engleski

Accuracy limitations of modified Zeiss interference comparator

In order to perform interferometric calibration of short gauge blocks with traceability to the definition of metre using I2 stabilized Helium Neon laser, several modifications were made to a Zeiss interference comparator. This instrument was used with Helium and Krypton isotope sources and is relatively widespread in National Metrology Institutes, so various modifications with different approaches were already performed on similar devices. The modifications we carried out include an optomechanical system for use of laser light sources, digital environment sensors, a digital camera for interferogram acquisition, and custom software for interferogram evaluation. In addition to I2 stabilized Helium Neon laser, a tunable Helium Neon laser was used as source of secondary wavelengths. The measurement uncertainty of the modified interferometer was evaluated, with significant improvements over the original system. Several features of the original Zeiss interferometer were not modified, namely the Kosters prism (used as a monochromator for Helium and Krypton sources) and the optical elements inside the interferometer. The quality of original beamsplitter, compensating plate and reference mirror is unknown, and could be compromised because of contamination and the age of the instrument. Due to the closed architecture of the device, only the summary influence of these elements to the measurement uncertainty could be established. To evaluate the possibilities of improving the measurement uncertainty, a new interferometer was developed based on the Twyman-Green configuration. This system provides an open platform for investigation of the influence of quality of optical components on interferogram quality. Preliminary measurements show that significantly better results can be obtained with new optical elements, indicating some deterioration of internal optics on Zeiss interferometer and providing a means for a better estimate of their quality. This insight could provide the accuracy limit of the Zeiss interferometer, based on the derived accuracy limit of the internal optical components.

Gauge block calibration; Optical interferometry; Interferogram analysis

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