1. Task T2 – Measurement of photo-elastic noise Goals: - observation of photo-elastic effects - dependence with temperature down to a few K - dependence with beam waist - test of low loss coatings LENS Florence: F Marin, M de Rosa, and F Marino LKB Paris: A Heidmann, O Arcizet, M Pinard, PF Cohadon, T Briant, and T Caniard

2. Theory Photo-thermal noise is due to the conduction inside mirrors and coatings of the heat induced by photon absorption. Theory:M. Cerdonio, L. Conti, A. Heidmann, M. Pinard, Phys. Rev. D 63, 082003 (2001) Theoretical investigation on light absorption effects of coatings in progress

3. Florence experiment Measurement of photo-thermal effect induced by an intensity-modulated light beam

4. Experimental results (large beam) Waist: 368  m Cut-off frequency : 3.25 Hz

5. Experimental results (small beam) Waist : 74  m Cut-off frequency : 80 Hz

6. Paris experiment Test and set-up of high-finesse cryogenic cavities Cavities with low-loss coatings suitable for cryogenic applications Measurement of photo-thermal effect at low temperature Using the experiment on opto- mechanical coupling with a cryogenic cavity and an intensity- modulated laser beam

7. Frequency locking and dual injection Second year improvements: Frequency locking via an AOM Dual beam injection in the cavity Cryogenic operation

8. Back-action noise simulated by an intensity modulation of the intense beam Observation of back-action cancellation Noise reduction by a factor 200  Paper in preparation