Adaptive Optics in the VLT and ELT era Laser Guide Stars

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Adaptive Optics in the VLT and ELT era Laser
Guide Stars
François Wildi Observatoire de Genève
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Outline of this short lecture

• 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 stars Main points of this lecture

• Laser guide stars are needed because there arent
  enough bright natural guide stars in the sky
• Hence YOUR favorite galaxy probably wont have a
  bright enough natural guide star nearby
• Solution make your own guide star
• Using lasers
• Nothing special about coherent light - could use
  a flashlight hanging from a giant high-altitude
  helicopter
• Size on sky has to be ? diffraction limit of a
  WFS sub-aperture
• Laser guide stars have PROS and CONS
• Pluses can put them anywhere, can be bright
• Minuses NGS give better AO performance than LGS
  even when both are working perfectly.
  High-powered lasers are tricky to build and work
  with. Laser safety is added complication.

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Two types of laser guide stars in use today
Rayleigh and Sodium

• Sodium guide stars excite atoms in sodium
  layer at altitude of 95 km
• Rayleigh guide stars Rayleigh scattering from
  air molecules sends light back into telescope, h
  10 km
• Higher altitude of sodium layer is closer to
  sampling the same turbulence that a star from
  infinity passes through

95 km
8-12 km
Turbulence
Telescope
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Reasons why laser guide stars cant do as well as
bright natural guide stars

• 1) Laser light is spread out by turbulence on
  the way up.
• Spot size is finite (0.5 - 2 arc sec)
• Can increase measurement error of wavefront
  sensor
• Harder to find centroid if spot is larger
• 2) For Rayleigh guide stars, some turbulence is
  above altitude where light is scattered back to
  telescope. Hence it cant be measured.
• 3) For both kinds of guide stars, light coming
  back to telescope is spherical wave, but light
  from real stars is plane wave
• Implications some of the turbulence around the
  edges of the pupil isnt sampled well

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Cone effect
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Scattering 2 different physical processes

• Rayleigh Scattering (Rayleigh beacon)
• Elastic scattering from atoms or molecules in
  atmosphere. Works for broadband light, no change
  in frequency
• Resonance Scattering (Sodium Beacon)
• Line radiation is absorbed and emitted with no
  change in frequency.

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Rayleigh Scattering

• Due to interactions of the electromagnetic wave
  from the laser beam with molecules in the
  atmosphere.
• The lights electromagnetic fields induce dipole
  moments in the molecules, which then emit
  radiation at same frequency as the exciting
  radiation (elastic scattering).

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Dependence of Rayleigh scattering on altitude
where the scattering occurs

• Product of Rayleigh scattering cross section with
  density of molecules is
• where P(z) is the pressure in millibars at
  altitude z, and T(z) is temperature in degrees K
  at altitude z
• Because pressure P(z) falls off exponentially
  with altitude, Rayleigh beacons are generally
  limited to altitudes below 8 - 12 km

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Rayleigh laser guide stars use timing of laser
pulses to detect light from Dz

• Use a pulsed laser, preferably at a short
  wavelength (UV or blue or green) to take
  advantage of ?-4
• Cut out scattering from altitudes lower than z by
  taking advantage of light travel time z/c
• Only open shutter of your wavefront sensor when
  you know that a laser pulse has come from the
  desired scattering volume Dz at altitude z

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Rayleigh laser guide stars

• GLAS Rayleigh laser guide star, La Palma.
  Current.
• MMT LGS. current

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Sodium Resonance Fluorescence

• Resonance scattering occurs when incident laser
  is tuned to a specific atomic transition.
• Absorbed photon raises atom to an excited state.
  Atom then emits photon of the same wavelength via
  spontaneous or stimulated emission, returning to
  the lower state that it started from.
• Can lead to large absorption and scattering
  cross-sections.
• Layer in mesosphere ( h 95 km, Dh 10 km)
  containing alkali metals, sodium (103 - 104
  atoms/cm3), potassium, calcium
• Strongest laser return is from D2 line of Na at
  589 nm.

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The atmospheric sodium layer altitude 95 km ,
thickness 10 km
Credit Clemesha, 1997
Credit Milonni, LANL

• Layer of neutral sodium atoms in mesosphere
  (height 95 km)
• Thought to be deposited as smallest meteorites
  burn up

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Rayleigh scattering vs. sodium resonance
fluorescence

• Atmosphere has exponential density profile

• M molecular mass, n number density, T
  temperature, k Plancks constant, g
  gravitational acceleration
• Rayleigh scattering dominates over sodium
  fluorescence scattering below h 75 km.

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Image of sodium light taken from telescope very
close to main telescope
Light from Na layer at 100 km
Max. altitude of Rayleigh 35 km
Rayleigh scattered light from low altitudes
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Overview of sodium physics

• Column density of sodium atoms is relatively low
• Less than 600 kg in whole Earths sodium layer!
• When you shine a laser on the sodium layer, the
  optical depth is only a few percent. Most of the
  light just keeps on going upwards.
• Natural lifetime of D2 transition is short 16
  nsec
• Cant just pour on more laser power, because
  sodium D2 transition saturates
• Once all the atoms that CAN be in the excited
  state ARE in the excited state, return signal
  stops increasing even with more laser power

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Sodium abundance varies with season

• At University of Illinois factor of 3 variation
  between December-January (high) and May-June (low)

• In Puerto Rico (Arecibo) smaller seasonal
  variation. Tropical vs. temperate?

