lectures 11 lasers used for guide stars wavefront errors
TRANSCRIPT
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Lectures 11 Lasers used for Guide Stars
Wavefront Errors from Laser Guide Stars
Projects: Discuss Performance Reqirements
Claire Max Astro 289, UC Santa Cruz
February 16, 2016
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Outline of laser guide star topics
✔ Why are laser guide stars needed?
✔ Principles of laser scattering in the atmosphere
✔ What is the sodium layer? How does it behave?
✔ Physics of sodium atom excitation
✔ I stopped here last lecture • Lasers used in astronomical laser guide star AO
• Wavefront errors for laser guide star AO
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Types of lasers: Outline
• Principle of laser action
• Lasers used for Rayleigh guide stars
• Lasers used for sodium guide stars
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Overall layout (any kind of laser)
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Principles of laser action
Stimulated emission
Mirror
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General comments on guide star lasers
• Typical average powers of a few watts to 20 watts – Much more powerful than typical laboratory lasers
• Class IV lasers (a laser safety category) – “Significant eye hazards, with potentially
devastating and permanent eye damage as a result of direct beam viewing”
– “Able to cut or burn skin” – “May ignite combustible materials”
• These are big, complex, and can be dangerous. Need a level of safety training not usual at astronomical observatories until now.
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Lasers used for Rayleigh guide stars
• Rayleigh x-section ~ λ-4 ⇒ short wavelengths better
• Commercial lasers are available – Reliable, relatively inexpensive
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Example: Frequency doubled Nd:YAG lasers
• Nd:YAG means “neodimium-doped yttrium aluminum garnet”
• Nd:YAG emits at 1.06 micron
• Use nonlinear crystal to convert two 1.06 micron photons to one 0.53 micron photon (2 X frequency)
• Example: Coherent’s Verdi laser – Pump light: from laser diodes – Very efficient – Available up to 20 Watts – Pretty expensive
» It’s always worrisome when price isn’t listed on the web!
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Types of Rayleigh guide star lasers
• SOAR: SAM
– Frequency tripled Nd:YAG, λ = 0.35 µm, 8W, 10 kHz rep rate
• LBT:
– Frequency doubled Nd:YAG, λ = 0.53 µm, 15 W each, 10 kHz rep rate
• William Herschel Telescope: GLAS
– Yb:YAG “disk laser” at λ = 0.515 µm, 18 W, 5 kHz
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Lasers used for sodium guide stars
• 589 nm sodium D2 line doesn’t correspond to any common laser materials
• So have to be clever: – Use a dye laser (dye can be made to lase at a range
of frequencies) – Or use solid-state laser materials and fiddle with
their frequencies somehow » Sum-frequency lasers (nonlinear index of refraction) » Raman scattering
– Latest development: Fiber lasers that use either sum-frequency mixing or Raman shifting or both
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Dye lasers
• Dye can be “pumped” with different sources to lase at variety of wavelengths
• Messy liquids, some flammable
• Poor energy efficiency
• You can build one at home! – Directions on the web
• High laser powers require rapid dye circulation, powerful pump lasers
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Dye lasers for guide stars
• Single-frequency continuous wave (CW): always “on” – Modification of commercial laser concepts
– Subaru (Mauna Kea, HI); PARSEC laser at VLT in Chile (now retired)
– Advantage: avoid saturation of Na layer
– Disadvantage: hard to get one laser dye jet to > 3 watts
• Pulsed dye laser
– Developed for DOE - LLNL laser isotope separation program
– Lick Observatory, then Keck Observatory
– Advantage: can reach high average power
– Disadvantages: potential saturation, less efficient excitation of sodium layer
– Use a lot of power
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Lick Observatory
Photo by Dave Whysong, NRAO
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Keck laser guide star
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Keck laser guide star AO Best natural guide star AO
Galactic Center with Keck laser guide star AO
Andrea Ghez, UCLA group
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Solid-State Lasers for Na Guide Stars: Sum frequency mixing concept
• Example: two diode laser pumped Nd:YAG lasers are sum-frequency combined in a non-linear crystal
• Kibblewhite (U Chicago and Mt Palomar), Telle and Denman (Air Force Research Lab), Coherent Technologies Incorporated (for Gemini N and S Observatories and Keck 1 Telescope)
(1.06 µm)-1 + (1.32 µm)-1 = (0.589 µm)-1
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Air Force laser at Starfire Optical Range
• Built by Craig Denman
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New generation of lasers: all-fiber laser (Toptica, Pennington and Dawson LLNL)
• Example of a fiber laser
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Toptica fiber laser (ESO, Keck 2)
Fiber laser Electronics
and cooling
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Advantages of fiber lasers
• Very compact
• Commercial parts from telecommunications industry
• Efficient: – Pump with laser diodes - high efficiency – Pump fiber all along its length - excellent surface to
volume ratio
• Two types of fiber lasers have been demonstrated at the required power levels at 589 nm (Toptica in Europe, Jay Dawson at LLNL)
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Questions about lasers?
