LISA Gravitational Reference Sensors - icecube.wisc.edu · for GRS Gap Measurement Optical Fiber...

TeV Particle Astrophysics II, Madison, WI 8/20061

GRSLISA_GRS_060830.pptLISALISA

LISA Gravitational Reference Sensors

TeV Particle Astrophysics IIUniversity of Wisconsin, Madison, August 28-31, 2006

Ke-Xun SunStanford University

For the LISA GRS Research Community



LISA: A Spacecraft Constellation

Each spacecraft houses two Gravitational Reference Sensors (GRS)

LISA has 6 GRS



GRS

Gravitational Reference Sensor (GRS) in the LISA Spacecraft

GRS is one of the core scientificinstruments the heart of LISA



LTP, LISA and BBO Configurations and Performance

Space Fleet LTP/ST-7 LISA BBO

Configuration

IMS/DRS test

1

20 cm

20 mW

N.A.

10-14 ms-2Hz-½

Observational Goal Discrete Sources Gravitational Wave

Relics

# of S/Cs 3 12

Arm Length 5x106 km 5x104 km

Laser Power 2 W 300 W

Strain Sensitivity h~10-21 h~10-23

Acceleration Limit 10-15 ms-2Hz-½ 10-17 ms-2Hz-½



LISA Baseline Performance



LISA Mid and Low Frequency RangeSensitivity Determined by GRS Performance

GRS

}IMS



• GRS consists of:– A freely-floating proof mass within a

housing,– Position measurement of the test mass

w.r.t. housing– Control of test mass orientation– Charge control subsystem

• Disturbance reduction from:– Solar magnetic field– Solar radiation pressure– Residual gas pressure– Thermal radiation pressure– Charging by cosmic rays– Spacecraft self-gravity– Spacecraft magnetic fields– Spacecraft electric fields

LTP Gravitational Reference Sensor

LTP Graphics thanks to Stefan Vitale



LTP Engineering Model Testing



LTP GRS Testing

LTP Graphics thanks to Stefan Vitale



Stanford Gravity Probe-B and Previous Flight• GP-B was launched April 2004

and started science measurement in August

• GP-B has experimentally measured frame-dragging effects

• GP-B experiences are an important assets to LISA-BBO R&D

• GRS based on Stanford experience with

TRIAD (Stanford/APL, 1972, < 5x10-11 m/s2 RMS over 3 days)

GP-B (Stanford, launch 2002, < 2x10-12 m.s-2/√Hz at 5x10-3 Hz )

• Position sensing, charge control from GP-B

• GP-B: 17 DOF• GP-B is a critical development

for high precision space flight, including LISA

– Drag-free technology– Cryogenics– Precision fabrication– Caging– Charge management



ST-7 Development at Stanford

Au/Pt PM

Housing with compound material

Vacuum system

Electronics and system integration



LISA Noise Sources

Additional leading term: Voltage Reference Instability



Cross Talk is the Leading Noise SourceAnyway to Reduce?

Cross talk due to : 1) Proof mass shape2) Two proof masses



Each Spacecraft has Two Cubic Proof Masses

Proof Mass

ProofMass

To and from Remote Spacecraft

Waveplate

Transmissive optics

Sensitive path

Great elaborated structure, but …• Interlinked scheme for re-correlation• Long sensitive path• Coupling throughout the system• dn/dT problem in transmissive optics

• Alignment coupling



Modular GRS has Only One Proof MassReduced Cross Talk

Modular GRS: The New LISA Baseline Architecture

LISA 5 (2004)

Amaldi 6 (2005)



A Standalone GRS Architecture Presented at LISA 5th Symposium

OutgoingLaser Beam

Proof Mass

Large gap

GRS Housing

Optical ReadoutBeam

Telescope

Incoming Laser Beam

Details Shown in Figure 2

• Single proof mass• Modularized, stand-alone GRS• GW detection optics external to

GRS• External laser beam not directly

shining on test mass (Just learned that LISA will follow! -- Heinzel talk before ours in this session)

• Internal optical sensing for higher precision

• Large gap for better disturbance reduction

• True 3-dim drag-free architecture

• Determine the geometric center and center of mass

Sun, Allen, Buchman, DeBra, Byer, CQG (22) 2005 S287-S296



An All-Reflective Configuration for Modular GRS

(b) (c) (d) (e)

Housing Wall

Thin, Double-sided, Dual Gratings

Laser for External Interferometer

Optical Fiber

To and from Remote Spacecraft

Optical source for GRS Gap Measurement

Optical Fiber

Detector

Reference Relay Region

1~2 cm gap(sensitive optical path)

