nicolas smith-lefebvre MIT GWDAW 2011 What did we learn? nicolas smith-lefebvre MIT GWDAW 2011 LIGO-G1100575

Outline Getting the most out of your OMC Handling a high power interferometer IFO IFO nicolas smith-lefebvre GWADW 2011

Coupling AS port into the OMC Pseudo-mathematical picture of an OMC Why is this important? What are the aspects and what was learned about them? nicolas smith-lefebvre GWADW 2011

The job of the OMC = Every bit of signal that is rejected counts as a loss nicolas smith-lefebvre GWADW 2011

Why does this matter? nicolas smith-lefebvre GWADW 2011

What to do?: OMC Alignment Sensing Problem: How to sense the alignment of the signal sideband field (the arm cavity mode) into the OMC? Poster child for “lesson learned” Going in, this problem was largely unexpected First “Beacon Alignment” now “Optimal Beacon Alignment” In my opinion, this is now a solved problem, only implementation questions remain. Being implemented now at GEO, Virgo+, H1 Squeezer Further reading: LIGO-P1100025 nicolas smith-lefebvre GWADW 2011

OMC Alignment Control Beam steering optics often also act as mode matching telescope There must be adequate gouy phase separation to avoid a singular control matrix This can be difficult for IFOs which add on an OMC (eLIGO, GEO) Easy if the good design is built in from the start nicolas smith-lefebvre GWADW 2011

OMC Mode Matching Difficulties are similar to alignment sensing: how to determine the mode of the arm cavity? These difficulties compounded because you can't easily measure or correct the mode matching in situ! Possibly limited shot noise sensitivity of H1 in S6. ALIGO “plan” is to get it right as designed If it doesn't work, maybe use the UFL dynamic modematching? nicolas smith-lefebvre GWADW 2011

Beam Jitter Noise Maybe the most important new noise source in eLIGO Lots of things were done to fight it: Set OMC alignment point to minimize coupling Improve isolation of steering optics (several times) Feedforward subtraction of magnetic field induced alignment fluctuations Change the beam “shape” to reduce coupling Beam jitter limited the sensitivity of eLIGO, some improvements of the aLIGO readout table are planned, but have we been convinced that beam jitter is gone? nicolas smith-lefebvre GWADW 2011

OMC Autolocking In eLIGO, the fast OMC length actuator did not have a full FSR of range, also other problems... This made autolocking difficult, operator intervention was often needed Likely had a negative impact on duty cycle nicolas smith-lefebvre GWADW 2011

High Power Handling Nothing too surprising, we expected this to be hard, it was. Thermal Compensation (TCS) solved stability problems, but not without difficulties New optomechanical- aware IFO alignment scheme nicolas smith-lefebvre GWADW 2011

Thermal Compensation It worked, but... Increasing power in the interferometer was a slow, onerous task Optimizing TCS delivered to the test masses required fine tuning Interferometer noise, and TCS efficiency depended strongly on TCS alignment Also fine tuning nicolas smith-lefebvre GWADW 2011

Optomechanical-aware Alignment Control IFO alignment begins as a hard problem (many degrees of freedom, sub-dominant sensing of some) Optomechanical effects add non-diagonal terms to the plant via Sidles-Sigg effect. Solution: Use the soft/hard basis for control filters Early modeling a big plus nicolas smith-lefebvre GWADW 2011

More General Lessons Borrowing technology from the future was a win-win eLIGO benefited with improved sensitivity aLIGO benefited with early testing of key technologies Let's do it again next time nicolas smith-lefebvre GWADW 2011

Summary OMC throughput will become even more important as squeezed light injection becomes more prevalent New alignment techniques have been developed Mode matching may work out of the box, if not we should have a definite plan (right now we don't) We should fight beam jitter with vibration isolation primarily nicolas smith-lefebvre GWADW 2011

Grazie! Further Summary eLIGO had a taste of the high powers to come in advanced IFOs TCS was a good answer to the problems of thermal stability, albeit slow and tedious Angular optomechanics proven tractable with the right control topology Grazie! nicolas smith-lefebvre GWADW 2011