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Annual Meeting Optical Society of America

Complex spatiotemporal dynamics in lasers

Jorge R. Tredicce, C. Green, E. J. D’Angelo, Lorenzo M. Narducci

Published Jan 1, 1990• 0 citations
Publisher

Abstract

It is well known that transverse intensity profiles of lasers may show a nontrivial spatial structure. Usually, depending on the degree of symmetry, the spatial patterns have been associated with modes of a passive resonator or combinations of them which cause oscillations in the laser intensity (mode beating).1 It was recently shown2 that the fluctuations may be due, instead, to the nonlinear coupling between atoms and the field. Here we show that a spontaneous symmetry-breaking process is also at the origin of the formation of complex spatiotemporal dynamics in which oscillations at a single frequency, quasiperiodicity, and chaos may coexist within in the same spatial structure. We compare our experimental results with those obtained from bifurcation theory and numerical integration of the Maxwell–Bloch equations. The transition from a simple (Gaussian) profile to a complex (turbulent) profile is studied by means of the average intensity patterns, frequencies, and phases of the local intensity and cross-correlation functions.

DOI
10.1364/oam.1990.thp1

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It is well known that transverse intensity profiles of lasers may show a nontrivial spatial structure. Usually, depending on the degree of symmetry, the spatial patterns have been associated with modes of a passive resonator or combinations of them which cause oscillations in the laser intensity (mode beating).1 It was recently shown2 that the fluctuations may be due, instead, to the nonlinear coupling between atoms and the field. Here we show that a spontaneous symmetry-breaking process is also at the origin of the formation of complex spatiotemporal dynamics in which oscillations at a single frequency, quasiperiodicity, and chaos may coexist within in the same spatial structure. We compare our experimental results with those obtained from bifurcation theory and numerical integration of the Maxwell–Bloch equations. The transition from a simple (Gaussian) profile to a complex (turbulent) profile is studied by means of the average intensity patterns, frequencies, and phases of the local intensity and cross-correlation functions.
Two-level and Raman lasers with wide-gain sections undergo spontaneous symmetry breaking beyond the first laser threshold resulting in an off-axis emission that, if stable, would correspond to a ring in the far field. The angle subtended by the emission to the laser axis depends on the laser detuning, polarization decay time, and size of the laser cross-section. Optical vortices, propagating fronts, and complex spatio–temporal patterns are predicted to emerge at the first and second thresholds. The intensity dependent detuning and pump laser transverse profile in the Raman laser offers a fascinating array of pattern forming possibilities involving coexisting dark and bright waves. When the polarization decays much faster than the cavity field and medium inversion, one usually adiabatically eliminates this variable from the Maxwell-Bloch equations. Inclusion of transverse effects in this limit, however, leads to spurious transverse instabilities that can be mistaken for numerical grid oscillations. Decoupling of the polarization variable from the electric field allows arbitrarily high transverse wavenumbers to grow.
We discuss a novel type of optical instability, which leads to the spontaneous formation of a stationary spatial structure, instead of to the onset of oscillations or pulsations. A passive system is contained in an optical cavity, to which we add two lateral mirrors. Under appropriate conditions, diffraction of radiation can induce the onset of a transverse-stripe structure in an initially uniform plane-wave field configuration. In this paper, we consider the case of a homogeneously broadened two-level system. The model on which our analysis is based is derived from the Maxwell-Bloch equations in the paraxial approximation, using the mean-field limit and assuming a large Fresnel number. We show that, with respect to the bistability parameter C, the threshold of the spatial instability coincides with the bistability threshold for absorptive, resonant optical bistability. The spatial instability requires, however, a detuned configuration and can arise both in the absence and presence of bistability. In the latter case, it can occur both in the lower and in the upper branch of the steady-state curve. The instability is caused by competition between different transverse modes, which realize a situation of spatial coexistence, different from the state of temporal coexistence which characterizes the spontaneous oscillations that arise from the previously known multimode instabilities. Finally, we discuss some points relevant for an experimental observation of this phenomenon.
A complex order parameter description of the spatiotemporal dynamics close to threshold for a single-longitudinal-mode, large-aspect-ratio laser with a weak injected signal is presented in one transverse spatial dimension and for a longitudinal cavity frequency smaller than the atomic resonance frequency. It is shown that, when the frequency of the external signal is tuned close to the atomic resonance frequency, the spatiotemporal dynamics of the laser may be described by two coupled Ginzburg-Landau equations with parametric terms analogous to those derived in hydrodynamics to study excitation of parametric waves in oscillatory convection. As a result of the parametric terms, which physically arise by means of a four-wave-intercation process mediated by the external signal, the bifurcating modes which are superimposed to the forced (homogeneous) mode may be either traveling or standing waves. In particular, it is shown that for small frequency detunings of the injected signal standing waves are preferred to traveling waves close to threshold. Stability analysis of traveling and standing waves beyond threshold within the amplitude equations reveals the emergence of both phase (Eckhaus) and amplitude instabilities.
Two-dimensional (2D) ``chaotic'' localized structures are observed in the transverse profile of light with a delay model of a ring cavity with strongly dispersive two-level atoms. Near nascent optical bistability, the 2D homogeneous solution undergoes a subcritical transition to a spatiotemporal chaos. The inhomogeneous pattern coexisting with the homogeneous solution is filled with large-intensity peaks, oscillating with time. In the subcritical domain, localized structures are observed, with isolated peaks more stable and more coherent than the ones in the filled structure. The order is easily recovered by a filtering procedure that reduces the wave-vector bandwidth.
We study the phase dynamics of a single-mode ring laser described by the complex Maxwell-Bloch equations. We identify three reference-frame frequencies and determine the properties of the field dynamics observed in these frames. In one of these reference frames, the phase jumps are always equal to \ensuremath{\pi}, irrespective of the detuning, while in another reference frame quasiperiodic field portraits reduce to periodic field portraits. We also apply the recent theory of Ning and Haken [Phys. Rev. Lett. 68, 2109 (1992)] to prove that the laser phase can be decomposed into a geometrical component that is frame invariant and a dynamical component that is frame dependent.
We investigate the interplay of multiwave-mixing processes with the microscopically coupled spatiotemporal light field and charge carrier dynamics in broad-area semiconductor laser amplifiers. Our theoretical description is based on extended spatially resolved Maxwell-Bloch equations including on the microscopic level spatiotemporal multiwave-mixing processes. Performing a third-order expansion in terms of the microscopic carrier distribution, we include in our description both the effects of population pulsations of the total carrier density at the beat frequency and the spatiospectral interactions of the light fields leading to gain nonlinearities as well as spatial and spectral hole burning. Our simulations show that in a broad-area semiconductor laser amplifier, spatiotemporal wave-mixing processes occur in both transverse and propagation directions of the multifrequency optical beams employed in a typical excite-probe configuration.