A theoretical nonlinear treatment of coupled quantum cascade lasers (QCLs) by a monolithic Talbot cavity all grown on the same chip is presented, analyzed and the results are compared to recent experiments. The model is capable of computing numerically the stability or instability of the supermodes of the coupled system and can capture possible bifurcations into pulsating intensities. The model is derived by using an equivalent ring laser model that contains several separated gain section all coupled by an integrated Talbot cavity. In the small signal gain limit it captures the threshold gain of the various supermodes and matches the results of previous calculations in the literature in the same limit.

Quantum cascade lasers (QCL) are semiconductor lasers based on ultrafast intersubband transitions with picosecond timescale that have become the most suitable laser sources from the mid-infrared to the THz range, due to their compactness, efficiency and high room temperature performances. In particular, high-power QCLs are powerful sources for optical countermeasures, including night vision blinding and missile out steering. This work investigates the nonlinear dynamical features of coupling of linear arrays of emitters in the so-called Talbot configuration for phase-locking operation using broad area emitters. These initial results are of paramount importance for creating future bright infrared sources with Watt-level power.

Mode creation and temporal response of broad-area quantum cascade lasers (BA-QCL) placed within an external feedback cavity are described in this publication. The critical feedback parameter becomes the mirror angle relative to the BA-QCL facet. With judicious angle choices, a plethora of curious modes can be created, each with their particular threshold and slope efficiency. These range from a nearly single far-field intensity peak to highly multimode emission similar to their diode counterparts. Dynamics are strongly dominated by transverse mode competition ranging for less than 20MHz to greater than 100MHz. When the mirror is parallel to the facet, higher frequency external cavity oscillations become undamped.

We perform a detailed numerical analysis of square-wave (SW) polarization switching in two semiconductor lasers with time-delayed, orthogonal mutual coupling. An in-depth mapping of the dynamics in the two-parameter plane coupling strength versus frequency detuning shows that stable SWs occur in narrow parameter regions that are localized close to the boundary of stability of the pure-mode solution. In this steady state, the two coupled lasers emit orthogonal polarizations. We also show that there are various types of SW forms and that stable switching does not need the inclusion of noise or nonlinear gain in the model. As these narrow regions of deterministic and stable SWs occur for quite different combinations of parameters, they could potentially explain the waveforms that have been observed experimentally. However, on the other hand, these regions are narrow enough to be in fact considered as experimentally unreachable. Therefore, our results indicate that further experimental statistical studies are needed in order to distinguish deterministic and stationary square waveforms from long transients because of noise.

Using advanced continuation techniques for dynamical systems, we elucidate the bifurcations leading to asymptotically stable square-wave pulsing and polarization mode switching in semiconductor lasers with mutual time-delayed and polarization rotating coupling. We find that the increase of coupling strength leads to a cascade of Hopf bifurcations on a mixed-mode steady state up to a transcritical bifurcation on a so-called pure-mode steady state where both lasers emit with the injected polarization state. From these successive Hopf bifurcations emerge time-periodic solutions that have a period close to the laser relaxation oscillation for weak coupling but a period close to twice the time delay for large coupling strength. The wave form of the time-periodic solutions also evolves from harmonic pulsing up to square-wave pulsing as has been observed recently in experiments.

We analyze the dynamics of two semiconductor lasers with so-called orthogonal time-delayed mutual coupling: the dominant TE (x) modes of each laser are rotated by 90 degrees (therefore, TM polarization or y) before being coupled to the other laser. Although this laser system allows for steady-state emission in either one or in both polarization modes, it may also exhibit stable time-periodic dynamics including square waveforms. A theoretical mapping of the switching dynamics unveils the region in parameter space where one expects to observe long-term time-periodic mode switching. Detailed numerical simulations illustrate the role played by the coupling strength, the mode frequency detuning, or the mode gain to loss difference. We complement our theoretical study with several experiments and measurements. We present time series and intensity spectra associated with the characteristics of the square waves and other waveforms observed as a function of the strength of the delay coupling. The experimental observations are in very good agreement with the analysis and the numerical results.

We study numerically the dynamics of VCSELs with polarization-rotated (PR) optical feedback or coupling, such that the natural lasing polarization of a VCSEL is rotated by 90 degrees and then is either feed back into the same laser, or is injected into another laser. In the latter case the coupling is symmetric and each laser receives PR injection. In the feedback scheme, we find stationary dynamics consisting on noisy, square-wave-like polarization switching with periodicity slightly longer than twice the feedback delay time, that degrade to (or alternate with) bursts of irregular and faster oscillations. In the coupling scheme, the square-waves are transient and decay towards one of the various possible stationary states. The influence of various model parameters on the duration of the stochastic transient time and on the lasers' dynamics in the stationary state is investigated.

We derive equations for the ASE intensity, decay time, and heat load. The crux of our development is frequency integration over the gain lineshape followed by a spatial integration over the emitters. These integrations result in a gain length that is determined from experiment. We measure the gain as a function of incident pump power for a multi-pass pumped Yb:YAG disk doped at 9.8 at.% with an anti-ASE cap. The incident pump powers are up to 3kW. Our fit to the measured gain is within 10% of the measured gain up to pump powers where the gain starts to flatten out and roll over. In this comparison we extract the gain length that turns out to be 43% of the pump spot size of 7mm.

Although pulsating and chaotic regimes in injection-locked semiconductor lasers have been described often in the literature, so far their relative abundance has remained poorly explored. Here, for two popular laser models, we report detailed Lyapunov phase (stability) diagrams characterizing the extension in parameter space of pulsating phases. Our phase (stability) diagrams discriminate regular from chaotic laser emissions and indicate where multistability is to be expected in injection-locked semiconductor lasers.

