Transcript G070148-00 - DCC

Large Aperture Dielectric Gratings for High Power LIGO
Interferometry
Jerald A. Britten, Hoang T. Nguyen, James D. Nissen, Cindy C.
Larson, Michael D. Aasen, Thomas C. Carlson, Curly R. Hoaglan
Lawrence Livermore National Laboratory
Patrick Lu, Ke-Xun Sun, Robert L. Byer
Stanford University
LSC/Virgo Meeting, Baton Rouge
March 19-22, 2007
Optics Working Group
UCRL-PRES-229023
G070148-00-Z
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This work was performed under the auspices of the United States
Department of Energy by the University of California Lawrence
Livermore National Laboratory under contract no. W-7405-Eng-48
Motivation
• LIGO needs reflective grating beamsplitters for high-power interferometry.
-
Transmissive optics suffer from thermal lensing.
> CO2 heating laser already required for ITM’s on initial LIGO.
> Advanced LIGO will require 830kW of circulating power in the arms. For a 6cm spot size,
that comes out to 30kW/cm2 of intensity.
• Significant investment and progress has been made in the development of
multilayer dielectric diffraction gratings for high-energy Petawatt-class laser
systems.
-
Projects such as LIGO are poised to take advantage of this capability.
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Laser Heating in Advanced LIGO
End test mass
Intracavity Power Level
830 kW
Laser
Power
after
Mode
Cleaner
125 W
Thermal lensing due
to dn/dT
Power
recycling
mirror
Input test mass
Laser
Mode
Cleaner
2.1 kW
@Beam
Splitter
Signal recycling
mirror (7%)
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All Reflective Grating Reduces Bulk Heating
Intracavity Power Level
830 kW
Littrow mounting.
-1 overlaps with
input
Laser Power after
Mode Cleaner 125 W
2.1 kW @Beam
Splitter
• Gratings used as
beamsplitter and input
couplers to arm cavities.
- No thermorefractive
aberrations. Thermoelastic
aberrations only.
- No bulk absorption. Surface
absorption only.
- Greater design flexibility:
allows the use of
nontransparent substrates,
which can be more
thermally conductive.
Input grating. Large aperture.
~80cm aperture required based on 67o
Littrow spread.
K.X. Sun, et. al, “All-reflective Michelson, Sagnac, and Fabry-Perot interferometers based
on grating beam splitters,” Optics Letters, 8(23), p. 567-9 (1998)
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Current Subject of Investigation
Advanced LIGO Arm Cavity
Diffraction efficiency > 99.5%
Light incident on grating at
Littrow angle (67o)
Littrow mounting causes
-1 diffracted order to
circulate in cavity.
Cavity finesse ~ 1200
830kW
circulating
power
HR End Mirror
The goal of this study is to build and test a working model of this configuration.
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LIGO’s Requirements for Gratings
• Large Aperture
• Low thermal aberrations
- 31.4 cm mirror diameter1.
- Gratings must be 83 cm based
on a 67o Littrow angle.
• High Efficiency
- Advanced LIGO: 830 kW
circulating in arm cavities2
- 6 cm spot size
- Intensity: 30kW/cm2
• Low Scattering
- 99.5% desired3
- Over large aperture for beam
quality
- Light scattering noise couples
seismic activity into signal
P. Barriga, et. al, “Numerical calculations of diffraction losses in advanced interferometric gravitational wave detectors,”
http://www.ligo.org/pdf_public/barriga.pdf
2 C. Zhao, et. al, “Compensation of Strong Thermal Lensing in High Optical Power Cavities,” gr-qc/0602096 v2
3 Miyakawa, et. al, “Measurement of Optical Response of a Detuned Resonant Sideband Extraction Interferometer,” LIGOP060007-00-R
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MLD grating fabrication process flow
resist
layer
stack view
1.70
1.50
1.25
1.00
0.75
0.50
0.25
0.00
-0.25
-0.50
-0.75
-1.00
-1.25
-1.50
-1.75
-2.00
-2.25
-2.50
-2.75
-3.00
-3.25
-3.61
0.00
0.20
0.40
substrate
0.57
Design
(efficiency,
E-field distribution, …)
Multilayer
oxide
deposition
substrate
substrate
Clean
Prime
Resist coat
Expose
Develop
Metrology
substrate
Transferetch (RIBE)
substrate
Cleaning &
Metrology
575 nm period
(only part of process
not done at LLNL)
During manufacture, optics are exposed to heat,
aggressive liquids, and vacuum processing.
