominant time-scale related to either linear cavity birefringence or relaxation oscillation, and with additional
period doubling or quasiperiodicity. Our work therefore not only reconciles previous experiments with different
conclusions on dynamical states, but also provides an improved understanding of the bifurcations underlying
the commonly used spin flip model for VCSEL - hence motivating new in depth experiments of polarization
dynamics at nanosecond time scale.
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