A Homodyne Optical Readout for Laser Interferometric Gravitational

A Homodyne Optical Readout for Laser Interferometric Gravitational

Title
A Homodyne Optical Readout for Laser
Interferometric Gravitational Wave Detectors
Tobin Fricke
PhD defense
October 14, 2011
1
LIGO-G1101153
Organization
⚫
⚫
⚫
⚫
⚫
⚫
Gravitational waves
LIGO & GW detectors
RF readout
DC readout
The output mode cleaner
Results
2
Organization II
3
Gravitational waves
⚫
⚫
⚫
⚫
⚫
predicted by general relativity
generated by accelerating mass
propagate at the speed of light
not yet detected directly
appear as a strain of spacetime
4
Gravitational wave sources
modeled
long-term
transient
unmodeled
"pulsars"
stochastic
background
binary
inspirals
supernova &
other bursts
5
Gravitational wave detectors
★ not to scale!
6
Network of detectors
Hanford, WA
LIGO
TAMA300
Mitaka, Japan
+ LCGT
Kamioka, Japan
LIGO
Livingston, LA
Ruthe, Germany
GEO600
VIRGO
Cascina, Italy
?
?
7
An interferometer of interferometers
It's interferometers
all the way down!
★ not to scale!
Terrapene carolina
8
Sensitivity (noise floor)
(Previous)
9
Enhanced LIGO
Construction begins 1994
Nov2005
Sep 2007
Initial
LIGO
July2009
Upgrade &
Commissioning
Oct 2010
Enhanced
LIGO
Advanced
LIGO
• Try out Advanced LIGO technologies
• Bet that increased sensitivity outweighs the downtime
exposure = time * (range)^3
Increase the laser power
Output mode cleaner
DC readout
New
New
New
New
Laser
input optics
Thermal Compensation
Alignment Control
10
Michelson
11
Fabry-Perot Michelson
12
Power-Recycled Fabry-Perot Michelson
13
Interferometer
A suitably polarized GW
produces differential
phase modulation in the
two arms, which interferes
constructively at the beam
splitter and exits at the
output port.
How to detect it?
?
14
Detection: frequency domain picture
HETERODYNE (RF)
optical frequency
-rf
-gw
+gw
+rf
-rf
-gw
+gw
+rf
HOMODYNE (DC)
optical frequency
laser carrier
15
Shot noise
laser
★ semiclassical picture
power
HETERODYNE (RF)
160
140
120
100
80
60
40
20
0
0
1
2
3
4
5
6
7
9
10
time (arbitrary)
HOMODYNE (DC)
power
8
90
80
70
realization (blue)
60
mean (white)
50
40
distribution (grays)
30
20
10
0
1
2
3
4
5
6
7
8
9
10
time (arbitrary)
16
power
Heterodyne shot noise
160
140
120
100
80
60
40
20
power - <power>
0
0
1
2
3
4
5
6
7
8
9
10
50
demodulation
waveforms
0
−50
0
1
2
3
4
5
6
7
8
9
The in-phase demoulation selectively samples the noisiest parts of the time series!
10
17
DC Readout vs balanced homodyne
Balanced homodyne
DC Readout
Both arms
held on
resonance
Arms
differentially
detuned
laser source
laser source
phase
modulator
Signal beam
+ Local oscillator
(coincident)
Signal beam
Local oscillator
beam
+
+
-
+
18
power at AS port
DC Readout: fringe view
0.5
PAS / PIN
0.4
0.3
0.2
0.1
0
−1000
−500
0
500
DARM offset [picometers]
1000
10 picometers
DARM length
19
The Coupled Cavity
Michelson
interferometer
with
power recycling
and
Fabry-Perot arms
power-recycling
cavity
arm cavity
(carrier and RF SB)
(carrier only)
3
1
2
transmission into
powe r recycl ing cavi ty [dB]
three mirror cavity
0
−20
−40
−60
−80
−1
10
0
10
1
10
2
frequency [Hz]
10
3
10
4
10
20
DC Readout
Looks pretty simple...
DARM_ERR
21
DC Readout promises
• fundamental improvement in SNR
• technical improvement in SNR
- perfect overlap of local oscillator and signal beams
- junk light removal by OMC
• improved laser and oscillator noise couplings
- exploit the amazing filtering ability of the interferometer
• Easier platform for squeezed light injection
• Easier to handle higher power
22
Junk Light
Hermite-Gauss modes
★ wikipedia
Laguerre-Gauss modes
★ wikipedia
23
DC Readout with OMC
Clean up the light at
the AS port with an
output mode cleaner.
