LGS AO photon return simulations
and laser requirements for the
Gemini LGS AO program
Céline d’Orgeville, François Rigaut
and Brent Ellerbroek
March 30, 2000
SPIE conference, Munich
1
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-Wattclass lasers
March 30, 2000
SPIE conference, Munich
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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
March 30, 2000
SPIE conference, Munich
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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
March 30, 2000
SPIE conference, Munich
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Laser power requirement
in the no-saturation limit
• Use small-signal “slope efficiency” numbers
• A first guess
1
– 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)
March 30, 2000
SPIE conference, Munich
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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
March 30, 2000
SPIE conference, Munich
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Two saturation effects
Normalized intensity
10 W
100 W
Spatial
S
A
T
U
R
A
T
I
O
N
10 W
100 W
Spot radius (cm)
March 30, 2000
Spectral
Frequency (MHz)
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Photon return vs.
laser power (both
at sodium layer i.e.
TBTO= TLLT= Tatmo= 1)
Mono/multimode
lasers give same
results at the 10-W
level
March 30, 2000
Photon return (Photon/cm2/s)
Efficiency comparison
between CW laser formats
No-saturation
limit
500 MHz
5 modes, 30 MHz
mode spacing
3 GHz
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power
(W)
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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
TLLT
Tatmo
Sodium column density
LLT diameter
1/e2 intensity diameter on LLT M1
Laser beam quality
LGS spot 1/e2 intensity diameter
March 30, 2000
= 0.6 / 0.8
= 0.9
= 0.8
= 2 109 cm-2
= 45 cm
= 30 cm
= 1.5 x DL
= 36 cm
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Photon return (Photon/cm2/s) vs.laser
output power and laser bandwidth
within the Gemini assumptions*
March 30, 2000
Gemini North
photon return
requirement
Laser bandwidth (MHz)
* FWHM = 36 cm,
TBTO= 0.6, TLLT= 0.9,
Tatmo= 0.8
= 160
photon/cm2/s
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Laser power (W)
10
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
March 30, 2000
SPIE conference,
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power
(W)
Optimum photon return (Photon/cm2/s)
CW laser bandwidth optimization
11
Photon return per Watt
of laser output power
March 30, 2000
Laser bandwidth (MHz)
Inefficient
spectral
format
(bandwidth
> 3 GHz)
X
Max.
efficiency
zone
Maximum
efficiency
at the
10-W level
X
X
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Laser
conference,
power (W)
Munich
Saturation
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Gemini North power requirements
for a LGS at zenith
Laser output power requirement
Laser temporal and
spectral characteristics
No-saturation
limit
Saturation
models
FWHM = 10 MHz
7.2 W
10.1 W
FWHM = 150-200 MHz
-
8.0 W
CW laser
Note: other laser formats (pulsed) are presented in the paper for
which the effects of saturation are much worse
March 30, 2000
SPIE conference, Munich
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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/adaptiveOptics/AOIndex.html
March 30, 2000
SPIE conference, Munich
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