Stefan Hild for the GEO600 team October 2007 LSC-Virgo meeting Hannover Homodyne readout of an...

Stefan Hild for the GEO600 team

October 2007

LSC-Virgo meeting Hannover

Homodyne readout of an interferometer with Signal Recycling

Stefan Hild LSC-Virgo Meeting 10/2007 Slide 2

Motivation for DC-readout (1)

Increased coupling of laser power noise.

Usually an output mode cleaner (OMC) is required.

Very sensitive to imbalances of the interferometer arms.


Motivation for DC-readout (2)

Reduced shot noise (no contributing terms from 2 times the heterodyne frequency)Reduction of oscillator phase noise and oscillator amplitude noiseStronger low pass filtering of local oscillator (due to PR cavity pole) Simplify the GW detector

Simpler calibration (GW-signal in a single data-stream, even for detuned SR) Simpler circuits for photodiodes and readout electronics Possibility to use photodiodes with larger area => reduced coupling of pointing Reduced number of beating light fields at the output photodiode => simpler

couplings of technical noiseRequires less effort for injecting squeezed light (=> useful precursor for GEO-HF)LO and GW pass the same optical system (identical delay, filtering, spatial profile) => This advantage is especially important for detectors with arm cavities.


DC-readout without OMC

102

103

10-10

10-9

10-8

10-7

Frequency (Hz)

Op

tica

l P

ow

er @

da

rkp

ort

(W

att

/ H

z1/2)

nfft/fs = 2.00 : navs = 60 : enbw = 0.75 : nsecs = 120

reference S5

10% MI midx

10% MI midx + TEM00 carrier

Turning down the radio frequency modulation (stable operation is possible with 10 times lower sidebands)

Dark port is dominated by carrier light (TEM00) from a 50 pm dark fringe offset

Disadvantage: Still some shot noise contribution from RF-sidebands.

Frequency [Hz]

Da

rkp

ort

po

we

r [W

/sq

rt(H

z)} Red. MI modulation

Heterodyne 550 Hz

Red. MI modulation+ carrier from dfo


Simulated shot noise: Homodyne vs Heterodyne detection

102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, tuned SRHeterodyne, tuned SR

DC-readout with tuned Signal-Recycling: - shape stays constant - overall level is reduced


102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, 550HzHeterodyne, 550Hz


DC-readout with detuned SR: - better peak sensitvity- shape is rotated => better at low freqs, worse at high freqs.



102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, 1kHzHeterodyne, 1kHz

DC-readout with detuned SR: - better peak sensitivity- shape is rotated => better at low freqs, worse at high freqs.



102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, tuned SRHeterodyne, tuned SRHomodyne, 550HzHeterodyne, 550HzHomodyne, 1kHzHeterodyne, 1kHz

1st Question: Can we confirm the rotation of the shape in our measurements?


‚Rotation‘ of the optical gain

102

103

105

Frequency [Hz]

Op

tical G

ain

[a.u

.]

102

103

-200

-100

0

100

200

Ph

ase [

deg

]

Frequency [Hz]

DC-readoutHeterodyne

Rotated shape of optical response confirmed by measurement:

Rotated shape of optical response can be understood by looking at the phases of the contributing light fields. => change of the optical demodulation phase.



102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, tuned SRHeterodyne, tuned SRHomodyne, 550HzHeterodyne, 550HzHomodyne, 1kHzHeterodyne, 1kHz

2nd Question: Can we confirm the change of the relative shape of tuned and detuned SR with DC-readout ?



102

103

10-22

10-21

Frequency [Hz]

Str

ain

[1

/sq

rt(H

z)]

Homodyne, tuned SRHomodyne, 1kHz

2nd Question: Can we confirm the change of the relative shape of tuned and detuned SR with DC-readout ?


102

103

10-22

10-21

Frequency [Hz]

Str

ain

sen

siti

vit

y [

1/s

qrt

(Hz)

]

Simulated shot noise, DC 1000Hz1/Optical Gain, DC 1000Hz, measuredSimulated shot noise, DC 0Hz1/Optical Gain, DC 0Hz, measured

Comparison of measured and simulated optical transfer function for DC-readout

The simulated optical transfer function for tuned and detuned SR wit DC-readout is reproduced by our measurements.


102

103

10-22

10-21

10-20

10-19

Frequency [Hz]

Str

ain

[1/s

qrt

(Hz)

]

Simulated shotnoise, DC-readout 550HzSimulated shotnoise, heterodyne 550Hzh(f), heterodyne 550 Hzh(f), DC-readout 550 Hz

Best sensitivity so far with DC-readout and a SR detuning of 550 Hz

Shot noise Increased at high freqs in DC-readout (with detuned SR)

Peak sensitivity roughly same as with heterodyne (2e-19m/sqrt(Hz))

Increasedtechnical

noise


Noise budget for DC-readout (detuned SR)

102

103

10-7

10-6

10-5

10-4

10-3

10-2

Frequency [Hz]

AS

D [V

/sqr

t(H

z)]

Noise projection for DC-readout with detuned SR

Mid visMID AA FB RotMID AA FB TiltSignal recycling longitudinal noiseLaser amplitude noise (BS ar)PR errorDark noiseModelled shot noiseSum of the noise

Laser power noise (LPN):

• is partly limiting at low frequencies

• overall seems to be less of a problem than initially expected

3rd Question: Do we understand the laser power noise coupling?


