Stefan Hild, Andreas FreiseUniversity of Birmingham

Roland Schilling, Jerome DegallaixAEI Hannover

January 2008, Virgo week, Pisa

Advanced Virgo:Wedges vs Etalon

Stefan Hild Virgo week January 2008 Slide 2

Motivation: Input mirror without wedge

Initial Virgo has no wedges in the input mirrors

The etalon effect could be used for adjusting the cavity finesse (compensating for differential losses)

If etalon effect is not controlled it might cause problems

Stefan Hild Virgo week January 2008 Slide 3

Motivation: Input mirror featuring a wedge

Used by initial LIGO

Reflected beams from AR coating can be separated from main beam => pick off beams provide additional ports for generation of control signals.

No etalon effect available.

Stefan Hild Virgo week January 2008 Slide 4

What to use for Advanced VIRGO?Etalon or Wedges ??

For AdV possibility to adjust cavity finesse gets more important (higher cavity finesse, DC-readout).

For AdV possibility to create more and better control signals seem desirable.

Is there a possibilty to have both for Advanced Virgo ??

Stefan Hild Virgo week January 2008 Slide 5

Advanced Virgo: waist inside the cavity

Increase beam size at mirrors => reduce thermal noise contribution of the test masses.

Move beam waist away from input test mass

Is there still an etalon effect in the (flat/curved) input mirror ?

Stefan Hild Virgo week January 2008 Slide 6

Etalon effect:flat/flat vs curved/flat

Flat/flat etalon: Perfect overlap of wavefronts

Curved/flat etalon: Mismatch of wavefront curvature Fortunately mirror curvature of a few km

is not so far from “flat”. Simulations show: a reduced etalon effect

in curved/flat input mirror is still present

Stefan Hild Virgo week January 2008 Slide 7

Etalon effect:flat/flat vs curved/flat

Flat/flat etalon: Perfect overlap of wavefronts

Curved/flat etalon: Mismatch of wavefront curvature Fortunately mirror curvature of a few km

are not so far “flat”. Simulations show: a reduced etalon effect

in curved/flat input mirror is still presentStill

we have to choose:

either wegde in input m

irror (P

ick-off beams available)

or no wedge in input mirro

r (Etalon effect available)

Stefan Hild Virgo week January 2008 Slide 8

IDEA: Wedges at input mirrors and etalon effect at end mirrors

Wedge at input mirrors: Allows for additional pick off beams (Concentrate on compensating thermal lensing in input mirror)

Use etalon effect at end test mass Replace AR-coating by a coating of about 10% reflectivity. Ideally use a curved back surface (same curvature as front). End mirror behaves similar to flat/flat etalon.

Stefan Hild Virgo week January 2008 Slide 9

Now let’s have a look

at numbers for

Advanced Virgo

Stefan Hild Virgo week January 2008 Slide 10

Starting with a single AdV arm cavity

Using a single AdV arm cavity (no IFO).

Figure of merrit = intra cavity power.

Parameters used:IM trans = 0.007IM loss = 50 ppmEM trans = 50 ppmEM loss = 50 ppmAR coatings = 0ppmIM curvature = 1910mEM curvature = 1910mInput = 1W

Parameters taken from these 2 documents:

Stefan Hild Virgo week January 2008 Slide 11

Influence of losses inside the cavity

Imperfection of optics (surface + coatings) might cause different losses in the arm cavities := differential losses.

What are the expected differential losses of AdV ? 5ppm? 50ppm?

A differential loss of 15ppm corresponds to a change of 2W intra cavity power in this example.

GOAL: Compensate 2W using etalon effect in end mirror.

Stefan Hild Virgo week January 2008 Slide 12

End mirror as curved etalon (optimal solution)

Simulation done with finesse.

Back surface of end mirror curved (1910m).

AR coating replaced by coating of 10% or 20% reflectivity.

R=0.1 allows adjustment range of 10W (65ppm)

R=0.2 allows adjustment range of 16W (95ppm)

Stefan Hild Virgo week January 2008 Slide 13

Etalon changes optical phase

When changing the etalon tuning the optical-phase changes as well. (noise!)

The two etalon surfaces build a compound mirror, whose position depends on the etalon tuning.

A single FSR of the etalon corresponds to about 3pm.

Stefan Hild Virgo week January 2008 Slide 14

Everything fine as long

Etalon matches the

specs…

… but what if not ??

=> need to check !!

Stefan Hild Virgo week January 2008 Slide 15

Optical design: Check system integrity for deviations from specs

A deviation in the refelctivity of the etalon coating: Only changes tuning range (no

problem)

A deviation in the curvature of the etalon surface: Imperfect wave front

overlap… Reduces tuning range … Beam shape distortions …

Analyzing “worst case” scenario: curved/flat etalon

Stefan Hild Virgo week January 2008 Slide 16

FFT-simulation of a flat/curved etalon

Using R. Schilling’s WaveProp, (http://www.rzg.mpg.de/~ros/WaveProp/)

Parameters: Field: 128x128 Computing 3000 roundtrips End mirror front:

50ppm transmission R_c = 1910m

End mirror back: 10, 20, 50% reflectivity R_c = flat

Stefan Hild Virgo week January 2008 Slide 17

WaveProp simulation

R=0.1 allows adjustment range of 3W (20ppm)

R=0.2 allows adjustment range of 5W (35ppm)

R=0.5 allows adjustment range of 9W (60ppm)

Stefan Hild Virgo week January 2008 Slide 18

Comparison of flat/flat and curved/flat etalon

For a curved/flat etalon the tuning range (etalon effect) is reduced by about a factor of 3.

Stefan Hild Virgo week January 2008 Slide 19

Comparison of WaveProp and finesse simulations

Waveprop and finesse are in excellent agreement.

WaveProp finesse

Stefan Hild Virgo week January 2008 Slide 20

Simulated beam inside arm cavity

Simulated beam shape inside the arm cavity (using finesse) Simulation done with etalon on resonance (worst case scenario) By eye: no change …

Curved/curved etalon Curved/flat etalon

Stefan Hild Virgo week January 2008 Slide 21

Changes the beam shape inside the arm cavity ??

Subtracted beams indicate the change in beam shape.

Residual light field is a factor of 60000 weaker than the intra cavity beam.

Stefan Hild Virgo week January 2008 Slide 22

Summary

Advanced Virgo CAN feature wedges in the input mirrors AND use the etalon effect at the end mirrors.

Proposed concept allows us to build ‘arm cavities with adjustable losses’.

A curved/curved etalon would be ideal (a curved/flat etalon should work as well, but with reduced tuning range).

Stefan Hild Virgo week January 2008 Slide 23

Outlook

Potential issues to be investigated:

How does misalignment of the etalon influence the alignment control signals:

For curved/curved etalon: probably fine (to 1st order) For curved/flat etalon: needs simulation

Check that optical-phase noise from fluctuations is no problem.

Need a control system for etalon tuning (error signal + actuator).

Need a value for the expected differential losses in AdV in order to choose the reflectivity of the etalon

Stefan Hild Virgo week January 2008 Slide 24

E N D

Stefan Hild Virgo week January 2008 Slide 25

It is important to compensate the differential losses

A differential loss of 70ppm causes already 100mW of waste light at the dark port.

increased shot noise !!

AdV simulation (dummy parameters)