LISA Interferometry
TeV II MeetingMadison, Wi
August 30th, 2006
Guido Mueller University of Florida
08/30/2006
Chandra: NGC6240
1. Super-massive Black Hole mergers
2. Extreme mass ratio Inpirals (EMRIs)
3. Galactic Binaries
LISA
Credit: Tod Strohmayer (GSFC)
Gravitational Wave
Sources
4. …
08/30/2006
LISA: Joint NASA/ESA project
Advanced LIGO
LIGO: NSF project
LISA vs. LIGO
EMRIs
08/30/2006
LISA Concept
LISA Concept:
3 Spacecraft in triangular formation 5 Gm distance betw. S/C Heliocentric Orbit Measure changes in distance with 10pm/rtHz accuracy!
Movie
08/30/2006
LISA
Technical Challenges:
1. How to build a gravitational reference sensor?
Need a non-accelerated proof mass
acceleration < 3x10-15 m/s-2 / rHz
2. How to do pm-Interferometry over 5 Gm?
Interferometry Measurement System (IMS)
08/30/2006
LISA Interferometry
Goal:
Measure distances with10 pm/rtHz accuracy
Basics:
Laser:• Wavelength: 1m• Power: 1 W
Telescopes:• f/1 - Cassegrain• Diameter: 40cm
Received power: ~100pW
08/30/2006
Arm lengths changeby about 50.000 kmduring 12 mts orbitor by ~ 1m/s.
The Main Problem
The Orbit Problem:
08/30/2006
Arm lengths changeby about 50.000 kmduring 12 mts orbitor by ~ m/s. Doppler shifts (~ MHz)
The Orbit Problem
08/30/2006
Arm lengths changeby about 50.000 kmduring 12 mts orbitor by ~ 1 m/s. Doppler shifts (~ MHz) Unequal arm lengths (frequency noise)
The Orbit Problem
08/30/2006
Arm lengths changeby about 50.000 kmduring 12 mts orbitor ~1 m/s. Doppler shifts (~ MHz) Unequal arm lengths (frequency noise) Telescope repointing (pointing noise)
The Orbit Problem
Very dynamic interferometer!
08/30/2006
High Gain Antennas
uN-Thrusters
LISA Concept
08/30/2006
Optical Benches
Proof Mass Housing
Telescopes
LISA Concept
08/30/2006
Main Components/Tasks:
1. Phasemeter
2. Laser Frequency Noise
3. Mechanical Noise (Solution: Engineering)
Interferometry
08/30/2006
Requirements:
• 2-20 MHz signal frequencies, changing by several MHz
• Frequency noise of 30Hz/Hz1/2 @ 1mHz
= 30000 cycl./Hz1/2 @ 1mHz
• need to be resolved with 10-5 cycles/Hz1/2 accuracy!
Dynamic Range of 9 orders of magnitude.
Phasemeter
08/30/2006
sin 2 mft t
sin 2 i iA t ft t
cos 2 mft t
NCO
m
H f
Feedback
I
Q
oA t
r t
o t
Input Tracks the Phase of RF signal with NCO
I/Q demodulation with tracking NCO
The JPL Phasemeter
08/30/2006
dynamic range ~109 @ 5 mHz
Requirement
Digitally tested dynamic range requirement.
– Digitally generated 3 independent, laser-like noise sources such that,
Phase 0 + Phase 1 - Phase 2 = 0
Equivalent Optical Setup
x107 zoom
(Results from Daniel Shaddock, Brent Ware, Bob Spero, JPL)
The JPL Phasemeter
08/30/2006
Requirements:
• Frequency noise of 30Hz/Hz1/2 @ 1mHz (for Phase meter)
• Free running laser: ~ 1MHz/Hz1/2 @ 1mHz
• Everything below 30Hz/Hz1/2 reduces requirements
on Phase meter
Solution:
• Frequency stabilization
• Time Delay Interferometry
Laser Frequency Noise
08/30/2006
Ground testing:
Two lasers independently stabilized to two reference cavities:
– References: 2 Zerodur spacers with optically contacted mirrors in ultra-stable vacuum chamber
– Pound Drever Hall stabilization scheme (Modulation/Demodulation)
1st Step: Stabilize to ultra-stable reference cavity:
• Baseline: ULE or Zerodur spacer ring cavity
Frequency Stabilization
08/30/2006
Similar Resultswith ULE spacers:• AEI Hanover• GSFC
Can we do better?
