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

2LIGO-G050471-00-Z

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 …

3LIGO-G050471-00-Z

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.

4LIGO-G050471-00-Z

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

5LIGO-G050471-00-Z

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 – ….

6LIGO-G050471-00-Z

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!

7LIGO-G050471-00-Z

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

8LIGO-G050471-00-Z

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

9LIGO-G050471-00-Z

IO Hardware

Modulators Mode Cleaner Faraday Isolator Stable Recycling Cavities

10LIGO-G050471-00-Z

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

11LIGO-G050471-00-Z

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

12LIGO-G050471-00-Z

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)

13LIGO-G050471-00-Z

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)

14LIGO-G050471-00-Z

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

15LIGO-G050471-00-Z

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

16LIGO-G050471-00-Z

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

17LIGO-G050471-00-Z

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)

18LIGO-G050471-00-Z

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!