Preliminary Thoughts on the optimal Arm Cavity Finesse of Advanced Virgo

Stefan Hild and A.Freise

Advanced Virgo meeting, December 2008

Preliminary Thoughts on the optimal Arm Cavity Finesse of Advanced Virgo

S. Hild Advanced Virgo, 18th of December 2008 Slide 2

Overview Advanced Virgo design: Many decissions have to been taken in near

future.

One decision with high impact on other subsystems is the choice of the Advanced Virgo arm cavity finesse.

This talk is a preliminary collection of aspects that need to be taken into account. Intention: to discuss if our considerations are okay with everyone…


Context

Advanced LIGO initially planned to go for an arm cavity finesse of ~1200.

The advanced Virgo reference design features an arm cavity finesse of 888.

Recently the Advanced LIGO team decided to lower the arm cavity finesse to 450.

One of the OSD tasks is to find the optimal arm cavity finesse for advanced Virgo.

Need to evaluate the benefit from lowering the finesse.


How to compare different arm cavity finesse values?

A change of arm cavity finesse goes hand in hand with a change of the optical power inside the arm cavities.

If we decrease the arm cavity finesse, the stored optical power will go done as well. => stronger shot noise contribution. => not a fair comparison.

One can compensate for the lower finesse by increasing the power recycling gain.

Proposal: If we change the arm cavity finesse we will always restore the intra cavity power by increasing the power recycling gain, thus we always compare configurations with ~750kW per arm.


Potential reasons for loweringthe finesse?

Sensitivity ???

Mirror losses ???

Dark fringe offset ???

Noise couplings from small Michelson ???

Thermal load of BS, ITM and CPs?

Lock acquisition ???

Losses inside the recycling cavities ???

Coating Brownian from ITMs ???

… anything else ???


Michelson sensitivity versus arm cavity finesse

In the initial detectors the arm cavity finesse determines the detector bandwidth:

Low finesse = large bandwidth

High finesse = best peak sensitivity

Is this also true for Advanced Virgo?


Sensitivity for finesse 888 and 444 Let’s see how the ADV sensitivity changes if we lower the arm

cavity finesse by a factor of 2.

Step 1:• double ITM transmission• double PR factor

Step 2:If we half the arm cavity finesse we also have to compensate the Signal Recycling parameters: • double Signal Recycling detuning• double SRM transmittance


The Advanced Virgo sensitivity is (within a certain) range independent of the arm cavity finesse !!

Sensitivity for finesse 888 and 444



Sensitivity …………………………………………………..independent

Mirror losses ???









Finesse and mirror losses

Advanced Virgo preliminary design assumes 37.5ppm loss per surface.

This is an ambitious goal. What happens if the losses turn out to be twice as much (75ppm)? Any influence of arm cavity finesse?

The sensitivity changes with the mirror losses independent of the arm cavity finesse.




Mirror losses ……………………………………………...independent









Dark fringe offset and arm cavity finesse

Consider imbalanced losses in the two arm cavities. => excess carrier light at the dark port.

Optical readout quadrature is determined by a combination of dark fringe offset and carrier light from differential losses.

To get best sensitivity we probably want to dominate the local oscillator by the dark fringe offset (need to check this…).

If strong light contribution from differential losses => need larger dark fringe offset => need to detect more light power.

Carrier light from differential losses at dark port proportional to arm cavity finesse.

In principle low finesse reduces the dark port power due to differential losses in the arm cavity. However, the dark port power from differential losses is rather small (<5mW) …need to put exact numbers on this ..





Dark fringe offset ……………………………………………...YES








Noise coupling from the small Michelson

All differential arm length noise inside the small Michel-son (MICH) gets suppressed by the arm cavity finesse.

Lower finesse => stricter requirements for:

Thermo refractive noise inside ITMs, CPs, BS.

Quietness of wedged optics (CPs? ITMs? BS?)

… etc …






Noise couplings from small Michelson ……………...NO







Thermal load of BS, CP and ITM substrates

Optical power inside the power recycling cavity is proportional to inverse of the arm cavity finesse.

Lowering the arm cavity finesse from 888 to 444 increases optical power in BS, CP and ITM substrates from 1.35kW to 2.7kW.

Any problem from thermal lensing? …need to check with TCS …

ITM probably okay, since dominated by coating absorbtion. (?)

How about thermal lenses in BS and CPs ?







Thermal load of BS, ITM and CPs …………..………...NO






Lock-acquistion and finesse

The capture range of arm cavities inverse proportional to the Finesse.

Lowering the arm cavity finesse would make lock acquisition easier.

… need some quantitative input from ISC …








Lock acquisition ………………………………………………...YES





Losses inside PRC and SRC

If there are unexpectedly high losses inside the PRC, then a high arm cavity finesse would be better.

If there are unexpectedly high losses inside the SRC, then a low arm cavity finesse would be better.

Advanced LIGO argumentation (from LIGO-T070303-00-D):

PRC losses can be compensated for by higher laser power. For equal PRC and SRC losses ALIGO favors a small finesse.

However, only ‘really’ strong losses (0.x%) inside the SRC seem to have a significant impact.

… need to put exact numbers on this for Advanced Virgo….









Losses inside the recycling cavities ………….…not clear




Coating Brownian and finesse Lower finesse => higher

transmittance of the ITM HR coating.

Lowering arm cavity finesse from 888 to 444:

increasing ITM transmittance from 0.007 to 0.014

might be able to get rid of 1 or 2 coating layer on ITM

Reduce coating Brownian of ITM

Might change overall coating noise contribution by a few %. … need to check exact numbers …

Coating Brownian noise of one mirror:









Losses inside the recycling cavities ………….…not clear

Coating Brownian from ITMs ………….………………...YES



Summary

For the choice of the optimal arm cavity finesse several aspects need to be taken into account:

PRO low finesse: Lock acquisition, coupling of differential losses to dark port power, ITM coating Brownian, (losses inside the SRC?).

CONTRA low finesse: Noise couplings from MICH to DARM, thermal load of BS, CP and ITM substrate.

Within a certain range of values (finesse between 300 and 1000) none of the aspects seem give a NO-GO argument.

Main Question: Are there any strong arguments from any other subsystems for choosing a particular arm cavity finesse.

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Preliminary Thoughts on the optimal Arm Cavity Finesse of Advanced Virgo