Transcript Slide 1

Locking tests with TCS
WG1 at Birmingham
July, 10th-11th 2008
Enrico Campagna
on the behalf of the
Locking/TCS group
INFN Sez. Firenze
European Gravitational
Observatory
Universita` di Urbino
1
Summary
A description of all the tests is in EGO working area under Virgo/locking g Activities g TCS

Thermal compensation system in Virgo



Why we need it
An overview
Tests

Step 12




10 mW
Higher powers
Etalon effect, Newton rings
Trying to balance the SBs



Step 8








1.3 W and up to 4.4 W
Scanning OMC temperature
Lock acquisition


with locking/alignment signals
with input power
700 mW from the beginning
Various power form step 6
Investigation of 11 Hz loop instabilities
Single free swinging cavity
Changing TCS annulus shape
Future
Conclusions
2
TCS in Virgo: why we need it

Absorbed laser power in the input optics



14 ppm in the WI
5 ppm in the NI
Evident effect during the lock acquisition

After reaching the DF we have to wait 10-15 min for the ITF to
thermalize


The cleaning with first contact polymer had no effect



Big transients on sidebands powers (great asymmetry)
The transient remained pretty much the same
Slight increase of mean SB power
TCS should

Give the optics a constant time behavior


make the locking signal more stationary
make the lock acquisition faster
3
TCS in Virgo: an overview





25 W CO2 laser (only on WI)
3-4 % on power meter g ph.diode
2 telescopes L1-L2 and L3-L5
Axicon: double conic lense
Ext. thermal camera
Beam dump
L5
Power meter
AXICON
Half waveplate
2" FM2
2" Hot mirror
L4
L3
Polarizer
1" Beam splitter
Aiming laser
LASY-20S
L1
L2
2" FM1
4
Step 12: 10 mW

With 10 mW we already have a strong effect on
SB balancing.
NOT EXPECTED, but observed many times, in
subsequent tests also
 Opposite to the std transient
 Due to central spot?
 Phase camera: clear effect on central area (movies)



Towards a flatter shape
MORE INVESTIGATION NEEDED
5
Step 12: higher powers


Up to some hundreds mW no significant differences
700 mW gives encouraging effects

SBs unbalancing as in a time-reversed std locking acquisition
transient







Indicates a possible reduction of thermal lensing
5 mins transient for each step in powers
B5_2f_ACq is decreasing a lot even if mean SB power is
increasing
Decrease in MICH offset g B5_DC increasing
B1_DC/B1p_DC increases indicating a cleaner matching of the SBs in
the OMC
West cavity finesse is significantly growing
More than 1 W


Upper SB starts going up
11 Hz instabilities (see later)
6
Step 12: cooling down and etalon effect

Switching TCS off after the test



The same 5-10 min fast trends
SBs start from very low values and then slowly increase
g long trend in the optical gain of NE, WE and BS


Clear slow trends on B5_2f_ACq and on
Gc_Driving_PRCL_NE-WE



correlated quite well with the horizon
WI heating g change in thickness g change in etalon g finesse
asymmetry g pure common modes couple to differential g PRCL
enters differently the df g gain beta changes
but PRCL noise not limiting the sensitivity
The same happens with B5 frequency noise coupling
7
More on etalon: Newton rings


From mirror temperature measurement (resonant mode
technique) rTM=0.3 K is not enough to explain the
etalon fringes
From FE symulation (M.Punturo)




Locally rT of few degrees
Differential radial reflectivity
Time dependent transmittivity
TCS with current configuration will never get rid of
thermal transient

Restyle it to allow for a central spot to heat as the Nd:Yag
laser while unlocked
8
Step 12: trying to balance the SBs
with locking and alignment signals

With locking signals



MICH offset has little effect and fast brings B5_DC to very low values
(the std way of balancing the SBs is not working)
B2_3f phase and B5 phase have no effect
With TCS off, B2_8MHz phase tuned to move B2_8MHz_ACq to 0




Lowering the PRCL offset




MICH offset goes to 0
It has no effects on B5_DC or B5_2f _ACq
But with TCS on nothing changes
SBs unbalance but
their power goes up
in another test at step 8 has no effect at all
Alignment signal of the input have no effects (at step 8)

MORE TESTS NEEDED
9
Step 12: trying to balance the SB with
input power

Two successful tests performed
Increase TCS power
 Increase up to 9.3 W IMC transmitted power in order to
balance the SBs
 Obvious gain in the B1 shot noise



Around 15% PIMCincrease gives a 7% shot noise reduction
No particular signal worsening detected
10
Moving the locking offsets at step 12


