Sensor Calibrations with the Improved Picodrift Interferometer

Abstract

To meet the growing demand for high precision measurement equipment, sensors with sub-nanometer resolution are becoming readily available. Because of its high precision, traceability and ease of use, the interferometer stands out for high precision measurements. However, the laser beam of an interferometer can be influenced by temperature and pressure fluctuations. Compensation of this error is possible by sensing temperature and pressure fluctuations while performing interferometric measurements. Alternatively, a refractometer running parallel with the interferometer can be used to do the same compensation with a higher accuracy. Additionally, errors caused by external sources such as vibrations, can be compensated by adopting a highly symmetrical design. Picodrift interferometer was developed and upgraded at VSL (Dutch Metrology Institute) as a calibration facility for high-precision sensors. First, the improved design of the picodrift interferometer is evaluated, implemented and tested allowing for a controlled measurement environment and high stability. The design requirements are determined to reduce the uncertainty caused by temperature and pressure fluctuations to picometer level in one hour. Based on those requirements, a vacuum system is implemented and tested to reach a pressure level in the ultra-high vacuum regime. During the implementation, necessary experiments and tests are performed to find leakages and the reasons for an undesired beam drift related to the pressure change. Additionally, an active temperature control system with passive shield is installed. Also, various kinematic coupling mount systems are designed and manufactured to provide a good positioning stability even in a noisy environment caused by a turbo pump. As the number of the interferometer channels has been doubled to perform measurements in both vacuum and atmospheric conditions, a well planned alignment procedure is developed to align the more sophisticated optical path. To characterize the performance of the new interferometer, a dead path measurement is performed. Second, to take advantage of the high precision and traceability, the interferometer is expanded to work as a calibration facility for external sensors of different sizes. A stable symmetrical calibration setup is designed to fit in a limited space for external measurements while minimizing uncertainty caused by temperature fluctuation and misalignment. An alignment procedure for the setup is developed. Eventually an example calibration is performed.