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LGS AO photon return simulations and laser requirements for the Gemini LGS AO program

LGS AO photon return simulations and laser requirements for the Gemini LGS AO program. Céline d’Orgeville, François Rigaut and Brent Ellerbroek. Gemini LGS AO program. Mid-2001 Gemini South 85-element curvature AO system with a 2-Watt CW commercial dye laser 2002-2003

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LGS AO photon return simulations and laser requirements for the Gemini LGS AO program

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  1. LGS AO photon return simulations and laser requirements for the Gemini LGS AO program Céline d’Orgeville, François Rigaut and Brent Ellerbroek SPIE conference, Munich

  2. Gemini LGS AO program • Mid-2001 • Gemini South 85-element curvature AO system with a 2-Watt CW commercial dye laser • 2002-2003 • Gemini North 12x12 Shack-Hartmann altitude-conjugated AO system (ALTAIR) • LGS upgrade with a 10-Watt-class laser • 2004 • Gemini South Multi-Conjugated AO system (MCAO) with 3 DMs and 5 LGSs created by a 50-Watt-class laser or 5x10-Watt-class lasers SPIE conference, Munich

  3. How do we set laser power requirements? 1/ Compute “photon return” requirement i.e. photon flux at the primary mirror of the telescope • Example of the Mauna Kea LGS AO system • Science drivers moderate Strehl = 0.2 - 0.3 @ 1.6 mm (H) • Full LGS AO code simulation  LGS magnitude  11 • Assumptions: atmospheric and optical transmissions, detector quantum efficiency  photon return  80 photon/cm2/s • Factor of 2 margin to account for: non ideal laser beam quality, miscellaneous aberrations  photon return requirement =160 photon/cm2/s SPIE conference, Munich

  4. How do we set laser power requirements? 2/ Assume atmospheric and optical transmission, assume sodium layer parameters and seeing 3/ Assume spatial, temporal and spectral characteristics of candidate laser 4/ Compute laser/sodium interaction efficiency 5/ Derive laser output power requirement from photon return requirement SPIE conference, Munich

  5. Laser power requirementin the no-saturation limit • Use small-signal “slope efficiency” numbers 1 • A first guess • gives order of magnitude for laser power requirements • enable comparison between different laser formats • But results do not include saturation effects which are more than likely to occur within small LGS spot diameters  Need a code including saturation effects 1 Telle et al., Proc. of the SPIE Vol. 3264 (1998) SPIE conference, Munich

  6. Saturation model for CW lasers • IDL code • Approach based on Doppler-broadened absorption cross-section of the sodium D2 line • Spectral and spatial saturation model • monomode, multimode or phase-modulated laser spectrum centered on D2 line highest peak • variable bandwidth, mode spacing and envelope shape • saturation per velocity group of sodium atoms (sodium natural linewidth = 10 MHz) • gaussian LGS spot profile • Compute photon return vs. laser power and spectral bandwidth SPIE conference, Munich

  7. 10 W SATURATION 100 W 10 W Normalized intensity 100 W Spatial Spectral Spot radius (cm) Frequency (MHz) Two saturation effects SPIE conference, Munich

  8. No-saturation limit 500 MHz Photon return (Photon/cm2/s) 5 modes, 30 MHz mode spacing Mono/multimode lasers give same results at the 10-W level 3 GHz Laser power (W) Efficiency comparisonbetween CW laser formats Photon return vs. laser power (both at sodium layer i.e. TBTO= TLLT= Tatmo= 1) SPIE conference, Munich

  9. Gemini specifications • We choose not to include the seeing contribution into the LGS spot size calculation in order for the LGS AO system to be laser-limited on very good seeing nights • LGS parameters: • TBTO = 0.6 / 0.8 • TLLT = 0.9 • Tatmo = 0.8 • Sodium column density = 2 109 cm-2 • LLT diameter = 45 cm • 1/e2 intensity diameter on LLT M1 = 30 cm • Laser beam quality = 1.5 x DL • LGS spot 1/e2 intensity diameter = 36 cm SPIE conference, Munich

  10. Gemini North photon return requirement = 160 photon/cm2/s Laser bandwidth (MHz) Laser power (W) Photon return (Photon/cm2/s) vs.laser output power and laser bandwidth within the Gemini assumptions* • FWHM = 36 cm, TBTO= 0.6, TLLT= 0.9, Tatmo= 0.8 SPIE conference, Munich

  11. Optimum bandwidth (MHz) Optimum photon return (Photon/cm2/s) Laser power (W) CW laser bandwidth optimization Gemini photon requirement (160 photon/cm2/s) met for a CW laser in the 8-10 W range with 150-200 MHz bandwidth X X SPIE conference, Munich

  12. X Inefficient spectral format (bandwidth > 3 GHz) Maximum efficiency at the 10-W level X Max. efficiency zone Laser bandwidth (MHz) Saturation X Laser power (W) Photon return per Wattof laser output power SPIE conference, Munich

  13. Gemini North power requirements for a LGS at zenith Note: other laser formats (pulsed) are presented in the paper for which the effects of saturation are much worse SPIE conference, Munich

  14. Conclusions • Do not underestimate the effect of saturation for LGS AO operation with small spot sizes • In the case of CW lasers, it is possible to balance saturation by increasing the laser spectral bandwidth • BUT increasing the laser spot size to balance saturation would be counter-productive in terms of the AO WFS signal-to-noise optimization • Most pulsed lasers show much more saturation • Gemini North (resp. South) laser power requirement is about 8 W (resp. 5x8 W) at zenith, up to 14 W (resp. 5x14 W) at 45º zenith angle • Paper available on Gemini/s web site:http://www.gemini.edu/sciops/instruments/adaptiveOptics/AOIndex.html SPIE conference, Munich

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