cascina, nov. 4 th, 2008 adv review 1 adv injection system e. genin european gravitational...

AdV review Cascina, Nov. 4th , 2008 1

AdV Injection systemE. Genin

European Gravitational Observatory


Scope of INJ subsystem

● The Injection system (INJ) of AdV takes care of the optics downstream of the high power laser, and of the interface of these optics with the laser and the ITF.

● The whole system must deliver :• a laser beam at the ITF input port with the required power and size.• a good quality beam (filtered by IMC cavity).• frequency and angular stability sufficient to reach AdV sensitivity goal.

● It should provide :• an EOM system able to modulate the laser beam in phase and amplitude

(for ITF longitudinal and angular control) at the output of the laser.• A vacuum compatible Faraday isolator (FI) able to withstand high laser

power (up to 250W) and other FIs to be installed in air.• Good quality waveplates and polarizers able to withstand high laser

power.


Main tasks

● In order to succeed in the completion of this subsystem, it has been divided into 5 tasks:• Optical design of the AdV INJ system :

– INJ Requirements: RF modulation, input power, beam jitter, IMC filtering, optical isolation, mode matching, scattering.

– Optical design activity : INJ General Optical Layout, IMC cavity (thermal effects and scattering study, angular control), beam pointing control system, Mode-Matching telescopes (MMT),…

• Input optics components selection and realization:– Faraday isolator (1 vacuum compatible +other FIs for laser and external injection benches).– Electro-optical modulators (for ITF sensing and control purpose).– Polarisers and waveplates.– Optics (low scattering in-air optics selection, Mode-Matching Telescope (MMT) optics selection,

New IMC mirrors selection,…).

• Input mode-cleaner : Mechanical design and realisation.– Design and realization of the mechanical parts related to the IMC on the Suspended Injection

Bench (in collaboration with INJ optical and mechanical design groups, SAT and PAY subsystems) and if needed on IMC end mirror suspension.

• Design and realisation of mechanics needed for INJ subsystem:

– In-air and in-vacuum injection benches mechanics : Optical bench selection, Optics mount design, Actuator selection.

• Design and realisation of electronics needed for INJ subsystem:– Define electronics needed for Advanced Virgo INJ system (sensors, actuators, driving electronics).– Modulation frequency electronics design and realisation.


Subsystem requirements

● General requirements of the subsystem have been given (using LIGO requirements and our experience in Virgo)

● INJ subsystem team is working on the definition of the requirements for all the systems (Mode cleaner and SIB automatic alignment, Beam pointing control system,…) a document gathering all these requirements for INJ subsystem is in preparation.


Reference solution

• In-air optics:– EOM system for IMC and ITF control– IMC mode-matching telescope.– Power adjustment system (for locking purpose)– Beam pointing control system– Steering optics

• In-vacuum optics:– Resonant Input Mode-Cleaner (IMC)– HP Faraday isolator– Mode-Matching Telescope (MMT)– PSL intensity stabilization photodiode– Reference cavity (RFC)– Steering optics


Status of the design : Faraday isolator

● Requirements :• Withstand high continuous laser power on long periods (250W).• Compensated in term of thermal lensing.• Compensated in term of depolarization.• Provide a good optical isolation (>40 dB; Impact of under vacuum FI isolation could be

checked on Virgo+).• Transmission > 95%.

● The solution has already been developped for Advanced LIGO by Institute of Applied Physics (Novgorod, Russia).

● A collaboration with IAP has started to design a Faraday isolator that fulfills AdV requirements and adapted to AdV Suspended Injection Bench.

Input beamInput beam

Rejected beam (ITF reflection)Rejected beam (ITF reflection)



● High power input optics R&D: Tests on TGG crystal have started.

● The gaussian laser beam is heating the TGG crystal and is responsible for:

• Thermal lensing.

• Depolarization.

• Modification of the faraday rotator polarization rotation angle (due to verdet constant change with temperature).

