Laser stabilisation for the measurement of thermal noise

In order to measure the thermal noise of a mirror suspended in a vacuum it is necessary for the length measurement error due to intensity and frequenc y noise of the probe laser to be reduced below the thermal noise level. Her e we report on an experiment to reduce the frequency and intensity noise of a 40mW Nd:YAG laser for this purpose. The frequency is stabilised using th e standard reflection locking technique. To stabilise the laser intensity a technique which uses the properties of an 'in loop' light field has been d eveloped. This technique is capable of suppressing the intensity noise belo w the shot noise limit without reducing the useful laser power. A servo bas ed on this technique has been designed and tested. The experimental results indicate that the laser noise can be reduced to a level which will allow a displacement sensitivity of 1.5 x 10(-19)m/root Hz for the detection of th ermal noise in a frequency band of 10 to 500Hz.