Guido Mueller University of Florida For the LIGO Scientific Collaboration
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Transcript of Guido Mueller University of Florida For the LIGO Scientific Collaboration
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LIGO-G050471-00-Z
Guido Mueller University of Florida
For the LIGO Scientific Collaboration
ESF Exploratory Workshop Perugia, Italy September 21st –23rd, 2005
Input optic requirements and
components for high power lasers
Adv. LIGO
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Table of Content
Input Optic for Advanced LIGO
Requirements for Adv. LIGO Layout
» Modulators» Mode cleaner» Isolator
Documents:LIGO-T020020-00-D IO-Subsystem Design Requirements DocumentLIGO-T020027-00-D IO-Subsystem Conceptual Design DocumentLIGO-T010075-00-D Advanced LIGO Systems DesignLIGO-T020097-0-D Auxiliary Suspended Optics Displacement …
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Advanced LIGO
PRM Power Recycling MirrorBS Beam SplitterITM Input Test MassETM End Test MassSRM Signal Recycling MirrorPD Photodiode
SILICA
40 kgChanges which affect the input optics:• Detuned Signal-recycling• Higher Laser Power• Increased Arm Finesse: T=0.5%• Decreased Recycling Cavity Finesse: T=6%
Iso.
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Requirements
Detuned Signal Recycling» Creates asymmetric RF-sidebands
– All demodulated signals are sensitive to phase between RF-sidebands and carrier (no technical noise suppression)
» For RF-sensing scheme: Modulation phase stability req.:– ISSB (10 Hz) < -92 dBc/Hz
– ISSB (100 Hz) < -140 dBc/Hz
– ISSB (1 kHz) < -163 dBc/Hz
» Compare with Rb Standard: PRS10 (Stanford Research)– ISSB (10 Hz) < -130 dBc/Hz
– ISSB (100 Hz) < -145 dBc/Hz
– ISSB (1 kHz) < -150 dBc/Hz– But that is for a 10MHz signal not 180MHz!
» Options:1. Lock to IFO 2. Reduce Frequency 3. DC-Sensing
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Requirements
Detuned Signal Recycling» DC-sensing (now baseline): RF signals are used only for auxiliary d.o.f.s
– Requirements unclear. Complicated function of locking scheme, cross coupling between channels, noise spectrum, and feedback bandwidth. But will be less difficult than in RF sensing.
» DC-sensing has additional advantages– Lower Shot noise– Less sensitive to laser frequency noise– Reduced requirements on high-frequency, high-power photo
detectors – ….
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Requirements
Higher Laser Power» Relative Intensity Noise (RIN):
– Generates technical RPN in arm cavities– Couples to asymmetry in arm cavity build-up– Only important for Carrier, sideband power noise does not create RPN!
» Requirement: 2x10-9 RIN/rHz @ 10 Hz on carrier intensity!– Stabilization will work with main laser beam (carrier + SBs)– Any change in the modulation index (SB power) will be undetected in the
intensity servo but will change carrier power and generate RIN» Generates a requirement for the stability of the modulation index:
< 10-10/ (f/Hz) 1/rHz (includes safety factor of 10) For 10-8 /rHz @ 10Hz Experimental tests on their way, but this is non-trivial!
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Requirements
Laser Beam Pointing at PR-mirror: Couples to misaligned mirrors Trade off between pointing and DC alignment
Measured in terms of 10-amplitude relative to 00-amplitude:
Optics Express, Vol13(18) pg.7118
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Requirements
Spatial Mode quality: 10-mode ~ misalignment (just discussed) BE-more (20+02 mode) ~ mode mismatch
» Depends on thermal lensing in main IFO (TCS-system) Content in all other modes should be below < 2% Power issue, no direct noise coupling expected (calculated)
Additional Requirements:See LIGO Documents mentioned on 2nd page
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IO Hardware
Modulators Mode Cleaner Faraday Isolator Stable Recycling Cavities
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Modulators
LIGO I modulators will not handle the increased laser power (losses and subsequent thermal lensing to high)
New materials: » KTP, KTA, RTA, RTP have high damage thresholds and high EO-
coefficients. » RTP has also very low optical and electrical losses. Measurements
at 50 W haven’t shown any measurable thermal lens. Long term (16d) measurements at ~100W did not show any degradation. Then laser failed.
