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Holographic light shaping through acousto-optic deflectors

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Universitat de Barcelona. Departament de Física Aplicada

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[eng] The invention of the laser in the 1960s triggered a new era of optical technologies which revolutionized many fields of industry and research. At the same time, a new demand for technologies emerged that allow to dynamically control the optical properties of a laser beam and to shape laser light into arbitrary patterns. 
This need led to the development of spatial light modulators, which are programmable diffractive elements that shape laser light into well-defined intensity distributions through optical phase or amplitude modulation. Their capability to display computer generated holograms made them an indispensable tool in a broad range of optics and photonics applications, because it allows to reconstruct optical wavefronts from digital models without the need of physical counterparts. This most versatile control over an optical wavefront enables complex functionalities such as holographic beam shaping, aberration correction, adaptive optics applications and optical micro-manipulation, to name only a few.
Various spatial light modulator technologies exist with inherent capabilities that define their possible applications. The high light efficiency and outstanding modulation capabilities of spatial light modulators based on liquid crystals (LC-SLMs) found a myriad of applications and represent the current standard for dynamic light modulation based on digital holograms. There are, however, several applications that require other capabilities than those offered by LC-SLMs. Ultra-fast light modulation, high laser power applications and high-quality pattern formation are difficult to achieve with liquid crystal devices, so that alternatives to this prevailing technology are needed.
An interesting candidate for high performance spatial light modulation are acousto-optic deflectors (AODs). These devices are commonly used for high-speed beam deflection, but it has also been demonstrated in few scattered precedents that they are capable of holographic light modulation. Acousto-optic deflectors are based on a considerably different technology than LC-SLMs, which complicates their use as full-fledged spatial light modulators. But at the same time, they provide promising light modulation capabilities, such as very high optical power thresholds, modulation rates of several kilohertz and a continuous (non-pixelated) wavefront modulation, which potentially make them a complement or even competitor to existing spatial light modulator technologies.
The scope of this thesis is the investigation and application of the holographic modulation capabilities of a spatial light modulator system based on acousto-optic deflectors. Such a system is realized in the frame of this work as a common optical Fourier transform setup with off the shelf hardware. The principal effort in the implementation is the development of calculation methods for acousto-optic holograms and the corresponding electronic driving signals, which on the one hand need to consider the specific capabilities and constraints of AODs, and on the other hand have to provide high light efficiencies of the displayed holograms and a high reconstruction quality of the formed patterns. Thus, the one-dimensionality of the acousto-optic modulation and the resulting separability restrictions for two-dimensional AOD modulation are discussed, and different reconstruction strategies for arbitrary two-dimensional patterns are investigated. Furthermore, various image degrading effects are analyzed, and corresponding correction methods are proposed. Especially the inherent reduction of coherent artifacts through the motion of acousto-optic holograms is analyzed in detail.
The capabilities of such implemented acousto-optic SLM are demonstrated by reconstructing arbitrary laser patterns with very high perceived image fidelity. Through the combination of different correction methods, also high-speed reconstructions of separable patterns at rates of several kilohertz are achieved, which eventually come into play in a structured illumination microscope.

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[spa] Los moduladores espaciales de luz (SLM) son herramientas optoelectrónicas versátiles que dan forma holográfica a la luz láser en patrones de intensidad complejos para su uso en una amplia gama de aplicaciones ópticas y fotónicas. Existe una variedad de tecnologías SLM, pero los moduladores espaciales de luz basados en cristales líquidos han prevalecido debido a su alta eficiencia lumínica y a sus excepcionales capacidades de modulación. Sin embargo, a esta tecnología le cuesta cumplir con los requisitos de las aplicaciones que necesitan velocidades de modulación en el régimen de los kilohercios, altas potencias de láser o proyección de patrones de alta fidelidad.
Un enfoque alternativo es la modulación espacial de la luz basada en deflectores acusto-ópticos. Estos dispositivos se utilizan comúnmente para la deflexión de un haz de luz a alta velocidad y son bien conocidos por sus excelentes tasas de modulación y su resistencia a altas potencias ópticas. Los deflectores acusto-ópticos también pueden modular espacialmente la amplitud y fase de un haz de láser, pero la volatilidad y unidimensionalidad de su modulación dificulta su uso en pie de igualdad con otros SLM.
En este trabajo, se investiga la modulación holográfica de la luz a través de deflectores acusto-ópticos y se desarrolla un marco metodológico que hace accesibles sus intrigantes capacidades de modulación espacial.
El sistema experimental consiste en dos AOD alineados perpendicularmente en una configuración de transformada óptica de Fourier, que es capaz de reconstruir patrones separables unidimensionales y bidimensionales. Se discute la restricción de separabilidad y se proponen estrategias para la generación de patrones bidimensionales arbitrarios. Los hologramas de Fourier se muestran en los AOD inyectando señales de direccionamiento moduladas complejas, que se adaptan a las propiedades particulares de la modulación de luz acusto-óptica.
Demostramos, tanto teórica como experimentalmente, que el movimiento de los hologramas acusto-ópticos reduce de forma inherente los artefactos coherentes que son omnipresentes en la holografía digital. Este fenómeno permite reconstruir holográficamente patrones de intensidad de muy alta calidad, indicados por valores de contraste de "speckle" medidos experimentalmente de tan solo 0.02. También se muestra que este efecto permite proyecciones a alta velocidad de patrones separables hasta 21 kHz mientras se mantiene una buena fidelidad en dichos patrones. Además, se presenta la aplicación del proyector de patrones universal implementado como fuente de iluminación estructurada para un microscopio confocal.

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Tesis Doctorals - Departament - Física Aplicada

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Holographic light shaping through acousto-optic deflectors