UF-results
Frequency Stabilization
RachelCruz
08/30/2006
Basic Idea: Lock laser frequency to LISA arm
Far S/C:Transponder(phase locked laser)
S(t) = (t-2)-(t) = 0!
Transfer function is zero at Fourier frequencies fN = N/2
Requires tailored feedback gain (~1/sqrt(f)) at and above f1 up to UGF High bandwidth, only limited gain
Laser frequency noise suppressed at all frequencies except at fN = N/2
Arm Locking
08/30/2006
Single Common Sagnac
Round-trip arm length
1 30mHzf
Difference between arms
1 3Hzf
Sagnac effect (rotation)
1 20kHzf
Different potential realizations:
Arm Locking
08/30/2006
Sagnac
Sagnac effect (rotation)
1 20kHzf
Sagnac:
• Allows high-gain, low-bandwidth feedback loop• Very simple design
Main disadvantage:• No redundancy: If one link malfunctions, the Sagnac signal is gone
Common arm locking is the baseline
Arm Locking
08/30/2006
Stabilized “Master”
Stabilized “Reference”
Phase-locked “Slave” Interferometer
& arm-locking
Arm Locking
08/30/2006
1 020
1 1LO
sAL
p p pS
G e
1
21 1
LOs
AL
p p
G e
Compare to LISA:
se
+ - 2
+ -
+ - + -
+ -
+ -
p1
p2 pLO
p0
GPL
GAL
LO
S20
S21
Arm Locking
08/30/2006
EPD using 25 MHz digitization rate, delay of 1.065ms or f1 = 939Hz
Arm Locking
Latest Arm Locking experiment at UF• currently limited by missing real time phasemeter
08/30/2006
Out-of-loop
Primary beat note demodulated to 10kHz
Phase of 10kHz signal measured using software phase meter.
Arm Locking
Latest Arm Locking experiment at UF• currently limited by missing real time phasemeter
08/30/2006
Out-of-loop
Primary beat note demodulated to 10kHz
Phase of 10kHz signal measured using software phase meter.
Arm Locking
Ira Thorpe
08/30/2006
Sb(t) - Sg(t) - Sb(t- 2τg) + Sg(t- 2τb)
First Generation X-combination:
Synthetic equal arm Interferometer!
Laser frequency stabilization
Time Delay Interferometry (TDI)
Requires to know the light travel times betw. S/C Ranging with 30m accuracy
Time Delay Interferometry
08/30/2006
(Nearly) full scale LISA signal
Limited by Transponder Noise
TDI Experiment
08/30/2006
5 orders suppression
Pha
se N
oise
[cy
cles
/rt(
Hz)
]
Frequency [Hz]
• Results currently limited by PLL performance
RachelCruz
TDI Experiment
08/30/2006
LISA Interferometry:
Requirements:
10 pm/rtHz in a dynamic 5 Gm interferometer
Key Technologies:
Phase meter
Laser frequency stabilization
– Reference Cavity
– Arm locking
Time Delay Interferometry
Summary
08/30/2006
ESA/EU:
ESA/Estec
Astrium, Germany
AEI Hanover
University Trento
University of Birmingham
University of Glasgow
…
NASA/US:
GSFC
JPL
University of Florida
JILA
Stanford
University of Washington
…
The End
Lets g
et din
ner!
Summary
+ many data analysis and theory groups
08/30/2006
LISA:
Remaining Challenges:
– How to move the telescope w/o distorting the measurements?
– Do we need to measure these distortions and correct for them?
– How to align the spacecraft to acquire lock?