Responsible for lack of FI isolationResponsible for lack of FI isolation

Responsible for mismatching on optical cavitiesResponsible for mismatching on optical cavities



● Thermal lensing.• 2 TGG crystals that are identical to the one used in Virgo FI tested up

to 84 W.• Using a code based on a finite element model, we can estimate the

crystal absorption (found to be around 1600-1800 ppm/cm) consistent with measurement performed on SIB faraday isolator.



● Measurement of depolarization in a TGG crystal up to 125 W by placing a TGG rod between 2 crossed polarizers.

From 70 W, depolarization γ (in dB) is proportional to P2 as already shown by Khazanov (Khazanov et al., IEEE Journ. Quant. Electr., Vol. 40, n. 10, (2004)).


● Measurement of faraday isolation change in air and in a vacuum tank up to 48 W.

As already seen in Virgo, FI isolation is worse in a vacuum than in air due to different thermal conditions (no convection in vacuum).This can be corrected by inserting a halfwave plate inside the faraday isolator to compensate for this rotation change (at 38 W, we rotated the half waveplate by 0.7° to improve the isolation by 3.5 dB).



Status of the design : EOM

● With 10 times more power respect to Virgo, thermal effects will become significant in EOM crystals: • We have to choose a material that is less absorbing than KTP currently used

in Virgo.• RTP crystal seems the most appropriate material (confirmed by Advanced

LIGO tests at high power).● Tests are planed using the High power test facility installed in EGO

optics lab.• Characterization of several electro-optic crystals that are available in

the lab (RTP, MgO doped LiNbO3 and KTP).• Plan to buy a KTP crystal that seems promising and has properties

close to RTP in term of absorption, damage threshold and EO coefficient.

• Work on the modulation electronics design will start soon.


AdV IMC

● Reference solution:• 144 m long resonant cavity.

● Next steps : IMC cavity simulation work in order to define all the specifications:• Finesse (coming from frequency, amplitude and beam jitter

filtering specifications), IMC locking frequency (given by ISC).• Coating absorption, flat mirrors substrate absorption.• End mirror surface flatness and roughness specifications to lower

internal scattering problems.• Study the way we hold the flat mirrors (independent suspension,

dihedron or 2 mirrors optically contacted).


AdV Suspended Injection Bench (SIB):preliminary design

● Optical design activity has started to check that it was possible to integrate the 2 mirrors of the non-degenerate power recycling cavity.

● The main constraint since we are not going to change the tower structure is the SIB bench dimension (tower aperture diameter = 1 m).


3D views

PRM2PRM2

North inputNorth input

SIBSIB


Technical risks

● Pollution of injection tower by actuators, mechanics and optical devices it could increase thermal effects particularly at the level of IMC flat mirrors and power recycling mirrors (in Virgo+, we are already experiencing this kind of troubles).

● We will have a lot of beam to manage (ITF reflection, PRM3 transmission for PR cavity control, IMC flat mirrors beam for control, SIB local control system beams) space constraints on the SIB, not that easy to put everything we want on this bench.


Plans for the next future

● Big steps towards INJ design completion: (due date : mid 2009)• Complete INJ subsystem requirements document (starting point of

the design work) : ready for the beginning of 2009.• IMC:

– IMC cavity geometry definition.– Take a decision about the necessity to suspend IMC flat mirrors,

keep a dihedron or try to optically contact 2 rectangular flat mirrors.• SIB:

– How to upgrade SIB suspension? Do we need to add 1 or 2 filters? Decisions have to be taken soon.

• Complete optical and mechanical layout for INJ system.


Plans for the next future

● High power input optics R&D:• In-vacuum high power compliant faraday isolator :

– design ready (beginning of next year)– a prototype should be ready for mid-end 2009.

• EOM :– tests on EO crystals should be done (end of 2008-beginning of

2009).– EOM modulation electronics design should start soon (a critical

point is the one concerning sidebands on sidebands generation). Hope to have a first prototype ready by mid 2009.

cascina, nov. 4 th, 2008 adv review 1 adv injection system e. genin european gravitational...

Documents