Title: Optical%20characterisation%20of%20VIRGO%20E.%20Tournefier%20ILIAS%20WG1%20meeting,%20Cascina%20January%2025th%20,2005
1Optical characterisation of VIRGOE.
TournefierILIAS WG1 meeting, CascinaJanuary
25th ,2005
- Introduction
- Beam matching
- Measurements of Fabry-Perot parameters
- Measurement of recycling gains
- Lengths of the recycling cavity
- Conclusion
2Optical parameters of the ITF
Contrast defect, CMRR
And the lengthes - Recycling length
lrec l0(l1 l2)/2 -
Asymmetry of the small Michelson ?l l1 - l2
3Why are we interested in these measurements ?
- The mirrors parameters (reflectivity, losses,
radius of curvature) have been measured in Lyon
and are within the specifications. - gt are the ITF optical parameters as expected
? - gt also important for the tuning of the
simulations - Finesse
- expected value from Rinput88 F50
- the rejection of the common mode depends on the
finesse asymmetry between the 2 FP cavities - Radius of curvature (ROC) of the end mirrors
- Important for the automatic alignment it uses
the Anderson technique - gt the first HG mode of the sideband must
resonate in the cavity - gt the modulation frequency depends on the
ROC
4Why are we interested in these measurements ?
- Losses (reflectivity) of the FP cavities
- expected to be 100ppm
- the recycling gain depends strongly on them
through Rcav - are they small enough ?
- Recycling gains
- with Rrec 92.2 we expect Grec 50
- does the recycling gain fit with the expected
losses? - we will soon change the recycling mirror
- Need to understand the actual gain in order to
define the reflectivity of the next mirror - Recycling length
- The sidebands must resonate in the recycling
cavity - Recycling length has to be tuned to the
modulation frequency - Contrast defect, CMRR are they small enough?
-
5Matching of the input beam to the ITF
tuning of the telescope length
Beam size and power
- The matching of the input beam parameters is done
by tuning - the length of the input telescope length
- The best matching maximizes the power stored in
the FP cavity - Note that the beam is astigmatic due to the
spherical mirrors of the telescope - a perfect matching cannot be reached
6Matching of the input beam to the ITF
- The monitoring of the beam shape at 3km vs the
telescope length allows to determine the input
beam parameters wx, wy,Rx,Ry - 94 of the beam power is
- coupled to the FP cavities
7Measurement of the Fabry-Perot parametersFinesse
(F) and radius of curvature (ROC)
- Use a single Fabry-Perot cavity with mirrors
freely swinging -
gt use the
transmitted power
Transmitted DC power
FSR
- Shape of the Airy peaks (FWHM)
- distance between 2 peaks (FSR)
- Finesse
FWHM
d02
Position of the first and second order modes gt
Radius of curvature of the end mirrors
8Measurement of the Fabry-Perot parameters
- Problem with real data the speed of the mirrors
is not constant - gt need to correct for the non-constant speed
- We know that between 2 peaks the cavity length
has changed by ?/2 -
-
gt deduce the cavity length
l(t) versus time -
- The cavity length is
modeled with l(t) A cos(wtp) (true on 1
period) - gt the speed and the
length of the cavity are known
?/2
9Measurement of the Fabry-Perot parametersFinesse
(F)
- Another difficulty for the finesse the Airy peak
is distorted by dynamical effects gt the FWHM is
not well defined and is speed dependent - Solution 1
- - Use the value of the speed measured
- - Simulate the Airy peaks for different
- values of F
- - Find the F value for which the simulation
- fits the best to the data
- Solution 2 use the ringing effect
- the amplitude and position of the peaks
- depend on the speed and on F
- gt Determine v and F by comparing data and
simulation
10Finesse measurements
- From the data taken with free FP cavities
- The finesse is extracted from a comparison of the
shape of the Airy peak between the data and
Siesta simulations - North West
- ringing effect, high speed cavity (method 2)
47 - (RNI 87.5)
- low speed cavity (method 1)
490.5 51 1 - (RNI 88.0 RWI 88.4)
- To be compared to Lyon measurements of mirror
reflectivities - RNI 88.2 RWI 88.3
50 51 -
- Good agreement with the coating measurements
- Note that the finesse can vary by /-2 effect
induced by thickness variation of the flat-flat
input mirror with temperature variation (not
observed yet)
Fabry-Perot effect in input mirror ?d gt ?F
11Measurement of the Fabry-Perot parametersRadius
of curvature of the end mirrors (ROC)
- Radius of curvature of the end mirrors
- Principle of the measurement on the data
- extract the ROC from the distance between the
first and second HG mode and the 00 mode (free
cavity) - difficulty the speed of the cavity is not
constant - Method
- use the position of the TEM00 modes to
- determine the length l(t) assuming
- l(t) A cos(wtp)
-
- 1/ Measure the time of the HG modes TEM00,
- TEM01, TEM02 t0, t1, t2 and deduce the
- distance between modes d0il(ti)-l(t0)
- 2/ extract ROC from d02 and d01
d02
12Measurement of the radius of curvature
- Results using this method
-
ROC(North) ROC(West) - From the data
- using 2nd mode
3550 20 m 3540 20 m - using 1rst mode
3600 40 m 3570 80 m - The ROC can be determined within 1-2
-
- From the map of the mirrors measured at Lyon
- -gt simulation of the cavity with the real
mirror maps, same method as on the data -
- using 2nd mode
3558 10 m 3614 10 m - using 1rst mode
3566 20 m 3643 20 m - Differences are expected the different modes do
not see the same radius of curvature - Data and simulation results differ by at most 70 m
13Do the ROCs fit with the modulation frequency ?
