A commercial Shack-Hartman wavefront sensor is being used in a test setup installed at the Wendelstein 40 cm
telescope to test methods for telescope alignment based on reverse optimization. Measured low-order Zernike
wave-fronts are being used to determine the misalignment of the telescopes optical elements. Then a procedure
to optimize wave-front performance by aligning the telescope secondary mirror is applied.
The setup contains a collimating optical system, the Shack-Hartman sensor and a guiding and acquisition
camera.
Today there exist different commercial and proprietary micro-optics measurement instruments for the characterization of micro-optical components and microlenses in particular. However there is often a lack of a complete quantitative optical characterization of the latter components. Therefore we will focus in this paper on the optical characterization of spherical microlenses. Moreover the results of the performed round robin within the European 6th FP Network of Excellence on Micro-optics "NEMO" will allow us to select the most appropriate instrumentation tools for characterizing refractive spherical microlenses.
High numerical aperture cylindrical micro-lenses are needed in collimating the laser light from laser bars in the near infrared. Diffraction limited performance of such collimation lenses can only be obtained if the surface shape deviates strongly from circular symmetry. Therefore, null tests make the use of diffractive optical elements (DOE) necessary. For performance test at the design wavelength the reference DOE produces an ideal cylinder wave which enables the compensation of the wave front coming from the micro-lens to a plane wave. The use of a DOE-master enables beside the null test geometry also a removal of the anamorphic distortion due to the cylinder geometry. The DOEs having a numerical aperture of 0.8 are produced on an e-beam machine. The measurement of the wave aberrations is done with the help of the phase shifting technique. Alignment aberrations are eliminated by a least square fit of suitable misalignment functionals derived from an analytic approximation. The shape of the micro-lenses is tested in reflected light showing surface defects directly. The cylinder symmetry allows
for a grazing incidence test using two DOE with nearly constant spatial frequency. There are no limitations concerning the test of high numerical aperture surfaces since the structure of the diffractive elements are parallel curves to the profile curve. The mean spatial frequency of the DOE defines the effective wavelength. Since we use a diffractive interferometer the effective wavelength is identical to the pitch of the DOE. Usually pitches between 4-10 μm are used resulting in a fringe sensitivity of 2-5 Μm. The test delivers the deviation of the surface from the ideal form. In the case of non-circular symmetry the DOE deviates from the axicon type DOE. In addition to the shape deviations also the radius of curvature at the vertex can be measured. The stage for the cylindrical micro-lens is equipped with a length measuring device using grating references from Renishaw providing a length increment of 0.1μm. With the axicon DOE there are two positions where nearly fluffed out fringes can be observed. Starting from the basic test position the lens can be moved until the vertex of the lens coincides with the focal line of the wave generated by the central part of the DOE. The distance between these two positions gives the radius of curvature for the vertex. Modern manufacturing of micro-lenses comprises also hot embossing in plastics or even into glass. Because of the small dimensions of the lenses and the required accuracy of the surface shape also tests of the impressing mould have to be carried out. The DOE approach enables also the test of the embossing form with the same sensitivity as the final test of
the lens. Measuring results for the two test methods will be given.
An interferometrical measurement method is presented for determining the quality of cylindrical micro lenses with a numerical aperture of up to 0.8 in transmitted light. In order to maximize the achievable accuracy, a null test configuration was chosen. The reference cylindrical waves are shaped by computer generated diffractive optical elements (DOEs), which are made by optical or e-beam lithography. The resulting wave front is analyzed by a fiber optic phase-shifting Mach-Zehnder-interferometer or a Shack-Hartmann wave front sensor. Besides the general setup, which is working in the near infrared (NIR), special aspects will be presented concerning the elimination of misalignment aberrations, the complete filtering of parasitic diffraction orders and the generation of an anamorphic optical transformation for increasing the lateral resolution perpendicular to the cylindrical axis. By means of experimental results the possibilities and accuracy of this technique are discussed.
Micro-cylinder lenses are used in beam shaping applications for laser light emitted by semiconductor laser bars. The radiation pattern is extremely asymmetric concerning the angle characteristic. There is a fast and a slow axis of the radiation field. The micro-cylinder lenses are mainly used to correct the fast axis field which makes high numerical aperture lenses necessary. The interferometric test of such fast cylinder lenses is rather involved especially since a deviation from circular symmetry of the meridian curve is mandatory in order to correct the on-axis aberrations. There is a need for surface testing during the manufacture process and performance testing in the final use of the lenses. Surface testing should be done in reflected light. We propose the use of a grazing incidence interferometer based on two diffractive optical elements as beam splitter and beam shaper. With this method arbitrary cylindrical surfaces can be measured with a repeatability of about 10nm rms. The performance test relies on interferometric measurements in transmitted light. To enable a null test a null lens or a diffractive null element is necessary. We report on a null test in transmitted light using a high aperture diffractive element at a wavelength of 780 nm.
In this paper we present our latest results on the fabrication and characterization of plastic microlenslet arrays using Deep Lithography with Protons (DLP) and highlight their geometrical dimensions, their surface profile and their uniformity. We also present quantitative information on their optical characteristics such as focal length and spherical aberration as measured with a Mach-Zehnder interferometer. Furthermore we demonstrate the flexibility of the DLP technology to fabricate arrays of microlenses that feature different pitches and different sags. Although the DLP technology is a valuable tool to rapidly prototype refractive micro-optical components, the approach is unpractical for mass-fabrication. We therefore introduce a replication technique, called vacuum casting, which is very appropriate when only a few tens of copies have to be made, and we bring forward the first quantitative characteristics of these microlens replicas.
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.
A single frame phase-shifting speckle-interferometer for quantitative measurements of out-of-plane-deformations is presented. Using a diffractive optical element, six phase- shifted interference patterns are generated and stored within the frame time of a single CCD-camera.
Speckle interferometry is a sophisticated technique for the non-contact, high precision measurement of form, deformation and vibration of technical surfaces. In this paper, a new single frame phase-shifting speckle interferometer for the quantitative measurement of out-of-plane-deformations is described and first measurements are presented. With a binary phase grating, six interference patterns are generated at a time. They are acquired within one frame time of a single CCD- camera, thus minimizing errors due to thermal fluctuations or vibrations. The use of fiber optics allows a flexible application of the system under industrial conditions and a compact design of the sensor head.
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