Astigmatism (optical systems) | EPFL Graph Search

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances. The term comes from the Greek α- (a-) meaning "without" and στίγμα (stigma), "a mark, spot, puncture". Forms of astigmatism There are two distinct forms of astigmatism. The first is a third-order aberration, which occurs for objects (or parts of objects) away from the optical axis. This form of aberration occurs even when the optical system is perfectly symmetrical. This is often referred to as a "monochromatic aberration", because it occurs even for light of a single wavelength. This terminology may be misleading, however, as the amount of aberration can vary strongly with wavelength in an optical system. The second form of astigmatism occurs when the optical system is not symmetric about the optica

Using effective medium theories to design tailored nanocomposite materials for optical systems

Toralf Scharf, Friedrich Daniel Werdehausen

Modern optical systems are subject to very restrictive performance, size and cost requirements. Especially in portable systems size often is the most important factor, which necessitates elaborate designs to achieve the desired specifications. However, current designs already operate very close to the physical limits and further progress is difficult to achieve by changing only the complexity of the design. Another way of improving the performance is to tailor the optical properties of materials specifically to the application at hand. A class of novel, customizable materials that enables the tailoring of the optical properties, and promises to overcome many of the intrinsic disadvantages of polymers, are nanocomposites. However, despite considerable past research efforts, these types of materials are largely underutilized in optical systems. To shed light into this issue we, in this paper, discuss how nanocomposites can be modeled using effective medium theories. In the second part, we then investigate the fundamental requirements that have to be fulfilled to make nanocomposites suitable for optical applications, and show that it is indeed possible to fabricate such a material using existing methods. Furthermore, we show how nanocomposites can be used to tailor the refractive index and dispersion properties towards specific applications.

2018

Femtosecond laser-based optomechanical processes for permanent and high-precision fine alignment of optical systems

Saood Ibni Nazir

Miniaturization has been at the forefront of scientific research in the past decade covering diverse areas such as electronics, mechanics, and optics. While 'small is beautiful' may be a vast generalization, the true benefits of miniaturization are especially evident in terms of performance, robustness, packaging density, and manufacturing cost. In this context, femtosecond laser processing of transparent dielectric materials has emerged as a versatile tool for designing complex shapes at the micro-scale. Its three-dimensional (3D) capability, single-step maskless fabrication approach, and material-independent nature bypass many limitations of traditional lithographic methods, thereby enabling the fabrication of micro-mechanical and micro-optical components in glass substrates, as well as their monolithic integration for optomechanical, optofluidic, and other such applications.The subject of this thesis is permanent high-precision alignment of micro-optical components. Here, we explore the use of femtosecond laser processing to fabricate pseudo-monolithic optical assemblies. Specifically, we integrate complex optomechanical functions on a single glass substrate and investigate novel non-contact alignment methods based on complex morphological events that occur within a laser-irradiated region. Due to the non-linear nature of laser absorption, the alignment method distinguishes itself from existing methods by its ability to introduce permanent and controlled sub-nanometer deformations in 3D glass micro-positioners, thereby forming a single monolith on which optical components are placed.First, we discuss some general design principles for micro-positioning elements. Although a plethora of examples can be found in literature, we establish certain constraints, based on which, some designs are chosen for further investigation. Next, we carry out component-level demonstrations by actuating the chosen designs using non-ablative femtosecond laser exposure. The laser exposure strategy, which is a crucial part of the alignment process, is also investigated and optimized. Later on, system-level integration is demonstrated in a fiber-coupling device by combining two alignment functions on a single substrate. Inside a compact design footprint, we achieve a near-theoretical coupling efficiency.Alignment sensitivity requirements vary from case to case and can range between sub-micron down to sub-nm levels. To investigate the resolution capability of femtosecond laser-enabled fine-alignment methods, we test this approach on a Fabry Perot cavity, typically designed for atom trapping applications, and that illustrates an application with extreme resolution requirements.Finally, as an essential parameter for sustainable packaging, we report on the long-term stability of laser-aligned optical systems. There, using a novel micro-tensile testing method, the room-temperature relaxation dynamics of fused silica under static stress conditions are explored for stress levels approaching 2 GPa. This work holds promise for both research and industrial applications. It enhances the potential of femtosecond laser micromachining by adding alignment functions, thereby enabling the fabrication of an 'all-glass optical motherboard'. The potential resolution of this method, combined with the intrinsic material properties of fused silica, are particularly interesting for the packaging of high-precision instruments, such as those for space applications.

EPFL2021

Self-tracking Solar Concentration

Volker Günter Zagolla

Solar concentration is intended to decrease the cost of expensive solar cell material by replacing the solar cell with a focusing optical system as the initial receiving aperture. This reduces the amount of solar cell material needed but adds the need of tracking to the entire system in order to keep the smaller solar cell at the focal spot. In state of the art concentrators, precise tracking (less than 1°) is needed in two directions to cover diurnal and seasonal variations. This adds to the cost of the system, diminishing the savings from the smaller solar cell, and consumes energy which in turn decreases the overall system efficiency. A self-tracking concentrator has the potential to increases the tolerances of the system (less precise tracking needed) and cover seasonal variations (only tracking in one dimension remains). Self-tracking makes use of the energy of the spectrum that is typically not utilized in PV, thus self-tracking does not consume an outside surplus of energy. During the thesis such a self-tracking system was developed. This thesis describes two concepts for the realization of a self-tracking concentrator, one of which is then developed into a working demonstration device with an angular acceptance of 32° (±16°). The work covers both the development of these two different mechanisms as well as an analysis of their performance as a single element and a discussion of their performance as part of a much larger device. The first concept, the micro-fluidic concentrator, uses vapor bubbles to efficiently couple light into a liquid waveguide (chapter 3), while the second concept, based on a phase change actuation uses the thermal expansion of paraffin wax to create a coupling feature at a waveguide (chapter 4). Based on the findings, the phase-change concentrator was then expanded into a self-tracking concentrator with a possible effective concentration of 10x (chapter 5). As a result this thesis shows the feasibility of a self-tracking concentrator and describes in detail the processes and techniques for its realization. Future work based on the designs presented in this thesis can optimize the concepts. Simulations show that a system based on this approach can achieve 150x effective concentration by scaling the system collecting area to reasonable dimensions (30 x 1 cm²). An optimized optical system can then also increase the acceptance angle of the self-tracking concentrator to the ±23° needed in order to cover seasonal variations. In addition, the presented concept is based on earth abundant, inexpensive materials, making it technologically and economically viable to extend it to much larger formats.

EPFL2015