- The modulation frequency has been tuned so that
it resonates in the input mode cleaner - (see Raffaeles talk)
- One sideband should also resonate in the FP
cavities for the 01 mode (Anderson technique) - the modulation frequency should correspond to the
Anderson frequency within 500Hz -
- The Anderson frequency is defined by the radius
of curvature of the end mirror - with the extreme values obtained from the
measurement or the simulation with real - maps
- - R3530m gt fAnderson 6264540 Hz
- - R3640m gt fAnderson 6263930 Hz
- OK with fmod 6264150 Hz
- fmod is different from the Anderson frequency by
at most 400Hz
14Measurement of the Fabry-Perot parameterslosses
(or cavity reflectivity)
rin
rcav
losses (L)
- The cavity reflectivity decreases with losses
- Losses on the cavity mirrors due to absorption
scattering - 10 ppm measured in Lyon
- But a simulation with real mirror maps gives
Rcav 98 - Expect non negligible losses
Rcav 98 ? L 600 ppm - with L round trip losses
- These losses might be due to mirror surface
defects.
15Tentative measurement of the cavity reflectivity
(losses)
- Use a freely swinging FP cavity
- - When the cavity goes through a resonance
- the reflected power is
- Pmin P0 x Rcav
- - Out off resonance the reflected power is
- Pmax P0
- gt Rcav (Pmax-Pmin)/Pmax
- Problems
- large dynamical effects
- gt need a very slow cavity
- the measurements seem very dependent on the
alignement - gt Some hints for Rcav 96-98 but no
clear measurement - gt indicates round trip losses of the
order of 500-1000ppm - gt Try to extract Rcav from the recycling gain
measurement
16Measurement of the recycling gains Gcarrier , GSB
- Recycling gain of the carrier
- Recycling gain of the sidebands
- Expected values (with Rcar, RSB1)
- Gcarrier 50 and GSB 36
- Measurement of the recycling gains
- Compare the power stored in the cavity
- with/ without recycling
- Can also use the reflected power
- to extract rcar
?l l1 - l2 ? 2?fmod
rSB, rcar
l1
rrec
l2
rITF
rSB, rcar
Pstored
Preflected
17Recycling gain of the carrier
- 1/ Comparing the power stored in the cavity with
and without recycling - Gcarrier (PVirgo/ Precombined )x TPR ? 30
- Equivalent to Rcav 97-98
- 2/ And with the reflected power the ITF
reflectivity - RITF PVirgo / Precombined ? 0.6
- Equivalent to Rcav 99
- Effect of higher order modes they are not
recycled - gt With 1/ the recycling gain for TEM00 is
underestimated gt Rcav also - gt With 2/ the ITF reflectivity is overestimated
gt Rcav also - Probably we have 97 lt Rcav lt 99 and
therefore losses around L300-600ppm - We should have better estimations when the
automatic alignment is implemented
Stored power (Watt)
Virgo
Recombined / TPR
18Recycling gain of the sidebands
- The stored power is demodulated at twice the
modulation frequency - A comparison of this power with and without the
recycling gives an estimation of the sidebands
gain - Gives GSB ? 20 equivalent to RSB ? 97
- Another method using the stored powered in
- Michelson, CITF and Virgo configurations
- gives the same result
- A simulation with real mirrors gives GSB ? 25
- Again we will have a better estimation when the
automatic alignment is implemented and with the
full input power
Stored power at 2xfmod (Watt)
Virgo
Recombined / TPR
19Measurement of the recycling mirror reflectivity
- The reflectivity of the recycling mirror rrec is
extracted from the measurement of the gain of the
central ITF (g0) - g0 1 / ( 1-rrec rin)
- g0 is obtained from the power stored in the
- central recycled interferometer
- g0 ? (PCITF / Pmich)
- rin is known precisely enough from the finesses
measurement rin 88.0/-0.5 - From g0 Rrec (92.0 /- 1.6) lt- limited
by power fluctuations due to alignment - Which agrees with the coating measurement made in
Lyon Rrec 92.2
20New PR mirror
- PR mirror will soon be changed
- monolitic mirror (resonances of the actual mirror
disturb the locking) - flat-flat mirror instead of curved-flat
- gt Change also the reflectivity ?
