M. Barton & S. Steplewski

Today we characterized an advanced LIGO AOSEM (Serial Number 003, without an IR filter) using a 3-axis translation stage in the electronics shop. The voltage from the photodiode was recorded as a function of position in each of the three different axes. As shown in the attached graphic, the X axis goes through the circular coil (longitudinal to the OSEM) the Y axis is along the LED beam path, and the Z axis is vertical.

We found the expected sigmoidal curve by moving the flag in the longitudinal (X) direction, and computed a rough slope of about -19.4 Volts/mm in the central linear region. We also saw that moving the flag along the Z axis gives a flat response over about 1 mm of range, even when we moved to different X offset values. This means that the Z position can be slightly off-center and the OSEM will still work well as a longitudinal sensor. However, when the Z position of the flag is moved to a larger offset the longitudinal sensing is reduced, as can be seen in the first graph green and yellow curves. We also looked at the Y axis and found pretty flat responses, even when varying X.