Ytterbium-doped fiber lasers are making impressive leaps in power production. Yet in spite of fiber’s large surface area to volume ratio which is beneficial for cooling, such power inevitably leads to high core temperatures that in turn affect the laser performance. In this paper, the temperature effects on the emission and fluorescence lifetime of ytterbium-doped optical fibers are investigated. From these the temperature dependent emission and absorption cross-sections are calculated.

The square-wave response of edge-emitting diode lasers subject to a delayed polarization-rotated optical feedback is studied experimentally and theoretically. Square-wave self-modulated polarization intensities of a period close to twice the delay τ of the feedback gradually appear through a sequence of bifurcations starting with a Hopf bifurcation (Gavrielides et al, Proc. SPIE 6115, to appear, 2006). In Gavrielides et al (submitted, 2006), squarewave solutions were determined analytically from the laser equations in the limit of large τ. A condition on the laser parameters was derived explaining why square-wave oscillations are preferentially observed for suffciently large feedback strength. In this paper, we concentrate on the relaxation oscillations that always appear at each intensity jump between the plateaus of the square-wave. We show analytically that if the feedback strength is progressively decreased, a bifurcation to sustained relaxation oscillations is possible for one of the two plateaus.

The response of a diode laser resulting from an incoherent delayed optical feedback is considered from numerical and experimental perspectives. We concentrate on a class of solutions that appear as regular square waveforms. A two-field model is used and the bifurcation diagram of these square-wave regimes is studied. Conditions under which they typically appear are determined. The roles of various parameters are examined, particularly with regard to the gains and losses of the two polarization modes. Numerical results are in close agreement with experiments.

The chaotic dynamics of a semiconductor laser subject to a delayed polarization-rotated optical feedback is investigated theoretically and experimentally. An extension of the usual one-polarization model is derived to account for two orthogonal polarizations of the optical field. The two-polarization model is motivated by observations of lag synchronization in our experiments using polarization-rotated optical feedback and unidirectional injection. Experimental data confirm the predictions of the two-field model. We also show that the two-polarization model can be reduced to the one-polarization model.

We report studies of SBS in optical fibers with the goal of using SBS phase conjugation as a passive beam combiner to build high power (>100 W cw) all fiber laser sources. We propose the development of a near infrared high power fiber laser by phasing two Er doped amplifiers in parallel using stimulated Brillouin scattering in a multimode fiber. We use a 1.5 p.m master oscillator-power amplifier configuration (MOPA) to generate SBS in a multimode fiber. The fiber amplifier consists of two Er doped multimode fiber amplifiers (diode pumped), in parallel, which will combine to generate SBS in the multimode fiber.

We investigate stimulated Brillouin scattering (SBS) threshold in single mode and multimode fibers in an all fiber network. The pump is a single mode fiber pigtail attached to a diode. We find the theory and experiment agree for both single mode and multimode GRIN fibers. We modify the bulk SBS threshold equation for use with fibers by properly accounting for mode sizes and modal dispersion.

Synchronized chaotic dynamics are investigated theoretically and experimentally in a system of unidirectionally-coupled semiconductor lasers subject to delayed, polarization-rotated optical feedback and injection. Experimental data in the time and frequency domains demonstrate chaos synchronization with a lag between transmitter and receiver equal to the injection time, also known as driving synchronization. The natural polarization mode of the transmitter is shown to synchronize most efficiently to the orthogonal state of the receiver which is being injected. A full two-polarization model is used for both lasers, and is in good agreement with polarization-resolved experimental measurements.

The main goal of this paper is the study of the stimulated Brillouin scattering (SBS) in multimode fibers, at 1550 nm wavelength cw operation, in order to build high power IR fiber lasers. Two theoretical models are considered, the usual plane wave model and a modal model, developed in this paper. The theoretical results for SBS threshold and SBS rellectivity are compared with the experimentally determined values. Good agreement was obtained when using the mode structure analysis.

We investigate a model of a fiber-ring laser, which includes nonlinear saturation and dispersion of gain, nonlinearity (saturation) of the splitter which releases outgoing solitons, and smooth modulation of the group-velocity dispersion (GVD) coefficient along the ring. A conclusion is that the gain dispersion (effective spectral filtering), splitting saturation, and GVD modulation are all necessary for stability of a pulse-circulation regime. Finite intervals are identified from which values of the outcoupling saturation parameter Γ and GVD-modulation rate γ must be chosen to secure stable periodic operation of the scheme and hence stable periodic generation of outgoing solitons. In an adjacent interval, period doublings and subsequent transition to a chaotic pulse-circulation regime occurs. If Γ or γ are too small or too large, only a continuous-wave state is generated. In the case when the gain is too strong, the ring resonator switches into a turbulent regime via a cascade of soliton splittings. Robust pulses are found too in the absence of the gain saturation.

The rate equations describing a laser with phase conjugate feedback are analyzed in the case of non-zero detuning. For low feedback rates and detuning, the stability diagram of the steady state is similar to the laser subject to injection. A stable steady state may loose its stability through a Hopf bifurcation exhibiting a frequency close to the relaxation oscillation frequency of the solitary laser. We also construct time-periodic pulsating intensity solutions exhibiting frequencies close to an integer multiple of the external cavity frequency. These solutions have been found numerically for the zero detuning case and play an important role in the bifurcation diagram.

This work focuses on the study of stimulated Brillouin scattering (SBS) in optical fibers with a goal of studying its applicability for phase conjugation of radiation from CW lasers, and of using SBS to demonstrate high power fiber lasers by coherent combination of beams from several fiber amplifiers. Results revealed that both phase conjugation and beam cleanup behavior can take place via SBS in the 4.4 km MM fiber used, depending on the divergence of the input beam and the input power level.