Homodyne readout of an interferometer with Signal Recycling Stefan Hild for the GEO600 team October 2007 LSC-Virgo meeting Hannover 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. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 2 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 noise Stronger 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 noise Requires 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. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 3 DC-readout without OMC nfft/fs = 2.00 : navs = 60 : enbw = 0.75 : nsecs = 120 reference S5 550 Hz Heterodyne -7 Red. modulation 10% MIMI midx Red. modulation 10% MIMI midx + TEM00 carrier + carrier from dfo -8 10 (Watt / Hz1/2) [W/sqrt(Hz)} Darkport Opticalpower Power @ darkport 10 -9 10 -10 10 2 10 Frequency [Hz] Frequency (Hz) 10 3 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. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 4 Simulated shot noise: Homodyne vs Heterodyne detection Strain [1/sqrt(Hz)] Homodyne, tuned SR Heterodyne, tuned SR 10 -21 10 -22 10 2 10 3 Frequency [Hz] DC-readout with tuned Signal-Recycling: Stefan Hild - shape stays constant - overall level is reduced LSC-Virgo Meeting 10/2007 Slide 5 Simulated shot noise: Homodyne vs Heterodyne detection Strain [1/sqrt(Hz)] Homodyne, 550Hz Heterodyne, 550Hz 10 -21 10 -22 10 2 10 3 Frequency [Hz] DC-readout with detuned SR: - better peak sensitvity - shape is rotated => better at low freqs, worse at high freqs. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 6 Simulated shot noise: Homodyne vs Heterodyne detection Strain [1/sqrt(Hz)] Homodyne, 1kHz Heterodyne, 1kHz 10 -21 10 -22 10 2 10 3 Frequency [Hz] DC-readout with detuned SR: - better peak sensitivity - shape is rotated => better at low freqs, worse at high freqs. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 7 Strain [1/sqrt(Hz)] Simulated shot noise: Homodyne vs Heterodyne detection 10 -21 10 -22 Homodyne, tuned SR Heterodyne, tuned SR Homodyne, 550Hz Heterodyne, 550Hz Homodyne, 1kHz Heterodyne, 1kHz 10 2 10 3 Frequency [Hz] 1st Question: Can we confirm the rotation of the shape in our measurements? Stefan Hild LSC-Virgo Meeting 10/2007 Slide 8 ‚Rotation‘ of the optical gain Optical Gain [a.u.] Rotated shape of optical response confirmed by measurement: 10 5 DC-readout Heterodyne Phase [deg] 200 10 2 10 3 Frequency [Hz] 100 0 -100 -200 10 2 10 3 Frequency [Hz] Rotated shape of optical response can be understood by looking at the phases of the contributing light fields. => change of the optical demodulation phase. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 9 Strain [1/sqrt(Hz)] Simulated shot noise: Homodyne vs Heterodyne detection 10 -21 10 -22 Homodyne, tuned SR Heterodyne, tuned SR Homodyne, 550Hz Heterodyne, 550Hz Homodyne, 1kHz Heterodyne, 1kHz 10 2 10 3 Frequency [Hz] 2nd Question: Can we confirm the change of the relative shape of tuned and detuned SR with DC-readout ? Stefan Hild LSC-Virgo Meeting 10/2007 Slide 10 Simulated shot noise: Homodyne vs Heterodyne detection Strain [1/sqrt(Hz)] Homodyne, tuned SR Homodyne, 1kHz 10 -21 10 -22 10 2 10 3 Frequency [Hz] 2nd Question: Can we confirm the change of the relative shape of tuned and detuned SR with DC-readout ? Stefan Hild LSC-Virgo Meeting 10/2007 Slide 11 Strain sensitivity [1/sqrt(Hz)] Comparison of measured and simulated optical transfer function for DC-readout 10 -21 10 -22 Simulated shot noise, DC 1000Hz 1/Optical Gain, DC 1000Hz, measured Simulated shot noise, DC 0Hz 1/Optical Gain, DC 0Hz, measured 10 2 Frequency [Hz] 10 3 The simulated optical transfer function for tuned and detuned SR wit DCreadout is reproduced by our measurements. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 12 Best sensitivity so far with DC-readout and a SR detuning of 550 Hz Strain [1/sqrt(Hz)] 10 10 10 10 -19 Simulated shotnoise, DC-readout 550Hz Simulated shotnoise, heterodyne 550Hz h(f), heterodyne 550 Hz h(f), DC-readout 550 Hz -20 -21 -22 10 2 3 10 Frequency [Hz] Increased technical noise Stefan Hild Peak sensitivity roughly same as with heterodyne (2e-19m/sqrt(Hz)) LSC-Virgo Meeting 10/2007 Shot noise Increased at high freqs in DC-readout (with detuned SR) Slide 13 Noise budget for DC-readout (detuned SR) • is partly limiting at low frequencies 10 • overall seems to be less of a problem than initially expected ASD [V/sqrt(Hz)] Laser power noise (LPN): 10 10 10 10 3rd Question: Do we understand the laser power noise coupling? Stefan Hild 10 -2 Noise projection for DC-readout with detuned SR Mid vis MID AA FB Rot MID AA FB Tilt Signal recycling longitudinal noise Laser amplitude noise (BS ar) PR error Dark noise Modelled shot noise Sum of the noise -3 -4 -5 -6 -7 2 10 10 3 Frequency [Hz] LSC-Virgo Meeting 10/2007 Slide 14 Understanding the LPN in DC-readout Mag [a.u.] measured data, SR 550 Hz Simulation, SR 550 Hz 10 0 10 2 10 3 Frequency [Hz] Good agreement between measurement and simulation !! Stefan Hild LSC-Virgo Meeting 10/2007 Slide 15 Summary Demonstrated DC-readout with tuned and detuned SignalRecycling (without OMC) Going to DC-readout changes the optical demodulation phase (rotated shape of optical response) Measurements and simulations agree pretty well: Optical response Laser 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) Stefan Hild LSC-Virgo Meeting 10/2007 Slide 16 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 Stefan Hild LSC-Virgo Meeting 10/2007 Slide 17 Additional slides Stefan Hild LSC-Virgo Meeting 10/2007 Slide 18 Output mode for positive and negative dfo: observation vs simulation negative dfo Stefan Hild dark fringe LSC-Virgo Meeting 10/2007 positive dfo Slide 19 Output mode for positive and negative dark fringe offset (dfo) positive dfo negative dfo Wave front radii of returning beams @ beam splitter: horizontal: north > east vertical: north < east Stefan Hild LSC-Virgo Meeting 10/2007 Slide 20 Realisation of tuned signal recycling For tunings < 250 Hz we cannot achieve a reasonable control signal. Signal amplitude [a.u.] 0 SR Mod = 9016395 Hz, tuning = 1000 Hz SR Mod = 9017065 Hz, tuning = 330 Hz -10 -9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9 10 Signal amplitude [a.u.] Position of MSR [nm] 0 -4 SR Mod = 9017065 Hz, tuning = 330 Hz SR Mod = 9017195 Hz, tuning = 200 Hz SR Mod = 9017395 Hz, tuning = 0 Hz -3 -2 -1 0 1 2 3 4 Position of MSR [nm] 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. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 21 2 different possibilities for going to tuned signal recycling 1. 2. Keep the modulation frequency and jump to center zerocrossing. Change the modulation frequency (corresponding to 0 Hz tuning) => only a single zerocrossing exists. Stefan Hild LSC-Virgo Meeting 10/2007 Slide 22 Laser intensity noise coupling for tuned and detuned SR Mag [a.u.] 10 measured data, SR 550 Hz Simulation, SR 550 Hz Simulation, tuned SR Simulation, tuned SR, adjusted mod freq Simulation, tuned SR, adjusted mod freq, lower mod index 1 10 0 10 2 10 3 Frequency [Hz] Stefan Hild LSC-Virgo Meeting 10/2007 Slide 23 Optical Gain [] Tuned DC with various dark fringe offsets Tuned DC, Tuned DC, Tuned DC, Tuned DC, 10 data set data set data set data set 3 2b 4 5 data set 2b: small –dfo data set 3: large –dfo 5 2 10 10 3 Frequency [Hz] 150 data set 4: small +dfo Phase [deg] 100 data set 5: large +dfo 50 0 -50 -100 2 10 10 3 Frequency [Hz] Stefan Hild LSC-Virgo Meeting 10/2007 Slide 24 Comparison of heterodyne 550 Hz, tuned heterodyne and tuned DC 10 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) -20 STRAIN [1/sqrt(Hz)] 10 -19 10 10 -21 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. -22 10 2 10 3 Frequency [Hz] Stefan Hild LSC-Virgo Meeting 10/2007 Slide 25 Combination of tuned SR and squeezing– an option for GEO HF? • Squeezed light is available for injection “Coherent Control of Vacuum Squeezing in the Gravitational-Wave Detection Band“, Vahlbruch et al, PRL 97, 011101 (2006) • Tuned Signal-Recycling operation was demonstrated „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. No need for long filter cavity ! Stefan Hild LSC-Virgo Meeting 10/2007 Slide 26
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