Focus distance modulated fibre-coupled confocal sensor for surface metrology
Final Report Abstract
A high-frequency focus distance modulated, fibre-coupled, confocal point sensor or distance sensor was developed, realised and investigated. In combination with a high-precision surface and coordinate measuring system (NMM-1), this makes it possible to measure continuous surfaces through a single lateral scan with simultaneous readjustment of the axial distance with the positioning stage. In contrast to conventional approaches of confocal microscopy, in which lateral scans are carried out at discrete axial heights, a huge reduction in the measurement time, depending on the axial measuring range, was achieved hereby and consequently a reduction in temperature-induced measurement errors due to lower temperature drifts. This was realized on the one hand by an innovative combination of a fibre-coupled lighting and detection with a tuneable acousti- cally gradient lens (TAG-lens) for high-frequency distance modulation of the focus. On the other hand, through a new type of signal processing with an FPGA-based lock-in amplification, which ensures a distance evaluation independent of the reflection properties and slope of the surface by forming the quotient of the 3rd and 2nd harmonic ampli- fication of the lock-in amplifier. By forming the quotient of the 2nd harmonic and an average value signal, a determination of the limits of the characteristic curves independent of the intensity of the input signal was shown. Furthermore, a method was investigated which enables the adjustment of the optical system. The optical performance of the entire system was demonstrated using various measurements on gratings and step-height standards. A correlation between the measured deviations and the recorded ambient temperatures were identified. These correlations could be validated and quantified by means of a concept specially designed for the sensor, consisting of high-resolution temperature sensors, whose traceability was ensured by calibration using DAkkS-calibrated comparison sensors. Furthermore, it was shown that with the help of a temperature chamber specially developed for the high-precision surface and coordinate measuring system NMM-1, the measurement deviations could be significantly reduced, so that with 25 re- peated measurements within 16 minutes with a temperature constancy below the resolu- tion of the Pt100 temperature sensors used (< 1 mK) resulted in a pointwise measurement repeatability (standard deviation) of 1.7 nm over the entire characteristic curve. The intermediate measurement precision of the sensor was determined over a period of 12 hours with a standard deviation of 3.6 nm at a maximum temperature fluctuation of the ambient temperature of 6.5 mK. Furthermore, a standard deviation of the position of the centre of the focus oscillation of 14 nm without correction of the temperature-related drifts and 9 nm with correction was determined over a period of 24 hours.
Publications
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High-speed focal-distance-modulated fiber-coupled confocal sensor for coordinate measuring systems. In: Applied Optics. 2018. 57(14), S. 3907–3914
Hausotte, T.; Gröschl, A. C.; Schaude, J.
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Improvements of a highspeed focus distance modulated fibrecoupled confocal sensor for nanocoordinate measuring systems. In: Sensoren und Messsysteme, 2018, VDE Verlag GmbH, Berlin, Offenbach, S. 657–661 – ISBN 978-3-8007-4683-5
Gröschl, A. C.; Köhnen, S.; Schaude, J.; Hausotte, T.
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Kontinuierliche laterale Abtastung von Oberflächen durch axiale Regelung des Arbeitsabstandes in einem Nanokoordinatenmessgerät mit einem hochfrequent fokusabstandsmodulierten, fasergekoppelten, konfokalen Punktsensor. In: tm – Technisches Messen. 2018, 85(S1): S. 26– 32
Gröschl, A. C.; Schaude, J.; Hausotte, T.
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Evaluation of the optical performance of a novel high-speed focal-distance-modulated fibre-coupled confocal sensor. In: Proc. SPIE 11056, Optical Measurement Systems for Industrial Inspection XI, 110560N (21 June 2019)
Gröschl, A. C.; Schaude, J.; Hausotte, T.
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Evaluation und Justierung eines neuartigen, hochfrequent fokusabstandsmodulierten, fasergekoppelten, konfokalen Punktsensors für axial geregelte Oberflächenmessungen mit einem Nanokoordinatenmessgerät. In: tm – Technisches Messen. 2019, 86(4): S. 216–226
Gröschl, A. C.; Schaude, J.; Hausotte, T.
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Evaluation und Korrektur thermischer Driften eines hochfrequent fokusabstandsmodulierten, fasergekoppelten konfokalen Punktsensors. In: tm – Technisches Messen. 2019, 86(S1): S. 117–121
Gröschl, A. C.; Schaude, J.; Hausotte, T.
