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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