Interferometer Configurations

Fujihara Seminar

May 2009

Kentaro Somiya

Caltech

Gravitational-wave detector

LASER

Photo-detector

GW

Base line (arm) = 3~4km

Proper location of mirrors to detect GWs

= Interferometer Configuration

Detector sensitivity

• Various kinds of noise• Shot noise is one of the limiting noise sources

total

Standard Quantum Limit

Shot noise

LASER

Phase fluctuation of photons [Quantum noise]

Reduction of shot noise

1. Increase the power2. Increase the signal3. Inject squeezing

keys of good configurations

1st generation detectors; type-I

signal

power

GW

GW power recycling (power )

Fabry-Perot cavity (power + signal )

Power-recycled Fabry-Perot Michelson interferometer (LIGO, Virgo, TAMA)

Both power and signal are enhanced.

1st generation detectors; type II

signal

power

GW

GW power recycling (power )

Dual-recycledMichelson interferometer (GEO)

signal recycling (signal )

Both power and signal are enhanced.

Power/Signal recycling

Power Recycling

Signal Recycling

Power recycling

improve the floor level

Signal recycling

improve the floor level and

narrower the bandwidth

Frequency response of an interferometer is determined by the floor level and the bandwidth.

2nd generation detectors

Nevertheless….

2G detectors will accommodate all threes.

Why do we need three kinds of cavity?

resonant

resonant

anti-resonant resonant

Resonant Sideband Extraction (RSE)

signal

power

decent power recycling (power ) signal extraction

(signal )

High finesse cavity (power ) (signal )

This system changes the power balance

[Mizuno 93]

Power balance

coating substrate

~300ppm heat absorption (=0.125W)

~1W cooling capability [safety factor of 8]

400W

400kW heat transfer

High finesse + low power-recycling (RSE) is suitable for LCGT

20K 20K

Detuning

SRMirror

SR: resonantRSE: anti-resonantdetune: intermediate

SR

RSE

detune

Signal at a certain frequency resonates

Narrow-band signal enhancement

Radiation-pressure noise

1st generation detector (50W at BS, FPMI)2nd generation detector (1kW at BS, RSE)

RP noise

Shot noise Standard Quantum Limit (SQL)

High precisionHeisenberg’s principle

Back action

Reduction of shot noise (high power) Radiation pressure noise

Sensitivity of a broadband detector is limited by the SQL

Radiation pressure in a detuned detector

signal(phase) signal

(phase+amp) laser + signal (amp) [radiation pressure]

signal (phase)

rotation due to the detuning

This loop makes an optical spring and allows us to overcome the SQL

Detuning in LCGT

SQL

thermal noise

broadband

detuned (opt. for BNS)

Observable range for Binary Neutron Stars Broadband RSE : SN=10 at 185Mpc Detuned RSE : SN=10 at 242Mpc

Better sensitivity by detuning!

Variation of the signal response w/ detuning[Ballmer 07]

Broadband

BHBH

NSNS

High Freq

• Reflectivity of the mirrors is fixed• Changing the SRM location and the input power• Tunable for each GW source

total noise level of AdLIGO

3rd generation detector

PRM

SRM

Baseline 5~10km ring cavity

Detuned Sagnac interferometer with Fabry-Perot arm cavities

speed-meter

measure x back action of p measure p no back action

Overcoming the SQL in a broad frequency band

[Chen 03]

Sensitivity curve of detuned Sagnac[Mueller-Ebhardt 09]

• Overcoming the SQL in a broad frequency band• Better than other methods in the optical-loss issue

20ppm loss/arm300ppm loss/arm

End-mirror cavity[Khalili 2005]

less coatings more coatings

Configuration to reduce thermal noise

anti-reso cavity

total thermal-noise level decreases

Replacing a mirror to a cavity…

Xylophone with SPI

• Top stage: MF 10-100Hz

• Mid stage: HF 100-10kHz

• Bottom stage: LF 1-10Hz

Low mirror thermal noiseHigh power

Extremely high powerLow optical loss + squeezing

Low power + very low temperatureLow suspension thermal noise

10km

10m

(suspension-point interferometer)

Summary

1st generation detector

2nd generation detector

3rd generation detector

• Power-recycled FP Michelson configuration• Reduction of shot noise

• Broadband/detuned RSE configuration• Reaching/overcoming the SQL

• Overcoming the SQL in a broad frequency band• Any ideas being welcome