Spatial heterodyne spectroscopy has become increasingly attractive for remote sensing of the atmosphere from microsatellites. Its outstanding light gathering power makes this technology particularly suitable for the detection of faint signals with minimal volume requirements. This paper is about an instrument, which was designed to measure the spectral shape of an atmospheric oxygen emission. The near infrared emission is observed in limb viewing geometry from space. The optical setup and specific characteristics of the design are presented. A focus is on the straylight behaviour of the system. In-field and out-of-field contributions are discussed. Straylight kernels are applied to expected background radiation fields with regard to performance-limiting factors of the system.
In the past, grazing incidence interferometry has been applied to plane, cylindrical, acylindrical and general rod-like surfaces using diffractive beam splitters. Here, we demonstrate that also rough convex steep rotationally symmetric aspherics can be measured along one meridian in a single step using diffractive beam splitters and phase shifting techniques. The measurement of rough surfaces is possible i.e. without the need to polish the surfaces, due to the large effective wavelength (here about 10μm) of the test. The whole surface can be measured by rotating it stepwise around its symmetry axis and measuring successive small meridian regions. These meridian regions have to be stitched together to get the whole surface. Besides the presentation of the measurement principle, simulation results of the procedure for aspherical specimens are given. The corresponding experimental setup with discussion of the measured phase distributions is presented and the misalignment analysis is performed for the special case of spherical objects under test.
A novel CubeSat payload for atmospheric research has been developed to study the temperature distribution in the mesosphere and lower thermosphere region. The payload consists of a small interferometer for the observation of airglow at 762 nm. Since the rotational structure of the O2 A-band emission follows the Boltzmann law, the ratio of different emission lines allows for temperature retrieval without the need of a precise absolute radiometric calibration of the instrument. Integrated in a 6U CubeSat, the instrument will perform simultaneous limb measurements between 60 km and 150 km globally. The agility of a CubeSat shall be used to focus the measurements on specific regions from different viewing directions. In order to achieve high spectral resolution and high optical throughput, a spatial heterodyne interferometer was chosen for detecting the rotational structure of the O2 A-band emission. The utilization of a two-dimensional imaging detector allows for recording these interferograms at various altitudes at the same time. Since this instrument type has no moving parts, it can be built as a solid block which makes it very attractive for atmospheric measurements especially from space. For highly spatially resolved images of the atmosphere in limb view mode, a high-quality front optics, which images the scene onto the refraction gratings of the interferometer, is required. In addition, a detector optics with minimal aberrations is needed to image the gratings onto the focal plane array. The optical design of the interferometer as well as the technical layout of front and detector optics are presented.
A highly miniaturized limb sounder for the observation of the O2 A-band to derive temperatures in the mesosphere and lower thermosphere is presented. The instrument consists of a monolithic spatial heterodyne spectrometer, which is able to resolve the rotational structure of that band. The SHS operates at a Littrow wavelength of 762 nm with a resolving power in the order of 10.000. Complemented by a front optics with an acceptance angle of less than ±1 degree and a detector optics, the entire optical system fits into a volume of about 1.5 liters. This allows this instrument to be flown on a three or six unit CubeSat. In this paper, we introduce the optical design and computer simulations on the expected performance of the instrument. The laboratory characterization of a prototype instrument, which has been built on university level, and the lessons learned are discussed.
In the past grazing incidence interferometry has been applied for plane, cylindrical, acylindrical and general rod-like surfaces using diffractive beam splitters. Here, we demonstrate that also rough convex steep rotational symmetric aspherics can be measured along one meridian in a single step using diffractive beam splitters and phase shifting techniques. Especially, rough surfaces can be measured with this method due to the large effective wavelength (here about 10μm) of the test. The extension to the whole surface can be attained by successive meridional measurements of the surface under test by azimuthal adjustments. The principle of the method is given, first simulated and experimental results are presented. The occurrence of the interferogram is discussed and the experimental evaluation on a single meridian including the unwrapped phase are shown. Furthermore the simulated results of aberrations caused by object misalignment and the first results for the elimination of adjustment aberrations are presented.