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Can see vertical distribution of Na atoms by
looking at laser return from side view
105 km
95 km
Propagation direction
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Time variation of Na density profiles over
periods of 4 - 5 hours
Night 1 single peaked
Night 2 double peaked
At La Palma, Canary Islands
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Next discuss line profile of D2 line, and
saturation of Na resonance transition

• Line profile determines what the linewidth of the
  laser should be, to get best return signal
• Line profile and atomic physics determine
  saturation level
• Beyond a certain incident laser flux, all the
  atoms that CAN be in the upper state ARE in the
  upper state.
• Laser return signal no longer increases as you
  increase incident laser power above the power
  corresponding to saturation.

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Doppler Broadening dominates line shape

• For gas in equilibrium _at_ temp. T, fraction of
  atoms with velocities between v and dv is given
  by Boltzmann distribution
• For atom moving at velocity v towards source,
  frequency of the radiation is shifted by
• Rewrite in frequency space as
• HWHM can be found from distribution in frequency
  space
• For sodium atom at 200 K, Doppler width is 1
  GHz 100 X larger than natural linewidth.

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Shape of Doppler-broadened sodiumNa D2 line in
mesosphere, T 200 K

• 1.8 GHz separation between 2 peaks due to
  hyperfine splitting of ground state
• FWHM 2.5 GHz
• Question if each naturally broadened line is 10
  MHz wide (natural linewidth), how many velocity
  groups are there within FWHM?

Answer 250 velocity groups within the Doppler
profile

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Result of previous calculation

• If sodium layer is illuminated with a single
  frequency laser tuned to the peak of the D2 line
  only a few per cent (of order 10 MHz/1GHz) of the
  atoms travel in a direction to interact at all
  with the radiation field.
• These atoms interact strongly with the radiation
  until they collide or change direction.
• Use multi-frequency laser in order to excite many
  velocity groups at once.
• Bottom line Saturation occurs at about Nsat a
  few x 1016 photons/sec.

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CW lasers produce more return/watt than pulsed
lasers because of lower peak power

• Lower peak power ? less saturation

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Keck requirement 0.3 ph/ms/cm2
3
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Laser guide stars Main points so far

• Laser guide stars are needed because there arent
  enough bright natural guide stars in the sky
• Hence YOUR favorite galaxy probably wont have a
  bright enough natural guide star nearby
• Solution make your own guide star
• Using lasers
• Nothing special about coherent light - could use
  a flashlight hanging from a giant high-altitude
  helicopter
• Size on sky has to be ? diffraction limit of a
  WFS sub-aperture
• Rayleigh scattering from 10 km, doesnt sample
  turbulence as well as resonant scattering from Na
  layer at 100 km. But lasers are cheaper and
  easier to build.
• Sodium laser guide stars must deal with
  saturation of atomic transition. Means you
  should minimize peak laser power. Some aspects
  of optimizing the Na return are not yet
  understood.

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Laser technology
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Types of lasers Outline

• Principle of laser action
• Lasers used for Rayleigh guide stars
• Serious candidates for use with Ground Layer AO
• Doubled or tripled NdYAG
• Excimer lasers
• Lasers used for sodium guide stars
• Dye lasers (CW and pulsed)
• Solid-state lasers (sum-frequency)
• Fiber lasers

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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 l-4 ? short wavelengths
  better
• Commercial lasers are available
• Reliable, relatively inexpensive
• Examples
• Frequency-doubled or tripled NdYAG lasers
• Nonlinear crystal doubles the frequency of 1.06
  micron light, to yield 532 nm light quite
  efficient
• Excimer lasers not so efficient
• Example Univ. of Illinois, l 351 nm
• Excimer stands for excited dimer, a diatomic
  molecule usually of an inert gas atom and a
  halide atom, which are bound only when in an
  excited state.

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Current Rayleigh guide star lasers

• SOAR SAM
• Frequency tripled NdYAG, ? 355 nm, 8W, 10 kHz
  rep rate
• MMT Upgrade
• Two frequency doubled NdYAG, ? 532 nm, 30 W
  total, 5 kHz rep rate
• William Herschel Telescope GLAS. Either
• YbYAG disk laser at ? 515 nm, 30 W, 5 kHz,
  or
• 25W pulsed frequency-doubled NdYLF (or YAG)
  laser, emitting at ? 523 (or 532) nm
• Both are in the literature. Not sure which was
  chosen.