– Stable materials and components:
• Laser switch, Fiber launcher, Vacuum system, Discharging, PAA actuator, …
Data Analysis challenges
– Galactic binaries create a GW “noise” floor
Does this sound different from other missions?
Summary
08/30/2006
LISA:
GRS:
– Will be flight tested in LTP around 2009/10
– LTP ground tests look very promising so far
Interferometry:
– Basic concepts of TDI, Arm-locking, clock noise removal are well understood
– Experimental tests at component level are progressing very well
– EPD unit enables detailed ground testing of TDI/AL
(Test as you fly, fly as you test)
Summary
08/30/2006
LISA:
Was considered a very challenging mission
No ground testing possible
No technology heritage for any of the major technologies:
– GRS
– Interferometry
– Data Analysis
Summary
08/30/2006
Out-of-loop measurement of primary beat note using frequency counter.
400x
Ira Thorpe
Arm Locking
08/30/2006
Prompt
Delayed
Prompt-Delayed First experimentalverification of TDI!
Rachel Cruz, Michael Hartman, UF
TDI Experiment
08/30/2006
UF technique:
Laser Phase replaced by beat note phase
Beat note phase delayed electronically (EPD).
LISA photodiodes replaced by electronic mixers.
LISA UF Simulator
Electronic Phase Delay
08/30/2006
System Date Hardware Max. Signal
Freq. # Chan. Max. Delay
Original Summer
2004 200 kHz PCI
card 30 kHz 2 80s
Current Summer
2005 Pentek 5 MHz 4 6s
Future Fall 2006
Pentek w/ PMs & NCOs
20 MHz 4 35s*
*Depends on resolution & BW
Electronic Phase Delay
08/30/2006
Short LISA HistoryShort LISA History
Foundation paper in 1984 by Bender, Faller, Hall, Hils and Vincent
Concept developed through
– Concept studies ‘84-’93
– ESA Pre-Phase studies ‘93-’98 (cf., PPA2 document)
– NASA Team-X study ‘98
– ESA Industrial Phase A Study ‘98-’00 (cf., FTR and STS documents)
– GSFC Project Office formed in ‘01, technology planning and development commenced.
– Flight demonstrations (LISA Pathfinder and ST-7) initiated in ‘00-’01
– NASA Formulation Phase began Oct. ‘04
– ESA Industrial Formulation Study begun at Astrium/Friedrichshafen Jan. ‘05, finished Phase I in Oct. ‘05
Concept has not significantly changed since PPA2 in 1998.
Current focus
– Architecture definition and refinement, design trade studies
– Technology development
– LISA Pathfinder and ST-7
Slide stolen from Robin ‘Tuck’ Stebbins LISA Symposium Talk
We entered Phase A late 2004!
08/30/2006
Mission StatusMission Status
• ST7 brings the least well-tested LISA instrumentation, DRS, to TRL level 9
• Preparations for 2010 launch will already greatly enhance
-Experience in building flight models
-Experience in tightly-coupled NASA/ESA cooperation
• Results from 2010 launch will be in time to inform formulation
1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4 1 2 3 4
ST-7
LISA
FY07 FY08 FY09 FY10 FY11
launchHW delivery
Phase C/D
Phase A (survival)Phase A Phase B
Phase E
Slide stolen from Colleen Hartman, LISA Symposium
08/30/2006
• Budget requirements have necessitated Beyond Einstein be sequential missions rather than parallel efforts
• Funding wedge for first BE mission start in 2009
• One of 3 will go first: LISA, Con-X, JDEM
• Special BE NRC panel in 2008-9
Mission StatusMission Status
From Colleen Hartman, LISA Symposium
Instead of two parallel lines of sequential missions
We hear you …
JDEM: Additional competition!
