• amplitude stabilization
• backscattering
• backscattering
• effetto Sagnac
• frequenza di Sagnac
• giroscopio laser
• He-Ne laser
• laser He-Ne
• optical-frequency stabilization
• ring laser gyro
• ring laser gyroscope
• Sagnac effect
• Sagnac frequency
• stabilizzazione d'ampiezza
• stabilizzazione della frequenza ottica

Ring laser gyroscopes are inertial sensor based on the Sagnac effect:
rotation causes the frequency of the two counter-propagating beam in the ring cavity to be shifted by an amount proportional to the angular velocity.
This shift (the Sagnac frequency) can be easily measured letting the two beams beat.
Applications of ring laser gyros range from inertial navigation system to geodesy, to test of fundamentals physics.
Several large laser gyros with high sensitivity have been developed in the last years.


This thesis presents the work done with a square ring laser with a side length of 1.40 m in the contest of the experiment G-pisa of the INFN.
This experiment may help improve the performance of the mirrors suspension of the gravitational wave antenna Virgo.
The laser design is based on instruments developed by the joint ring laser
working group in New Zealand and Germany.
It is a helium-neon laser working on the 474 THz line of neon.
Earth rotation is enough to bias the Sagnac frequency to 110 Hz.
The capacitive-coupled discharge exciting the laser, whose stabilization was specially designed, is the flagship of this experiment.

In the first apparatus the operation was quite unstable with the laser gyro often blind to rotation.
Long term operation of the laser was limited by the contamination of the gas.

The laser have been extensively tested.
Diagnosis of the discharge shows the prominent role of hydrogen as contaminant, leading to the installation of getter pumps.
Ring-down time was measured to check the quality factor of the laser cavity.
Investigation of the mode structure proved an invaluable tool to understand the behaviour of the laser.
The laser usually works single mode, but unusually good Sagnac signal can be achieved with the laser working multimode.
These analysis shows that the gyro is mostly limited by split modes: independent mode jump of counter-propagating beams causing the gyro to be intermittently blind to rotation.

Stabilization of the beam intensity has been implemented to obtain more stable operations.
Active stabilization of the optical frequency is designed, developed and tested in order to avoid mode jump and improve the performance of the gyro.
A piezo-electric transducer is used to move one of the four mirrors, acting on the perimeter of the cavity.
This kind of stabilization brings the laser against the subtle effect of backscattering of light of each beam in the other.
A simple theoretical treatment of the backscattering and of its influence on the Sagnac effect is reported to explain acquired data, followed by some numerical simulations.
Results obtained with this setup are reported: achieved optical frequency stability, prolonged operation (till now of several hours), consequences on the Sagnac frequency (periodic pulling) and rotation sensitivity.