Optical Gyroscopes for Ground Tilt Sensing in Advanced LIGO

Introduction

The low frequency isolation of suspensions for Advanced LIGO will be achieved using seismometers, whose signals are fed forward into active hydraulic units (HEPI). The sensitivity of the seismometers to horizontal displacements is good enough to achieve the required isolation. However at very low frequencies, couplings of rotation or ground tilt, into horizontal seismometer signals, become problematic.

For a horizontal seismometer the ratio of sensitivity to rotation to sensitivity to horizontal motion is given by:

attachment:equation1.png

Below some frequency we may expect that the response of the seismometers will become dominated by tilt. By using a rotation sensor in parallel with the seismometers, it will be possible to remove the rotation component of the signal that is fed forward to the active stage.

Laser based gyroscopes operate on the sagnac principle ( http://en.wikipedia.org/wiki/Sagnac ) whereby the path length for light travelling round a ring is altered as it rotates. In a sagnac interferometer, beams sent in opposite directions round the rind are interfered at the output, giving a beat frequency that is proportional to the rotation rate.

attachment:lasergyro.png