Understanding the LPN in DC-readout

Good agreement between measurement and simulation !!

102

103

100

Ma

g [

a.u

.]

Frequency [Hz]

measured data, SR 550 Hz

Simulation, SR 550 Hz


Summary

Demonstrated DC-readout with tuned and detuned Signal-Recycling (without OMC)

Going to DC-readout changes the optical demodulation phase (rotated shape of optical response)

Measurements and simulations agree pretty well:Optical responseLaser intensity noise coupling

Achieved a displacement sensitivity of 2e-19m/sqrt(Hz) (currently worse sensitivity than in heterodyne readout)

Laser power noise is not as bad as rumors suggest (due to filtering of PR cavity pole)


Where to go in future ??

Best shot noise at low and high frequencies.

This combination of SR tuning and DC-readout would allow an ‚easy‘ implementation of squeezed light (no filter cavity necessary to get full benefit)

See talk by S.Chelkowski @ QND-meeting


Additional slides


positive dfodark fringenegative dfo

Output mode for positive and negative dfo: observation vs simulation


Output mode for positive and negative dark fringe offset (dfo)

positive dfo

negative dfo

Wave front radii of returning beams @ beam splitter:

horizontal: north > eastvertical: north < east


Realisation of tuned signal recycling For tunings < 250 Hz we cannot achieve a reasonable control signal.

Developed a new technique: We ‘kick‘ MSR in a controlled way to jump to tuned SR, where a reasonable control signal can be obtained again.

MSR is caught at the tuned operating point again.

-10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 1010

0

Sig

na

l a

mp

litu

de

[a.u

.]

Position of MSR [nm]

-4 -3 -2 -1 0 1 2 3 4

0

Sig

na

l a

mp

litu

de

[a.u

.]

Position of MSR [nm]

SR Mod = 9017065 Hz,tuning = 330 HzSR Mod = 9017195 Hz,tuning = 200 HzSR Mod = 9017395 Hz,tuning = 0 Hz

SR Mod = 9016395 Hz,tuning = 1000 HzSR Mod = 9017065 Hz,tuning = 330 Hz


2 different possibilities for going totuned signal recycling

1. Keep the modulation frequency and jump to center zerocrossing.2. Change the modulation frequency (corresponding to 0 Hz tuning)

=> only a single zerocrossing exists.


Laser intensity noise coupling for tuned and detuned SR

102

103

100

101

Ma

g [

a.u

.]

Frequency [Hz]

measured data, SR 550 HzSimulation, SR 550 HzSimulation, tuned SRSimulation, tuned SR,adjusted mod freq

Simulation, tuned SR,adjusted mod freq,lower mod index


Tuned DC with various dark fringe offsets

102

103

105

Frequency [Hz]

Op

tica

l G

ain

[]

102

103

-100

-50

0

50

100

150

Ph

ase

[deg

]

Frequency [Hz]

Tuned DC, data set 3

Tuned DC, data set 2b



data set 2b:small –dfo

data set 3: large –dfo

data set 4: small +dfo

data set 5:large +dfo


Comparison of heterodyne 550 Hz, tuned heterodyne and tuned DC

102

103

10-22

10-21

10-20

10-19

Frequency [Hz]

ST

RA

IN [

1/sq

rt(H

z)]

DER_DATA_H (nominal S5= 550 Hz, heterodyne) 2007-05-16 03:20:30 -

TUNED SR, heterodyne readout (P-quad), (Sept 2006)

TUNED SR, DC-readout (4513)DC 0Hz (simulated shot noise)

Hetero 0Hz (simulated shot noise)

Hetero 550Hz (simulated shot noise)

While in the two heterodyne cases the sensitivity is close to simulated shot noise at 2 kHz,

this is not the case for tuned DC.


Combination of tuned SR and squeezing– an option for GEO HF?

• Squeezed light is available for injection

• Tuned Signal-Recycling operation was demonstrated

No need for long filter cavity !

„Demonstration and comparison of tuned and detuned Signal-Recycling in a large scale gravitational wave detector“ , S Hild et al, CQG. 24 No 6, 1513-1523.

“Coherent Control of Vacuum Squeezing in the Gravitational-Wave Detection Band“, Vahlbruch et al, PRL 97, 011101 (2006)

Stefan Hild for the GEO600 team October 2007 LSC-Virgo meeting Hannover Homodyne readout of an...

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