The Institute for Atmospheric and Environmental Research at the University of Wuppertal and the Institute of Energy and Climate Research Stratosphere at Research Center Juelich developed a CubeSat payload for atmospheric research. The payload consists of a small interferometer for the observation of airglow near 762 nm. The line intensities of the oxygen A-band are used to derive temperatures in the mesosphere and lower thermosphere region. The temperature data will be used to analyze dynamical wave structures in the atmosphere. The interferometer technology chosen to measure the ro-vibrational structure of the O2 atmospheric band near 762 nm is a spatial heterodyne interferometer originally proposed by Connes in 1958. It can be designed to deliver extraordinary spectral resolution to resolve individual emission lines. The utilization of a two-dimensional imaging detector allows for recording interferograms at adjacent locations simultaneously. Integrated in a six-unit CubeSat, the instrument is designed for limb sounding of the atmosphere. The agility of a CubeSat will be used to sweep the line-of-sight through specific regions of interest to derive a three-dimensional image of an atmospheric volume using tomographic reconstruction techniques.
The Institute for Atmospheric and Environmental Research at the University of Wuppertal and the Institute of Energy and Climate Research Stratosphere (IEK-7) at the Research Center Juelich developed a novel CubeSat payload for atmospheric research. The payload consists of a small spectrometer for the observation of airglow at 762 nm. The line intensities of the oxygen A-band are used to derive temperatures in the mesosphere and lower thermosphere (MLT) region. The temperature data will be used to analyze dynamical wave structures in the atmosphere which have become increasingly important for the modeling of the climate system. Integrated in a 6U CubeSat, the instrument needs a highly accurate attitude determination and control system (ADCS) for limb sounding of the atmosphere. The agility of a CubeSat shall be used to sweep the line-of-sight through specific regions of interest to derive a three-dimensional image of an atmospheric volume using tomographic reconstruction techniques.
The spectrometer technology chosen to measure the ro-vibrational structure of the O2 atmospheric band at 762 nm is a Spatial Heterodyne Interferometer (SHI) originally proposed by Pierre Connes in 1958. The throughput of an SHI is orders of magnitude larger than of a conventional grating spectrometer of the same size. It can be designed to deliver extraordinary spectral resolution to resolve individual emission lines. The utilization of a two-dimensional imaging detector allows for recording interferograms at adjacent locations simultaneously. Since an SHI has no moving parts, it can be built as a monolithic block which makes it very attractive for remote sensing, especially from space.
We are motivated by the question if scanning laser projection with low speckle noise is possible. Scanning laser projection requires “instantaneous” speckle reduction, within a few nanoseconds – meaning that no moving diffusors can be used. We will argue that instantaneous speckle reduction is possible by conversion of spatial coherence to spatial incoherence - but nature demands for a compensation. The cost can be estimated via the information theoretical concept “channel capacity”, which incorporates the etendue as well as the signal-to-noise ratio. We will show that an optical system with low spatial coherence (=low speckle noise) must provide significantly more degrees of freedom than a coherent imaging system. The consequence for the technical optical system is serious: significant speckle reduction can only be achieved by an excessively large projection aperture. This is not just a sophistic consideration, it seriously restricts the design of scanning laser projectors.
Speckle interferometry is an optical metrology technique for characterizing rough surfaces. In one application, the
deformation of a specimen under a load may be determined by comparing measurements before and after the load is
applied. Owing to the surface roughness, however, the results are impaired by phase singularities, leading to a strong
noise in the measurement results. Usually, filtering and smoothing operations are performed to reduce the noise.
However, these procedures also affect the underlying systematic phase and are therefore disadvantageous. Instead, we
examine incoherent averaging, a physical procedure, to reduce the number of phase singularities in the first place. We
tailor the spatial coherence of the light using extended light sources of continuous or multipoint shape, achieving
smoother phase distributions. The mechanism behind the reduction process involves subtle effects like enhancing phase
singularity correlations in the fields before and after the deformation takes place.
This publication presents a novel interferometric method for the simultaneous measurement of the phase and
state of polarization of a light wave with arbitrary, in particular locally varying elliptical polarization. The mea-
surement strategy is based on variations of the reference wave concerning phase and polarization and processing
the interference patterns so obtained. With this method, that is very similar to the classical phase shifting
interferometry, a complete analysis of spatially variant states of polarization and their phase fronts can be done
in one measurement cycle. Furthermore, a direct analysis of specimens under test regarding birefringence and
the impact on the phase of the incoming light can be realized. The theoretical description of the investigated
methods and their experimental implementation are presented.
