Dual Recycling in GEO 600

H. Grote, A. Freise, M. Malec for the GEO600 team

Institut für Atom- und MolekülphysikUniversity of Hannover

Max-Planck-Institut für Gravitationsphysik

9. July 2003

9. July 2003 Hartmut Grote

Dual Recycling Concept

two recycling cavities enhance independently the carrier light and the signal sidebands

shot noise limited sensitivity is improved (typically by a factor of 102 to 103)

MPR

MSR

9. July 2003 Hartmut Grote

Adjustable Sensitivity

Frequency [Hz]

Str

ain

sens

itivi

ty [

1/sq

rt(H

z)]

Therm. noise

TMSR = 0,1%

TMSR = 1%

TMSR = 5%

9. July 2003 Hartmut Grote

Dual Recycling Configuration

PR cavity control (25kHz)

Michelson control (300Hz)

SR cavity control (400/40 Hz)

9. July 2003 Hartmut Grote

PR Error Signal

PR Tuning [Degree]

Mic

hels

on T

unin

g [D

egre

e]

9. July 2003 Hartmut Grote

Problem: Lock Acquisition

PR has to be locked first, but state space is limited

Lock Michelson in largely detuned SR state (larger state space)

MI optical gain depends on SR tuning

SR tuning [Degree]

Opt

ical

gai

n [A

rb.]

Michelson error signal

Signal Recycling error signal

9. July 2003 Hartmut Grote

SR Error Signal

Small capture range (0.5nm)

SR error signal is sensitive to various parameters

SR demodulation phase

Laser frequency with respect to PR cavity

Alignment

Michelson deviation from dark fringe

9. July 2003 Hartmut Grote

Michelson Deviation

MSR offset from operating point [nm]

SR

err

or s

igna

l [A

rb.]

0 nm

+1 nm

-1 nm

-2 nm

Michelsondeviationfrom dark fringe

9. July 2003 Hartmut Grote

Lock Acquisition

Dark fringe power

Intra cavity power

MI ESD feedback

MI slow feedback

SR error signal

SR feedback

MI error signal

9. July 2003 Hartmut Grote

Summary and Outlook

Established dual recycling lock at largely detuned operating point

Get more frequent acquisitions

Decrease pitch noise of pendulums

Normalize Michelson gain

“search“ for SR mirror by sweeping the operating point

Get stable DR locks including SR autoalignment

Tune signal recycling to nominal GW band in lock

9. July 2003 Hartmut Grote

9. July 2003 Hartmut Grote

Next Steps

PR + MI lockedPR + MI locked normalize MI gain to make system more robust during SR acquisitionnormalize MI gain to make system more robust during SR acquisition

simultanious ramping of SR modulation frequency and demodulationphase to search for SR fringe

simultanious ramping of SR modulation frequency and demodulationphase to search for SR fringe

Signal Recycling autoalignmentSignal Recycling autoalignment

Tuning down to GW band in lockTuning down to GW band in lock

9. July 2003 Hartmut Grote

MI error signal slope

Nominal operating points

Lock acquisition operating points

9. July 2003 Hartmut Grote

More experimental issues

SR error signal is sensitive to various parameters

SR demodulation phase

Laser frequency with respect to PR cavity

Alignment

Michelson deviation from dark fringe

9. July 2003 Hartmut Grote

9. July 2003 Hartmut Grote

Lock Acquisition

Dark fringe power

Intra cavity power

MI ESD feedback

MI slow feedback

SR error signal

SR feedback

MI error signal

9. July 2003 Hartmut Grote

9. July 2003 Hartmut Grote

Laser

Mode CleanersCommon arm length:(laser frequency stabilsation)

Pound-Drever-Hall

Dual Recycling Control

14.9 MHz

9 MHz

Differential arm length:(gravitational wave signal)

heterodyne detection Schnupp modulation

Signal-Recycling control: a separate modulation frequency reflected beam from AR coating

to laser frequency 37.2 MHz

9. July 2003 Hartmut Grote

Tuneable sensitivity

9. July 2003 Hartmut Grote

GEO Status July 2003

Michelson Interferometer

Laser

Mode Cleaners

2 W 1 W

~ 10 mW

300 W atBeam Splitter

MPR

final optics

test optics

MSR

9. July 2003 Hartmut Grote

Mode Healing

power recycling only:

Each recycling cavity minimises the loss due to mode mismatch of the respective other

with signal recycling:

9. July 2003 Hartmut Grote

Mode Healing

Mode healing

Mirror curvature compensation

MSR with T=1% yields almost perfectmode healing

9. July 2003 Hartmut Grote

Control Sidebands (SR) Resonance condition in the detuned, coupled, 4-mirror cavity:

fSR= 72 ·FSRSRC 9MHz

Detuning the modulation frequency and/orthe demodulation phase changes the SR operating point

9. July 2003 Hartmut Grote

SRC Error Signal

Example operatingpoint for 200 Hz detuning

9. July 2003 Hartmut Grote

From Power- to Dual Recycling

MPR

MSR

MI

Optical signals change because control sidebands and higher order TEM modes experience a coupled, 4-mirror cavity.

MI arm length difference: 10 cm cavity length difference: 9 cm

Transfer function for control sidebands must be maximised for various possible configurations:

9. July 2003 Hartmut Grote

SRC Error Signal

Largely detuned operating point for lock acquisition

9. July 2003 Hartmut Grote

Detuned Dual Recycling

Zeit [s]

Michelson Ausgang

PR Resonator

Michelson Fehlersignal

ESD Feedback

IM Feedback

SR Fehlersignal

SR Feedback

9. July 2003 Hartmut Grote

Power Recycling

End mirrors with imperfectradius of curvature

beamsplitter: „tilt“motion

9. July 2003 Hartmut Grote

Power Recycling Signals