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 efficiency5/ 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

Phot

on re

turn

(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 spacingMono/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)

Lase

r ban

dwid

th (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

ba n

d wid

th ( M

Hz )

Opt

imum

pho

ton

retu

rn (P

hoto

n/cm

2 /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)

Lase

r ban

dwid

th (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

X Saturation

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