Common applications using optical chaos in a semiconductor laser include, among others, random number generation and chaos-encrypted communications. They rely on chaos of high dimension with a large bandwidth and a high entropy growth rate to achieve good results. Optical chaos from a semiconductor laser with conventional optical feedback (COF) is typically used as the primary source of chaos. Additional enhancing techniques are used to enlarge the chaos bandwidth. In this contribution, we show experimentally how using phase-conjugate feedback (PCF) can naturally produce a chaos of higher bandwidth than COF. PCF is an alternative to COF which consists of feeding the conjugate of the optical output back into the laser cavity, with a time-delay. Thanks to an oscilloscope with a fast sampling rate, and a large bandwidth, we were able to measure and observe the time-resolved frequency dynamics with a good precision. In the regime of low-frequency fluctuations (LFF), where dropouts of optical power occur randomly, we were able to compare the difference in dynamics before and after a dropout, for PCF and COF. In the range of attainable reflectivities, we measured a bandwidth increase of up to 27 % with PCF when compared to COF. Interestingly, we found that high-frequency dynamics are enabled before dropouts in PCF, where it was theoretically shown that the system jumps between destabilized self-pulsing states at harmonics of the external-cavity frequency, the so-called external-cavity modes (ECMs). This observation tends to confirm that ECMs in PCF are indeed fundamentally different than ECMs in COF, where they are simple steady-states. Finally, we believe that the enhancing techniques used with COF could also be used with PCF to obtain even wider chaotic bandwidths. These results could lead to studies about the dimension and the entropy growth rate of chaos from a laser diode with PCF.
Thanks to the band-gap engineering of quantum confined semiconductor materials and the development of semiconductor-based saturable absorber mirrors, recent years have seen the development of compact and low-cost external-cavity laser diodes generating pulses at several tens of GHz. The physics of the bifurcation leading to selfpulsation leads however to an intrinsic limitation: the fundamental repetition rate is fixed to and limited by the externalcavity round-trip time. By contrast, we demonstrate here that an external-cavity diode laser may generate fundamental self-pulsating dynamics at harmonics of the external-cavity frequency, when a phase conjugate mirror replaces the conventional mirror. As is known from theory, a laser diode with phase conjugate external feedback supports a single stationary solution that bifurcates to self-pulsating dynamics of increasing frequency when increasing the amount of light reflected back to the laser diode. The self-pulsation frequency then increases in step of the external-cavity frequency as one increases the feedback strength. We provide here the first experimental evidence of such harmonic external-cavity fundamental self-pulsation. As a proof-of-concept, we generate experimentally a self-pulsating dynamics at twice and three times the fundamental external-cavity frequency using an edge-emitting laser with a self-pumped ring-cavity photorefractive phase conjugator. Numerical simulations also predict stable higher harmonics.
Phase-conjugate optical feedback (PCF) has been largely used as a way to stabilize and reduce the linewidth of laser emission but is also known to generate complex dynamics including self-pulsation and chaos. In contrast to the large number of theoretical works, there have been only few experiments reporting on nonlinear dynamics from PCF. Most importantly, experiments so far have not addressed the peculiarities of the PCF dynamics in comparison with dynamics observed from conventional optical feedback (COF). We report here experimentally and theoretically on two chaotic dynamics that relate to the peculiar dynamical properties of a laser diode with PCF. First, we find a chaotic dynamics that resembles the so-called low-frequency fluctuations (LFF) of a laser diode with COF, i.e. the output power shows abrupt dropouts at randomly distributed time-intervals followed by a slower recovery. Although the LFF in PCF shows similar statistical properties to those observed in the LFF in COF, they originate from a distinctively different bifurcation scenario. Increasing the PCF strength the laser diode shows successive bifurcations to time-periodic solutions at the frequency of the external cavity and multiples - also called 'external-cavity modes' (ECMs). In contrast to COF the PCF laser system shows no steady state for large enough feedback strength. Following the destabilization of several such ECMs to chaotic attractors, the dynamics shows a transition to a global attractor connecting the chaotic ECMs and that explains the sequence of power dropouts and recoveries. In addition we show how the bifurcations on these self-pulsing ECMs generate dynamics with extreme events, i.e. pulses with peak intensities well above the average value of the peaks in the output power and that show properties similar to the rogue waves in hydrodynamics. This is the first demonstration of temporal extreme events in a time-delayed optical system.
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.
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.
Thermal and optical properties of Buried structures laser are simulated in the case where we include
a waveguide layer beneath the Multi Quantum Well's. The role of this waveguide is twofold. On one hand It
attracts the optical field on lower side of the active waveguide where the losses are low due to n doped side of
the laser. On the other hand, we can improve the coupling coefficient to the fiber by increasing the divergence
of the beam together with its waist increase. Finite element software is used to calculate thermal resistance of
the structures. Beam Propagation method is use for optical modeling. We optimize active waveguide
composition and thickness for various multi-quantum wells number. Optical modeling is used to check the
single mode operation of the structure together with beam shape optimization according to the objectives.
Waveguide parameters are optimized to reduce the thermal resistance of investigated structures. Trade of
between thermal and optical properties is discussed.