LIGO NSF review, 11/10/05 1 AdLIGO Optical configuration and control Nov 10, 2005 Alan Weinstein for AdLIGO Interferometer Sensing and Control (ISC) and

LIGO NSF review, 11/10/05 1

AdLIGOOptical configuration and

control

Nov 10, 2005

Alan Weinsteinfor AdLIGO Interferometer Sensing and Control (ISC)

and the 40 meter lab


AdLIGO optical configuration and control

Problem: If the current Initial LIGO optical configuration (power-recycled Michelson with Fabry-Perot arms) is retained in AdLIGO, the increased laser power (needed for better sensitivity in the high-frequency shot-noise-limited regime) will put intolerable thermal load on the transmissive (absorptive, lossy) optics in the power recycling cavity (BS, ITM substrates).

Solution: increase the finesse (optical gain) of the F-P arms, decrease the gain in the PRC.

40 KG FUSED SILICA

ADVANCED LIGO LAYOUTADVANCED LIGO LAYOUT

BS Beam SplitterITM Input Test MassETM End Test MassPD PhotodiodePRM Power Recycling MirrorSRM Signal Recycling Mirror


Advanced LIGO optical configuration

Detuning

PRM

BS

FP cavity

FP

ca

vit

y

Laser

GW signal

Power

Problem: Increasing the finesse of the arms causes the cavity pole frequency to decrease, leading to reduced bandwidth for GW signal.

Solution: resonant sideband subtraction! the PRM acts to increase the optical gain of the arms, for the carrier. the SEM acts to decrease the optical gain of the arms, for the GW

signal sidebands – the signal sidebands are resonantly extracted out the asymmetric port.

This decouples the problem of storing the carrier power (CARM+PRC) from extracting the signal (DARM+SEC), allowing us to optimize both for best quantum-limited response to signal, and apportionmentof optical gain / thermal load.

SEM


Tuning the signal response

rITM

SR

RSE

rCC

RSE SRtuned(narrow band)

= 2kls = 4ls(fcarr+fsig)/c

The red curve corresponds to r = rITM, ie, no SR mirror

Better solution: Detuned signal extraction optimizes the signal extraction for a signal frequency away from DC, allowing us to resonantly enhance the response at, say, 40 Hz, shaping the frequency response to optimize sensitivity in the presence of other noise sources (thermal, seismic.. which are overwhelming near DC). By choosing the phase advance of the signal (fcarr+fsig) in the signal recycling cavity, can get longer (SR) or shorter (RSE) storage of the signal in the arms:


Using DR to optimize sensitivity

Now we can independently tune hDC and fpolarm to optimize sensitivity(eg, hug the thermal noise curve)


101

102

103

10-24

10-23

10-22

Frequency (Hz)

Str

ain

No

ise,

h(f

) /H

z1/2

Newtonian background,estimate for LIGO sites

Seismic ‘cutoff’ at 10 Hz

Suspension thermal noise

Test mass thermal noise

Unified quantum noise dominates at most frequencies for fullpower, broadband tuning

Optimize detuned RSE response, in the presence of other noise sources, to maximize BNS range.

Projected Adv LIGO Detector Performance

10 Hz 100 Hz 1 kHz

10-22

10-23

10-24

10-21

Initial LIGO

Advanced LIGO

Str

ain


Control of the AdLIGO optical configuration

Problem: the detuned signal extraction, on top of the power-recycled Michelson with Fabry-Perot arms, is a very complicated optical configuration. The current Initial LIGO sensing scheme has no hope of acquiring lock and controlling the mirrors in AdLIGO.

» The Initial LIGO scheme uses one pair of RF sidebands for PDH reflection locking of the arms and PRC, and Schnupp transmission locking for the Michelson. The signals for the short degrees of freedom (PRC and MICH) would be overwhelmed by the large signals from the arms, if the arms weren’t tightly controlled: gain hierarchy.

Solution: enhance the length signal extraction by using two pairs of RF sidebands, used in clever ways.

» The signals for the short degrees of freedom (PRC, MICH and SEC) can be completely decoupled from the large signals from the arms; the length sensing matrix is much more diagonal, no gain hierarchy needed. Expect control to be more robust!


AdLIGO signal extraction scheme

Arm cavity signals are extracted from beat between carrier and f1 or f2.

Central part (Michelson, PRC, SRC) signals are extracted from beat between f1 and f2, not including arm cavity information.

f1-f1 f2-f2

Carrier (Resonant on arms)

• Single demodulation• Arm information

• Double demodulation• Central part information

Mach-Zehnder installed to eliminate sidebands of sidebands.

Only + f2 is resonant on SRC. Unbalanced sidebands of +/-f2 due

to detuned SRC produce good error signal for Central part.