Spherical PM.Diameter 2~10 cm

(a)

Detector



Diffractive Optics Simplifiesfor LISA Baseline with Cubic Proof Mass

• Compact construction• Displacement + Angular sensing

The only sensitive optical path

SEM Image of Grating

Grating fabrication studies- Funded by JPL DRDF- The progress is ahead of schedule- e-beam lithography at Stanford

Nano-Science Facility- Grating pattern on dielectric

demonstrated- Grating on noble metal in progress



• Multi-mirror design

• Telescope main heavy primary mirror and tube fixed

• Steering direction by moving (rotating) lighter tertiary and quaternary mirrors

• Assign coarse and fine adjustments to different mirrors

• Add CCD imager for seeking acquisition

CCD imager

Webb Space Telescope Multi-Mirror Steering Scheme

From interferometer laserto remote S/C

From remote S/C



The Single Proof Mass GRS ArchitectureIt is now adopted by BBO

Presented at BBO Working Group Meeting, Collocated with LISA 5th

Symposium, ESTEC, Netherlands, July 2004

BBO new design as of December 2004, presented by JPL at Texas Symposium at Stanford



“Strap Down” (Modular GRS) Has Become the New LISA Interferometer Baseline



Comparison of Control ComplexityStand-alone GRS Simplifies Control

Mission & DOF Counts

GPB ST-7 LISA BBO(New)

Stand-alone GRS LISA/BBO

Displacement DOF

6

Angular DOF

Other DOF

Total DOF 19 13 10

Single S/C Decoupled DOF counts 3+7

Time to setup experiment

3 mo. Mission 3 mo.

12 mo. (?)

? Shorter than coupled scheme

120(New BBO)

3

1

Total Fleet DOF 17 18 57 156 30 (LISA)

15 9 9 6

2 9 9 6

1 1Single SC



An Alternative Also Considered by LISA

• LISA perceived a similar approach

• Single proof mass• Fiduciary mirror based beam

steering• Simpler than half/half

separation scheme

System and Technology Report, July 2000,A.4.4, P270, Laser Metrology Harness



Baseline Spacecraft Structure Before Modular GRS

Graphics thanks to Ulrich Johann



New Proposed Structure after Strap-Down (Modular GRS)

Graphics thanks to Ulrich Johann, EDS Astrium



Structure Using Single Proof Mass

• Cleaner structure

• Smaller volume

• Smaller rocket

Graphics thanks to Ulrich Johann



Developing Modular GRS

Thermal control

Center of Mass Measurement

Mass distributionMoment of Inertia measurement

Optical displacement sensing

Grating design and fabrication

External Interferometry

UV LED charge management system

Electromagnetic modeling

Surface studies

Grating angular sensor



Grating Cavity Experimental Setup

Graham Allen, Ke-Xun Sun, and Robert L. Byer, “A Grating Cavity as a Displacement Sensor in Drag-Free Satellites”, LISA 6th Symposium, 19-23 June 2006, Goddard Space Flight Center, Greenbelt, MD



Cavity Sweep Results

• 0.12 mV/nm slope– mV/◊Hz stability of DC amplifiers

• 33 dB RF SNR– 26 pm noise floor– Limited by resistor noise

• 400 mW delivered via fiber– 71 mW reflected signal– Higher power result of weaker than

expected side-band signal

• 55 nm linear range• Greater than 60% back-coupling

into fiber• Higher order mode clearly visible

– New grating may fix



Recent Grating Angular Sensing Experiment with Two Detectors

Custom Grating 935 lines/mm

NPRO

Quad Det.

Scope

S.A.

Nθ-1 ~ -840

6 cm

Quad Det.

• Simple construction• No extra optics• No other uncertainty

and noise

Ke-Xun Sun, Patrick Lu, and Robert L. Byer, “A robust, symmetric grating angular sensor”, LISA 6th Symposium, 19-23 June 2006, Goddard Space Flight Center, Greenbelt, MD



Enhancement of Angular Sensitivity

0 10 20 30 40 50 60 70 80 90100

101

102

103

Diffraction Angle of +1 Order [degree]

Enh

ance

men

t of

An

gula

r Sen

sitiv

ity

• Grating angle amplification

2cos( )'cos( )

inm in

m

P K Pθθ

⎛ ⎞= ⎜ ⎟

⎝ ⎠

• Beam cross section compression

K. Sun, S. Buchman, and R. L. Byer, “Grating Angle Magnification Enhanced Angular and Integrated Sensors for LISA Applications,” accepted for publication at J. Phys. C. Special issue of Almadi 6 Conference on Gravitational Waves



