March 30, 2000 SPIE conference, Munich 1
LGS AO photon return LGS AO photon return simulations and laser simulations and laser
requirements for the Gemini requirements for the Gemini LGS AO programLGS AO program
Céline d’Orgeville, François Rigautand Brent Ellerbroek
March 30, 2000 SPIE conference, Munich 2
Gemini LGS AO programGemini 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
March 30, 2000 SPIE conference, Munich 3
How do we set laser power How do we set laser power requirements?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 m (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 4
How do we set laser power How do we set laser power requirements?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 5
Laser power requirementLaser power requirementin the no-saturation limitin 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)
March 30, 2000 SPIE conference, Munich 6
Saturation model for CW Saturation model for CW laserslasers
• 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 7
Two saturation effectsTwo saturation effects
Spatial
Spot radius (cm)
Nor
mal
ized
inte
nsity
10 W100 W
Spectral
Frequency (MHz)
SATURATION
10 W
100 W
March 30, 2000 SPIE conference, Munich 8
Pho
ton
retu
rn (
Pho
ton/
cm2 /
s)
Laser power (W)
Efficiency comparisonEfficiency comparisonbetween CW laser formatsbetween CW laser formats
Photon return vs. laser power (both at sodium layer i.e. TBTO= TLLT= Tatmo= 1)
No-saturation limit
500 MHz
3 GHz
5 modes, 30 MHz mode spacing
Mono/multimode lasers give same results at the 10-W level
March 30, 2000 SPIE conference, Munich 9
Gemini specificationsGemini 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
March 30, 2000 SPIE conference, Munich 10Laser power (W)
Las
er b
andw
idth
(M
Hz)
Photon return (Photon/cmPhoton return (Photon/cm22/s) /s) vs.laser output power and laser vs.laser output power and laser
bandwidth within the Gemini bandwidth within the Gemini assumptions*assumptions*
* FWHM = 36 cm, TBTO= 0.6, TLLT= 0.9, Tatmo= 0.8
Gemini North photon return requirement
= 160 photon/cm2/s
March 30, 2000 SPIE conference, Munich 11Laser power (W)
Opt
imum
ban
dwid
th (
MH
z)
Opt
imum
pho
ton
retu
rn (
Pho
ton/
cm2 /
s)
CW laser bandwidth CW laser bandwidth optimizationoptimization
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, Munich 12Laser power (W)
Las
er b
andw
idth
(M
Hz)
Photon return per WattPhoton return per Wattof laser output powerof laser output power
XInefficient spectral format (bandwidth > 3 GHz) Max.
efficiency zone
Maximum efficiency at the 10-W levelX
XSaturation
March 30, 2000 SPIE conference, Munich 13
Gemini North power Gemini North power requirements for a LGS at requirements for a LGS at
zenithzenithLaser output power requirement
Laser temporal andspectral characteristics No-saturation
limitSaturation
models
FWHM = 10 MHz 7.2 W 10.1 WCW laser
FWHM = 150-200 MHz - 8.0 W
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 14
ConclusionsConclusions
• 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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