1 large aperture dielectric gratings for high power ligo interferometry lsc/virgo meeting, baton...
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Large Aperture Dielectric Gratings for High Power LIGO Interferometry
LSC/Virgo Meeting, Baton Rouge March 19-22, 2007
Optics Working Group
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
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-48UCRL-PRES-229023 G070148-00-Z
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• 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.
Motivation
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Laser Heating in Advanced LIGO
End test mass
Signal recycling mirror (7%)
Power recycling mirror Input test mass
Intracavity Power Level 830 kW
Laser Power after Mode Cleaner 125 W
2.1 kW @Beam Splitter
Thermal lensing due to dn/dT
Mode Cleaner
Laser
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All Reflective Grating Reduces Bulk Heating
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)
Littrow mounting. -1 overlaps with input
• 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.
Intracavity Power Level 830 kW
Laser Power after Mode Cleaner 125 W
2.1 kW @Beam Splitter
Input grating. Large aperture. ~80cm aperture required based on 67o Littrow spread.
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Current Subject of Investigation
830kW circulating power
Light incident on grating at Littrow angle (67o)
Littrow mounting causes -1 diffracted order to circulate in cavity.
Advanced LIGO Arm Cavity
Diffraction efficiency > 99.5%
The goal of this study is to build and test a working model of this configuration.
HR End Mirror
Cavity finesse ~ 1200
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LIGO’s Requirements for Gratings
•Large Aperture- 31.4 cm mirror diameter1.- Gratings must be 83 cm based
on a 67o Littrow angle.
•Low thermal aberrations- Advanced LIGO: 830 kW
circulating in arm cavities2
- 6 cm spot size- Intensity: 30kW/cm2
•High Efficiency- 99.5% desired3
- Over large aperture for beam quality
•Low Scattering- Light scattering noise couples
seismic activity into signal
1 P. Barriga, et. al, “Numerical calculations of diffraction losses in advanced interferometric gravitational wave detectors,” http://www.ligo.org/pdf_public/barriga.pdf2 C. Zhao, et. al, “Compensation of Strong Thermal Lensing in High Optical Power Cavities,” gr-qc/0602096 v23 Miyakawa, et. al, “Measurement of Optical Response of a Detuned Resonant Sideband Extraction Interferometer,” LIGO-P060007-00-R
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1
0.995445
0.996
0
0.498
99.0
95.0
90.0
0.6
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
Groove depth0.80.3 0.4 0.5 0.6 0.7
substrate
CleanPrimeResist coat
substrate
resistlayer
ExposeDevelopMetrology
Transfer-etch (RIBE)
substrate
Cleaning &Metrology
During manufacture, optics are exposed to heat, aggressive liquids, and vacuum processing.
substrate substrate
Multilayeroxidedeposition
1.70
-3.61
-3.25
-3.00
-2.75
-2.50
-2.25
-2.00
-1.75
-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
0.570.00 0.20 0.40
stack view
Design(efficiency,E-field distribution, …)
(only part of process not done at LLNL)
575 nm period
MLD grating fabrication process flow
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0.00
1.00
2.00
3.00
4.00
5.00
90 92 94 96 98 100
Average Diffraction Efficiency (%)
RM
S D
iffr
ac
tio
n E
ffic
ien
cy
(%
)
In the past 18 months LLNL has produced 77 production gratings @ 1740 l/mm
Apertures from 140 mm to 800 mm
Over 11 m2 of grating surface with average efficiency > 96%
For use wavelengths from 1017 nm to 1064 nm
LIGOproject
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Ratioing scanning photometry setup @ LLNL for efficiency measurements
500 mW CW 1064 nm laser
Fused silica window
Reference detector
Signal detector
Fused silica wedge
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
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#011 (200x100 mm):99.2% Ave, 0.3% RMS
#021 (170x100 mm):99.3% Ave, 0.2% RMS
0.90 0.92 0.94 0.96 0.98 1.00histogram of 10K data points
histogram of 10K data points
Design spec of >99.5% is achievable
1740 line/mm HfO2/SiO2 grating on BK7 substrates
>99% diffraction efficiency gratings have been delivered to Stanford
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Resolution 991 x 1005Wedge 1.000PV 0.1765 wvPVq(99%) 0.1509 wvRMS 0.0292 wvStrehl 0.967
011 CW 021 CW
Resolution 991 x 1005Wedge 1.000PV 0.1310 wvPVq 0.0837 wvRMS 0.0175 wvStrehl 0.988
CW orientationw/ S/N up
Gratings exhibit flat diffracted wavefront
Diffracted Wavefront at Littrow angle for 1053 nm (66.4o)
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GSI5_06_04_19_221034_EFF_XLS0.90 0.92 0.94 0.96 0.98 1.00
#005 (800x400 mm) @ 1053 nm, 72.5o: 97.3% Ave, 0.7% RMS
#011: 97.1% Ave @ 1053 nm, 72.5o:, so this grating could be >99% for LIGO conditions
LLNL has delivered 13 productiongratings at this size (800x400 mm)
LIGO aperture required has been demonstrated for other projects
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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
Shack-Hartman Wavefront Sensor
He-Ne Laser
4x Beam Expander
LIGO 8 W NPRO oscillator with 2 stage 4 passes rod amplifier
Mini Slab Power Amplifiers
(300 W diode pump)Modecleaner
30W Single Frequency
TEM00 1064nm laser
MLD Grating
Max power was 34.5 W in a 1.5mm spot: ~2 kW/cm2
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Thermal testing of 100 mm diameter LLNL witness grating
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3mm HeNe Probe Beam
removed Tip, Tilt, and Piston
PV: 59.3 nm, Rms: 11.2 nm
11 W, 0.6 kW/cm2
PV: 53.4 nm, Rms: 9.4 nm
Grating wavefronts measured at two power densities show no difference
34.5 W, ~2 kW/cm2
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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
LLNL Dielectric Grating on Translation Stage
Single Frequency6 W YAG laser 1064 nm wavelength
Circulator
Servo Electronics
Intracavity Power ~1200 W
EO Modulator
Cavity end-mirror with piezo
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Thermal aberration measurement at higher power planned for near future
LLNL Dielectric Grating on Translation Stage
Shack-Hartman Wavefront Sensor
He-Ne Laser
Beam Expander
Single Frequency100 W YAG laser 1064 nm wavelengthOr Fiber Laser 100 W
Circulator
Servo Electronics
Intracavity Power ~20 kWPower Density > 30 kW/cm2
Interferometric Sensor CCD
EO Modulator
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Conclusions
• Capability exists to manufacture large-aperture multilayer dielectric diffraction gratings for demanding LIGO applications
- >99% diffraction efficiency in Littrow mount
- kW power-handling capability
- 80+ cm apertures