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Description
The position and angular control of LIGOs suspended optics, and the damping of their mechanical resonances, is essential for the operation of the detector. Currently, this is achieved using OSEMs, which combine a shadow displacement sensor and electromagnetic voice-coil actuator. However, the noisefloor of these sensors is limiting the motion of the optics on the triple and quadruple suspensions at frequencies above a few Hz and is beginning to limit the GW sensitivity at 10 Hz. To achieve the design sensitivity of A+ LIGO, and enable A# and future detectors, lower noise suspension displacement sensors are needed.
We present a new beam deflection displacement sensor, which can be retrofit into the existing OSEM bodies, dubbed the QOSEM. The sensor achieves a noisefloor of 4 $\frac{\text{pm}}{\sqrt{\text{Hz}}}$ above 1Hz, with a dynamic range of 1 mm, providing a factor of 10 improvement in sensitivity over the current shadow sensors. Additionally, it features a 2D readout, further increasing readout sensitivity, particularly to angular motion. We will discuss the design and characterisation of the sensor, and detail its performance on LIGOs newly installed beamsplitter suspension.