LISA Interferometry

TeV II MeetingMadison, Wi

August 30th, 2006

mueller@phys.ufl.edu

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