13.1 Microoptics and freeform optical beam shaping

Abstract

Microoptics by now has developed into a versatile concept and technological approach for the implementation of optical elements and compact optical systems. In the first part of this chapter we discuss the fundamental aspects of microoptics as well as some technological background. After introducing the concept of diffractive optics as well as the most important technologies for the fabrication of refractive microlenses we focus on microoptical systems integration. Being a specific branch of microsystems technology, microoptics is specifically well suited for the implementation of complex hybrid microsystems. This is of specific interest for the implementation of optical elements with active or adaptive functionality which is discussed in the subsequent section. Optical beam shaping as well as laser diode beam shaping are the most outstanding applications for microoptical elements or systems combining a variety of optical functions in order to realize the complex optical task. In the second part of this chapter we thus focus on these specific applications. After categorization of the various approaches to optical beam shaping we discuss the concepts for laser diode beam shaping. Throughout the last few decades microstructures have gained a constantly growing importance in technical and specifically also optical systems [07Gia]. From the perspective of optical applications this has been driven by the desire to control or shape light beams or wavefronts in a more and more complex manner. Although the term “tailoring light” recently has been used in the context of nanostructures and nanooptics it can also be used to describe the motivation for the development of microoptics. The origin for the extensive use of microstructures in optical beam shaping elements and for optical systems optimization can be found in the 1960ies in the context of information optics. At that time spatial filtering techniques have been extensively investigated for the design of specific transfer functions of optical systems (so called Optical Transfer Function (OTF) synthesis). To this end a variety of techniques for complex optical beam shaping such as computer-generated holography and kinoform optical elements have been invented. These mark the beginning of the remarkable development of diffractive optics and microoptics which has been supported by the revolution triggered by the use of lithographic techniques for microfabrication [00Men, 84Iga, 97Her, 03Sin]. Even after decades of successful research, development, as well as commercialization of microoptical components and systems a precise definition of what is meant by microoptics is still a matter of discussion. Some important groups in the field define microoptics as components and systems with optical functionality partially resulting from structures with dimensions in the range of a few to several 100s of micrometers. In that sense almost any optical element or system has to be considered a “microoptical element” since at least the generally applied optical coatings benefit from (sub-)micrometer-sized layer thicknesses. A more generally applicable definition of microoptics refers to the close relationship to microsystems and their fabrication technology. Figure 13.1.1 illustrates this relationship between the various “micro-”technologies such as microelectronics, microfluidics, and micromechanics. All these approaches have the common focus on miniaturization, which is immediately obvious and intuitively related to the prefix “micro-”. More importantly,