Sensitivity comparison with TCS off/on/unb-sb
TCS power-noise budget (E.Tournefier)
From noise measurement
 Stating a 1/f
 matching 596 Hz line

Thanks to Edwige
11
Step 8: up to 4.4 W



With the same configuration as during the transient (MICH on
B5_ACq and PRCL on B2_3f_ACp)
USB reaches very low values
(SFP reconstruction fails)
Looking at phase camera:



At about 1.3 W the ITF behaves like a “cold” one
Should correspond to equal input mirror RoCs
Long unlockable period


Only two locking trials
Saturation of PR_zCorr signal but with different behhaviors:



Step 6.5 unlock (25-30 Hz oscillation of PRCL loop)
Step 7.5 unlock (145 Hz oscillation on all the loops)
Not enough to conclude anything
12
Step 8: scanning OMC temperature

In the normal state the

total SB power in the df is about 27.9 mW



With 1.5 W TCS laser power

53.1 mW (doubled!)




87% TEM00
13% Laguerre
Going to 2W slightly increases the total power
SBs strongly unbalanced


46% TEM00
54% Laguerre
rotating B2_3f phase (for best P/Q at PRCL 62 Hz line) of 12 degrees it
unlocks
Possible explanation:


LSB has a good Gaussian shape ( g high recycling gain)
USB has almost completely a Laguerre shape ( g almost no recycling
gain)

Without the new phase camera there is no way to prove it.
13
Lock acq.: 700 mW from the beginning

Starting with an already heated WI mirror with 700 mW


B5_2f power starts from higher values at the beginning of the lock
acquisition and decreases slower than usual.
At the MICH/PRCL mixing now there is no powers step


At the MICH PRCL driving matrix reshuffling signals seem to come back
to the “normal” configuration


8MHz more or less the same as before, B2_3f is changed by the TCS
TCS has a clear effect on the ITF working point



B2_3f_ACp responds in a different way to MICH displacement
It is not clear what really happens
Is it a good thing?
MORE TESTS NEEDED
STARTING WITH LOW Nd:Yag POWER
14
Lock acquisition: with various power
from step 6

Modifying the lock acquisition in order to
survive in the old dying high-power state
DARM on B1p
 As soon as ITF in df (on B8_ACp) we switch on the
TCS at various powers
 No evident effects

15
Investigation on 10-11 Hz instabilities





At step 12 “resonance” appearing on MICH and PRCL loops
Similar instability triggered by BS-marionette reallocation
Beta servo non working well around 10 Hz (different shape from
PRCL to B1)
Measurements on PRCL OLTF while moving the MICH driving
term to PR show something around 10 Hz
Not well balanced BS marionette re-allocation?




Measure BS TFs Ma=MariogB1, RM=RMassgB1, zGc=z_GcgB1
Ma/RM gives the filter to be used for reallocation ( but fz<0 )
zGc/RM actuation unbalancing to be compensated
TO BE IMPLEMENTED
BS
PR
=
0.707
0
MICH
-2.33
-1
PRCL
16
Single free swinging cavity



We locked the two arms (step 1) and then
We induced several unlock (up to 0.9 W TCS power)
Look at WI free swinging cavity

estimate the elastic deformation (RoCs) of the WI HRcoating surface from measuring higher order mode (TEM02)
frequency shift (R.Day/B.Swinkels)




Main peak identification
Sub-peak identification
Frequency shift
Compare the result with what is expected (0.3 % shift)

Slight shift detected (about 5 time less then expected)
17
Unlockable period after single cavity
tests

5 unlocks at step 5.5


Pretty much at the moment of reaching the dark fringe
Similar behaviors




Unlock due to B5_d2_ACp (SSFS) fast oscillation
B5_d2_ACp dirty since some seconds (except #2)
#4 very dirty DARM
2 unlocks at step 6.5 (#5 and #8)


30 Hz DARM oscillation
B5_d2_ACp saturation
18
Changing the TCS laser shape

Std rIN=5 cm, rOUT=20 cm
increase of about 40% the rIN
 same laser power
 No visible effect on transient signals

Not the actual shape
Not the actual shape
DL3-Axidecreased
DL3-Axiincreased
19
Conclusions

To be understood:

Origin of 10 mW effect




if it is a physical problem on the system should ABSOLUTELY be solved
(we want to perform accurate locking test with low power)
What signal to control SB balancing (alignment…)
Free swinging cavity null result
To be done:






Central spot TCS for avoiding Newton rings
Tests with low Nd:Yag power
Phase camera 1 installation to understand the actual SB behavior
New BS marionette re-allocation filter implementation
Trying to survive in the old dying high power state
Tests varying the annulus shape
20
21