08/30/2006
Optical Bench
PMto/from far SC
Fiber to/from Second Bench
from LaserBench
Phase Meter 2Phase Meter 1 Phase Meter 3
08/30/2006
Optical Bench
from LaserBench
Phase Meter 1Bench A:
PM1A: 1(t) - 2(t) + fibernoise
Bench B:
PM1B: 1(t) - 2(t) - fibernoise
PM1A + PM1B = 2 [1(t) - 2(t)]
• Independent of fiber noise• Used to phase lock local lasers• Allows to compare both
Interferometer arms
Like having a beam splitter in a Michelson Interferometer
Fiber to/from Second Bench
Only works if OPL in fiber is independent of propagation direction!
08/30/2006
Optical Bench
Laser
BS
/4/2
Pol
Pol
/4
Fiber
Polarization Sagnac Interferometerfor Optical Fiber Tests at UF
Parallel tests in Glasgow, Hanover
08/30/2006
Optical Bench
PM 2 – PM 1 : Distance PM - SC
PMto/from far SC
Fiber to/from Second Bench
from LaserBench
Phase Meter 2Phase Meter 1 Phase Meter 3
08/30/2006
Optical Bench
PMto/from far SC
Fiber to/from Second Bench
from LaserBench
Phase Meter 2Phase Meter 1 Phase Meter 3
PM 3: Distance SC – SC How?
08/30/2006
Optical Bench
PMto/from far SC
from LaserBench
Phase Meter 3
Phase Meter 3 on S/C 2 and 3:• Used to Phase lock local laser
To Laser frequency actuator PLL
PM 3A – PM 1A
08/30/2006
LISA
Master S/C Slaved S/C
Slaved S/C
08/30/2006
Optical Bench
PM
to/from far SCfrom LaserBench
Phase Meter 3
PM 1A – PM 3A
= 1(t)-1(t-21)+GW1
(~Unequal Arm MI)
• Dominated by Laser frequency noise f :1000 cycl./rtHz noise
Phase Meter 3 on Master S/C 1:
08/30/2006
Primary HardwarePrimary Hardware
Key Features
- 4 Channels
- 14-bit ADC
- 16-bit DAC
- 1 GB SDRAM
- 100 MHz sampling
- 5 FPGAs
- PowerPC processor
- Ethernet, serial, VME
08/30/2006
Limited to 33Ms/s
Primary HardwarePrimary Hardware
Key Features
- 4 Channels
- 14-bit ADC
- 16-bit DAC
- 1 GB SDRAM
- 100 MHz sampling
- 5 FPGAs
- PowerPC processor
- Ethernet, serial, VME
08/30/2006
NS/NS merger (MNS ~ 3x1030kg ~ 1.4 MSun)
1. Smallest Distance: dmin ~ 20km (2xDiameter of NS)
We receive about P=1..100mW/m2 from each binary!Like full moon during a clear night!
2. Potential Energy: E = - GM2/d ~ 3x1046J
3. Newton: f (d=100km) ~ 100 Hz, f (d=20km) ~ 1 kHz
4. Takes about 1s to get from 100km to 20km
5. During that second nearly half of the Potential Energy is radiated away!
6. Assume binary is in the Virgo cluster (15 Mpc ~ 6x1024 m)
Gravitational WavesGravitational Waves
08/30/2006
Gravitational WavesGravitational Waves
We can see the moon, why haven’t we seen Gravitational Waves yet?
GW-Amplitude: h=L/L is
G/c4 = 10-45s2/kg m
08/30/2006
Gravitational WavesGravitational Waves
We can see the moon, why haven’t we seen Gravitational Waves yet?
GW-Amplitude: h=L/L is
G/c4 = 10-45s2/kg m
Our example (f=400Hz):
Or 1am over 1km
Answer:
Space is stiff
08/30/2006
LISALISA
LISA will probe space and time
at the forming edges of black holes listening to the sounds of vibrating spacetime:
– the booming roar of supermassive black holes merging
– the chorus of death cries from stars on close orbits around black holes
– and the ripping noise of zipping singularities
Copied from: Beyond Einstein: from the big bang to black holes
Even the NASA-folks were a little excited about LISA
Unfortunately, LISA will be unmanned and not on Mars …
08/30/2006
Ground-based Simulator:
1. First Generation of Experiments
• Frequency-stabilized lasers
• Arm-locking
• Time Delay Interferometry (TDI)
2. Future Experiments
• Doppler shifts
• Clock noise, laser com.