DARM_ERR
24
OMC design
beam
steering
mirrors
beam from
interferometer
bowtie cavity
fast and
slow
length
actuators
two
DC
photodiodes
two quadrant
photodiodes (QPDs)
monolithic, suspended, in-vacuum
Sam Waldman et al
25
Dither Locking
no first-order response
at maximum
180 degrees
out of phase
in phase
1. put in small dither sinusoid
2. demodulate output at same freq
== error signal!
26
OMC Length Control
Cavity length dithered at ~10 kHz via PZT actuator
PZT offloaded onto slow, long-range thermal actuator
DARM
27
OMC Alignment Control
The mode cleaner will clean the modes if you can
identify what mode you want to keep.
Initial idea: maximize transmission through the OMC
28
Junk light confuses simple servo
00 mode
00 + 01
01 mode
transmission
versus beam
pointing
01 mode leads
the servo astray
29
Drumhead Beacon Dither
Idea: Tag the photons in the arm by modulating the ETM
Excite the test-mass
drumhead mode (9 kHz)
Dither the "tip tilt" mirrors
at low frequency (~3 Hz)
detect power in
drumhead mode
demodulate at
dither frequency
M.Evans/Nicolás(LHO)
30
The eLIGO interferometer
Michelson
interferometer
with
power recycling
and
Fabry-Perot arms
reflected
port
phase
modulator
laser source
radiofrequency
oscillator
input
mode
cleaner
input
faraday
isolator
output
faraday
isolator
open
port
3%
heterodyne
97%
output mode cleaner
homodyne
31
Commissioning
MAY 2008
200
R
MBE
E
C
DE
9
32
Noise Couplings
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Oscillator amplitude
Oscillator phase
Laser intensity
Laser frequency
amplitude
relative to input field
carrier
upper RF sideband
lower RF sideband
audio frequency
sidebands
frequency (relative to carrier)
Ref: J. Camp, et al., J. Opt. Soc. Am. A/ Vol. 17, No. 1/January 2000
33
Oscillator Amplitude noise
−10
10
−11
10
meters per SB RIN
RF model
−12
10
RF measurement
−13
10
−14
10
DC measurements
DC models
−15
10
−16
10
100 Hz
1000 Hz
34
Oscillator Phase noise
−9
10
RF measurement
−10
10
−11
meters per radian
10
DC measurements
−12
10
−13
10
−14
10
−15
10
−16
10
100 Hz
1000 Hz
35
Anatomy of intensity noise coupling
laser intensity noise coupling to DC readout
0
carrier
Michelson
transmission
−20
Watts per Watt [dB]
carrier + sidebands
carrier only
sidebands only
−40
carrier AM
recycling
gain
−60
cancellation
−80
sideband AM
−100
−120
−1
10
0
10
1
10
2
frequency [Hz]
10
3
10
10
36
Anatomy of intensity noise coupling II
laser intensity noise coupling to DC readout
0
carrier + sidebands
carrier only
sidebands only
Watts per Watt [dB]
−20
−40
−60
−80
−100
−120
−1
10
0
10
1
10
2
frequency [Hz]
10
3
10
10
37
Laser intensity noise
38
Laser frequency noise
−12
10
RF model
+20pm
−13
10
-15,-20
-5pm
meters per HZ
−14
10
−15
10
+10pm
+5pm
−16
DC models
10
−17
10
−18
10
100 Hz
1000 Hz
39
Shot noise
40
Shot noise II
−16
10
−17
meters per Hz1/2
10
−18
10
−19
10
−20
10
100 Hz
1000 Hz
41
Summary
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Installed OMC and set up DC readout
Commissioned control systems for OMC and DC readout
Measured and modeled noise couplings
Modeled and verified shot-noise performance
paper: http://arxiv.org/abs/1110.2815
strain
spectral
density
1e-21
Detector noise floor
(lower is better)
O
1/rtHz
Ini
t
1e-22
En
LIG
l
a
i
h
O
IG
L
d
ce
an
1e-23
va
Ad
O
LIG
d
e
nc
frequency
10 Hz
100 Hz
1 kHz
Enhanced LIGO:
25% increase in range,
factor of 2 in volume.
Lots of experience with
Adv LIGO technologies.
walrus without tusks
42
Thanks for listening!
Thanks
for
listening!
Special thanks to
Gaby González, Valera Frolov,
Rana Adhikari, Adrian Melissinos, and
everyone who worked on Enhanced LIGO.
43
DC Readout: phasor view
optical gain:
How do we choose the DARM offset?
• Must be much greater than residual DARM displacement
• Must overcome contrast defect and electronics noise
• But not excessively detrimental to power recycling
In practice: turn the knob to get the best sensitivity
44