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

1
LGS AO photon return simulations and laser
requirements for the Gemini LGS AO program

• Céline dOrgeville, François Rigaut
• and Brent Ellerbroek

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

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 _at_
  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

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

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)

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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

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Two saturation effects
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Efficiency comparisonbetween CW laser formats
Photon return vs. laser power (both at sodium
layer i.e. TBTO TLLT Tatmo 1)
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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

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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

11
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
12
Photon return per Wattof laser output power
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
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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/adaptiveO
  ptics/AOIndex.html

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Saturation model forhigh repetition rate lasers

• Uses analytical formula given by Milonni et al. 2
• Photon return saturates as ln(1Ipeak/Isat)
• Ipeak proportional to laser power and inversely
  proportional to LGS spot area, pulse length and
  repetition rate
• Same assumptions for Gemini as before
• The spot size assumption has a major influence on
  the laser power requirement, however reducing
  saturation by increasing spot size would be
  counter-productive in terms of WFS SNR
  optimization.
• 2 P. Milonni et al., JOSA A, Vol. 15, No. 1, pp.
  217-233 (Jan. 1998)

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Pulsed laser with 100 ns pulses at 30 kHz
repetition rate
17
Photon return vs. power and rep. rate for a 100
ns-pulse laser
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Gemini photon return vs. pulse length, rep. rate
and power