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Rayleigh guide stars in planning stage

• LBT (planned)
• Possibly 532nm Nd in hybrid design with lower
  power Na laser at 589nm
• Cartoon courtesy of Sebastian Rabien and Photoshop

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Lasers used for sodium guide stars

• 589 nm sodium D2 line doesnt 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 crystals (nonlinear index of
  refraction)

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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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Two types of dye lasers used for sodium laser
guide stars

• Dye solution is circulated from a large reservoir
  to the (small) lasing region. Types of lasing
  region
• Free-space dye jet
• Dye flows as a sheet-like stream in open air from
  a specially-shaped nozzle
• Can operate CW (continuous wave) - always on
• Average power limited to a few watts per dye jet
• Contained in a glass cell
• Dye can be at pressure gtgt atmospheric
• Very rapid dye flow ? can remove waste heat fast
  ? can operate at higher average power

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Dye lasers for guide stars

• Single-frequency continuous wave (CW) always
  on
• Modification of commercial laser concepts
• At Subaru (Mauna Kea, HI) PARSEC laser at VLT in
  Chile
• Advantage avoid saturation of Na layer
• Disadvantage hard to get one laser dye jet to gt
  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
• Efficiency dye lasers themselves are quite
  efficient, but their pump lasers are frequently
  not efficient

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Keck laser guide star
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Keck dye laser architecture

• Dye cells (589 nm) on telescope pumped by
  frequency doubled NdYAG lasers on dome floor
• Light transported to telescope by optical fibers
• Dye master oscillator, YAG lasers in room on dome
  floor (Keck)
• Main dye laser on telescope
• Refractive launch telescope

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PARSEC dye laser at the VLT, Chile

• Under the Nasmyth platform
• More compact than Lick and Keck lasers (I
  think...)

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Solid-State Lasers for Na Guide Stars Sum
frequency mixing concept

• Two diode laser pumped NdYAG lasers are
  sum-frequency combined in a non-linear crystal
• Advantageous spectral and temporal profile
• Potential for high beam quality due to non-linear
  mixing
• Good format for optical pumping with circular
  polarization
• Kibblewhite (U Chicago and Mt Palomar), Telle
  (Air Force Research Lab), Coherent Technologies
  Incorporated (for Gemini N and S Observatories
  and Keck 1 Telescope)

(1.06 mm)-1 (1.32 mm)-1 (0.589 mm)-1
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Air Force Research Labs sum-frequency laser is
the farthest along, right now

• Sum-frequency generation using nonlinear crystal
  is done inside resonant cavity
• Higher intensity, so increased efficiency of
  nonlinear frequency mixing in crystal
• Laser producing 50W of 589 nm light!

Telle and Denman, AFRL
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Air Force Research Lab laser seems most efficient
at producing return from Na layer

• Why?
• Hillman has theory based on atomic physics
  narrow linewidth lasers should work better
• Avoid Na atom transitions to states where the
  atom cant be excited again
• More work needs to be done to confirm theory
• Would have big implications for laser pulse
  format preferred in the future

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Future lasers all-fiber laser (Pennington, LLNL
and ESO)

• Example of a fiber laser

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Potential advantages of fiber lasers

• Very compact
• Uses commercial parts from telecommunications
  industry
• Efficient
• Pump with laser diodes - high efficiency
• Pump fiber all along its length - excellent
  surface to volume ratio
• Disadvantage has not yet been demonstrated at
  the required power levels at 589 nm

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Working with lgs
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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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Sky coverage is determined by distribution of
(faint) tip-tilt stars

• Keck gt18th magnitude

From Keck AO book
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LGS Hartmann spots are elongated
Sodium layer
Laser projector
Telescope
Image of beam as it lights up sodium layer
elongated spot
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Elongation in the shape of the LGS Hartmann spots
Representative elongated Hartmann spots
Off-axis laser projector
Keck pupil
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Keck Subapertures farthest from laser launch
telescope show laser spot elongation
Image Peter Wizinowich, Keck
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LGS spot elongation due to off-axis projection
hurts system performance
From Keck AO book
Ten meter telescope
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For ELTs new CCD geometry for WFS being
developed to deal with spot elongation
CW Laser
Pulsed Laser
Sean Adkins, Keck
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Polar Coordinate Detector

• CCD optimized for LGS AO wavefront sensing on an
  Extremely Large Telescope (ELT)
• Allows good sampling of a CW LGS image along the
  elongation axis
• Allows tracking of a pulsed LGS image
• Rectangular pixel islands
• Major axis of rectangle aligned with axis of
  elongation

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Pixel Island Concept
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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)
• ?sFA2 ( D / d0)5/3

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Dependence of d0 on beacon altitude
from Hardy

• One Rayleigh beacon OK for D lt 4 m at l 1.65
  micron
• One Na beacon OK for D lt 10 m at l 1.65 micron

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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 stilt2
  ( ? / ?tilt )5/3
• New focus anisoplanatism sFA2 ( D /
  d0 )5/3
• Laser spot larger than NGS smeas2 (
  ?b / SNR )2

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Main Points

• Rayleigh beacon lasers are relatively
  straightforward to purchase, but 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 LGSs
• Tilt anisoplanatism, cone effect, larger spot