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Transcript of Studies of Thermal Loading in Pre-Modecleaners for Advanced LIGO Amber Bullington Stanford...
Studies of Thermal Loading in Pre-Modecleaners for Advanced LIGO
Amber BullingtonStanford University
LSC/Virgo March 2007 MeetingOptics Working Group
G070084-00-Z
2
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
3
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
4
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
5
‘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
6
‘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
7
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%
8
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
10
‘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
11
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
12
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)