1. Advanced Virgo: Optical Simulation and Design
Extra Slides
Andreas Freise for the OSD Subsystem
Advanced Virgo review

2. How to design the interferometer from scratch?
Phase 1: each task provides draft layouts or requirements and specifications based on an educated guesses about the possible results from other tasks
Phase 2: results must be exchanged and a common, consistent design developed through an iterative process: most tasks finish only at the end of the entire design process!!
We are currently in Phase 2 which lasts effectively until construction starts.
A. Freise
Advanced Virgo review

3. Optical Layout
Near-symmetric 3 km long arm cavities (power 760 kW, finesse 900)
Non-degenerate Power Recycling cavity (build up factor >20, power on BS 2.7 kW)
125W laser power injected to interferometer
Non-degenerate Signal Recycling cavity (SRM transmission 11%)
Main detection port: high power photodiode in vacuum
Possible optical readout ports for control
A. Freise
Advanced Virgo review

4. Arm cavity geometry Science driver: Coating Brownian noise
Coating Brownian noise of one mirror:
beam radius on mirror
Absolute beam size should be as large as possible.
A. Freise
Advanced Virgo review

5. Arm cavity geometry Method: Near symmetric mirror curvatures
Beam size is determined by mirror radii of curvature:
Asymmetric coating thickness leads so a slight asymmetry for the radii of curvatures:
coating thickness
~3 m
~6 m
A. Freise
Advanced Virgo review

6. How to decide on Beam Size ?
Sensitivity
Advanced Virgo needs to have a sensitivity pretty close to Advanced LIGO.
Need to make the beams as large as possible!
Cavity stability
Large beams means pushing towards instability of the cavity.
Cavity degeneracy sets limit for maximal beam size
Mirror size
The maximum coated area might also impose a limit for the beam size.
Clipping losses require coating size times the beam radius.
Consider beam sizes of up to 6.5cm.
A. Freise
Advanced Virgo review

7. Choice of ROCs/beam size: Sensitivity vs Mode-non-degeneracy
In general mode-non-degeneracy and sensitivity go opposite.
Asymmetric ROCs are beneficial:
For identical mode-non-degeneracy (parallel to arrows in lower plot) and even slightly increased senstivity we can reduce the beam size in the CITF from 6 to 5.5 cm.
A. Freise
Advanced Virgo review

8. Sensitivity with symmetric ROCs
With 6cm radius and 1530m ROC: Advanced Virgo obtains about 150 Mpc.
For comaprison: Advanced LIGO will achieve a 180 to 200 Mpc.
A. Freise
Advanced Virgo review

9. Executive summary: Beam Geometry
Advanced Virgo needs to have a sensitivity competitive with Advanced LIGO in order to contribute to any network analysis.
This requires very large beam sizes (close to instability).
Trade off decision taking into account:
Sensitivity
Mode non-degeneracy
Mirror size / clipping losses
The current design features:
Beam sizes of 5.5 cm (IM) and 6.5 cm (EM).
The corresponding ROCs are 2% off instability.
The resulting sensitivity is about 30% worse than Advanced LIGO.
A. Freise
Advanced Virgo review

10. Sensitivity for finesse 888 and 444
The ideal Advanced Virgo sensitivity is (within a certain) range independent of the arm cavity finesse !!
A. Freise
Advanced Virgo review

11. Potential reasons for lowering the finesse?
Sensitivity ………………………………………………………..independent
Mirror losses …………………………………………………...independent
Coupling of diff losses to dark port power ………independent
Noise couplings from small Michelson ……………...NO
Thermal load of BS, ITM and CPs …………..………...NO
Lock acquisition ………………………………………………..independent
Losses inside the recycling cavities ………………..…YES
Coating Brownian from ITMs ………….…………….. independent
A. Freise
Advanced Virgo review

12. Executive summary: Arm Cavity Finesse
Current value for the Advanced Virgo arm cavity finesse is 880.
Advanced LIGO will use about 400 (original aimed at 1200)
LGCT plans to use 1600.
At the moment there is no strong argument to change this value.
However, in case new or updated information appears, we can perform a new trade-off decision.
The main arguments considered in such trade-off process are:
Signal loss inside the signal recycling cavity
Suppression of noise from the central interferometer
Thermal load of the central interferometer
Lock acquistion (currently not)
A. Freise
Advanced Virgo review

13. Signal Recycling
Science driver: Sensitivity optimisation (quantum noise reduction)
The Signal Recycling (SR) mirror in the interferometer output makes the detector resonant for a certain signal frequency in order to reduce quantum noise. SR allows us to tune the sensitivity curve - also during operation.
SR is a mature optical technique developed over 10 years in the GEO collaboration and currently planned for all second-generation detectors.
Optimize parameters of SR mirror:
microscopic position (tuning)
power transmission
A. Freise
Advanced Virgo review

14. Signal Recycling Technical constraint: Fundamental noise sources
A vacuum tank for the Signal Recycling mirror is already present in the Virgo vacuum system.
The potential of the detector optimisation is limited by thermal noise above ~ 30 Hz and ultimately gravity gradient noise at lower frequencies.
A. Freise
Advanced Virgo review

15. Signal Recycling Method: Optimize NS-NS inspiral range
The SNR for a NS-NS inspiral is given by the integral with a known frequency dependence:
The SR parameters are tuned in order to maximise the accumulated signal.
SNR=
SR tuning: 0.15 rad
SR transmission: 11%
A. Freise
Advanced Virgo review

