Optical Configuration 03.11.2008 Advanced Virgo Review Andreas Freise for the OSD subsystem

Optical Configuration

03.11.2008 Advanced Virgo Review

Andreas Freise for the OSD subsystem

A. Freise Advanced Virgo Review 03.11.2008 Slide 2

Optical Simulation and Design

Scope of the OSD subsystem:

Optical design of the core interferometer (Michelson interferometer and Recycling cavities)

Core tasks:

Arm cavity geometry Arm cavity finesse Geometry of mirror and beam splitter substrates Power Recycling cavity Signal Recycling cavity Auxiliary beams and scattered light Optical simulations


Design Strategy

Design drivers and constraints:

Interferometer design must be improved so that the sensitivity increase by a factor 10 can be achieved The positions and size of the vacuum tanks must be largely kept unchanged The envisaged joint operation with Advanced LIGO determines the timing

Strategy

Take existing Advanced LIGO design as start configuration Adapt parameters within the given constraints Provide always a consistent set of optical parameters to other subsystems Automate design tools so that parameter evolution can be done with low effort and high accuracy


Optical LayoutNear-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)

Non-degenerate Signal Recycling cavity (SRM transmission 11%)

125W laser power injected to interferometer

Main detection port: high power photodiode in vacuum

Possible optical readout ports for control


Arm cavity geometry

Science driver: Coating Brownian noise

beam radius on mirror

Coating Brownian noise of one mirror:

Absolute beam size shouldbe as large as possible.


Arm cavity geometry

Technical constraint: Mirror radius of curvature

In order to increase the beam the radii of curvature must be pushed close to the point of an instable cavity. `Corrective coatings' could be employed to achieve precise values (the accuracy of the metrology at LMA for radii of curvatures is ~0.1%). We have chosen the absolute value to be 2% away from cavity instability.

In a symmetric 3km cavity thiswould mean Rc > 1530.

mode non-degeneracy


Arm cavity geometry

Technical constraint: Mirror size

Beam radius should be five to six times smaller than the coating diameter to keep the clipping losses below 1ppm (including a safety margin).

Coating size is limited by manufacturing process.

Mirror size: 35cmBeam size: 5 to 7cm


Arm cavity geometry

Method: Near symmetric mirror curvatures

coating thickness

Beam size is determined by mirror radii of curvature:

3 m 6 m

Asymmetric coating thickness leads so a slight asymmetry for the radii of curvatures:


Non-degenerate Recycling cavities

Science driver: Signal loss due to scattering into higher-order modes

Thermal effects or misalignmentsscatter light into higher-order modes so that optical signal is lost. Non-degenerate cavities can 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

First design options



Technical constraint: Size of vacuum system limits length of Recycling cavities

PRM2 in PR tower PRM2 in PR tower No change for BS,IMX or IMY No change for BS,IMX or IMY

PRM1 and PRM3in injection tower PRM1 and PRM3in injection tower

Non-degenerate cavities must be longer than the Rayleigh range of the beam. The arm cavity beam has a Rayleigh range of 200m.



Method: Focusing elements inside the Recycling cavities

Use beam propagation calculus to optimize radii of curvatures of the optical elements inside the recycling cavities for a given total length and single-trip Gouy phase.

Design procedure has been established, first draft design for Power Recycling cavity:

IMX

Length total: 24 to 25 m

limx = 10 cm lx = 5.5 m lprm3 = 10 m lprm2 = 4.5 mlprm1 = 4.5 m

component PRM1 PRM2 PRM3 BS IMX (AR)

Rc [m] 2.1 -1.8 30 10.5

w [mm] 1.8 2.2 16.5 38.5 50


Signal Recycling Science driver: Sensitivity optimisation (quantum noise reduction)

The Signal Recycling (SR) mirror in the interferometer output makes the detector resonant fora certain signal frequency inorder to reduce quantum noise.

SR allows us to tune the sensitivity curve - also during operation.

SR is a mature optical techniquedeveloped over 10 years in theGEO collaboration and currently planned for all second-generation detectors.

Optimize parameters of SR mirror: microscopic position (tuning) power transmission


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 detectoroptimisation is limited by thermalnoise above ~ 30 Hz and ultimately gravity gradient noiseat lower frequencies.


Signal Recycling

Method: Optimize NS-NS inspiral range

The SNR for a NS-NS inspiralis given by the integral with a known frequency dependence:

The SR parameters aretuned in order to maximise

the accumulated signal.

SNR=

SR tuning: 0.15 radSR transmission: 11%


Summary

Current baseline:

arm cavity geometry fixed mirror substrate geometry fixed beam splitter geometry

currently being adapted to new beam size

recycling cavity design chosen, parameters to be evaluated

arm cavity finesse to be optimized


Next steps

Geometry of core optics:

Preliminary design has been proposed and will now be checked in detail for consistency Optical parameters will be refined in an iterative process, based on a study of their impact on other subsystems

Arm cavity finesse:

The finesse of the arm cavities is one of the dominant scaling factors for the influence of noise from the small interferometer (BS, IM, PRM, SRM) A trade-off study for the arm cavity finesse is strongly linked to the R+D program concerning mirror surface losses


...end


Example of Gouy phase and beam size


Searching the parameter space

RC,IMX=10m

Gouy Phase

Beam Size


Searching the parameter space

RC,IMX=flat

Gouy Phase

Beam Size

Documents

Optical Configuration 03.11.2008 Advanced Virgo Review Andreas Freise for the OSD subsystem