Transcript Technologies for the Future of interferometric detectors

Technologies for the Future of interferometric
detectors
C. Nary MAN
UMR 6162, Observatoire Cote d’Azur, BP 4229, 06304 Nice Cedex 4, France
•Introduction : fundamental limits of ground-based detectors
•Possible solutions in the MF range:
High power lasers
new materials for optics
controls of optics behaviour
•A lot of ideas to extract better signals with sophisticated
configurations of signal recycling….
•Other optical configurations ……
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Sensitivity curve and fundamental limitations
Pendulum thermal noise
Mirror thermal noise
Shot noise 20W
Seismic wall
Quantum limit
Gravity gradients
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Issues in MF range
 f 
1
 .
h˜shot( f )  1.08 x 10 . f pole. 1  
f pole   .Srec .PBS
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Shot noise limit :
- directly by laser power
- indirectly by optical imperfections
24
Mirror thermal noise limit :
- Q of test-mass (substrate, coatings)
- T of test-mass, M of test-mass
Increase laser power but increase also thermal effects
(radiation pressure problem : larger masses )
hth 
k BT
M .Q
h˜rad. press 
1
h .P
2
M.Larm. f 2 3 c
New materials for mirrors, high Q even at low T, large size, optical quality
Coatings of high Q ?
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High power single-frequency laser
Stringent demands on frequency stability 10-6 Hz/√Hz (of ground-based detectors):
> 500 W ?
50 W
Front End
Low power
master
1-3 W
Medium
power slave
Power stages
50 W
•Rod systems (LZH)
•Stable-unstable slab oscillator
(Adelaide)
•MOPA type (Stanford)
•Ceramic laser
•Fiber laser
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Rod Laser systems
LZH: Laser medium is rod, end-pumped by fibre-coupled diode lasers, good wall-plug
efficiency, delivers @ 20W
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LZH: Power scaling of End Pumped rod to 100W
Modeling :
• 100 W of output power will be achieveable
•aberrations , to be compensated for
•aberrations comparable in end pumped and transversally pumped rod
Realized (02):
•4 diode boxes have been set up (1200 W of pump power)
•temperature stabilization
•pump light homogenization has been demonstrated
•45 W single mode and 75 W multi mode laser has been demonstrated (single rod, no
compensation)
Mitsubishi: > 200 W achieved in TEM00 output with transverse diode-pumped rod laser
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Adelaide 100 W slab laser configuration
Nd:YAG slab pumped by 520 W fibre-coupled diode lasers
Resonator stable in the zig-zag H direction, unstable in V direction
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Stanford MOPA design
amplification goal > 100W with 2 zig-zag slab amplifiers and 20W master oscillator
27 W stable operation achieved at 1st stage
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High power lasers: ceramic lasers
Ceramic laser : any size (23 cm long max for YAG
xtals, twice this length for ceramic), any shape, high
Nd doping, mass production…
first Nd:YAG ceramic laser gives 300 mW output
(Ikesue et al. in 1995)
98 in Japan, development of highly transparent
Nd:YAG ceramic: efficiency comparable to single
xtal lasers , 1.5 kW cw output ( Ueda et al,2001)
1.46 kW obtained in multimode
operation with YAG ceramic
Quality of the beam has to be worked out
Wavefront quality, distributions of Nd ions to be compared vs xtals…
Possibility of having Nd:Y2O3 ceramic where thermal conductivity twice of YAG
with similar thermal expansion coef.
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High power lasers: Fiber lasers
Erbium doped Silica
Ytterbium doped all glass (eff > 80%)
Ytterbium doped Silica (eff 85%)
Used as power amplifier with NPRO,
emits 20 W on single-frequency output
(Jena, 2001)
Possibility of scaling up to 100 W with
9m fiber.
Fiber lasers based on rare-earth doped
silica: very high output powers up to 2 kW
cw operation in June 02 (IPG Photonics).
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Substrate for future mirrors
low absorption material with good conductivity, high Q, good optical quality ….
