1. Variable reflectivity signal-recycling mirror and control
Stefan Goßler
for the experimental team of
The ANU
Centre of Gravitational Physics

2. Outline Tuned and detuned signal recycling Introduction of VRSM
Layout of the initial experiment
Error signals
First results
Carrier/Subcarrier offset phase-locking control scheme
Summary
David Rabeling, Malcolm Gray, David McClelland
Control and VRSM

3. Signal Recycling I
Signal recycling can be used to enhance SNL sensitivity
of gravitational wave detectors within certain bandwidth:
Tuned SR: SRC is tuned on resonance with carrier
Requires broadband mode: low finesse of SRC
Detuned SR: SRC is detuned from resonance with carrier
Enables narrowband mode: high finesse of SRC
Microscopic tuning of SRM position alters the tuning
Changing the reflectivity of SRM alters the bandwidth
Control and VRSM

4. Signal Recycling II
Control and VRSM

5. 125 W input Detuned quant T=0.1% Detuned total T=0.1% Tuned total T=7%10
-25
-24
-23
-22
-21
-20
h(f)
[1/sqrt Hz] 1 2 3 4
Detuned total T=0.1%
Tuned total T=7%
Tuned quant T=7%
Detuned quant T=0.1%
Frequency [Hz]

6. Complication Rotational stage Cantilever springs 360 µm Coilholder for
180 mm
280 mm
420 mm
360 µm
Upper masses
Coilholder for
local control
Activ
stack
isolator
200 µm
Steel wire loops
162 µm
Intermediate
masses
Mirror 2,9 kg
150 mm
Magnet-coil
actuator
3 mm

7. Complicaton: Solution: A mirror with a reflectivity
Changing the reflectivity of the
signal-recycling mirror causes
a down-time of the GWD for
a substantial period of time!
A mirror with a reflectivity
that can be varied on demand:
a VRSM
Solution:
Control and VRSM

8. VRSM Can be realised by: Fabry-Perot cavity
Thermal tuning of an etalon
Kawabe, Hild et al.
Downside: Slow and lateral thermal
gradients the substrate
Michelson interferometer
deVine, Shaddock, McClelland
Downside: Very complex system
Fabry-Perot cavity
Strain, Hough
Downside: Complex reflectivity
Control and VRSM

9. Simplification of Layout
Signal-recycling
mirror
Power-recycling
Arm-cavity
Michelson
interferometer
`Michelson´
Variable reflectivity
signal-recycling
Dual recycled
Michelson interferometer
with arm cavities
Control and VRSM

10. Simplification of Layout
Signal-recycling
mirror
Power-recycling
Arm-cavity
Michelson
interferometer
`Michelson´
Variable reflectivity
signal-recycling
Collapse
Michelson interferometer
into one mirror
Control and VRSM

11. Variable reflectivity
Introduction of VRSM
Signal-recycling
mirror
Power-recycling
Arm-cavity
Michelson
interferometer
`Michelson´
Variable reflectivity
signal-recycling
Control and VRSM

12. Experimental Layout ~ Control of SRC via in-phase error signal
shifter
PBS R1 R2 R3
190 MHz
Local
Oscillator
Laser
l/2
RF output
SRC
FSR =
380 MHz V
RSM
30 GHz
Signal ~
Quadrature
errror signal
Lowpass
filter
Servo
Isolator
In-phase
modulator
l/4
Control of SRC via in-phase error signal
Control of VRSM via in-quardature error signal
Control and VRSM

13. Error Signals Control and VRSM
Amplitude and phase response of a narrow linewidth cavity
Amplitude and phase response of a broad linewidth cavity
1.0
1.0
0.8
Amplitude
0.8
0.6
Amplitude
Phase
0.6
0.4
Phase
0.4
0.2
0.2
[Arb. units]
0.0
0.0
[Arb. units]
-0.2
-0.2
-0.4
-0.4
-0.6
-0.6
-0.8
-0.8
-1.0
-1.0
-200
-150
-100
-50 50
100
150
200
-200
-150
-100
-50 50
100
150
200
Frequency [MHz]
Frequency [MHz]
PDH error signal for a narrow linewidth cavity
PDH error signal for a broad linewidth cavity
0.5
0.4
0.4
In quadrature
0.3
In quadrature
0.3
In phase
0.2
In phase
0.2
0.1
0.1
[Arb. units]
0.0
[Arb. units]
0.0
-0.1
-0.1
-0.2
-0.2
-0.3
-0.4
-0.3
-0.5
-0.4
-200
-150
-100
-50 50
100
150
200
-200
-150
-100
-50 50
100
150
200
Frequency [MHz]
Frequency [MHz]
Control and VRSM

14. Experimental layout ~ Control of SRC via in-phase error signal
shifter
PBS R1 R2 R3
190 MHz
Local
Oscillator
Laser
RF output
SRC
FSR =
380 MHz V
RSM
30 GHz
Signal ~
Quadrature
errror signal
Lowpass
filter
Servo
Isolator
In-phase
modulator
l/2
l/4
Control of SRC via in-phase error signal
Control of VRSM via in-quardature error signal
Control and VRSM

15. First results...
...of a linear three mirror coupled cavity when varying the reflectivity of the VRSM from
R=93% over R=87% to R=63%
Theoretical frequency response
Experimental frequency response
1.0
1.0
0.9
0.9
0.8
0.8
0.7
0.7
0.6
0.6
T/Tmax
0.5
T/Tmax
0.5
0.4
0.4
0.3
0.3
0.2
0.2
0.1
0.1
0.0
0.0
370
375
380
385
390
395
400
405
410
370
375
380
385
390
395
400
405
410
Frequency [MHz]
Frequency [MHz]
Control and VRSM

16. Carrier/Subcarrier ~ VRSM cavity Phase modulator Power recycled
Michelson
interferometer
Carrier
laser
VRSM
cavity ~
In quadrature
Frequency
offset
In phase
Output
modecleaner
Sub-carrier
laser
Phase
modulator
Readout
Control and VRSM

17. Frequency Offsets Control and VRSM
VRSM reflectivities for some carrier-subcarrier offsets
Norm. Refl.
Effect on signal cavity
Norm. Trans.
Control and VRSM

18. Frequency Offsets II Control and VRSM Norm. SRC reflection
1.0
0.9
0.8
Norm. SRC reflection
0.7
Norm. VRSM reflection
0.6
0.5
0.4
0.3
0.2
0.1
Narrowband mode
Broadband mode
Norm. refl.
Norm. refl.
Control and VRSM

19. Summary VRSM and Control
The use of a VRSM eases the change of bandwidth,
avoiding long down-times of the detector
A new way to control the SRC including a VRSM by using
an auxiliary laser that is offset phase locked to the carrier laser
This control scheme is compatible with the injection of squeezed light
First experiments using a linear three mirror cavity are carrried out
A more complex experiment using a dual-recycled Michelson
interferometer with arm cavities is in preparation
Control and VRSM