16. Non-degenerate Recycling cavities
Science driver: Signal loss due to scattering into higher-order modes
First design options
Thermal effects or misalignments scatter light into higher-order modes so that optical signal is lost. Non-degenerate cavities reduce this effect.
Commissioning experience shows that degenerate cavities cause problems for control signals. Y. Pan showed in 2006 that also GW signal is lost.
Degenerate cavity
A. Freise
Advanced Virgo review

17. Non-degenerate recycling cavities for Advanced Virgo?
Motivation:
sensitivity: reduce higher-order mode content in signal sidebands. Several years of R+D by the LSC indicate that a marginally stable SRC can lead to significant loss of GW signal. See documents listed on OSD IRC website.
commissioning: reduce higher-order mode content in RF sidebands. Virgo commissioning has been slowed down a number of times due to issues related to the marginally stable PRC. Even though this is now mostly under control it, an RSE interferometer will pose new challenges and we should learn from previous mistakes.
relax specifications for TCS.
reduce beam size for detection and injection system
A. Freise
Advanced Virgo review

18. What is a non-degenerate cavity?
The term non-degenerate means that not all spatial modes resonate at the same time, i.e. when the TEM00 mode is resonant, the TEM01 generally would not be resonant. In short: a non-degenerate cavity is very much like a mode-cleaner cavity.
If the TEM00 mode is resonant, the resonance of a TEMnm mode is determined by its round-trip phase:
with n,m the mode indices and Psi the round-trip Gouy phase. The mode is resonant if the phase is a multiple of 2*pi. The Gouy phase is the main parameter in this design!
A. Freise
Advanced Virgo review

19. Virgo Advanced Virgo Telescope mirrors New PR mirror marginally stable
non-degenerate
PR Recycling cavity
5 meter
marginally stable
PR-Recycling cavity
New PR mirror
Telescope mirrors
A. Freise
Advanced Virgo review

20. A possible optical layout
PRC design similar to Advanced LIGO
Beam waist
Beam size
w=???
Beam size w=55mm
A. Freise
Advanced Virgo review

21. Option 1: preliminary design
Pro: small beams on PRMs, tolrances for PRMs relaxed
Con: strong astigmatism via BS, divergent auxiliary beams of IMX/IMY
A. Freise
Advanced Virgo review

22. Option 1 parameters
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Advanced Virgo review

23. Option 2: recycling mirrors next to BS
Pro: compatible with vacuum apertures, current favourite with PAY
Con: larger astigmatism, maybe not compatible with new TCS design
A. Freise
Advanced Virgo review

24. Option 2 parameters
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Advanced Virgo review

25. Option 3: large beams passing BS
Pro: very long PRC
Con: beam 'touches' BS, PRM suspended next to monolithic suspension
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Advanced Virgo review

26. Option 3 parameters
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Advanced Virgo review

27. Option 4: tiny BS
Pro: small BS, possible alternative options for TCS compensation plates
Con: telescopes are effectively in the interferometer arms (can create differential effects), PRC and SRC have same Gouy phase
A. Freise
Advanced Virgo review

28. Option 4 parameters
A. Freise
Advanced Virgo review

29. Summary Option 1 needs further investigation into astigmatism
Option 2 possible, pending further PAY/TCS input
Option 3 possible but beam is 'touching' BS in current vacuum setup
Option 4 possible, needs further investigation
Current favourite for baseline design: Option 2
A. Freise
Advanced Virgo review

30. Tolerances for lengths and ROCs
The telescopes inside the recycling cavities are strongly focusing, and are thus require a very precise setup. A first tolerancing analysis has been performed for cavity stability, the criterion was to keep the change in Gouy phase smaller than +-10 deg. The table below shows exemplary the results for Option 2:
The numbers are slightly misleading: Advanced LIGO has shown that e.g. a wrong ROC can be compensated by changing the telescope length. Work in progress.
A. Freise
Advanced Virgo review

31. Astigmatism and losses
NDRC layouts can show significant astigmatism if not designed carefully. This can affect the detector performance in three ways:
'differential' astigmatism creates a round trip loss inside PRC and SRC, which causes a loss of recycling gain and also signal loss
Off-axis telescopes create astigmatic eigenmodes for PRC and SRC, this makes the coupling to IMC and OMC more difficult
More importantly it reduces the coupling between PRC, SRC and the arm cavities which probably causes a drop in recycling gain and signal loss
APC has developed a model for these losses based on ABCD matrices (using Matlab). See document on OSD IRC page for preliminary results: Option 3 seems to show lowest losses, Option 2, 4 seem feasible.
A. Freise
Advanced Virgo review

32. Pick-off beams from IM and CP
Pick-off or auxiliary beams originate at AR coated surfaces (BS, CPs, IMX/IMY)
Must be either detected or dumped, i.e. need to be guided through free apertures
This plot is only to show effects qualitatively. A proper CAD drawing is required…
A. Freise
Advanced Virgo review

33. Extraction of pick-off from BS-AR
Pick-off from BS-AR coating needs to separated from the main beam before being reflected back into the interferometer from the Signal-Recycling mirror.
IDEA: Extract the BSAR-pickoff close to the beam waist between SRM2 and SRM3 (could work even without BS wedge).
(Layout for option 1 shown on the left)
A. Freise
Advanced Virgo review

34. ...end
A. Freise
Advanced Virgo review