Signal Spectrum for 4 mW Incident Power Signal Noise floor lower than 1 nrad/Hz1/2

• PZT displacement 10 nm• Grating rotation 0.5 μrad• 4 mW input power• Noise floor level ~1 nrad/ /Hz1

• Potential applications:– Telescope orientation – Fiber collimator orientation– Coarse Frequency stabilizatio



Grating Fabrication for Grating Angular Sensor

• Transfer-imprinting of gold gratings– This is performed by pressing a

dielectric grating into a gold surface with force sufficient to exceed the yield stress of gold

– Centimeter-sized dielectric gratings were fabricated with e-beam lithography on quartz wafers.

– Gold gratings had 933 lines/mm and 300nm of depth.

– Various duty cycles have been demonstrated.

– 275nm depth 50% duty cycle gold gratings have been measured to have 26% diffraction efficiency in the +/-1 orders and 36% efficiency in the 0th

order.

Dielectric grating, 50% duty cycle (AFM image)

Imprinted gold grating, 50% duty cycle (SEM)

Dielectric grating 25% duty cycle (SEM)

Patrick Lu, Ke-Xun Sun, and Robert L. Byer, “Methods of Fabricating Grating Patterns on Dielectric and Metal Surfaces”, LISA 6th Symposium, 19-23 June 2006, Goddard Space Flight Center, Greenbelt, MD



Grating Duty Cycle Variation

Chrome etch mask 25% duty cycle.

Chrome etch mask 75% duty cycle.



Pendulum Angular Measurement

● Measure inertia to below 1 part in 104

− Includes mass property uncertainties

Grating Angular Optical Sensing*:● Angular magnification● -1 ≈ 81° , 0 ≈ 16.8°● Independent of vertical motion

Ke-Xun Sun, Saps Buchman, and Robert Byer. Grating Angle Magnification Enhanced Angular and Integrated Sensors for LISA Applications. Journal of Physics: Conference Series, 32:167-179, 2006.

Quad Photodetector:● High sensitivity● Accurate zero crossing

Position Sensing Detector:● Large dynamic range● 1V ≈ 0.25° pendulum rotation



Gravitational Self-AttractionQ = 1500f = 2.715 Hz

FFTresolution < 1 mHz



Displacement and Angular MeasurementsMeet LISA Sensitivity Requirements

Configuration Measurement Performance

Fiber Michelson Interferometer

Displacement 10 nm/Hz1/2

1 nm/Hz1/2

Grating CavityDirect Detection

Displacement(3 mW)

30 pm/Hz1/2

Grating CavityRF Detection

Displacement(0.1 mW)

10 pm/Hz1/2

Direct Reflection Angular <10 μrad/Hz1/2

Grating Two Side Detection

Angular(4 mW)

1 nrad/Hz1/2

Grating Single Side Detection

Angular(8 mW)

10 nrad/Hz1/2

Free Space Michelson

Displacement



Center of Mass Measurement

CM

GC*

R

r

�

�

x

�



Center of Measurement

● Initial measurement accuracy: 2-3 m

● Expect <1 m accuracy in future



Deep UV LED Key CharacteristicsLED will be the UV Source Flown in LISA

UV LED Charge Management System– Weight: 0.3 kg (including electronics)– Electrical power ~1.5 W (0.1 W/LED. Est. power for control electronics ~1.2 W)– Fast switching (<0.1 μS), wide selection of modulation frequency and duty ratio– AC charge management at frequencies out-of signal band (e.g. at cap bridge freq.)– Various AC charge management techniques for better performance• Peak wavelength: 257.2 nm, comparable to Hg line 254 nm• FWHM: 12.5 nm, good photoemission for Au coatings• Total UV power: 0.144 mW, sufficient for charge management



Positive and Negative AC Charge TransferUV LED and bias voltage modulated at 1 kHz



UV LED Power and Spectral Stability Tests For AC Charge Management

UV LED Operation > 2700 Hours UV LED Spectrum Stable



Developing the Modular GRS



Summary

• LISA GRS technology made much progresses– LTP ground testing performance met requirements– LTP launch is still on schedule

• Novel LISA GRS architecture– Modular GRS (“Strap Down”) is adopted as the new

LISA IMS/GRS baseline– Single proof mass configuration under serious

considerations– Much progress in modular GRS development

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