K.Somiya

1
Caltech 40m Current Issues
Kentaro Somiya
University of Florida 2004. 10. 19
K.Somiya
2
Contents

• Introduction Advanced LIGO and the 40m
• Part I Lock acquisition of the 40m
• Part II Frequency noise and Mach-Zehnder noise

3
LIGO and AdLIGO
Comparison of the quantum noise sensitivity.
Detuned RSE technique improves the sensitivity.
4
Detuned RSE
RSEResonant Sideband Extraction
Detune the SR cavity from the carriers resonant
point. ( Broadband RSE)
Additional mirror at the dark port. Signal
Recycling Mirror
Totally 5 degrees of freedom to be controlled
L, L-, l, l-, and ls.
5
40ms role as the final prototype of AdLIGO
6
Part I Lock Acquisition
7
Two frequency modulation scheme
PRFPMI (4 DOFs)
PR-BRSE (5 DOFs)
f2 brings the ls signal.
Carrier reso in the arms and the PRC f1
reso in the PRC 33MHz f2
reso in the PR-SRC 166MHz
Carrier reso in arms and the PRC f1
reso in the PRC
The central part is locked only by SBs.
8
Lock Acquisition
Lock the central DR part somehow. Lock the
central DR part only by RF SBs. Lock the arms
by the carrier.
9
Lock Acquisition of the central part
It has turned out to be quite difficult.
Carrier BP BP, except l- signal Signals at the BP l
Carrier BP BP, except l- signal Signals at the DP l- (no DC)
33MHz BP BP and DP (mostly) (a little) Signals at the BP l, l-, and ls
33MHz BP BP and DP (mostly) (a little) Signals at the DP l, l-, and ls
166MHz BP DP, and DP BP Signals at the BP l, l-, and ls
166MHz BP DP, and DP BP Signals at the DP l, l-, and ls
No combination brings the l- signal well isolated
from the others.
10
Example l at the BP

• Four SBs resonate at different points
• Good signal obtained when f2 resonates
• Signal for f2 resonance has an opposite polarity
• Disturbed when f2 and f2 resonate at the same
  point
• Signals for ?f1 resonance can be cancelled by
  proper DDM phases, which coincide with the phases
  for symmetric signal

11
Dither Locking
10kHz mechanical modulation on l- Its like a SB
injected from the DP. All the 10kHz SB returns
to the DP except the l- signal component.
Carrier BP BP, except l- signal Signals at the BP l
Carrier BP BP, except l- signal Signals at the DP l- (no DC)
Dither 10kHz DP DP, except l- signal Signals at the BP l- (no DC)
Dither 10kHz DP DP, except l- signal Signals at the DP ls
The error signal taken from the beat of 10kHz,
divided by the power at the pick-off port, shows
a clear l- signal.
12
Locking the PRM-SRM cavity
After locking the l-, the condition is simple.
The PRM follows the swinging SRM Then, once ls
is locked, well recover l 0º.
13
Successful Locking
Lock now Control later l- dither
_at_AP ? DDM_at_AP l 33_at_SP ?
DDM_at_SP ls DDM_at_SP ? DDM_at_PO
ls lock at -5.2 sec l- lock at -5.3 sec l lock
at -5.6 sec
14
Part II Mach-Zehnder Noise
15
Disturbance by sub-sidebands
Original concept Real
world
SB of SB (sub-SB)
Carrier
f1
-f1
f2
-f2

• Sub-sidebands are produced by two series EOMs.
• Beats between carrier and f2 f1 disturb the
  central part.

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
Port Dem. Freq. L? L? l? l? l s
SP f1 1 -1.4E-8 -1.2E-3 -1.3E-6 -6.2E-6
AP f2 1.2E-7 1 1.4E-5 1.3E-3 6.5E-6
SP f1 ? f2 7.4 -3.4E-4 1 -3.3E-2 -1.1E-1
AP f1 ? f2 -5.7E-4 32 7.1E-1 1 7.1E-2
PO f1 ? f2 3.3 1.7 1.9E-1 -3.5E-2 1
Sub-sidebands should be removed.
16
Mach-Zehnder to eliminate the sub-SBs
Mach-Zehnder interferometer no sub-sidebands
Series EOMs sub-sidebands are generated
29MHz EOM has been moved to inside the MZ.
But this MZ introduces additional noise on the
frequency.
17
MZ differential motion noise
f1
Ca
f2

f1
Laser
PC1

PC2
f2
Differential Mode
Common Mode
Disturbance of orthogonality between the carrier
and the SB.
There are three paths via which MZ noise
contributes to L-.
18
Three paths of MZ noise to L-

1. Direct coupling with contrast defect component
2. Via frequency stabilization system of the MC
3. Via frequency stabilization system of the L

This is not a problem with the DC readout
scheme. Seijis calculation says this is not a
problem even with RF readout and if the
finesse differs for 10.
This can be suppressed by the FSS of L.

• (1) and (3) are the same problems as RF phase
  noise of the EOM.
• Freq noise should be calculated to see if (2) and
  (3) limit the sensitivity.

19
Frequency noise of detuned RSE
What we have

• J.Camp calculated frequency noise for a PR-FPMI.
• J.Mason extended the calculation with the SR
  cavity.

What we should add

• Radiation pressure effect caused by freq noise
  sideband.
• Storage time difference will be larger with
  higher finesse arms.
• RMS cavity fluctuation converts freq noise to
  amplitude noise
• and it appears as freq noise because of the
  detuning.

20
Calculated frequency noise TF of the 40m
GW Signal gtgt two peaks Freq noise around
Carrier gtgt two peaks gtgt spectrum shows no
dips Freq noise around RF SB gtgt flat gtgt
spectrum shows two dips
21
Requirement of freq fluctuation after MC
Goal sensitivity divided by freq noise TF
22
Frequency stabilization servo preliminary
x laser freq noise y MC displacement noise z
MZ differential noise
G gtgt 40m MC servo G, H gtgt TAMA servo (just for
example)
23
MC sensitivity requirement
This is hopefully not a big problem since the
total noise level is limited by PSL noise at
high frequencies.
24
MZ sensitivity requirement
MZ noise will limit the sensitivity at 1001kHz.
25
Is there a way to prove the contribution of MZ
noise?

• We have seen the offset voltage on the MC error
  signal when
• MZ is not locked to the bright fringe.
• We can measure the MZ noise on the reflected
  light of the MC,
• although the contribution is less than on the
  transmitted light.

gtgt next page
x laser freq noise, y MC motion noise, z MZ
diff noise F finesse and LPF of the MC, G VCO
gain, H MCL gain
26
Comparison of MC noise and MZ noise on the MC
reflected light
true only around this region, where FSS gain is
high enough

• We are able to measure the TF from MZ noise to
  MC noise.
• We cannot see the direct contribution on the
  noise spectrum.

27
How to reduce the MZ noise?

• Installation of a phase-correcting pockels cell
  in a MZ arm.
• Alternative to the MZ Virtual Mach-Zehnder
  ref. P.Beyersdorfs document

28
Conclusion

• The lock acquisition scheme with the dither
  locking has been
• developed and we have succeeded in locking the
  central part.
• So far the PRC is locked to the carrier, and the
  next step is
• to lock the PRC to the sidebands.
• Frequency noise of DRSE has been analytically
  calculated.
• We have installed the MZ to remove
  sub-sidebands, but it
• introduces additional MZ noise via freq noise.
• There are a few ways to reduce the MZ noise and
  shall be
• tested in the 40m.