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Outline of laser guide star topics
✔ Why are laser guide stars needed?
✔ Principles of laser scattering in the atmosphere
✔ What is the sodium layer? How does it behave?
✔ Physics of sodium atom excitation
✔ Lasers used in astronomical laser guide star AO
• Wavefront errors for laser guide star AO
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Laser guide star AO needs to use a faint tip-tilt star to stabilize laser spot on sky
from A. Tokovinin
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Effective isoplanatic angle for image motion: “isokinetic angle”
• Image motion is due to low order modes of turbulence – Measurement is integrated over whole telescope
aperture, so only modes with the largest wavelengths contribute (others are averaged out)
• Low order modes change more slowly in both time and in angle on the sky
• “Isokinetic angle” – Analogue of isoplanatic angle, but for tip-tilt only – Typical values in infrared: of order 1 arc min
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Tip-tilt mirror and sensor configuration
Telescope
Tip-tilt mirror Deformable mirror
Beam splitter
Beam splitter
Wavefront sensor
Imaging camera
Tip-tilt sensor
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Sky coverage is determined by distribution of (faint) tip-tilt stars
• Keck: >18th magnitude
1"
0
271 degrees of freedom 5 W cw laser
Galactic latitude = 90° Galactic latitude = 30°
From Keck AO book
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“Cone effect” or “focal anisoplanatism” for laser guide stars
• Two contributions:
– Unsensed turbulence above height of guide star
– Geometrical effect of unsampled turbulence at edge of pupil
from A. Tokovinin
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Cone effect, continued
• Characterized by parameter d0
• Hardy Sect. 7.3.3 (cone effect = focal anisoplanatism)
σFA2 = ( D / d0)5/3
• Typical sizes of d0 ~ a few meters to 20 meters
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Dependence of d0 on beacon altitude
• One Rayleigh beacon OK for D < 4 m at λ = 1.65 micron
• One Na beacon OK for D < 10 m at λ = 1.65 micron
from Hardy
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Effects of laser guide star on overall AO error budget
• The good news: – Laser is brighter than your average natural guide star
» Reduces measurement error – Can point it right at your target
» Reduces anisoplanatism
• The bad news: – Still have tilt anisoplanatism σtilt
2 = ( θ / θtilt )5/3 – New: focus anisoplanatism σFA
2 = ( D / d0 )5/3 – Laser spot larger than NGS σmeas
2 ~ ( 6.3 / SNR )2
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Compare NGS and LGS performance
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Main Points
• Rayleigh beacon lasers are straightforward to purchase, but single beacons are limited to medium sized telescopes due to focal anisoplanatism
• Sodium layer saturates at high peak laser powers
• Sodium beacon lasers are harder: – Dye lasers (today) inefficient, hard to maintain – Solid-state lasers are better – Fiber lasers may be better still
• Added contributions to error budget from LGS’s – Tilt anisoplanatism, cone effect, larger spot
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Projects:
• Investigable science questions
• Resulting performance requirements