Optical Spring Experiments With The Glasgow 10m Prototype Interferometer

1. Optical Spring Experiments With The Glasgow 10m Prototype Interferometer Matt Edgar

2. Talk Outline • The significance of optical springs • The Glasgow 10m prototype interferometer • The optical spring experiment • Experimental aims • Experimental design • Coupled cavity control scheme • Modelled results • Future work and conclusions

3. The Significance of Optical Springs • Future detectors will make use of significant strength optical springs • Advanced LIGO - utilising detuned signal recycling. • Optical Bar / Optical Lever - achieve optical rigidity. These techniques are possible approaches towards sub-SQL performance over at least some of the detection band.

4. The Glasgow 10m Prototype Interferometer • 10m vacuum tank system • Triple-stage suspensions • Passive seismic isolation system • Fused-silica mirrors • 2W Nd:YAG 1064nm Laser light • Sophisticated digital controller

5. The Optical Spring Experiment • Experimental aims: • Examine properties of optical spring in a fully suspended environment. • Create an optical spring in a coupled cavity configuration – analogous to recycling cavities. • Explore the interactions with the control system. • Digital control, variable input power. • Inform the design of ISC for the AEI Hannover prototype system.

6. Optical Spring Experiment • Experimental design: • Triple stage light-weight suspension design featuring passive eddy-current damping. • 100g end test mass (with fused silica mirror). • Detuned Fabry-Perot cavity - optical spring. • Three mirror coupled cavity configuration.

7. Coupled Cavity Control Scheme Modified Pound-Drever-Hall technique • Amplitude and Phase Modulation • PM sidebands @ 18MHz used to derive arm cavity length-sensing signal. • PM sidebands @ 10MHz, and AM sidebands @ 14.525MHz used to derive recycling cavity length-sensing signal. • Flexible scheme to decouple the control signals of the two cavities. • Already tested on cavity with 2.7kg test masses [1]. • Allows us to detune one cavity and maintain decoupling. • Error signals are fed back to the PZT and temperature of laser to control AC and to recycling mirror EM actuators to control PRC. [1] Techniques in the optimization of length sensing and control systems for a three-mirror coupled cavity 2008 Clas Quan. Grav., Huttner et al

8. Modelled Results • Several approaches have been undertaken to simulate the optical spring dynamics within a three mirror coupled cavity. • Optickle modelling (takes account of radiation pressure effects) • Matlab and Simulink (observe the effective pendulum dynamics within feedback loops)

9. Measured Results We have started measuring transfer functions of our experimental setup by injecting longitudinally onto the end test mass using EM actuators and monitoring the response. We also plan to measure the effect of cavity detuning on the power recycling feedback signal to characterise the level of decoupling between the two cavities. We expect experimental results soon!

10. Future Work and Conclusions • Swap the new suspension and light-weight mirror with ITM. • Closer to the optical-bar topology • Proof-of-principle experiment to verify it’s operation in a fully suspended prototype scale environment. • Move the end mirror, use a separate local readout system to sense the movement on light-weight ITM. • We need sensitive measurements, but not that sensitive, to measure the optical bar transfer function on prototype scale, based on initial calculations. In conclusion, we have rapidly designed, manufactured, and installed the suspension for investigations with optical springs. Preliminary data looks very promising and we expect to confirm the optical spring behaviour and to characterise its interaction with the controls.