Desing and development of Multichannel Optical Systems for Imaging and Nonimaging Applications.
Autor: Joao Mendes Lopes
Director: Juan Carlos Miñano
From the beginning of history of science and technology, humankind has sought inspiration from the wonders created by Nature and natural evolution. In the last century, several key inventions were inspired by biological mechanisms found in diverse organisms in Nature: from the invention of Velcro - inspired by the way burdock burrs get attached to fur - up to the development of new energy technologies. The fins of humpback whales were the inspiration to create more powerful wind turbines, and the wave vortices, produced by schools of swimming fish, inspired a new optimization of spatial disposition of wind farms.
Optics is no exception. One of the great challenges of optical design is to achieve optical systems ever smaller, with larger field of view and acceptance. Natural evolution found a solution in the vision system of many invertebrates, such as a fly - the compound eye. A compound eye consists of a large number of extremely small vision systems on a curved macro surface, capturing a large field of view, while maintaining their dimensions small. The concept of compound eye has been adapted for optical design, and in the last quarter of century different kinds of multichannel systems were developed. Similarly to how a compound eye works, light splits and is transmitted through a number of different channels and then recombined, either optically or electronically.
Multichannel systems have met applications in both imaging and nonimaging optics.
In imaging applications, arrays of multi aperture optics have been researched for achieving miniaturization, and for achieving high resolution in specific sectors of the field of view. The segmentation of the field of view has the potential to break the usual trade-off between focal length and field of view, and leads to a reduction of the imaging system's total track length. Multichannel design has been applied in technologies such as vision sensors, head mounted displays and cameras, among others.
Nonimaging optics is a branch of optics that deals with the efficient transfer of light between a source and a receiver. It is the best approach for designing solar concentrators and illumination systems, among other applications. The name comes from the fact that there is no requirement to create an image of the source, and the only concern is the efficient light transfer, which provides a larger freedom to design systems.
The multichannel approach in nonimaging optics has been used both in illumination as in Concentrated Photovoltaics (CPV) applications by combining it with advanced methods such as the Simultaneous Multiple Surface (SMS) design method and Köhler integration for designing freeform surfaces.
Concentrated Photovoltaics (CPV) is a technology that consists of a concentrator optical system that focuses sunlight onto a photovoltaic cell. The main goal of this technology is to decrease the Levelized Cost of Electricity (LCOE). The strategy to achieve this goal is based on two approaches: (i) by concentrating the sunlight, it is possible to dramatically decrease the amount of photovoltaic cell area, since concentrating optics is used to focus sun light onto a much smaller solar cell (optical materials are much cheaper than solar cells) ; (ii) as the amount of photovoltaic cell area decreases by a concentrator factor, it is possible to use high efficiency cells, which typically are too expensive to be used without concentration in terrestrial applications.
CPV systems have typically a primary optical element (POE) and a secondary optical element (SOE). On the multichannel approach, the POE divides the incoming bundle of light into multiple ones, and each sector of the SOE has to manage a correspondingly smaller field of view, and provide a smaller magnification. This approach provides both a larger acceptance angle and a potential for an increased magnification as well.
A different type of multichannel optics is diffractive lenses, where the control and generation of wavefronts is achieved by segmenting initial wavefronts and redirecting the segments using interference and phase control. Diffractive optical elements (DOE) have had applications in several different types of optical systems. The first applications were spectroscopic ones. Other applications followed, such as beam splitters, laser-beam shapers, optical interconnectors, etc. The application to imaging systems is more recent. In these configurations, DOEs have the potential to enhance the performance (by controlling chromatic and field curvature aberrations, decreasing size and weight, achieving atomic resolution (<1nm), etc).
The aim of this thesis is to explore the multichannel approach in both imaging and nonimaging applications, using both geometric as well as diffractive optics. The multichannel approach is combined with the use of freeform surfaces to achieve the maximum performance.
This thesis is divided in four chapters:
Chapter 1 establishes the basics by presenting the fundamental concepts required to< understand the following chapters. Basic definitions of geometrical, imaging and non imaging optics are covered, as well as an introduction to multichannel systems and freeform surfaces. SMS and Köhler integration design methods are also explained. Each chapter was written to be self-sustainable, so some concepts that are only used in a specific chapter were left out of chapter 1, and they are presented in their respective chapters.
Chapter 2 describes the development of an optical concentrator for a high concentration CPV system, for multijunction photovoltaic cells. Based on a multichannel configuration and Köhler integration, a 9-fold Fresnel-Köhler (FK9) was developed. The motivation for the development of FK9 is to create a system not only capable of overcoming the current limitations of CPV systems, but also to pave the way for systems in the future with four, five and six junction solar cells. FK9 presents a 9-channel configuration, each channel performing Köhler integration, providing a uniform spectral and spatial irradiance distribution on the solar cell and a large acceptance angle. The Chapter establishes the fundamentals of CPV, introduces the FK9’s design procedure, and demonstrates, through raytrace simulations, its high performance in key figures of merit: high concentration factor, large tolerances, high optical efficiency, and uniform irradiance on the solar cell. A comparative study between concentrators demonstrates that FK9 can sustain a high performance even at extremely high concentration levels (2000x) and ambient temperatures (45ºC). This Chapter also introduces the Fresnel Lens with Variable Focal Point (FL-VFP) as a new design to avoid light crosstalk between different channels, a major issue in multichannel systems. FL-VFP avoids that light impinging in extreme points of the lens crosstalk to other channels of the system, enhancing the performance and acceptance angle of FK9.
Chapter 3 describes an extension of the SMS method, a geometrical optics design method, to design diffractive optical surfaces. This method involves the simultaneous and direct (no optimization) calculation of diffractive and refractive/reflective surfaces. Using the phaseshift properties of diffractive elements as an extra degree of freedom, two rays for each point on each diffractive surface are controlled. The concept of Generalized Diffractive Oval is introduced (in 3D geometry), and the diffractive SMS method (in 2D) is presented as a sequential application of the Generalized Diffractive Oval design procedure. The method can decrease the number of elements of a system, thus decreasing size and weight. Design examples of a 3D freeform diffractive surface coupling two wavefronts and a polychromatic hybrid 2D lens controlling three wavefronts with different wavelengths have been successfully implemented.
Chapter 4 explores the experimental characterization of a double-channel freeform lens, for a head mounted display imaging application. The optical design methods and fabrication machinery for freeform optics have reached a point where the surfaces can be manufactured with high precision. Yet, accurate measurements of the resultant surfaces are still a standing challenge to the surface metrology community. The discontinuities between channels represent an even larger challenge. In this Chapter, two different cutting-edge freeform metrology technologies are used to measure the lens surfaces. A characterization was made, including roughness and topography measurements. The real and design lenses performance were compared through polynomial interpolation and simulations.