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.