Publication: Inestabilidades en láseres de estado sólido y gran apertura
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2016-02-18
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Universidad Complutense de Madrid
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Abstract
En los laseres de gran apertura, en los que la sección del medio activo es mayor que la cintura del haz gaussiano y el numero de Fresnel toma valores superiores a la decena, las pérdidas por difracción de los modos transversos de alto orden son relativamente reducidas, por lo que es frecuente que estas soluciones para las ecuaciones de Maxwell-Bloch en aproximación paraxial sean capaces de autoampliarse y participar en la dinámica transversa. La interacción no lineal entre modos transversos da lugar a la aparición de distintas inestabilidades espacio-temporales. En los láseres de estado s olido y alta ganancia estos comportamientos frecuentemente coexisten con otras formas de inestabilidad, generandose una dinámica de gran complejidad e interés. En esta memoria se recogen varios trabajos dedicados al estudio de las propiedades diná micas de un sistema l aser de estas características, operado en régimen libre y bombeado mediante dos l amparas de destello alimentadas por un pulso de alta intensidad. En el primero de estos trabajos se analiza la aparición de modos transversos no ortogonales. Dada la alta similitud entre los patrones experimentales y diferentes familias de modos electromagnéticos transversos (TEMs por sus siglas en ingles), de nidos para cavidades vacías y régimen de onda continua, es frecuente asumir que propiedades como la ortogonalidad de los modos can onicos son heredadas por los modos experimentales. Sin embargo en nuestro sistema la ortogonalidad no se cumple, como demuestra la persistencia de batidos de modos transversos en el espectro de la potencia total emitida por el laser. El an álisis del per l de uorescencia inducido por las lámparas de destello en el interior de un medio activo de Nd:YAG, as como la realización de simulaciones numé ricas en las que este per l es modelado en dos casos (uno simétrico y otro con una variación lateral del 10 %) nos permite relacionar la asimetría detectada en el bombeo experimental con la violación de la ortogonalidad en los modos resultantes. Este resultado amplía la lista de fen ómenos cuya capacidad para producir la desortogonalizaci ón del patron transverso es conocida...
In broad-area lasers (those where the section of the active medium is greater than the Gaussian beam waist and the Fresnel number takes values higher than ten) di raction loses of high-order transverse modes are relatively small, so frequently these solutions for the Maxwell-Bloch equations in paraxial approximation are able to self-amplify and participate in transverse dynamics. The nonlinear interaction between transverse modes gives rise to di erent spatialtemporal instabilities. In solid-state high-gain lasers these behaviors often coexist with other forms of instability, generating dynamics of great complexity and interest. This dissertation is a compilation of studies on the dynamical properties of such type of laser systems when operated in free regime and pumped by two ash lamps powered by a high intensity pulse. In the rst of these studies the appearance of non-orthogonal transverse modes is analyzed. Given the strong similarity between the experimental patterns and the di erent families of transverse electromagnetic modes (TEMs), de ned for empty cavities and continuous wave operation, it is common to assume that properties like orthogonality of the canonical modes are inherited by experimental ones. However, in our system the orthogonality is not met, as evidenced by the persistence of transverse mode beatings in the spectrum of the total power emitted by the laser. Indeed, we are able to relate the asymmetry detected in the experimental pumping with the violation of orthogonality in the resulting modes by analyzing the uorescence pro le induced by the ash lamps inside an active medium of Nd:YAG and performing numerical simulations, in which this pro le is modeled in two cases. This result extends the list of phenomena whose ability to produce non-orthogonality of the transverse pattern is known. In the second study we compared the transverse dynamics sustained along the laser pulse in di erent Nd3+ doped active media. As local intensity measurements and instantaneous transverse patterns show, when mounted within the same experimental system the three materials compared evolve di erently according to the type of broadening of the gain curve. Despite showing very di erent population inversion redistribution rates, materials with inhomogeneous broadening maintain a somehow orderly structure throughout the emission time, even when it is extended by changing the pumping circuit con guration. On the other hand, in the active medium with homogeneous broadening a dynamic transition is observed. After this transition the resulting pattern has a cluttered appearance and a very low spatial correlation between di erent points located across the width of the beam...
In broad-area lasers (those where the section of the active medium is greater than the Gaussian beam waist and the Fresnel number takes values higher than ten) di raction loses of high-order transverse modes are relatively small, so frequently these solutions for the Maxwell-Bloch equations in paraxial approximation are able to self-amplify and participate in transverse dynamics. The nonlinear interaction between transverse modes gives rise to di erent spatialtemporal instabilities. In solid-state high-gain lasers these behaviors often coexist with other forms of instability, generating dynamics of great complexity and interest. This dissertation is a compilation of studies on the dynamical properties of such type of laser systems when operated in free regime and pumped by two ash lamps powered by a high intensity pulse. In the rst of these studies the appearance of non-orthogonal transverse modes is analyzed. Given the strong similarity between the experimental patterns and the di erent families of transverse electromagnetic modes (TEMs), de ned for empty cavities and continuous wave operation, it is common to assume that properties like orthogonality of the canonical modes are inherited by experimental ones. However, in our system the orthogonality is not met, as evidenced by the persistence of transverse mode beatings in the spectrum of the total power emitted by the laser. Indeed, we are able to relate the asymmetry detected in the experimental pumping with the violation of orthogonality in the resulting modes by analyzing the uorescence pro le induced by the ash lamps inside an active medium of Nd:YAG and performing numerical simulations, in which this pro le is modeled in two cases. This result extends the list of phenomena whose ability to produce non-orthogonality of the transverse pattern is known. In the second study we compared the transverse dynamics sustained along the laser pulse in di erent Nd3+ doped active media. As local intensity measurements and instantaneous transverse patterns show, when mounted within the same experimental system the three materials compared evolve di erently according to the type of broadening of the gain curve. Despite showing very di erent population inversion redistribution rates, materials with inhomogeneous broadening maintain a somehow orderly structure throughout the emission time, even when it is extended by changing the pumping circuit con guration. On the other hand, in the active medium with homogeneous broadening a dynamic transition is observed. After this transition the resulting pattern has a cluttered appearance and a very low spatial correlation between di erent points located across the width of the beam...
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Tesis inédita de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, Departamento de Óptica, leída el 11-01-2016