- The actual PR mirror has a reflectivity RPR
92.2 - The reflectivity can be increased in order to
increase the recycling gains - It should not be too close to the cavities
reflectivity in order to avoid phases rotations
which will complicate the lock acquisition - gt keep RPR lt Rcav for the carrier and the
sidebands - FP effect in flat-flat mirror gt need to be
carefull with the AR side coating - the real PR reflectivity has to be defined
including this effect - gt We decided to increase the PR reflectivity
from 92 to 95
21Measurement of the lengths lrec , ?l
- Why do we need to know these lengths?
- The recycling length lrec should be tuned to the
modulation frequency ( the SB should resonate) - The length asymmetry ?l gives the transmission of
the sidebands - These lengths are known from the tower positions
at /- few cm. - Can we measure them using demodulation phase
tuning of the dark fringe signal ? - - if lrec is wrong
- the optimum demodulation phase used for the
recombined and the recycled ITF will be
different - - ?l the optimum demodulation phase for
the West cavity and for the North cavity should
be different by ?? ? ?l/c - A precision on ?? of 0.1o will give 1.3 cm
on ?l -
- gt Still to be investigated
22Contrast defect
- In the recombined configuration, the power on the
dark fringe is given by - Pdf P0 ( J02(m) (1-C)/2 2J12 (m) T )
- Where T is the sidebands transmission T sin2(?
?l/c) 0.013 - Minimum power observed on dark fringe Pdf 6.5
?W -
gt Pdf / P0 3 10-4 - Power on the bright fringe
P0 45 mW - But the contribution from the sidebands is not
negligible - 2 P0 J12 (m) T (6.5 ?2 ) ?W
( m is not precisely known) - P0 J02(m) (1-C)/2 lt 2 ?W and 1 C lt 10-4
- The same exercise on the full Virgo configuration
gives the same result - gt The contrast defect seems quite good 1 C lt
10-4
23Commom mode rejection ratio (CMRR)
- The common mode noise (for example frequency
noise) is not completely canceled by - the interference on the dark fringe the
remaining contribution reflects the - asymmetry of the 2 arms ( finesse, losses,..)
gt CMRR - Some measurements have been in the recombined
configuration (no recycling) during C4 run (june
2004) - - The photodiode used for the frequency
- stabilisation had high electronic noise (n).
- - The frequency stabilisation introduced this
- noise in the ITF as frequency noise (??).
- - This noise was seen on the dark fringe as a ?L
- ?L ?? x (?/ L) x CMRR
24Commom mode rejection ratio (CMRR)
- Propagation of the electronic noise introduced by
the frequency stabilisation to the sensitivity - The CMRR is estimated at high frequency (gt few
kHz) CMRR ? 0.5 - More studies are going on with some frequency
noise lines injected during the C5 run
25Conclusion
- The measurement of the mirrors reflectivities
(recycling, input mirrors) with the ITF data fits
with the expectations - The losses in the FP arent precisely known but
seem not negligible - L 500 ppm
- The recycling gains will be better known when the
automatic alignment is implemented and the
measurement easier with the full input power - Gcarrier 30 (expected 50)
- GSB 20 (expected 36)
- The contrast and the CMRR are quite good 1 C lt
10-4 and CMRR lt 0.5