• GW-signals
Long Term Goal: Provide realistic data streams with injected GW signals
UF BenchtopUF Benchtop
08/30/2006
The MissionThe Mission
Current Design of single tube:
08/30/2006
GRS-ChallengesGRS-Challenges
A few (obvious) forces pushing the PM:
Lorentz Force:
Charged PM moving in variable solar magnetic field
– Charge Control (UV-light, continuous or every ~10-20h?)
Magnetic Force:
Magnetic Susceptibility couples to magnetic fields
– Gold Platinum Alloy: m
~ 0 (Problem: Grains in PM have variable m)
Self-Gravity from S/C:
1kg mass 10cm from PM gives a gradient of 10-7m/s2/m
– S/C motion < 10nm/rHz (Design of S/C, N-Thrusters)
08/30/2006
GRSGRS
A few (not so obvious) forces pushing the PM:
Patch Fields:
Crystal Boundaries create voltage potentials
Gas pressure noise:
Gas hitting the PM from both sides
– ma ~ PT requires T < 10-4K/rHz and P < 10-8torr
Thermal photon pressure:
Black Body Radiation from walls
– ma ~ T requires T < 10-4K/rHz
…
08/30/2006
Define the timing error as: || shiftEPD
The delay time of the EPD, just as the optical delay time of the LISA arm, will not fall exactly at one of the sampling points of the data stream.
sampt2
1max
Timing ErrorTiming Error
08/30/2006
Suppression LimitSuppression Limit
)()()( tttS pp |)(~
|)sin(2|),(~
| min fffS p
•The timing error in the experiment < Δτmax = ½ tsamp = 6.25 μsec
•Interpolation can be used to reduce the timing error
•Experimental results appear to hit another noise source at ~5x10-5 cycles/rt(Hz)
Pha
se N
oise
[cy
cles
/rt(
Hz)
]
Frequency [Hz]
Smin(f,Δτmax)
Exp. Time-delayed Comb.
08/30/2006
Two-Arm ExperimentTwo-Arm Experiment
08/30/2006
Data Analysis ChallengeData Analysis Challenge
The signals
– 3 Hz sampling of 18 beat signals
– Time Delay Interferometry (TDI) algorithms to remove laser and clock frequency noise
– Auxiliary ‘sciencekeeping’ data (solve for PM motion)
More than 10,000 interfering GW signals.
Signals have to be
– Identified, separated, tracked, and subtracted from data stream
Source direction can be determined
– Frequency and amplitude modulation from orbital Doppler shifts
– Phase modulation from time-of-flight across antenna
08/30/2006
Master laser
Reference laser
LISA Simulator with 1 Laser on each S/C.
LISA Benchtop
08/30/2006
S12(t) = 20(t-21)-10(t)
S21(t)S23(t)
S13(t)
S32(t)
S31(t)
S12(t)
Master laser
Reference laser
LISA Benchtop
08/30/2006
S12(t) = 20(t-21)-10(t)
S21(t)=0S23(t)
S13(t)
S32(t)
S31(t)=0
S12(t)
Master laser
Reference laser
LISA Benchtop
08/30/2006
Note: All optical path are common mode Insensitive to Optical path length changes!
LISA Benchtop
08/30/2006
LISA Benchtop
08/30/2006
PD
PD
Current Setup: • Cancels all optical path length changes.• No GW-signals or Doppler shifts
PD
PD
PZTFuture Setup:• Split Optical Path • Doppler shift can be added in the EPD unit• GW-signal can be added via PZT• Sensitive to acoustic noise
• Will be moved in Vacuum
LISA Benchtop
08/30/2006
S21(t)=0S23(t)
S13(t)
S32(t)
S31(t)=0
S12(t)
Master laser
Reference laser
• Common Arm-locking• Sagnac Arm-locking• …
LISA Benchtop