Experimental investigation of dynamic photo-thermal effect

In an optical interferometer, a part of the laser power is absorbed by the mirrors and gives rise to surface displacements through thermal expansion. The position measurement sensitivity is, therefore, limited by the shot noise of the absorbed radiation. This phenomenon is called the photo-thermal effect and its typical frequency dependence and size can span over several orders of magnitude, depending on the mirror material and temperature, hence it is crucial to have an accurate model extending over such a large range. We present an extensive experimental investigation of dynamic photo-thermal effects, covering more than seven decades in frequency and including finite mirror size effects (low frequency) and coating effects (high frequency) and showing the dependence on the beam waist. A specific extension of the basic model is being developed that can well explain the results.