Optical%20characterisation%20of%20VIRGO%20E.%20Tournefier%20ILIAS%20WG1%20meeting,%20Cascina%20January%2025th%20,2005

1
Optical 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

2
Optical 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
3
Why 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

4
Why 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?

5
Matching 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

6
Matching 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

7
Measurement 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
8
Measurement 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
9
Measurement 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

10
Finesse 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
11
Measurement 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
12
Measurement 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

13
Do 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

14
Measurement 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.

15
Tentative 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

16
Measurement 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
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Recycling 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
18
Recycling 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
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Measurement 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

20
New 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

21
Measurement 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

22
Contrast 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

23
Commom 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

24
Commom 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

25
Conclusion

• 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