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Physical Review Letters

Phase Instabilities in the Laser Vector Complex Ginzburg-Landau Equation

Maxi San Miguel

Published Jul 17, 1995• 75 citations
Publisher

Abstract

The interplay between the polarization state of light and transverse effects in lasers is analyzed through an amplitude equation description of an atomic transition between spin sublevels. Linearly polarized traveling waves are found, whose stability is restricted by a phase instability associated with the direction of polarization. The instability persists for polarization stabilized lasers. Novel states of laser light such as standing waves with a spatially periodic linear polarization or coexisting traveling waves with different wave numbers and circular polarizations are also found.

DOI
10.1103/physrevlett.75.425

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The interplay between the polarization state of light and transverse effects in lasers is analyzed through an amplitude equation description of an atomic transition between spin sublevels. Linearly polarized traveling waves are found, whose stability is restricted by a phase instability associated with the direction of polarization. The instability persists for polarization stabilized lasers. Novel states of laser light such as standing waves with a spatially periodic linear polarization or coexisting traveling waves with different wave numbers and circular polarizations are also found.
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We address polarization instability in a freely decaying polariton condensate created by 2-ps-long linearly polarized laser pulses in the upper sublevel of the lower-polariton (LP) branch in a GaAs-based microcavity with reduced symmetry. The generated linearly polarized condensate is found to lose its stability at excitation densities above the threshold value: it passes into the regime of inner Josephson oscillations with strongly oscillating circular and diagonal linear polarization degrees, as well as monotonically decreased oscillations in linear polarization accompanied by a gradual increase in the condensate of the low-sublevel component. These phenomena occur with a relatively small decrease in the total polarization and spatial coherence of the spinor condensate. At high LP densities, the LP-LP interaction leads to the nonlinear Josephson effect. All effects are found to be well reproduced by the model based on spinor Gross-Pitaevskii equations. The cause of the instability was clarified by considering a simplified model of the spinor 0D oscillator: it is spin anisotropy of the LP-LP interaction. The threshold density was shown to increase with decreasing difference $\mathrm{\ensuremath{\Delta}}\ensuremath{\alpha}$ of the constants of the interparticle interaction of LPs with identical and opposite spins as ${\ensuremath{\delta}}_{l}/\mathrm{\ensuremath{\Delta}}\ensuremath{\alpha}$, where ${\ensuremath{\delta}}_{l}$ is the LP level splitting. The reduction in the linear polarization is connected with the fact that the LPs escaping from the condensate with oscillating circular polarization carry off more energy than from the linearly polarized one.
Instabilities in the interaction of counterpropagating laser beams in a nonlinear medium can have important consequences for nonlinear optical processes such as optical bistability and phase conjugation. We have performed a theoretical study of the polarization characteristics of two counterpropagating waves in an isotropic Kerr medium in order to determine the stability of the multivalued solutions found by previous workers.1 Our method involves numerical integration of the coupled-amplitude equations describing the spatial and temporal evolution of the counterpropagating vector waves.2 Our results show that the multiply stable solutions for the output polarizations of the two vector fields show hysteresis, and that the order in which the two fields are turned on determines which solution is reached. For even higher input intensities we find that the output polarizations fluctuate in time.
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Of the Lorenz-type instabilities expected in optical systems, the single-mode instability in optical bistability has been the subject of detailed theoretical and experimental work by our groups. A treatment employing the plane-wave and mean-field limits of the Maxwell-Bloch equations predicts a rich variety of higher-order bifurcations including period doublings and intermittency.1 An extension of this work to include the transverse dependence of the intracavity field reveals no significant qualitative differences between the plane-wave theory and the Gaussian beam theory as regards the stability boundaries obtained from a linearized analysis.2 The instability persists in the presence of a Gaussian transverse profile and the boundaries are not radically modified compared with those obtained from the plane-wave theory. However, numerical integration of the Maxwell-Bloch equations in the single transverse mode approximation does not show the wealth of higher-order dynamic states present in the plane-wave case. An experimental investigation of optical bistability for two-level atoms in a single transverse, single longitudinal mode cavity exhibits limit cycle oscillation with stability boundaries that agree well with the boundaries obtained from the work in Ref. 2. However, for the range of values C < 300, |Δ| < 5, |θ| < 50, X ~ 102, the experimental results do not show the diversity of higher dynamic states expected from the plane-wave theory, in agreement with our numerical results. In addition the measurements support the existence of a single transverse mode oscillation. The sum of our theoretical and experimental work provides a quantitative statement for the role of the transverse effects in optical instabilities.
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.