We report on a position sensing system for quasistatic MEMS mirrors using a point source LED imaged onto the backside of the mirror. The position signal is detected by a 4-quadrant-diode and can be used for digital closed-loop control. The resulting resolution is about 13 bit at a sampling frequency of 500 ksps. The whole detection system is integrated in a package enclosing the MEMS mirror, the optical beam path as well as the amplifier circuit of the 4-quadrant-diode. Based on similar control principles, a laser steering system for searching and tracking of moving objects, marked by retro-reflectors, will be presented. During an initial search, the laser steering system identifies objects by their initial position within a detection range between 1.5 m and 5 m distance and a maximum detection angle between 20 degrees and 72 degrees. The system follows the detected object on random trajectories by minimizing the zero-position error of the back reflected beam on a 4- quadrant-diode. Angular velocities of more than 100 degrees per arcsec have been measured.
HERSCHEL's 3.5 m primary mirror will be passively cooled to T ~ 80 K in the L2 orbit. In order to reduce the effects of the remaining high thermal background on the sensitive far infrared detectors (60..210 μm), a focal plane chopper is a vital element in the entrance optics of the imaging and spectroscopic instrument PACS. A gold coated 32 × 26 mm2 plane mirror, suspended by two flexural pivots and driven by a linear motor, allows for precise square wave chopping with up to 9° throw at a frequency 10 Hz with a position accuracy of 1 arcmin. The power required at T ~ 4 K is about 1 mW. The chopper has undergone an extensive qualification programme, including 650 million cold chop throws, 15 cold-warm-cold thermal cycles, 3-axis 26 G-vibration at T ~ 4 K etc. Five models were built and thoroughly tested; the flight model of the chopper is now integrated into the flight model of PACS, ready for the HERSCHEL/PLANCK launch in 2008 by an ARIANE5 rocket and the following 5-year mission.
During the last few years, the understanding of pupil plane manipulation capabilities to achieve an enhanced image formation process and its theoretical limitations becomes more and more important. Particularly the increased potential of computer calculations and digital signal detection makes it possible to simulate this kind of manipulations very fast and efficient. At the same time there is the opportunity to change the optical system such, that a digital reconstruction of the image gives a higher amount of information. An analysis of the optical transfer function as an important parameter of imaging quality with special interest in extended depth of focus is presented. The performance of different pupil plane masks is illustrated in comparison with standard optical systems. This means the basic features like depth of focus, resolution and contrast were derived and the limitations are shown. The mathematical principle of extended depth of focus with pupil manipulation is described and demonstrated with exemplary calculations. Furthermore, the relation between a given optical transfer function and the matching pupil function is shown. A robust and iterative algorithm is presented to calculate a pupil mask for a desired optical transfer function.
By introducing sinusoidal deformations on piezoelectric crystals using surface acoustic waves with spatial frequencies > 30 mm-1 and amplitudes > 1 nm new optical diffractive elements can be generated. For high diffraction efficiencies it is necessary to produce amplitudes of the SAW as high as possible or to use light wavelength as short as possible. Conventional diffraction gratings are fixed in amplitude and spatial frequency whereas both can be varied for this new type of gratings. The gratings can operate in several environments such as N2, He gas or vacuum and be remote controlled. Therefore they are a very flexible tool. First results on these gratings with frequencies up to 200 MHz and amplitudes in the order of 5 nm are shown.
The basic principle and implementation of a near-field technique developed at ESTEC for measuring stray and backscattered light in optical telescopes are summarized. The measurement problems posed by transmit-receive antenna telescopes for optical intersatellite communication systems are described; the inadequacies of conventional methods for ground-based testing of such telescopes are explained; and the theoretical basis, instrumentation, and calibration of the present method are discussed. In this approach, a 40-nsec-pulsed laser-diode source is employed, and the intensity of the backscattered and stray light is measured by a microchannel-plate photomultiplier during the time of flight of the pulse from the optics assembly to a light trap.
KEYWORDS: Polarization, Near field, Telecommunications, Telescopes, Laser communications, Space telescopes, Diffraction, Near field optics, Free space optical communications, Laser applications
A technique developed at ESTEC for the measurement and theoretical estimation of optical polarization in the antenna telescopes of space optical communication systems (e.g., the ESA SILEX intersatellite system using 825-nm semiconductor lasers) is described. The design methods used to achieve high channel separation ratios in the SILEX telescopes are recalled; the failure of straightforward measurement techniques in practical ground-based testing is explained; and a solution based on the mathematical relationship between far-field and near-field intensity distributions is outlined. The primary instrument used in this method is a scanning ellipsometer.
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