Lgs ao photon return simulations and laser requirements for the gemini lgs ao program
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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

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


Gemini lgs ao program
Gemini LGS AO program the 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


How do we set laser power requirements
How do we set laser power requirements? the Gemini LGS AO program

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


How do we set laser power requirements1
How do we set laser power requirements? the Gemini LGS AO program

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


Laser power requirement in the no saturation limit
Laser power requirement the Gemini LGS AO programin 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)

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Saturation model for cw lasers
Saturation model for CW lasers the Gemini LGS AO program

  • 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

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Two saturation effects

10 W the Gemini LGS AO program

SATURATION

100 W

10 W

Normalized intensity

100 W

Spatial

Spectral

Spot radius (cm)

Frequency (MHz)

Two saturation effects

SPIE conference, Munich


Efficiency comparison between cw laser formats

No-saturation limit the Gemini LGS AO program

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


Gemini specifications
Gemini specifications the Gemini LGS AO program

  • 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

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Photon return photon cm 2 s vs laser output power and laser bandwidth within the gemini assumptions

Gemini North photon return requirement the Gemini LGS AO program

= 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

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Cw laser bandwidth optimization

Optimum bandwidth (MHz) the Gemini LGS AO program

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

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Photon return per watt of laser output power

X the Gemini LGS AO program

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

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Gemini north power requirements for a lgs at zenith
Gemini North power requirements for a LGS at zenith the Gemini LGS AO program

Note: other laser formats (pulsed) are presented in the paper for which the effects of saturation are much worse

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Conclusions
Conclusions the Gemini LGS AO program

  • 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

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