Speckle interferometry is a well established technique for the optical characterization of rough objects, with the quantification of deformations as one particular application of interest. Owing to its common path property, a speckle-shearing interferometer is often the natural choice as a setup. Like other speckle techniques, however, speckle-shearing interferometry suffers from the existence of phase singularities present in the speckle patterns. Phase singularities introduce ambiguities into the phase unwrapping process and make this evaluation step highly sophisticated. In this work, we attempt to reduce the number of phase singularities by physical means, i. e. by applying an incoherent averaging of multiple, mutually independent speckle intensities. The effect of the incoherent averaging on the number of phase singularities has been investigated theoretically, by computer simulations, and experimentally. To obtain high contrast fringes in connection with a shearing setup, which would not be the case for a simple extended light source, a periodically structured light source with a period matched to the shear distance is applied. It turns out that the number of phase singularities may indeed be reduced, but only to a certain extent.
A lateral shearing interferometer usually provides the slope data of a wave front under test along one direction. For the
complete reconstruction of the wavefront, two slope datasets along different directions are required. Based on diffractive
gratings, a simultaneous measurement of bothdata sets can be carried out. Two possible realizations are presented using a
polarization signature and a partially coherent light source.
Interferometric surface tests of stigmatic aspherics can be carried out in a null test configuration. Null tests require
reference null elements either plane or spherical surfaces or both. A parabolic reflector transforms a plane into a spherical
wave which converges to the focus of the paraboloid. Therefore, a spherical ball lens or a steel ball can be placed into the
focus enabling a double-pass geometry for the null test. Here a Fizeau interferometer geometry has been selected in order
to guarantee invariance against polarization distortions under the assumption that radially polarized laser light is used for
the interferometer. Radial polarized light is necessary to mimic a Hertzian dipole field. Due to the extreme solid angle
produced by the paraboloid the alignment of the setup is very critical and needs auxiliary systems for the control.
Aberrations caused by misalignments are removed via fitting of suitable functionals provided through ray-trace
simulations. It turned out that the usual vector approximations fail under these extreme circumstances. Test results are
given for a paraboloid with 2mm focal length transforming a plane wave into a near dipole wave comprising a solid angle
of about 3,4π.
A quite simple numerical model for the wave-optical simulation of the interference in a grating lateral shearing
interferometer with a periodic light source and a large lateral shear is presented. Aberrations of the collimating lens will
generate a spatially varying modulation in the interference pattern. The model assumes that the light source itself is
completely spatially incoherent so that only the light from each point of the light source has to be propagated wave-optically
through the optical system. Then, the intensity distributions of all light source points in the detector plane can
just be added. The simulations are compared to theoretical calculations of partial coherence theory and also to
experimental results.
The cylinder geometry allows for grazing incidence tests of optical surfaces let it be micro- or macro cylinder lenses. By using diffractive beam splitter and beam shaping elements a grazing incidence interferometer can be built where the zero diffraction order is used as reference beam and one of the first orders as probe beam for the cylinder surface. Circular as well as noncircular meridian curves can be dealt with by shaping the DOE structure accordingly. The structure of the DOE are parallel curves to the meridian providing a constant spatial frequency which alleviates the lithographic process. The fringes indicate surface deviations from fringe to fringe by p/2 where p is the pitch of the DOE. A description of a micro- and a macro-interferometer together with measuring results will be given.
Two different methods for the measurement of cylindrical lenses will be presented in this paper. The first method uses the principle of grazing incidence interferometry. A computer generated diffractive optical element (DOE) generates the wave fronts which impinge under grazing incidence onto the surfaces to be tested. The light is reflected at the surfaces and diffracted at a second DOE identical to the first one. The 0-th diffraction order of both DOEs is used as reference wave. The deviations of up to three surfaces (front-side, backside and one of the border sides) of a cylindrical lens from their ideal shape can be measured simultaneously. Additionally, the orientation of these three surfaces with respect to each other are determined. The second method measures the aberrations of a cylindrical lens in transmitted light by using an interferometer of the Mach-Zehnder type. The cylindrical wave of the lens under test is compensated by a DOE which generates a plane wave if the incident wave is an ideal cylindrical wave. So, the wave aberrations of the cylindrical lens can be measured. The set-up is designed for cylindrical lenses with a high numerical aperture of up to 0.8. The principles of both methods and first experimental results will be presented.
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