High Precision Sensor Probe for Beam-Tilt Alignment Based on a Quadrant Photodetector

This paper describes the development of a laser beam positioning sensor based on a quadrant photodetector. The sensor was a critical component of a wider alignment system. It was developed to meet the stringent 45 μrad precision requirement for the orientation of the optical interferometry cavity for the length measurement in the new calculable cross-capacitor at the National Measurement Institute, Australia. This system has been used to align the optical cavity to the capacitor’s axis to within 110 μrad, which is corrected for, with an uncertainty of 6 μrad, well within the required precision.