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Studies of Thermal Loading in Pre-Modecleaners for Advanced LIGO
Amber BullingtonStanford University
LSC/Virgo March 2007 MeetingOptics Working Group
G070084-00-Z
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Outline
Thermal Loading Experiment» Ring cavity known as pre-modecleaner (PMC)
– Same geometry as cavity used in the PSL
» Goal: Achieve resonant thermal load akin to Advanced LIGO
– Use calibrated absorption loss– Reach absorption comparable to Advanced
LIGO ~12 mW per suspended MC optic for
0.5 ppm coating absorption – Compare results to models
Results» Onset of thermal loading» Thermal limits
For the future» Further tests» Suggestions
A Pre-Modecleaner
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Experimental Setup
NPRO4-rod
amplifier
LIGO 10 W MOPA
4-rod amplifier
External Amplifier
Mode-matching lenses
Low FinessePre-Modecleaner
Pre-Modecleaner under test
Beam Analysis-Transmission Monitor-Data Acquisition-CCD
Reflection locking electronics
Reflection locking electronics
Mode-matching lensesPhotodiode
Modal Analysis-’Mode Analyzer’ - Scanning modecleaner
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Pre-Modecleaner Properties
Dominant thermal load in coatings» Low-loss coatings coated by Research Electro-Optics» Absorption loss measured by PCI
– Flat I/O optics: 1.4 ppm
– Curved optic: 0.4 ppm
‘Lossy’ pre-modecleaner» Low-loss fused silica input/output optics» Rear optic: low-loss coating on absorptive substrate
– IR absorbing glass: KG5
– KG5 optic coating transmission: 81 ppm
‘Low-loss’ pre-modecleaner» REO coatings on BK7 substrates
Resonances of different polarizations do not overlap
– Low Finesse (p-pol) ~ 330
– High Finesse (s-pol) ~ 5000
Fused Silica
BK7
BK7
KG5
‘Lossy’ Pre-modecleaner
‘Low-loss’ Pre-modecleaner
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‘Lossy’ Pre-Modecleaner – Low Finesse
Increase input power in steps» Analyze transmitted TEM00
content
Higher order mode overlap with TEM00
» Occasional overlap seen as power is increased
» Roll-off in TEM00 content at high absorbed power
Transmitted CCD Image – ‘Lossy’ PMCCCD Image – Mode Analyzer PMC
Transmitted TEM00 Mode Content
0.8
0.85
0.9
0.95
1
1.05
0 10 20 30 40 50
Absorbed Power, mW
Fra
ctio
n T
EM
00
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‘Lossy’ PMC Thermal Limit
Thermal Limit» Transmission linear up to 5 W
input power (Pabsorbed = 40 mW)
» For Pabsorbed > 40 mW, higher order modal content degrades transmitted beam quality
» For Pabsorbed > 47 mW, constant locked transmitted power not achieved
Transmitted Beam Reflected Beam Mode Analyzer Transmission‘Lossy’ PMC Images
Input Power = 6 W, Pabsorbed = 47 mW
'Lossy' PMC Transmission
0
1
2
3
4
5
0 1 2 3 4 5 6 7
Input Power, W
Tra
nsm
itte
d P
ow
er, W
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Thermally Induced Power Fluctuation
Power fluctuation from thermal effects
» Lossy PMC: Thermal cycling induces periodic fluctuation of transmitted power
Transmitted mode images1: near maximum power
2: during power decline
3: near minimum power
1 2 3
Transmitted Power Fluctuation
0
1
2
3
4
5
6
0 0.02 0.04 0.06 0.08 0.1 0.12 0.14 0.16
Time, s
Po
wer
, W
29 Hz
41%
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Modal Analysis
Higher order mode overlap with TEM00
» Change in cavity g-factor: g = 1 - L/R» Change in Radius of Curvature (ROC)
of KG5 optic New g factor gives thermally
distorted radius of curvature, Rhot
» Cold cavity g factor = 0.79– Rcold = 1 m
» Example: TEM00/TEM11,0 overlap– Hot cavity g factor = 0.83– Rhot = 1.2 m
Calculate absorbed power from Rhot, compare with expected absorption
» Pabsorbed = (4s)/
Input Power
Calculated Absorbed Power
Expected Absorbed Power
Observed Mode
1 W 13 mW 9 mW LG23
4 W 23 mW 30 mW TEM11,0
> 4.5 W 39 mW 40 mW LG12
TEM11,0
TEM00
Cold g
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‘Lossy’ Pre-Modecleaner – High Finesse
Strong higher order mode coupling
» First observation at 4.25 mW absorbed power
– Several TEMmn, large m+n, modes
– 96 % TEM00 output
Degradation of TEM00 output» Increased absorbed power
– Multiple higher order mode coupling
– Sometimes one dominant higher order mode
» Lowest TEM00 transmission 57% at 12 mW absorbed power
Transmitted Beam, Pabs ~ 4 mW Mode Analyzer
Mode Analyzer, Pabs ~ 10 mW
PZT drive
TEM00
Higher order modes
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‘Low-loss’ Pre-Modecleaner
REO coatings on BK7 substrates
High finesse» Higher order modes at a few
absorbed powers, starting at Pabs = 5.1 mW
» Rollover in transmission > 20 mW absorbed power
Low Finesse» First higher order mode coupling
noted near max available input power, Pabs = 5.1 mW
Transmitted TEM00 Content, High Finesse
0.920.930.940.950.960.970.980.99
11.01
0 5 10 15 20 25
Absorbed Power, mW
Fra
ctio
n T
EM
00
'Low-Loss' PMC Transmission, High Finesse
0
0.5
1
1.5
2
2.5
3
3.5
0 2 4 6 8 10 12
Input Power, W
Tra
nsm
itte
d P
ow
er, W
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Other Issues
Reduced transmission in high finesse for both pre-modecleaners» 30% coupling, expect closer to 80%
» Scatter loss from curved optic
» Response to thermal load agrees with expected absorption
Melody» Mode-based interferometer thermal modeling tool
» Limited agreement with data– Thermal loading of ‘Lossy’ PMC in low finesse– Differing behavior for thermal loading in high finesse
Melody shows sharp coupling decline without higher order mode coupling
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Summary
Transmission degradation from higher order mode coupling» Stronger coupling in high finesse case» Large thermal load may induce power fluctuations
Recommendations based on observations» Coating absorption <= 1ppm» Higher finesse -> lower absorption
– From experiment, < 4 mW– Scale results to other substrate materials
» Aperture to suppress large order modes– Suspended modecleaner
» Consider alternative geometry– 4-bounce pre-modecleaner?
Advantage: reduce mode overlap possibility Disadvantage: not polarization sensitive
For the future … » Test pre-modecleaners with sapphire and undoped YAG optical substrates» Design an all-reflective modecleaner using gratings
– Expand substrate possibilities (e.g. silicon)