Development for Observation and Reduction of Radiation Pressure Noise

GWADW 2010 in Kyoto, May 19, 2010

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Development for Observation and Development for Observation and Reduction Reduction

of Radiation Pressure Noiseof Radiation Pressure Noise

T. Mori, S. Ballmer, K. Agatsuma, S. Sakata,S. Reid, O. Miyakawa, A. Nishizawa, K. Numata,H. Ishizaki, A. Furusawa, S. Kawamura, N. MioGraduate School of Frontier Science, the University of TokyoNational Observatory of Japan


OutlineOutline1. Objectives and scope of the experiment2. Setup and Experiment3. Problems and Solutions4. Future plans, Summary

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1. Objectives and Scope1. Objectives and ScopeObjectives

Observe radiation pressure (RP) noiseReduce radiation pressure noise

measurement of ponderomotively squeezed vacuum fluctuations at the best homodyne phase

ScopeFabry-Perot cavities with tiny mirrors and high Finesse

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Radiation pressure noise

Shot noiseSQL

Radiation pressure noise reducedat some homodyne phase


Ponderomotive squeezingPonderomotive squeezingSqueezing ponderomotively

Correlating amplitude fluctuation with phase fluctuation via back actionDisplacement changed by back-action of laser light and

vacuum fluctuationsSqueezing with frequency dependence

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Laser light and vacuum fluctuations

displacementLaser light and

ponderomotively squeezed vacuum


Conceptual designConceptual design Main interferometer: Fabry-Perot Michelson

interferometer with tiny mirrors (20mg) and high Finesse (10000)

Output light homodyne detected with local oscillator

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Homodyne detection

Fabry-Perot Michelson interferometer

Tiny mirrors

High finesse

signalnoise h : homodyne phase

signal×noise


Noise budgetNoise budget

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Laser power 200 mW

Injected laser power 120 mW

Finesse 10000

End mirror mass 20 mg

Diameter of end mirror 3 mm

Thickness of end mirror 1.5 mm

Front mirror mass 14 g

Reflectivity of end mirror 99.999 %

Reflectivity of front mirror 99.94 %

Optical loss 50 ppm

Beam waist of end mirror 340 m

Mechanical loss of substrate

10-5

Mechanical loss of coating 4×10-4

Length of silica fiber 1 cm

Diameter of silica fiber 10 m

・ Observation of RPN; 1×10-17 [m/rtHz]@1 kHz・ Ponderomotive squeezing 　　　　 of 6 dB

Observation and reduction of RP noise between 300Hz and 1kHz


2. Setup: Small mirror and 2. Setup: Small mirror and pendulumpendulum

Silica fiber attached on 20mg mirrorGlued with UV cured resin

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Diameter: 3mm

Silica fiber of 10m in diameterFor lower suspension thermal noiseNow 20m: due to availability

20 mg mirror with a diameter of 3mm

Effective diameter ~2mmFlat mirror


Initial setup: single Fabry-Perot Initial setup: single Fabry-Perot cavitycavityFabry-Perot cavity

Cavity length: 60mm, Finesse:1400Front mirror: fixed mirror mounted on PZTEnd mirror: suspension by a double pendulum

Middle mass (aluminium): eddy-current dampedAll the core optics are on one optical table

Table is suspended by a double pendulum

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End mirror

Front mirror

Fabry-Perot cavity


Assembling of a pendulumAssembling of a pendulum Very large yaw fluctuation prevented the stable locking Damping magnet: optimized for damping the yaw motion

instead of the damping for the displacement (longitudinal) motion

We could lock the cavity with this configuration(hopefully higher Eddy-current damping)much less trouble with yaw-instabilities

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uniform magnetic field (top view)


3. Characteristics of the cavity3. Characteristics of the cavity

succeeded in the stable locking UGF: ~1kHzPhase margin:

~80deg.Limited by the

mechanical resonance @4.5kHz

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Open loop transfer function

Gain

Phase

dela

y


SensitivitySensitivity

Estimated from the error signal

Not optimized yetElectric noise not

eliminated

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Free running laser frequency noise (in theory)

Previous measurement (20080410)


4. Problem: Yaw instability4. Problem: Yaw instabilityTechnically: prevents the stable locking, and makes the alignment

very difficult and painfulTheoretically: actuated by the radiation pressure

Resonant axis is changed from center of mirror because of mirror fluctuationsYaw mode fluctuation is increased by radiation pressure → angular anti-

spring effect (S. Sakata et. al., PRD81, 064023(2010))Can be actuated easily: because of single silica fiber

Torque by radiation pressure is a factor of 1000 larger than restoring force (for final cavity power)

Angular control system needed

Now constructing

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Incident light

Front mirror

End mirror


Negative g-factor is better, but..Negative g-factor is better, but..Another solution for the yaw instabilityFor yaw motion: in general always stable & unstable

mode If input mirror is controlled, small mirror becomes

- unstable for positive g-factor- stable for negative g-factor

flat mirror: always unstable If enough budget, we will try a curved one

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g=(1-L/R1) * (1-L/R2)0<g<1 for optical stability

L > R2


Problem: Q –factorProblem: Q –factor

Q-factor: previously measuredFiber: 10mworse than the expected value

Pendulum mode : Q= 7360 @ 5 Hz

Thermal noise: slightly lower than designed RP noise

1st violin : Q=3975 @ 1567 Hz

Clamping section might be problem

We will measure this again, and investigate them

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Future plansFuture plansNow installing;

Laser power: 200mW -> 500mWwith frequency and amplitude stabilization system

Input mirror with angular control system

Now preparing;accurate measurement of the value of the system;

Q-factor of the torsion and pendulum modeclarify where mechanical loss comes from

->Build a FP Michelson interferometer

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SummarySummary

Goal: Observation and Reduction of radiation pressure noiseKey point: suspended 20 mg mirror by 10 m diameter silica

fiber

Cavity locked stably at low power improvement on the yaw motion damping

Future planLaser power upgradeStabilization of the light sourceNew input mirror suspension for yaw motion control systemBuild a full interferometer

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Thank you!Thank you!

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Development for Observation and Reduction of Radiation Pressure Noise

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