Cryogenic optical shadow sensors for gravitational wave detectors

Displacement sensors have a variety of applications within gravitational wave detectors. The seismic isolation chain of the LIGO core optics utilises optical shadow sensors for their stabilisation. Future upgrades, such as LIGO Voyager, plan to operate at cryogenic temperatures to reduce their thermal noise and will require cryogenic displacement sensors. We present the results of simulations and experimental tests of the shadow sensors embedded in the Birmingham Optical Sensors and Electromagnetic Motors (BOSEMs) to establish whether BOSEMs are suitable candidates for cryogenic applications. We determine that the devices can reliably operate at 100 K, and an improvement in their performance is seen due to the improved quantum efficiency of the LED by a factor of 2.7. The shot noise improvement due to the electronic changes of the readout’s amplifier (satellite amplifier) results in a shot noise enhancement from_______ at room temperature,_______ to under cryogenic conditions.