» Requires additional long term, high power testing but looks OK. Parallel vs. complex Modulation:
» Cross products (SB on SB) generated in serial modulation might need to be reduced:
– Parallel modulation in Mach-Zehnder– Complex modulation using additional AM-modulator
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Modulator
Sources:
RTA-Crystals:
• Raicol in Israel
Complete Modulator:
• Self made, need probably 3/IFO + spares
• Also collaborate with New Focus to build their modulator around our RTA crystals
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Modulation Schemes
Serial Modulation:
Parallel Modulation
Complex Modulation
Frequency
Problem: SB on SB modulationHas same frequency than SB-SB beat
Frequency
(modulate also at SB-SB frequency with opposite sign)
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Mode Cleaner
Requirements: Length Stability < 3.6x10-15 (Hz/f) m/rtHz for (f<1kHz) (RPN?) Mode cleaning Angular Stability:
Current Design: Triangular Cavity
» Flat mirrors at Input and Output near MC waist» Curved mirror at acute angle ROC=26.9m (cold), expect 27.9m (hot)
L = 33.2m (Roundtrip), FSR = 9 MHz Finesse = 2000 (current design)
» Was driven by pointing from laser (overestimated pointing)» Will probably be reduced (My best guess: 600)
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High-power Faraday isolators
Possible Problems: Depolarization reduces isolation
efficiency Thermal lensing reduces spatial mode
quality
Depolarization: Two novel optical architectures with two
Faraday crystals and wave plate (b) or Quartz Rotator (c)
Developed by IAP, Nizhni Novgorod, Russia
Thermal Lensing: Compensated with material with opposite
dn/dT, preferably using a crystal, not a glass
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High Power Faraday Isolator
Pr
QR
H H
Pt
HP Faraday isolator design uses quartz rotator:- Developed at IAP, Russia- 33dB at 180W laser powerDesign with thermal compensation (still with FK51 glass):- No significant lensing up to 90WCurrently under test at LZH
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Stable Recycling Cavities
Current Baseline: Recycling Cavities are only marginally stable
» Essentially flat-flat cavities» Will increase scatter of
RF-SB and GW-signal into higher order modes
Option: Stable Recycling Cavity
» Move mode matching telescope into Recycling cavity
» Stabilizes the Recycling cavities and reduces losses into higher order modes
Add TCS and we should have very small problems with Thermal Deformations
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Summary
Input Optics for Advanced LIGO: Faraday Isolator, Modulator expected to be able to handle thermal
noise w/o degrading the beam quality significantly Mode Cleaner should be fine, no thermal degradation expected
» Careful with frequency noise driven by technical RPN Mode matching problems related to thermally distorted IFO eigenmode
(stable recycling cavities might help) Pointing requirements seem to be within reach Stability of Modulation phase seem to be OK for DC-sensing
» Likely driven by frequency stabilization servo
My main concern: Stability of Modulation index (RIN in carrier field) Unknown spatial mode in main IFO (Greg Harry: TCS)
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Summary
Input Optics for Advanced LIGO: Faraday Isolator, Modulator expected to be able to handle thermal
noise w/o degrading the beam quality significantly Mode Cleaner should be fine, no thermal degradation expected
» Careful with frequency noise driven by technical RPN Mode matching problems related to thermally distorted IFO eigenmode
(stable recycling cavities might help) Pointing requirements seem to be within reach Stability of Modulation phase seem to be OK for DC-sensing
» Likely driven by frequency stabilization servo
My main concern: Stability of Modulation index (RIN in carrier field) Unknown spatial mode in main IFO (Greg Harry: TCS)
Warning: This is my opinion and NOT shared by the everybody!