1. Variable reflectivity signal-recycling mirror and control Stefan Goßler for the experimental team of The ANU Centre of Gravitational Physics

2. Outline Control and VRSM Tuned and detuned signal recycling Introduction of VRSM Layout of the initial experiment Error signals First results Carrier/Subcarrier offset phase-locking control scheme David Rabeling, Malcolm Gray, David McClelland Summary

3. Signal Recycling I Control and VRSM Signal recycling can be used to enhance SNL sensitivity of gravitational wave detectors within certain bandwidth: Microscopic tuning of SRM position alters the tuning Changing the reflectivity of SRM alters the bandwidth Tuned SR: SRC is tuned on resonance with carrier Requires broadband mode: low finesse of SRC Detuned SR: SRC is detuned from resonance with carrier Enables narrowband mode: high finesse of SRC

4. Signal Recycling II Control and VRSM

5. 125 W input

6. Activ stack isolator Complication Mirror 2,9 kg 150 mm Magnet-coil actuator Steel wire loops 162 µm Intermediate masses 200 µm 360 µm Rotational stage Upper masses Coilholder for local control 3 mm Cantilever springs Cantilever springs 180 mm 280 mm 420 mm

7. Complicaton: Control and VRSM Changing the reflectivity of the signal-recycling mirror causes a down-time of the GWD for a substantial period of time! A mirror with a reflectivity that can be varied on demand: a VRSM Solution:

8. VRSM Can be realised by: Thermal tuning of an etalon Kawabe, Hild et al. Downside: Slow and lateral thermal gradients the substrate Downside: Very complex system Michelson interferometer deVine, Shaddock, McClelland Fabry-Perot cavity Strain, Hough Control and VRSM Downside: Complex reflectivity

9. Simplification of Layout Control and VRSM Signal-recycling mirror Power-recycling mirror Arm-cavity Michelson interferometer `Michelson´ mirror Signal-recycling mirror `Michelson´ mirror Variable reflectivity signal-recycling mirror Dual recycled Michelson interferometer with arm cavities

10. Simplification of Layout Control and VRSM Signal-recycling mirror Power-recycling mirror Arm-cavity Michelson interferometer `Michelson´ mirror Signal-recycling mirror `Michelson´ mirror Variable reflectivity signal-recycling mirror Collapse Michelson interferometer into one mirror

11. Introduction of VRSM Control and VRSM Signal-recycling mirror Power-recycling mirror Arm-cavity Michelson interferometer `Michelson´ mirror Signal-recycling mirror `Michelson´ mirror Variable reflectivity signal-recycling mirror

12. Experimental Layout Control and VRSM Control of SRC via in-phase error signal Control of VRSM via in-quardature error signal

13. Control and VRSM -200-150-100-50050100150200 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 [Arb. units] -200-150-100-50050100150200 1.0 0.8 0.6 0.4 0.2 0.0 -0.2 -0.4 -0.6 -0.8 -200-150-100-50050100150200 0.5 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 -0.5 -200 -150-100-50 050100150 200 0.4 0.3 0.2 0.1 0.0 -0.1 -0.2 -0.3 -0.4 Amplitude Phase In quadrature Error Signals Amplitude and phase response of a narrow linewidth cavityAmplitude and phase response of a broad linewidth cavity PDH error signal for a narrow linewidth cavityPDH error signal for a broad linewidth cavity Frequency [MHz] Amplitude Phase In phase Frequency [MHz] [Arb. units] In quadrature In phase

14. Experimental layout Control and VRSM Control of SRC via in-phase error signal Control of VRSM via in-quardature error signal

15. First results... Control and VRSM 370375380385390395400405410 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 370375380385390395400405410 1.0 0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0.0 Frequency [MHz] Theoretical frequency responseExperimental frequency response T/T max...of a linear three mirror coupled cavity when varying the reflectivity of the VRSM from R=93% over R=87% to R=63%

16. Carrier/Subcarrier Control and VRSM Power recycled Michelson interferometer In quadrature In phase Frequency offset Carrier laser Sub-carrier laser VRSM cavity Output modecleaner Readout Phase modulator Phase modulator ~

17. Frequency Offsets Control and VRSM VRSM reflectivities for some carrier-subcarrier offsets Effect on signal cavity Norm. Trans. Norm. Refl.

18. Frequency Offsets II Control and VRSM Norm. VRSM reflection Norm. refl. 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 Norm. SRC reflection Narrowband mode Broadband mode

19. Summary VRSM and Control Control and VRSM A new way to control the SRC including a VRSM by using an auxiliary laser that is offset phase locked to the carrier laser First experiments using a linear three mirror cavity are carrried out The use of a VRSM eases the change of bandwidth, avoiding long down-times of the detector This control scheme is compatible with the injection of squeezed light A more complex experiment using a dual-recycled Michelson interferometer with arm cavities is in preparation