Advanced Virgo: Optical Simulation and Design Extra Slides Andreas Freise for the OSD Subsystem 24.03.2009 Advanced Virgo review

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 5 times the beam radius. Consider beam sizes of up to 6.5cm. A. Freise Advanced Virgo review 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

A possible optical layout PRC design similar to Advanced LIGO Beam waist Beam size w=??? Beam size w=55mm A. Freise Advanced Virgo review 24.03.2009

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 24.03.2009

Option 1 parameters A. Freise Advanced Virgo review 24.03.2009

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 24.03.2009

Option 2 parameters A. Freise Advanced Virgo review 24.03.2009

Option 3: large beams passing BS Pro: very long PRC Con: beam 'touches' BS, PRM suspended next to monolithic suspension A. Freise Advanced Virgo review 24.03.2009

Option 3 parameters A. Freise Advanced Virgo review 24.03.2009

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 24.03.2009

Option 4 parameters A. Freise Advanced Virgo review 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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 24.03.2009

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