ETMy

ETMx

ITMy

ITMxBSPRM

SRM

4km

4k

mf2

f1


5 DOF for length control

: L=( Lx Ly) / 2

: L= Lx Ly

: l=( lx ly) / 2

=2.257m: l= lx ly = 0.451m

: ls=( lsx lsy) / 2

=2.15m

Port Dem. Freq.

L L l l l s

SP f1 1 -3.8E-9 -1.2E-3 -1.3E-6 -2.3E-6

AP f2 -4.8E-9 1 1.2E-8 1.3E-3 -1.7E-8

SP f1 f2 -1.7E-3 -3.0E-4 1 -3.2E-2 -1.0E-1

AP f1 f2 -6.2E-4 1.5E-3 7.5E-1 1 7.1E-2

PO f1 f2 3.6E-3 2.7E-3 4.6E-1 -2.3E-2 1

Signal Extraction Matrix (in-lock)

Common of armsDifferential of armsPower recycling cavity

MichelsonSignal recycling cavity

Laser

ETMy

ETMx

ITMy

ITMxBS

PRM

SRM

SPAP

PO

lx

ly

lsx

lsy

Lx =38.55m

Finesse=1235

Ly=38.55m

Finesse=1235Phase Modulationf1=33MHzf2=166MHz

T =7%

T =7%

GPR=14.5


Full lock of AdLIGO optical configuration at the 40 Meter prototype

Problem: this is all fine in theory, but does it work in practice?» Apparently so! We can use this scheme to acquire lock and control the

40m prototype interferometer, even during the noisy daytime.

» Full lock acquisition is now relatively routine, and robust.

» The optical response is precisely as expected, including the resonant enhancement (at the 40m, this is at 4 kHz; in AdLIGO, it will be at ~40 Hz) as well as the optical spring (in the 40m, at ~50 Hz).

» In the process, we have learned a great deal about the intricacy of the AdLIGO optical design, and how to control it.

» Exploiting the enhanced controls and more diagonal sensing matrix, we are developing deterministic lock acquisition procedures, step-by-step approaches to take the wait and chance out of lock acquisition.


The path to full RSE at the 40m

Carrier33MHz166MHz

Oct. 2004Oct. 2004Detuned dualrecycled Michelson

Nov. 2004Nov. 2004Arm lock with offset in common mode

Oct. 2005Oct. 2005RSE

ITMy

ITMxBSPRM

SRM

ETMx

ETMy

Shutter

Shutter

Reducing offset


Frequency sweep of optical spring

~1900W

~270W


Optical spring and Optical resonance in differential arm mode of detuned RSE

• Optical gain of L- loopDARM_IN1/DARM_OUT divided by

pendulum transfer function

• Optical spring and optical resonance of detuned RSE were measured.

• Frequency of optical spring depends on cavity power, mass, detuning phase of SRC.

• Frequency of optical resonance depends on detuning phase of SRC.

• Theoretical line was calculated using A. Buonanno and Y.Chen’s equations.-150

-100

-50

0

50

100

150

Pha

se[d

eg]

102 3 4 5 6 7 8 9

1002 3 4 5 6 7 8 9

10002 3 4 5 6 7

Frequency[Hz]

60

40

20

0

-20

Mag

[dB

]

Measured data Theoretical line

Measured optical gain of arm differential mode in detuned RSEOct 22, 2005


Optical spring in E2E

• Calculated by time domain simulation

• No length control• Lock lasts ~0.7sec, so

statistics at low frequency is not good.

• Simple length control required

• Calculation time ~5min using DRMI summation cavity


But will it work in AdLIGO?

AdLIGO has » 4 km arms (longer storage time)» quadruple pendulums (much quieter, but also much less actuation

force on the test mass)» advanced seismic isolation (much quieter)

No problems are foreseen, but we must extrapolate from 40m to AdLIGO with detailed simulation -> e2e.

Will maintaining lock be more difficult in Adv LIGO? We might expect it will be significantly easier, because of the much quieter seismic platform, powerful multiple pendulum isolation, and more diagonal length sensing plant.» This is an inference that must be verified through tests (LASTI) and

simulation (e2e).


Differences betweenAdvLIGO and 40m prototype

100 times shorter cavity length Arm cavity finesse at 40m chosen to be = to AdvLIGO ( = 1235 )

» Storage time is x100 shorter

Control RF sidebands are 33/166 MHz instead of 9/180 MHz» Due to shorter PRC length, less signal separation

LIGO-I 10-watt laser, negligible thermal effects» 180W laser will be used in AdvLIGO.

Noisier seismic environment in town, smaller isolation stacks» ~1x10-6m at 1Hz

LIGO-I single pendulum suspensions» AdvLIGO will use triple (MC, BS, PRM, SRM) and quad (ITMs, ETMs)

suspensions.

Documents

LIGO NSF review, 11/10/05 1 AdLIGO Optical configuration and control Nov 10, 2005 Alan Weinstein for AdLIGO Interferometer Sensing and Control (ISC) and