A position-sensitive detector for the advanced LIGO suspensions

A simple optical displacement sensor consisting of an infrared LED source, a photodiode detector, and an occluding 'flag', has been investigated for potential use in the Advanced LIGO gravitational wave detectors. A number of different commercially available LEDs and photodiodes were tried in the displacement sensor, and an optimal pair was found. A surprising geometrical effect was noted which allowed the residual noise level to be reduced at low frequencies, permitting a displacement sensitivity of ~10-10 m/√Hz to be attained at and above frequencies in the region of 2 Hz.