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This paper presents a fibre-based low coherence interferometric sensor developed by FOGALE nanotech. Based on the proven principle of partial coherence interferometry the sensor works as a comparator of optical path lengths. The optical path lengths along the optical axis in the measurement interferometer arm containing a target object are compared with the optical path lengths in an internal delay line. Multiple, partially reflecting surfaces of the target can be detected during one scan of the delay line.
Standard measurement ranges are between a few mm up to 400 mm (optical thickness). Longer measurement ranges are available on request. The measurement zone can be placed at a distance of up to several meters away from the instrument's exit. The sensor reaches an absolute accuracy on position measurements down to ±100 nm over the full measurement range. The system has been successfully applied in industry (glass and optical) as well as in research environments, e.g. for the dimensional metrology of large astronomical telescopes. This paper focuses on an innovative application of the sensor in the optics manufacturing industry.
The paper starts with a description of the measurement technique, the system hardware concept, the detection and signal processing scheme. We present a modeling-based approach for the dimensional metrology of optical components (e.g. single lenses, windows, prisms) or complete, mounted systems where the positions of all individual elements can be detected. A comprehensive propagation model including dispersion and phase effects is used to extract the distances from the optical path differences. Prior to a measurement, the optimum tailoring of the measurement beam is obtained by a simulation of the beam propagation through the target object. This ensures that each surface to be measured delivers a sufficiently strong signal. To illustrate our approach we present its application in the fabrication process of complex optical systems where "global" metrology of complete systems can be performed with a very high accuracy in a short time.
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