1. nicolas smith-lefebvre MIT GWDAW 2011
What did we learn?
nicolas smith-lefebvre MIT GWDAW 2011
LIGO-G

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

3. 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

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

5. Why does this matter?
nicolas smith-lefebvre GWADW 2011

6. 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-P
nicolas smith-lefebvre GWADW 2011

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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