Transcript Folie 1 - DCC

HOMODYNE AND HETERODYNE READOUT OF A SIGNALRECYCLED GRAVITATIONAL WAVE DETECTOR
S. Hild, H. Grote, J. Degallaix, A. Freise, M. Hewitson, H. Lück, K.A. Strain, J.R. Smith and B. Willke
Max-Planck-Institut für
Gravitationsphysik
(Albert-Einstein-Institut)
LIGO-G070443-00-Z
Rotation of optical response (for detuned SR)
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
10
5
DC-readout
Heterodyne
200
Phase [deg]
Reduced shot noise (no contributing terms from 2 times the heterodyne
frequency)
Optical Gain [a.u.]
Motivation for DC-readout
10
Frequency [Hz]
-100
-200
10
2
10
3
Frequency [Hz]
The predicted rotation of the detector response
is confirmed by the measurements.
C GW+ GW-
This phenomenon can be explained by the
opposite phase of the two heterodyne sidebands.
Increased coupling of laser power noise.
MI+
MI-
f<< 550 Hz
0
0
0
0
180
f>>550 Hz
0
0
180
0
180
Detuned Signal-Recycling (550Hz)
Usually an output mode cleaner (OMC) is required.
10
Definitions
Optical gain
Transfer function from differential displacement to signal at the detection point.
ASD [V/sqrt(Hz)]
10
10
10
Noise projection for DC-readout with detuned SR
-2
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
Readout system
10
• heterodyne: LO from RF sidebands (Schnupp modulation)
10
Strain [1/sqrt(Hz)]
Very sensitive to imbalances of the interferometer arms.
• detuned: carrier is off resonance in SR-cavity (550 Hz or 1 kHz)
3
0
LO and GW pass the same optical system (identical delay, filtering, spatial profile) =>
This advantage is especially important for detectors with arm cavities.
• tuned: carrier is resonant in SR-cavity
10
100
Requires less effort for injecting squeezed light (=> useful precursor for GEO-HF)
Tuning/detuning of the Signal-Recycling cavity (microscopic length)
2
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
-6
• DC-readout / homodyne: Carrier from dark fringe offset serves as LO
10
-7
2
10
10
3
10
Frequency [Hz]
DC-readout in GEO without OMC
How to achieve
DC-readout?
nfft/fs = 2.00 : navs = 60 : enbw = 0.75 : nsecs = 120
-22
10
Laser power noise limits the sensitivity at
some frequencies below 300 Hz.
Above 300 Hz laser power noise seems
not to be a problem.
2
3
10
Frequency [Hz]
Increased
technical
noise
Shot noise
Increased in
DC-readout
Roughly same as
with heterodyne
(2e-19m/sqrt(Hz))
reference S5
Heterodyne
550 Hz
-7
10
Red.
modulation
10%
MIMI
midx
Tuned Signal-Recycling
Red.
modulation
10%
MIMI
midx
+ TEM00 carrier
-8
10
measured data, 550 Hz
Simulation, 550 Hz
Simulation, tuned SR
Simulation, tuned SR, adjusted mod freq
Simulation, tuned SR, adjusted mod freq, lower mod index
1
10
Mag [a.u.]
Optical Power @ darkport
[W/sqrt(Hz)}
power
Darkport
)
(Watt
/ Hz1/2
+ carrier from dfo
0
10
-9
10
Tuned SR is realized by using a fast
jumping technique.
Two different operation modes: resonant
and non resonant RF modulation frequency.
2
3
10
10
Frequency [Hz]
Simulation of Laser power noise
-10
10
3
Frequency
Frequency (Hz)[Hz]
Turning down the radio frequency modulation (stable
operation is possible with 10 lower sidebands)
Dark port is dominated by carrier light (TEM00) from a
50 pm dark fringe offset
+ offset (large)
+ offset (small)
- offset (small)
- offset (large)
-19
10
5
Phase [deg]
200
10
detuned SR: A “rotation”
of the detector response is
observed, when going from
heterodyne to DC-readout
3
-21
10
2
10
3
Frequency [Hz]
Strain [1/sqrt(Hz)]
 DC-readout gives a better
peak sensitivity than heterodyne readout, independent of
the SR tuning.
10
0
10
 Simulations were performed
with FINESSE.
2
-21
Summary
We demonstrated a DC-readout scheme
without output mode cleaner in GEO600.
DC-readout and heterodyne detection has
been compared for several SignalRecycling tunings.
 For
-22
10
10
2
10
Frequency [Hz]
2
3
10
10
Frequency [Hz]
Optical gain increases with the size of the dark fringe offset.
Optical gain for + and – dark fringe offset have 180 degree different phase.
Sensitivity seems to independent of sign and size of the dark fringe offset.
Homodyne, 550Hz
Heterodyne, 550Hz
Homodyne, 1kHz
Heterodyne, 1kHz
Homodyne, tuned SR
Heterodyne, tuned SR
10
-20
10
100
-100
Simulated
shot noise
+ offset (large)
+ offset (small)
- offset (small)
- offset (large)
10
STRAIN [1/sqrt(Hz)]
2
10
Optical Gain [a.u.]
10
3
Using DC-readout a displacement sensitivity of 2·10-19m/sqrt(Hz) is achieved.