LIGO-G040767-00-D

LIGO- I:LIGO- I:Breaking the Sensitivity BarrierBreaking the Sensitivity Barrier

March 2nd, 2004Rana Adhikari MIT

LIGO-G040767-00-D

OUTLINEOUTLINE

What is the noise in the interferometers now? How its going to get better... Where does the SRD Curve come from? Can we ever reach the design sensitivity?

!#@?#$&%!! What do we do about it?

LIGO-G040767-00-D

The Noise in LivingstonThe Noise in Livingston

LIGO-G040767-00-D

The Noise in HanfordThe Noise in Hanford

by S. Ballmer

LIGO-G040767-00-D

Near Term Noise Reduction:Near Term Noise Reduction:Phase Noise IPhase Noise I

Bad Recycling Cavity --> TEM00 SB doesn't get through.The SB that gets through is junk. Makes shot noise, no signal.Low laser power into the interferometer; laser's are weak, transmissions of mode cleaners not high.

LIGO-G040767-00-D

Near Term Noise Reduction:Near Term Noise Reduction:Phase Noise IIPhase Noise II

Thermal CompensationApply heat to make T_SB > 75%Tweak up lasers for > 8W into the MCREFL signal goes down with heat: Implement NRSB scheme to increase phase sensitivity at REFL port. Reduces frequency noise.POB signal goes up like (T_SB)2: 20x reduction of POB noise.

LIGO-G040767-00-D

Near Term Noise Reduction:Near Term Noise Reduction:Oscillator Phase NoiseOscillator Phase Noise

Oscillator noise is OK: 4 x 10-7 rad/rHzCoupling is ~100X higher than design.LO & RF SB follow different paths.Custom low noise OCXO: 1 x 10-8 rad/rHzLO filter to symmetrize the RF paths.

LIGO-G040767-00-D

The LIGO-I Sensitivity Goal:The LIGO-I Sensitivity Goal:SRD CurveSRD Curve

Seismic: “Standard” spectrum x(f) � f-2

filtered by an ideal stack and a 1 Hz pendulum.

SUS Thermal: Viscous damped harmonic oscillator. h(f) � f-2

Shot Noise: Mediocre optics -> Recycling Gain = 30 Contrast = 3 x 10-3

LIGO-G040767-00-D

Measurements:Measurements:Seismic NoiseSeismic Noise

Ground noise is higher than expected.

0.1 < f < 4 Hz excess @ LLO: Hydraulic Pre-Isolator

f > 5 Hz narrow features from HVAC at both sites. Being investigated at LHO.

Motion at top of the piers is 10-100x morethan ground motion at some frequenciesfrom 10-100 Hz!

Studies at LASTI say that its mostly tilt.

LIGO-G040767-00-D

Measurements:Measurements:Seismic NoiseSeismic Noise

Take accelerometer measurements of pier top.

Propagate through the Hytec BSC Stack Model and the suspension model to estimate the bottom line.

LIGO-G040767-00-D

Measure lots of test mass Q's (i.e. get someone else to do it)

Use highest Q to set lower limit on internal loss of the fused silica substrate.

Use Dennis' FEA voodoo to get the loss angle of the coating.

Use formula from A. Gretarsson's thesis to get effective test mass displacement (Yuri Levin's Method).

Method was verified by K. Numata's experiment: PRL (2004), blah,blah

Results: �

Substrate ~ 1 x 10-7 �

Coating ~ 2 x 10-4

Qualitatively and quantitatively different from summing up the normal modes. Some of the normal modes are limited by gunk which doesn't necessarily produce strain noise.

Measurements:Measurements:Test Mass Thermal NoiseTest Mass Thermal Noise

LIGO-G040767-00-D

Used to be viscous damping x(f) ~ f-2. We know now that its structure damping x(f) ~ f-5/2.phi of wire can be as low as 3e-4 to 15e-4.Measurements of linewidths ~ crappy.Ringdowns better.By displacing the beam by ~1 cm on the mirror surface the thermal noise from the bending of the bottom part of the wire can be minimized.plots ...

Measurements:Measurements:Suspension Thermal NoiseSuspension Thermal Noise

LIGO-G040767-00-D

Measurements:Measurements:Suspension Thermal NoiseSuspension Thermal Noise

Assumes NO loss except for in the ends of the suspension wire. Good clamps, no rubbing.

High upper limit of 1.3 x 10-3 set by measuring in lock linewidth of violin modes.

Linewidths limited by temperature drift.

Ringdowns do not have this problem.

LIGO-G040767-00-D

Measurements:Measurements:Shot NoiseShot Noise

Measured recycling gains (45-50) are higher than the design (30). Limited by the excess scatter loss on the test masses (~70 ppm/mirror).

Measured contrasts (L1 = 3e-5, H1 = 7e-4) better than SRD (3e-3).

New output mode cleaner will remove all higher order contrast problems. Residual contrast defect will be TEM00 defect (6e-6) due to arm imbalance.

RF Modulation depth then lowered to optimize SNR.

Gives Gamma ~ 0.15. Gamma ~0.45 in original design.

Also assumes 100% sideband transmission to AS port. Means perfect thermal compensation and all SB power in TEM00.

LIGO-G040767-00-D

Noise:Noise:S(>4) RunS(>4) Run

Identical Thermal Noise assumed for both IFOsMore seismic noise in Hanford than in Livingston !Could be better ?

LIGO-G040767-00-D

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I. Where does this curve come from?? Lost in the mists of time + doesn't matter. So lets guess.

A. Seismic

Assumptions:

1. Ground noise is 1e-8 /f^2 m/rHz (sort of true)

2. 1 Hz pendulum (its really 0.75 Hz)

3. 1/f^8 at 12 Hz (not really, better above 30 Hz)

B. SUS Thermal

What it is: When we swing the pendulum, the wire bends at the top and the bottom, producing some heat through the wire's internal friction. Symmetry of hte FD theorem says that this is also a point at which thermal forces can move the test mass.

1. Assumed to be a viscously damped harmonic oscillator so having a 1/f^2 shape. In reality its a 1/f^2.5 and the loss in music wire is a little better than expected.

2. Measured Q's give a wire loss angle of < 1.3e-3. True wire loss is probably more like 0.5e-3, but we don't know yet about the small amplitude loss due to the clamps. Need to do ringdowns with full IFO at a few frequencies.

C. Internal Thermal 1. Was previously calculated by looking at modal Q's and doing a weighted sum to get the GW output signal. 2. Levin's method uses more direct application of FD theorem. 3. Kenji's measurement verifies Levin's approach. 4. Gregg's measurement of modal Q's can be used to determine seperately the substrate phi and the coating phi. Initial measurements agree with guesses. phi_s = 1e-7 & phi_c = 2e-4.

D. Shot Noise 1. SRD makes some conservative assumptions (good thinking). a. Contrast 3e-3. b. Power recycling gain = 30

2. Now we have some real numbers. a. Contrasts: L1 = 3e-5 H1 = 8e-4 b. Recycling gain 45-50 c. Output Mode Cleaner will give us C < 1e-5. (my guess)

3. Adjust modulation depth to maximize SNR.