Fused Silica (today substrate):
•
Absorption: best quality has 0.7 ppm/cm
•
Numata et al (Amaldi 01): measured Q of 13 kinds of FS, Q = 7.105 to 4.107 : no simple
correlation with known specs, seems to increase with annealing process…
•
Homogeneity and roughness of polishing: meet specs
Sapphire:
•
Absorption : around 20 ppm/cm, vary following samples
•
Q = 6.5x 107 at room temperature and low temperatures behavior studied extensively,
•
but direct measurement of thermal noise necessary
•
Homogeneity: need to be improved by factor 5 to 10 (Caltech, CSIRO)
Silicon:
•
Used in reflection only (suitable for all-reflective interferometers)
•
Q around 2x108 confirmed for a variety of samples, thermal noise improves at low T
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New Candidate Materials for mirrors: CaF2
(VIRGO, Elba 2002 )
Low absorption, high resistance to thermal
& mech shocks, high Q, good candidate for
cryogenic solution
(Silicate bonding not working )
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Coatings: optical performances (1)
Optical performances achieved today in Virgo-SMA:
1992
1994
Absorption at 633 nm
20
ppm
10 ppm
<5
ppm
4 ppm
Absorption at 1064 nm
-
2-3
ppm
0,5
ppm
0.6 ppm
Scattering at 633 nm
50
ppm
5 ppm
1,2
ppm
2 ppm
0,6
ppm
4 ppm
over
F150 mm
Scattering at 1064
nm
-
2000 to Virgo
4 ppm
Wavefront
-
-
-
3.8 nm
rms over
F150mm
Components
diameter
25
mm
50 mm
25 mm
350 mm
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Coatings : optical performances (2)
80 mm high reflectivity
mirror wavefront before
and after corrective coating
Ion Source
SiO2 target
Mask
Sputtered
Atoms
Robot
Y
X
Mirror
Interferometer
Wavefront control
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Coatings : mechanical loss
•Levin (98) showed coatings could be a limiting source of loss
•Preliminary measurements at Glasgow, Stanford & Syracuse: fcoating = 2.5 x10-4
•To be used in avanced/future detectors, loss factor < 10 –5
•Coating program initiated to measure thin and thick substrates with different number of
coating layers , ….
•Loss factor at low T (Yamamoto, Elba 02): fcoating < 10- 4 without change of reflectivity
First conclusions:
•First interface between layers is not dominant source of loss
•Interfaces between multi-layer are not dominant source of loss
•Interface substrate-coating is not a signicant source of loss
•Ta2O5 layer has higher loss than SiO2
•What is the way forward?
Other high index materials than Ta2O5?
•Will it be a trade-off between absorption and mechanical loss ?
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Thermal effects
Thermal lensing of test-mass:
•large efforts to reduce thermal lensing by reducing absorption in sapphire, but not very
reliable ? (Fejer 2001 LSC, Blair 97, Benabid 00)
•Tomaru et al (Amaldi 01) reported efficient reduction of thermal lensing in the
cryogenic sapphire mirrors
Wavefront distorsion of optical components:
•Active wavefront corrections via direct thermal actuation are being developed at MIT
•R&D to measure aberrations (Shack-Hartmann type sensors, and correct with
deformable mirrors (Stanford) the wavefront distorsion of high power lasers.
•Reshaping of laser beams with intracavity deformable mirrors
•Reshaping of laser wavefronts with deformable mirrors outside the lasers
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Compensation of wavefront deformations
Mirror heating with outer ring and scanned beam heating (MIT)
Ottaway PAC 12
M.Zucker LSC meeting 02
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Laser cooling of solids
1929: anti-Stokes fluorescence is basis of optical refrigeration cycle.
Three-level atom example:
•Laser pumps atom from E2 to E3
•Radiative deexcitation from E3 to E2
•Fluorescence from E3 to E1
=> absorption of a phonon E2-E1
=> decreasing the thermal energy
60 ’s: GaAs, Nd:YAG,…
90 ’s; Yb doped ZBLAN: up to 48°C
(Los Alamos)
E3
Radiative transitions
Laser pumping
E2
Phonon absorption
E1
Cooling a 3-level atom
Applications to GW detectors:
•Identify materials also with high Q, high homegeneity
•Recycle the anti-Stokes fluorescence to remove its th.effects out of the solid
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All-reflective interferometers
Advantages:
•Higher light power because no bulk absorption
•Use of test mass materials giving lower thermal noise
such as xtal silicon
Experimental demonstration in 98 by Sun & Byer
in a Sagnac configuration
Drawbacks come from use of gratings:
•Conversion of laser frequency noise to pointing
noise: retroreflecting compensator
•Laser center frequency drift < max deviation
•Distort spatial profile of diffracted beam
•Scattered light
Improvement needed
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Future detector: with thermal correction/compensation
Single - 50W
frequency
front end
High Power
stages (with 500W
deformable
mirror)
Wavefront
sensor
Pre-modecleaner
Wavefront correction
Faraday isolators
Phase modulators
Long Input
mode cleaner
Power
stabilisation
Correction by Deformable mirrors
+ signal recycling
configurations
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Future detector: all-reflective Sagnac
Single - 50W
frequency
front end
High Power
stages (with 500W
deformable
mirror)
Wavefront
sensor
Pre-modecleaner
Faraday isolators
Phase modulators
Wavefront correction
Long Input
mode cleaner
Power
stabilisation
Correction by Deformable mirrors
Transmission port
M2
+ thermal
compensation
of mirrors
SR
M3
grating
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M1
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Intelligent digital controls
•Digital electronics to monitor and control the complex seismic isolation
(gain and phase re-adjusted automatically with the drift /ageing of mechanics
due to environment…..)
Low noise digital electronics for all position controls (test-mass, laser beam,
beam shape, beam pointing, etc…)
•Fast digital electronics to lock the laser parameters (frequency, amplitude)
•Neural networks to manage all the controls , from the locks sequence, the
automatic relocks of each servo, the electronic gain/phase adjustments due to
the ageing of mechanical actuators, etc….., also the kind of signal extraction ?
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