Diffractive Optical Elements for Tracking and Receiving in Optical Space Communication Systems

Abstract

The design of first, generation free space laser communication systems is based on laser diodes with output powers in the order of 100 mW [1]. The data rate transmission is in the order of 100 Mbit/s. This leads to terminals with large transmitter and receiver telescope diameters and, consequently, to high terminal mass and dimensions. The optical systems are usually designed with refractive lenses and reflective mirrors. Alternatives are planar diffractive optical elements (DOEs). By relying on diffraction and interference rather than on reflection and refraction, unique and novel properties can be realized. Almost any structure shape, including non–rotationally symmetric aspherics, can be manufactured, which provides all degrees of freedom for the design. Other interesting aspects of DOEs are their low weight, their strong dispersion, and the possibility to make segmented elements, large arrays of elements, beamsplitters, and polarizers. These properties are useful for many applications of DOEs in space, including: filters for image data processing [2], beam shaping [3, 4], and antireflection structures [5, 6]. Furthermore, the combination of refractive and diffractive surfaces (hybrid elements) offers new possibilities for optical design. The negative dispersion of DOEs can be used to compensate the chromatic aberrations of refractive lenses [7, 8]. Hybrid elements can also be used to compensate the temperature induced variations of their mounting system [9, 10]. Diffractive optical elements for space applications must comply with a number of requirements, including mechanical, thermal and optical stability [8]. Suitable techniques for realizing the microstructures in space qualified materials are based on a variety of high resolution lithographic and optical processes [11].