New signal extraction schemewith harmonic demodulation
for power-recycled Fabry-PerotMichelson interferometers
Koji Arai, Masaki AndoA, Shigenori MoriwakiB,Keita KawabeA, and Kimio TsubonoA
National Astronomical Observatory
A

Department of Physics, University of Tokyo

B

Department of Advanced Materials Science, University of Tokyo

Introductionl New scheme to extract control signals
for power-recycled Fabry-Perot Michelson~ Based on frontal modulation~ Demodulation at the 3rd harmonic frequency (3fm)~ Control signals for RM and BS

l Principle~ Beating of 1st and 2nd order modulation sidebandsl Advantages~ Insensitivity to arm cavity motions~ Robust extraction: amplitudes and signs of the signals-- less dependent on the optical parametersl Experimental tests on the 3m suspended prototype

Harmonic Demodulation Scheme
Demodulating reflected light at the 3rd harmonic frequency
I Q
Phase
Shifter

LASER

3fm
demodulation
I
3fm

Q
3fm

δl + δl −δL +LP

Faraday
Isolator

EOM

SY
IQ

δl + δl −δL +L−=LP−LI
L+=LP+LI
l− = lP−lIl+ = lP+lIlPPOlIASIQ

δL +δL −LI

Problems of conventional schemel Difficulty in separating δl+ from δL+~ because of the phase enhancement by the arm cavities~ signal ports are ~100 times more sensitive to δL+ than to δl+l The δl+ and δl− signals can disappear
The δl+ signal vanishes when G0 = G1 .
The δl− signal vanishes when G0 is maximized.
Recycling gain
G0 : for the carrier
G1 : for the 1st order sidebands

Principle
lPhotocurrent at the 3fm ~ beating of SB2 and SB-1
modulation
sidebands

3fm

frequency of light
SB-3 SB-2 SB-1 CarrierSB1 SB2 SB3lSB2s are not resonant with the IFO
Effect of SB2
Emphasized at the reflection port
The amplitudes and the signs
Less dependent on the couplings of CA and SB1

Advantages
lEasy to implement~ no addtional modulation

lContribution of δL+ is reduced~ in comparison with the fm demodulation

lRobust extraction of δl+ and δlAmplitudes of the signals~ less dependent on the optical parameters
Signs of the signals~ do not depend on the optical parameters~ never change during lock acquisition

lOperating the IFO without the pick-off mirror~ the δL+ signal is extracted at the reflection port

Experimental tests on the 3m prototype
l3m prototype power-recycled interferometerat University of Tokyo.A=JKHAIEnd Tank(Inline)
Center Tank
Recycling Mirror
Tank
End Tank(Perpendicular)

Input Optics

Suspended Mirrors
Double pendulums
Enclosed in chambers
Recycling Gain
~3.8 (RR=0.63)~5.4 (RR=0.81)
Light source:
Nd:YAG
1064 nm, 42 mW
Arm length:2.95 mArm finesse:~240
1999/9

Harmonic Demodulation for the 3m40MHz
FaradayIsolator

EOM

SY340MHz120MHzfrequency
tripler

120MHzδl + δl −frequency
tripler

POASIQ40MHz

δL +40MHzδL −Frequency of the local oscs. is tripled by tuned
amplifiers with nonlinearly driven transistors

Experimental achievementsl Automatic lock / stable operationl Reduction of dL+ contribution~ Sensitivity to dL+ --- 20 times reduced.

l Robust extraction of dl+ and dl~ Modest optical gain variations during lock --- confirmed.
~ Sign of the dl- signal --- never been flipped.

l Operation without pick-off inside the recycling caivty~ Stable operation with recycling gain of 5.4

Locking acquisition

Reduction of dL+ contributionlSensitivity to dL+ has been 20 times improvedby the 3fm schemeδl+ (3fm demod.)δl+ (fm demod.)

δL +
δL +

Robust extraction of dl+ and dllSensitivity variation during lock acquisition
fm δl+

RecycledMichelson

RecycledMichelson+
One Arm

Recycled
Fabry-Perot
Michelson

RMI −> RFPMI

1 / 7.6fm δl-

1 / 10.7

3fm δl+1 / 1.7

3fm δl-1 / 1.9

Robust extraction of dl+ and dll Sign flipping of δl- during lock / by change in optical param.
Tests by changing the transmissivity ofpick-off mirror inside of the recycling caivty.

The sign of δl- by the 3fm demod. never changes

IFO operation without pick-offl Stable operation of the 3m without pick-off mirror
I Q

3fm
demodulation
I
3fm

Q
3fm

δl + δl −SY

Pick-off

ASQI

δL +

δL −

IFO operation without pick-offl Stable operation of the 3m without pick-off mirror
Recycling Gain for the carrier(RR = 0.81)
G0 = 5.4 0.05
(almost critical coupled)

Inline arm
Perpend. arm

IFO operation without pick-offl Noise caused by the pick-off was eliminatedFurther reduction of dL+
Erase the 3rd order sidebands at the reflection port!!!
l by adjusting the opt. param.
~ The best separation ratio (dL+ / dl+) of about 0.5

l by applying weak 3fm modulation

Discussionl Shot-noise level of the 3fm signals are usually worse
than those of the signals with the conventional scheme.
(measured with modulation depth m of 0.7)

SummarylHarmonic demod. for extraction of δl+ signals~ Demodulation at the 3rd harmonic frequency
lExperimental tests on the 3m prototype~ Stable lock has been achived~ Inherent insensitivity to δL+ (20 times improved)~ Robustness of the signal amplitudes during lock~ Sign of δl- does not depend on the optical parameters.
~ Stable operation without pick-off mirror (rec. gain of 5.4)