Adv LIGO Input optic requirements and components for

Adv. LIGO Input optic requirements and components for high power lasers ESF Exploratory Workshop Perugia, Italy September 21 st – 23 rd, 2005 LIGO-G 050471 -00 -Z Guido Mueller University of Florida For the LIGO Scientific Collaboration

Table of Content Input Optic for Advanced LIGO § Requirements for Adv. LIGO § Layout » Modulators » Mode cleaner » Isolator Documents: LIGO-T 020020 -00 -D LIGO-T 020027 -00 -D LIGO-T 010075 -00 -D LIGO-T 020097 -0 -D LIGO-G 050471 -00 -Z IO-Subsystem Design Requirements Document IO-Subsystem Conceptual Design Document Advanced LIGO Systems Design Auxiliary Suspended Optics Displacement … 2

Advanced LIGO Changes which affect the input optics: • Detuned Signal-recycling • Higher Laser Power • Increased Arm Finesse: T=0. 5% • Decreased Recycling Cavity Finesse: T=6% 40 kg SILICA Iso. PRM BS ITM ETM SRM PD LIGO-G 050471 -00 -Z Power Recycling Mirror Beam Splitter Input Test Mass End Test Mass Signal Recycling Mirror Photodiode 3

Requirements § Detuned Signal Recycling » Creates asymmetric RF-sidebands – All demodulated signals are sensitive to phase between RFsidebands and carrier (no technical noise suppression) » For RF-sensing scheme: Modulation phase stability req. : – ISSB (10 Hz) < -92 d. Bc/Hz – ISSB (100 Hz) < -140 d. Bc/Hz – ISSB (1 k. Hz) < -163 d. Bc/Hz » Compare with Rb Standard: PRS 10 (Stanford Research) – – ISSB (10 Hz) < -130 d. Bc/Hz ISSB (100 Hz) < -145 d. Bc/Hz ISSB (1 k. Hz) < -150 d. Bc/Hz But that is for a 10 MHz signal not 180 MHz! » Options: 1. Lock to IFO LIGO-G 050471 -00 -Z 2. Reduce Frequency 4 3. DC-Sensing

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 – …. LIGO-G 050471 -00 -Z 5

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: 2 x 10 -9 RIN/r. Hz @ 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: Ø d. G < 10 -10/G (f/Hz) 1/r. Hz (includes safety factor of 10) For G=0. 1 d. G< 10 -8 /r. Hz @ 10 Hz Experimental tests on their way, but this is non-trivial! LIGO-G 050471 -00 -Z 6

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: LIGO-G 050471 -00 -Z 7 Optics Express, Vol 13(18) pg. 7118

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 2 nd page LIGO-G 050471 -00 -Z 8

IO Hardware § § LIGO-G 050471 -00 -Z Modulators Mode Cleaner Faraday Isolator Stable Recycling Cavities 9

Modulators § LIGO I modulators will not handle the increased laser power (losses and subsequent thermal lensing to high) § New materials: » KTP, KTA, RTP have high damage thresholds and high EOcoefficients. » RTP has also very low optical and electrical losses. Measurements at 50 W haven’t shown any measurable thermal lens. Long term (16 d) measurements at ~100 W 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 LIGO-G 050471 -00 -Z 10

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 LIGO-G 050471 -00 -Z 11

Modulation Schemes § Serial Modulation: Problem: SB on SB modulation Has same frequency than SB-SB beat Frequency § Parallel Modulation § Complex Modulation Frequency (modulate also at SB-SB frequency with opposite sign) LIGO-G 050471 -00 -Z 12

Mode Cleaner Requirements: § Length Stability < 3. 6 x 10 -15 (Hz/f) m/rt. Hz for (f<1 k. Hz) (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. 9 m (cold), expect 27. 9 m (hot) § L = 33. 2 m (Roundtrip), FSR = 9 MHz § Finesse = 2000 (current design) » Was driven by pointing from laser (overestimated pointing) » Will probably be reduced (My best guess: 600) LIGO-G 050471 -00 -Z 13

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/d. T, preferably using a crystal, not a glass LIGO-G 050471 -00 -Z 14

High Power Faraday Isolator l/2 QR Pt Pr H H HP Faraday isolator design uses quartz rotator: - Developed at IAP, Russia - 33 d. B at 180 W laser power Design with thermal compensation (still with FK 51 glass): - No significant lensing up to 90 W Currently under test at LZH LIGO-G 050471 -00 -Z 15

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 LIGO-G 050471 -00 -Z 16

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) LIGO-G 050471 -00 -Z 17

Summary Wa rn Optics for Advanced LIGO: Inputin g: § Faraday Isolator, Modulator expected to be able to handle thermal Th noise w/o idegrading the beam quality significantly si sm § Mode Cleaner should be fine, no thermal degradation expected yo § § § » Careful with frequency noise driven by technical RPN p in ion Mode matching problems related to thermally distorted IFO eigenmode an (stable recycling cavities might help) d. N Pointing requirements seem to be within reach OT Stability of Modulation phase seem to beh for DC-sensing s OK ar » Likely driven by frequency stabilization servo e db yt he My main concern: § Stability of Modulation index (RIN in carrier field) § Unknown spatial mode in main IFO (Greg Harry: TCS) LIGO-G 050471 -00 -Z 18 ev ery bo dy !