Unifying Frequency Combs in Active and Passive Cavities: Temporal Solitons in Externally Driven Ring Lasers.

Frequency combs have become a prominent research area in optics. Of particular interest as integrated comb technology are chip-scale sources, such as semiconductor lasers and microresonators, which consist of resonators embedding a nonlinear medium either with or without population inversion. Such active and passive cavities were so far treated distinctly. Here we propose a formal unification by introducing a general equation that describes both types of cavities. The equation also captures the physics of a hybrid device-a semiconductor ring laser with an external optical drive-in which we show the existence of temporal solitons, previously identified only in microresonators, thanks to symmetry breaking and self-localization phenomena typical of spatially extended dissipative systems.

Dynamics of a broad-band quantum cascade laser: from chaos to coherent dynamics and mode-locking.

The dynamics of a multimode quantum cascade laser, are studied in a model based on effective semiconductor Maxwell-Bloch equations, encompassing key features for the radiation-medium interaction such as an asymmetric frequency dependent gain and refractive index as well as the phase-amplitude coupling provided by the linewidth enhancement factor. By considering its role and that of the free spectral range, we find the conditions in which the traveling wave emitted by the laser at the threshold can be destabilized by adjacent modes, thus leading the laser emission towards chaotic or regular multimode dynamics. In the latter case our simulations show that the field oscillations are associated to self-confined structures which travel along the laser cavity, bridging mode-locking and solitary wave propagation. In addition, we show how a RF modulation of the bias current leads to active mode-locking yielding high-contrast, picosecond pulses. Our results compare well with recent experiments on broad-band THz-QCLs and may help in the understanding of the conditions for the generation of ultrashort pulses and comb operation in mid-IR and THz spectral regions.

Intensity noise behavior of an InAs/InGaAs quantum dot laser emitting on ground states and excited states.

We experimentally and numerically study the amplitude stability of an InAs/InGaAs quantum dot laser emitting simultaneously on ground states (GSs) and excited state (ESs) at center wavelengths of 1245 and 1168 nm, respectively. The stability is quantified by a spectrally resolved noise current analysis that is dependent on the laser injection current. We find a non-monotonic behavior of the amplitude noise which shows a reduction of up to 4 dB when the GS and ES emit simultaneously. Simulations based on a rate equation model confirm the reduction in noise and suggest the cascaded GS and ES carrier paths as the relevant underlying mechanism.

Dissipative Phase Solitons in Semiconductor Lasers.

We experimentally demonstrate the existence of nondispersive solitary waves associated with a 2π phase rotation in a strongly multimode ring semiconductor laser with coherent forcing. Similarly to Bloch domain walls, such structures host a chiral charge. The numerical simulations based on a set of effective Maxwell-Bloch equations support the experimental evidence that only one sign of chiral charge is stable, which strongly affects the motion of the phase solitons. Furthermore, the reduction of the model to a modified Ginzburg-Landau equation with forcing demonstrates the generality of these phenomena and exposes the impact of the lack of parity symmetry in propagative optical systems.

Multimode regimes in quantum cascade lasers with optical feedback.

We study the instability thresholds of the stationary emission of a quantum cascade laser with optical feedback described by the Lang Kobayashi model. We introduce an exact linear stability analysis and an approximated one for an unipolar lasers, who does not exhibit relaxation oscillations, and investigate the regimes of the emitter beyond the continuous wave instability threshold, depending on the number and density of the external cavity modes. We then show that a unipolar laser with feedback can exhibit coherent multimode oscillations that indicate spontaneous phase-locking.

QCL-based nonlinear sensing of independent targets dynamics.

We demonstrate a common-path interferometer to measure the independent displacement of multiple targets through nonlinear frequency mixing in a quantum-cascade laser (QCL). The sensing system exploits the unique stability of QCLs under strong optical feedback to access the intrinsic nonlinearity of the active medium. The experimental results using an external dual cavity are in excellent agreement with the numerical simulations based on the Lang-Kobayashi equations.

A concomitant and complete set of nonvolatile all-optical logic gates based on hybrid spatial solitons.

