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March 30, 2000SPIE conference, Munich1 LGS AO photon return simulations and laser requirements for the Gemini LGS AO program Céline d’Orgeville, François Rigaut and Brent Ellerbroek

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March 30, 2000SPIE conference, Munich2 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

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March 30, 2000SPIE conference, Munich3 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 m (H) Full LGS AO code simulation LGS magnitude 11 Assumptions: atmospheric and optical transmissions, detector quantum efficiency photon return 80 photon/cm 2 /s Factor of 2 margin to account for: non ideal laser beam quality, miscellaneous aberrations photon return requirement = 160 photon/cm 2 /s

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March 30, 2000SPIE conference, Munich4 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

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March 30, 2000SPIE conference, Munich5 Laser power requirement in 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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March 30, 2000SPIE conference, Munich6 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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March 30, 2000SPIE conference, Munich7 Two saturation effects Spatial Spot radius (cm) Normalized intensity 10 W 100 W Spectral Frequency (MHz) SATURATIONSATURATION 10 W 100 W

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March 30, 2000SPIE conference, Munich8 Photon return (Photon/cm 2 /s) Laser power (W) Efficiency comparison between CW laser formats Photon return vs. laser power (both at sodium layer i.e. T BTO = T LLT = T atmo = 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

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March 30, 2000SPIE conference, Munich9 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: –T BTO = 0.6 / 0.8 –T LLT = 0.9 – T atmo = 0.8 –Sodium column density= 2 10 9 cm -2 –LLT diameter= 45 cm –1/e 2 intensity diameter on LLT M1 = 30 cm –Laser beam quality= 1.5 x DL –LGS spot 1/e 2 intensity diameter= 36 cm

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March 30, 2000SPIE conference, Munich10 Laser power (W) Laser bandwidth (MHz) Photon return (Photon/cm 2 /s) vs.laser output power and laser bandwidth within the Gemini assumptions* * FWHM = 36 cm, T BTO = 0.6, T LLT = 0.9, T atmo = 0.8 Gemini North photon return requirement = 160 photon/cm 2 /s

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March 30, 2000SPIE conference, Munich11 Laser power (W) Optimum bandwidth (MHz) Optimum photon return (Photon/cm 2 /s) CW laser bandwidth optimization Gemini photon requirement (160 photon/cm 2 /s) met for a CW laser in the 8-10 W range with 150-200 MHz bandwidth X X

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March 30, 2000SPIE conference, Munich12 Laser power (W) Laser bandwidth (MHz) Photon return per Watt of laser output power X Inefficient spectral format (bandwidth > 3 GHz) Max. efficiency zone Maximum efficiency at the 10-W level X X Saturation

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March 30, 2000SPIE conference, Munich13 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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March 30, 2000SPIE conference, Munich14 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

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March 30, 2000SPIE conference, Munich15 Saturation model for high repetition rate lasers Uses analytical formula given by Milonni et al. 2 –Photon return saturates as ln(1+I peak /I sat ) –I peak 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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March 30, 2000SPIE conference, Munich16 Pulsed laser with 100 ns pulses at 30 kHz repetition rate

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March 30, 2000SPIE conference, Munich17 Photon return vs. power and rep. rate for a 100 ns-pulse laser

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March 30, 2000SPIE conference, Munich18 Gemini photon return vs. pulse length, rep. rate and power

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