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In the past 18 months LLNL has produced 77
production gratings @ 1740 l/mm
Apertures from 140 mm to 800 mm
RMS Diffraction Efficiency (%)
Over 11 m2 of grating surface with average efficiency > 96%
5.00
4.00
3.00
2.00
LIGO
project
1.00
0.00
90
92
94
96
98
100
Average Diffraction Efficiency (%)
For use wavelengths from 1017 nm to 1064 nm
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Ratioing scanning photometry setup @ LLNL for
efficiency measurements
Grating and HR
on XZ translation stage
•Ratio HR to beam in air at
beginning and end of test for absolute scale
•Ratio grating to HR at same location for every
pass while scanning grating over beam
Signal detector
Fused silica window
Reference detector
Fused silica wedge
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500 mW CW 1064 nm laser
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>99% diffraction efficiency gratings have been
delivered to Stanford
1740 line/mm HfO2/SiO2 grating on BK7 substrates
#011 (200x100 mm):
99.2% Ave, 0.3% RMS
histogram
of 10K data points
#021 (170x100 mm):
99.3% Ave, 0.2% RMS
0.90 0.92 0.94 0.96 0.98 1.00
histogram
of 10K data points
Design spec of >99.5%
is achievable
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Gratings exhibit flat diffracted wavefront
Diffracted Wavefront at Littrow angle for 1053 nm (66.4o)
011 CW
Resolution
Wedge
PV
PVq(99%)
RMS
Strehl
021 CW
991 x 1005
1.000
0.1765 wv
0.1509 wv
0.0292 wv
0.967
Resolution
Wedge
PV
PVq
RMS
Strehl
991 x 1005
1.000
0.1310 wv
0.0837 wv
0.0175 wv
0.988
CW orientation
w/ S/N up
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LIGO aperture required has been demonstrated for
other projects
#005 (800x400 mm) @ 1053 nm,
72.5o: 97.3% Ave, 0.7% RMS
LLNL has delivered 13 production
gratings at this size (800x400 mm)
0.90 0.92 0.94 0.96 0.98 1.00
GSI5_06_04_19_221034_EFF_XLS
#011: 97.1% Ave @ 1053 nm, 72.5o:,
so this grating could be >99% for LIGO conditions
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Byer Group Laser Lab
NPRO Oscillator and Rod Amplifiers
100~200 W Amplifiers
Laser lab is being set up for high
power cw laser heating
characterization of LLNL gratings
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Initial thermal testing of 100 mm diameter LLNL MLD
witness grating
Modecleaner
Mini Slab
Power Amplifiers
(300 W diode pump)
30W Single
Frequency
TEM00
1064nm laser
He-Ne
Laser
LIGO 8 W NPRO
oscillator with 2 stage
4 passes rod amplifier
Shack-Hartman
Wavefront Sensor
4x Beam Expander
Max power was 34.5 W in a
1.5mm spot: ~2 kW/cm2
MLD Grating
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Thermal testing of 100 mm diameter LLNL witness
grating
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Grating wavefronts measured at two power densities
show no difference
removed Tip, Tilt, and Piston
11 W, 0.6 kW/cm2
34.5 W, ~2 kW/cm2
PV: 59.3 nm, Rms: 11.2 nm
PV: 53.4 nm, Rms: 9.4 nm
3mm HeNe Probe Beam
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Next Steps
• Fiber laser and amplifiers to enable 100 W of single
mode light
• Cavity configuration to enable thermal tests to
reach ~30 kW/cm2 for centimeter-scale beams
• Large-aperture efficiency measurements will be
made at Stanford to corroborate LLNL
measurements
• Scatterometer measurements, including back
leaks
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Efficiency/Finesse Measurements planned for near
future
Single Frequency
6 W YAG laser
1064 nm wavelength
Cavity end-mirror
with piezo
EO Modulator
Circulator
Servo
Electronics
Intracavity
Power ~1200 W
LLNL Dielectric Grating
on Translation Stage
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Thermal aberration measurement at higher power
planned for near future
Single Frequency
100 W YAG laser
1064 nm wavelength
Or Fiber Laser 100 W
Shack-Hartman
Wavefront Sensor
He-Ne Laser
Beam
Expander
Interferometric
Sensor CCD
EO Modulator
Circulator
Servo
Electronics
Intracavity
Power ~20 kW
Power Density
LLNL Dielectric Grating > 30 kW/cm2
on Translation Stage
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Conclusions
• Capability exists to manufacture large-aperture multilayer dielectric
diffraction gratings for demanding LIGO applications
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>99% diffraction efficiency in Littrow mount
-
kW power-handling capability
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80+ cm apertures
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