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18 June 2007 On-axis, non-contact measurement of glass thicknesses and airgaps in optical systems with submicron accuracy
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This paper presents the fibre-optics low coherence interferometric sensor LISE and its applications in the optics manufacturing industry. The sensor works as a comparator of optical group delays. The group delay along the optical axis of the probe interferometer arm containing an object, for example a lens assembly, is compared with the group delay of the reference arm containing a movable delay line. The light source is a super luminescent diode (SLD) emitting at 1.31 &mgr;m with a coherence length of typically 25 &mgr;m. Thanks to the limited temporal coherence of the source, multiple surfaces of the object can be detected during a single scan of the delay line. Measurement ranges are between a few mm up to 600 mm (optical thickness). The measurement range can be placed at a working distance of up to several meters away from the instrument's exit. Two classes of accuracy exist. While the standard system has an absolute accuracy of ±1 &mgr;m for position and distance measurements, the second generation, high-accuracy system reaches an accuracy of better than ±200 nm for distance measurements while maintaining the ±1 &mgr;m accuracy for position measurements. The paper starts by explaining the measurement principle in Section 1. The following Section 2 describes the system design and the individual system components. The definition and validation of the absolute accuracy are discussed in Section 3, followed by a description of the complete detection procedure for the high accuracy system in Section 4. In its final Section 5 the paper gives examples of applications in the optics manufacturing industry. This description ranges from the centre thickness measurement of lenses to the "global" on-axis metrology of completely mounted optical systems such as objectives where all lens thicknesses and airgaps are measured without touching or disassembling the optical system. Practical considerations concerning alignment, focusing of the measurement beam, model-based signal identification, dispersion and longitudinal resolution are discussed.
© (2007) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Rainer Wilhelm, Alain Courteville, Fabrice Garcia, and François de Vecchi "On-axis, non-contact measurement of glass thicknesses and airgaps in optical systems with submicron accuracy", Proc. SPIE 6616, Optical Measurement Systems for Industrial Inspection V, 66163P (18 June 2007);

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