Autocollimators are excellent measuring tools for many applications, such as adjustments or characterizations of precision mechanics, optics and instruments. Autocollimators have several characteristic error sources due to misalignment or nonideality of the collimator lens, the light source and the sensor. To fully characterize errors related to simultaneous engagement of both measurement axes of the autocollimator a two directional angle generator is necessary.
In this paper, VTT MIKES interferometric 2-directional small angle generator (I2D-SAG) with updated model for calibration of autocollimators is described. It generates angles around 2 orthogonal axes with high accuracy. The maximum range of the I2D-SAG is ±1000” for both axes. In calibration of a high quality autocollimator a standard uncertainty below 0.01” can be reached.
In addition, preliminary results of a comparison of I2D-SAG and PTB Spatial Angle Autocollimator Calibrator (SAAC) instrument are presented. This is the first comparison between 2D autocollimator calibration systems. In this comparison, a good agreement was reached in characterization of an electronic autocollimator over 900”×900” range.
Online measurement of diameters and concentricities of optical fibre layers, and the coating layer in particular, is one of the challenges in fibre manufacturing. Currently available instruments can measure concentricity and diameter of layers offline, and are not suitable for precise monitoring or control of the manufacturing process in real time. In this work, we use two laser beams, positioned orthogonally to illuminate the fibre from two sides, and calculate deviations from the expected geometry by analysing the scattering pattern. To measure the diffraction pattern we use two 8K linear array detectors, with the scattered light incident directly on the sensors. Each detector is capturing approximately 90° angular range directly behind the fibre. The two measurement channels are positioned at different heights. The scattered pattern is modelled mathematically with finite-element and Fourier-modal methods, with various diameter and concentricity deviations. The sensitivities of the changes in the scattering pattern are identified in respect to these deviations. Since calculations are computationally intensive, the sensitivities are pre-calculated in advance, and the realtime measurement is based on pattern recognition. The symmetry of the pattern is used to differentiate between diameter and concentricity variations. We performed online measurements with the prototype instrument in production conditions, and show that this method is sensitive enough to measure deviations of under 1 µm in diameter and concentricity of the coating layer.
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