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 Gryoscopes

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. They are commonly used in applications where the requirement is for a small rugged unit capable of measuring relatively large rotation rates. However research is also taking place into more sensitive gyroscopes for geophysical measurements, such as in the following papers:

attachment:ringlaser1.pdf attachment:ringlaser2.pdf

The systems shown in these papers are ring lasers, where the gain medium is actually within the ring. It is also possible to build a system where the excitation is external to the ring, with the laser being injected into the cavity from outside. This can be seen in the following papers:

attachment:external_excitation1.pdf attachment:external_excitation2.pdf

We plan to follow a similar route, using a laser external to the cavity. The system will look something like this:

attachment:lasergyro.png

In this diagram, the laser would be locked to the cavity length for the clockwise beam. A Pound-Drever-Hall locking scheme will be used, necessitating sidebands which we put onto the laser beam using an EOM. More about PDH locking can be found here:

attachment:PDH.pdf

The counter-clockwise beam is locked to the cavity by frequency shifting using AOM. The beat frequency of the two modulation signals is then dependent on rotation rate, attachment:omega.png , and is given by:

attachment:rotation.png

Useful Reading

1. Ring Laser Gyro for Tilt Sensing in Virgo++ attachment:RomeGyro.pdf

II. Virgo g-Laser gyro. attachment:RomeGyro-doc.pdf