Amaldi6 Okinawa

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Flat top beam interferometerto depress mirror
thermal noise

• Juri AGRESTI Erika DAMBROSIO
• Riccardo DESALVO Danièle FOREST
• Patrick GANAU Bernard LAGRANGE
• Jean-Marie MACKOWSKI Christophe MICHEL
• Jean-Luc MONTORIO Nazario MORGADO
• Laurent PINARD Alban REMILLIEUX
• Barbara SIMONI () Marco TARALLO ()
• Phil WILLEMS
• LMA Lyon/EGO Caltech/LIGO
• () Thesis on Mex-Hat mirrors

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Why do we need a flat top (mesa) beam profile

• Thermal noise is a fundamental limiting factor
  for GWID sensitivity
• Gaussian beams sample a relatively small fraction
  of the mirror surface
• Thermal noise (all kinds) is depressed by
  averaging on larger fraction of the mirror

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Flat Top (Mesa) Beam profiles Compared with
Gaussian beams
The advantages The price you pay
Profiles normalized for Same Integrated power
Higher peak power
Steep rim
Slow exponential fall
Aspheric profile
Steeper fall
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Flat Top (Mesa) Beam Design
Flat mesa beam profiles require rimmed
Mexican Hat mirror profiles
Please see P. Beyersodorf poster
The beam intensity profile is flat and then falls
off much faster than a Gaussian The beam diameter
at the mirror can be as large as 50 of the
mirror diameter The mirror is not spherical!
Must be manufactured with a special procedure
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Thermal noise reduction using Mesa Beam
(evaluation for Advanced LIGO mirrors)
2x
Gaussian beam
Mesa beam
Beam rad. (cm)
mirror
mirror

• Advanced LIGO Evaluation conditions
• The beam radius is dynamically adjusted to
  maintain a fixed diffraction loss 1ppm
• The mirror thickness is also dynamically adjusted
  as a function of the mirror radius to maintain a
  fixed 40 Kg mirror mass.

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Comparison between Gaussian and Mesa beam
Gaussian beam Mesa beam
Constraint 40 Kg mirror
Substrate Thermoelastic Coating
Brownian Substrate Brownian Coating Thermoelastic
mirror
Overall thermal noise budget
Gain factor
mirror

• Calculation at the frequency 100 Hz

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Simulated Mex Hat cavity modes
Flat TEM 00 Beam profiles homologous to Gaussian
profiles But confined by the mirrors rim
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How can we build such a strange mirror profile?

• A two step process

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Mirror construction
Start from a flat substrate 99 of the
mirror Profile is generated with a
Dead-Reckoning Deposition step A carefully
profiled mask between the SiO2 ion source
and the rotating substrate, calculated to deposit
the required thickness where needed
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Mirror construction
Then the mirror profile generated by the
first step is interferometrically measured A
map of its deviation from the ideal profile is
generated The deviations are corrected under
numerical control with a SiO2 molecular beam
pencil
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What beams can we expect from the new mirrors

• The mirrors built are at the lowest limit of
  manufacturable dimensions
• Larger mirrors much easier!!
• The test Mex-hat test mirrors are not perfect
• The maps of the actual test mirrors have been
  used to calculate the expected best beam profile

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Simulated beam profiles Mirror map 5008
12-10-8 m
Deviation from ideal profile
2-10-8 m
Deviation from ideal profile
2.5 10-8 m
2-10-7 m
80
80 mm
40 mm
40
After tilt 1micro rad
Simulation (as mapped)
Ideal beam
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The test setup

• We built a 7 m long Fabry Perot Cavity F 60
• Inject Gaussian beam
• Monitor the shape of the transmitted light
• Cavity cleans the mode into a Flat top profile
• Learn of ease of use and ease of control
• Do not measure the actual thermal noise

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The test setup

• A rigid, folded, suspended Fabry Perot Cavity

Flat folding mirror
Thermal shield
Spacer plate
Flat input mirror
2x 3.5 m
INVAR rod
Vacuum pipe
MH mirror
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Suspension System
GAS spring
wires
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Suspension wires
Vacuum pipe
Thermal shield
Spacer plate
INVAR rod
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What are the instrumental limits

• The FP laser cavity was first operated with a
  spherical mirror
• The deviations from Gaussian profile give an
  estimation of the instrumental limits (10)

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Real MH mirrorsComparison data-simulation
Beam scanner measurement
Simulation tilted mirror
Perfect alignment simulation
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Mex-Hat
Mode 1,1
Gaussian
No confinement
Rim confinement
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Initial study ofbeam deviationsfrom Flat Profile
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• Work to do. . . .
• compare beam profiles for the three MH prototypes
  to evaluate effect of imperfections
• measure angular sensitivity

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ConclusionsThermal Noise Prospects

• Most of TN originates from the Tantala in
  coatings
• Doped Tantala gives x1.25 (v1.5)
  improvements
• x2 may be possible with other dopants
• Mex-Hat mirror profile give x 1.8
  improvements
• Optimized coating structure x 1.37
  improvements
• Overall x 3.1 TN improvements
• Does this mean x 3.1 range improvement????

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Thermal Noise Prospects
NS-NS inspiral Reach 193 Mpc 251 Mpc x
1.3 Detection Event Rate x 2.2 Require Higher
Beam power or Quantum tricks to take advantage of
lower thermal noise
Mex-Hat mirror profile optimized coatings
structure doped Tantala
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