We theoretically demonstrate the realization of a complete canonical set of all-optical logic gates (AND, OR, NOT), with a persistent (stored) output, by combining propagative spatial solitons in a photorefractive crystal and dissipative cavity solitons in a downstream broad-area vertical cavity surface emitting laser (VCSEL). The system uses same-color, optical-axis aligned input and output channels with fixed readout locations, while switching from one gate to another is achieved by simply varying the potential applied to the photorefractive crystal. The inputs are Gaussian beams launched in the photorefractive crystal and the output is a bistable, persistent soliton in the VCSEL with a 'robust' eye diagram and large signal-to-noise ratio (SNR). Fast switching and intrinsic parallelism suggest that high bit flow rates can be obtained.

Observation of electro-activated localized structures in broad area VCSELs.

We demonstrate experimentally the electro-activation of a localized optical structure in a coherently driven broad-area vertical-cavity surface-emitting laser (VCSEL) operated below threshold. Control is achieved by electro-optically steering a writing beam through a pre-programmable switch based on a photorefractive funnel waveguide.

Intrinsic stability of quantum cascade lasers against optical feedback.

We study the time dependence of the optical power emitted by terahertz and mid-IR quantum cascade lasers in presence of optical reinjection and demonstrate unprecedented continuous wave (CW) emission stability for strong feedback. We show that the absence of coherence collapse or other CW instabilities typical of diode lasers is inherently associated with the high value of the photon to carrier lifetime ratio and the negligible linewidth enhancement factor of quantum cascade lasers.

Self-mixing in multi-transverse mode semiconductor lasers: model and potential application to multi-parametric sensing.

A general model is proposed for a Vertical Cavity Surface Emitting Laser (VCSEL) with medium aspect ratio whose field profile can be described by a limited set of Gauss-Laguerre modes. The model is adapted to self-mixing schemes by supposing that the output beam is reinjected into the laser cavity by an external target mirror. We show that the self-mixing interferometric signal exhibits features peculiar of the spatial distribution of the emitted field and the target-reflected field and we suggest an applicative scheme that could be exploited for experimental displacement measurements. In particular, regimes of transverse mode-locking are found, where we propose an operational scheme for a sensor that can be used to simultaneously measure independent components of the target displacement like target translations along the optical axis (longitudinal axis) and target rotations in a plane orthogonal to the optical axis (transverse plane).

Carboxamides and hydrazide of glycopeptide antibiotic eremomycin. Synthesis and antibacterial activity.

Carboxamides and hydrazide of glycopeptide antibiotic eremomycin were obtained by a direct reaction of the carboxy group of eremomycin with an appropriate amine or hydrazine using diphenyl phosphorazidate as a condencing agent. Eremomycin hydrazide was also obtained by hydrazinolysis of the eremomycin methyl ester. Use of dicyclohexylcarbodiimide or 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide for amidation led to the corresponding eremomycin ureides. The ESI-MS data indicate that eremomycin and its amides exist as dimers. The carboxamide, methylamide and benzylamide of eremomycin were as active against Gram-positive bacteria as the parent antibiotic, and the methylamide, benzylamide and hydrazide were almost an order of magnitude more active than eremomycin against Staphylococcus epidermidis clinical isolates in vitro. Amide of eremomycin as well as ureides were devoid of histamine liberating properties, which demonstrates that protection of the carboxyl group leads to a decrease in the allergenic properties.

Could cavity solitons exist in bidirectional ring lasers?

We show numerically that bright and dark cavity solitons can be obtained in bidirectional class A ring lasers only if the phase invariance of the electromagnetic field is broken. The phase invariance symmetry is responsible for the existence of phase waves, which generate long-range interactions destroying the property of independence among otherwise localized structures. We improved the usual model describing such types of lasers, and we prove that it leads to genuine cavity solitons.

On homoclinic snaking in optical systems.

The existence of localized structures, including so-called cavity solitons, in driven optical systems is discussed. In theory, they should exist only below the threshold of a subcritical modulational instability, but in experiment they often appear spontaneously on parameter variation. The addition of a nonlocal nonlinearity may resolve this discrepancy by tilting the "snaking" bifurcation diagram characteristic of such problems.

Proposed resolution of theory-experiment discrepancy in homoclinic snaking.

In spatially extended Turing-unstable systems, parameter variation should, in theory, produce only fully developed patterns. In experiment, however, localized patterns or solitons sitting on a smooth background often appear. Addition of a nonlocal nonlinearity can resolve this discrepancy by tilting the "snaking" bifurcation diagram characteristic of such problems.