1. Optical readout for a resonant gw bar

2. Old setup

3. New alignment mirrors system

4. New optical bench

5. Alignment prisms Optical fiber /2 plate Mode-matching lenses Auxiliary cavity /4 plate

6. DUAL sensitivity target Laser power = 7 W Finesse = 10 6 S xx = 10 -45 m 2 /Hz … but With a waist of w = 1 mm: S Br = 5·10 -44 m 2 /Hz S rp = 8·10 -41 m 2 /Hz We need a waist of w > 20 cm !!!!

7. Folded Fabry-Perot (FFP) M1 M2 M3 M4 D Signal:  N Brownian noise:   N Radiation pressure:  N·F (constant) Displacement noise:  1/F  N Linewidth (  bandwidth):  1/(N·F) (constant) F. Marin, L. Conti, M. De Rosa: “A folded Fabry-Perot cavity for optical sensing in gravitational wave detectors”, Phys. Lett. A 309, 15 (2003)

8. FFP for dual cylinder } shot-noise limited sensitivity } radiation pressure effect Fixed total length: 2.3 m

9. Prototype of FFP fabricated -Two parallel rows of mirrors on independent oscillating masses, with resonance frequencies of 1 kHz and 2 kHz -Three possible configurations: - 2 mirrors (simple FP) - 9 mirrors - 17 mirrors

10. Calculated response to modulated laser power

11. Photo-thermal effect: direct measurements

12. Model for Photo-thermal + radiation pressure displacements

13. High power: -Bistability -Kramers model for jump probability (wip)

14. Intermediate power: -Hopf bifurcation -‘New’ dynamics (similar to FitzHugh-Nagumo) -Self oscillations F. Marino, M. De Rosa and F. Marin, to be published on Phys. Rev. E

15. Q = 5 Q = 20 Q = 10

16. Q = 10 3 Q = 10 4 Q = 10 5

17.  The probability of a noise-induced state jump is non-zero  In any case, a tight locking is probably necessary  Several QND schemes are difficult to be implemented  Comparing two cavities, the laser must be in tight resonance with both cavities  - two laser beams and heterodyne ?? (but the requirement on the phase noise of the reference tunable rf oscillator is too stringent: -200dBc @ 2-5 kHz) - locking the cavities (at least one) ?? (but high dynamic range and low noise are not easily obtained)