Automated calibration system for laser interferometers

摘要

For dimensional measurements and positioning, laser interferometers are often used to obtain highly accurate readings. To service their reliability, a system of laser interferometry standards is made available at the National Measurement Laboratory to offer instrument calibration as well as to ensure their traceability to the SI units. Owing much to their great sensitivity, the accuracy of laser interferometers undergoing calibration is strictly influenced by surrounding conditions, especially the correction parameter in the refractive index of air - a complex combination of ambient temperature, humidity, and atmosphere pressure. In order to minimize the deviation, an automated calibration system is constructed by employing a computer-controlled driver stage to perform linear displacement and data acquisition in the absence of operator intervention. A supplementary displacement measurement sub-system was set up to serve as an independent control on the stage. An average of 12 data points were taken at each predefined positions along the 20-m travel to support the comparison between the original system and the automated one. Several test runs from the calibration operation showed a standard deviation of 2.4 * 10~(-8) for the automated system and 6.5 * 10~(-8) for the previous. In addition to lower operational cost, experimental data also indicated improved calibration reliability benefited from the automation.