Laser Guide Stars, Part 2 Lecture 11

1
Laser Guide Stars, Part 2Lecture 11

• Claire Max
• UCSC
• February 12th, 2008

2
Outline of two lectures on laser guide stars

• 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
• Sky coverage for laser guide stars determined by
  accuracy of tip-tilt correction
• Wavefront error terms with laser guide stars

3
Atomic processes for two-level atom

• Einstein, 1916 atom interacts with light in 3
  ways
• Spontaneous emission
• Stimulated emission
• Absorption

Graphics credit Wikipedia
4
Principle of detailed balancing

• Einstein coefficients A21 B21 B12 are fixed
  probabilities associated with each atom. Dont
  depend on state of the gas of which the atoms are
  a part.
• At equilibrium net change in the number of any
  excited atoms is zero. Losses and gains due to
  all processes balance.
• Bound-bound transitions have detailed balancing
  net exchange between any 2 levels will be
  balanced.

5
Check units
atoms
(cm3 Hz / erg) sec-1

• Bnm U(?) Nn Amn Nm Bmn U(?) Nm

ergs / cm3 Hz
sec-1 per atom
6
Saturation effects in the Na layer, from
Kibblewhites paper

• Consider a 2 level atom which initially has
  ground state n containing Nn atoms and an empty
  upper state m. Atom is excited by radiation field
  tuned to transition
• ? Em-En/h, h? gtgt kT
• In equilibrium BnmU(?) Nn AmnNm BmnU(?)Nm
• Amn is Einstein's coefficient A ( 1/lifetime
  in upper state). Bnm Bmn Einsteins B
  coefficient.
• U(?) is the radiation intensity in units of
  Joules/cm3 Hz

7
Saturation, continued

• Solve for Nm Nn Bnm U(?) / BnmU(?) Amn
• If we define the fraction of atoms in level m as
  f and the fraction in level n as (1-f) we can
  rewrite this equation as
• f Bmn U(?) (1 - f )/ (BmnU(?) Amn)
• f 1/2 Amn/ BmnU(?)
• This equation shows that at low levels of
  radiation U(?) the fraction of atoms in the upper
  level is BmnU(?)/Amn .
• As the radiation density increases, fraction of
  atoms in upper level saturates to a maximum level
  of 1/2 for an infinite value of U (?).
• Define a saturation level as radiation field
  generating 1/2 this max
• Usat(?) Amn/2Bmn

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Saturation, continued

• The ratio Amn/Bmn is known from Planck's black
  body formula and is equal to 8?h?3/c3 joules
  cm-3 Hz
• The intensity of the radiation field I(?) is
  related to U(?) by
• I(?) U(?) c watts/cm2 Hz
• Isat ? 9.48 mW/cm2 for linearly polarized light
  
• In terms of photons Nsat a few x 1016
  photons/sec.

9
Saturation curve for 2-level atom

• When U Usat, half the atoms are in the upper
  state
• When U 3 Usat, three quarters of the atoms are
  in the upper state

10
CW lasers produce more return/watt than pulsed
lasers because of lower peak power

• Lower peak power ? less saturation

3
Keck requirement 0.3 ph/ms/cm2
3
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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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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 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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Frequency doubled NdYAG lasers

• NdYAG means neodinium-doped yttrium aluminum
  garnet
• NdYAG 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 Coherents Verdi laser
• Pump light from laser diodes
• Very efficient
• Available up to 18 Watts
• Expensive
• Its always worrisome when price isnt listed on
  the web!

17
Early Rayleigh guide stars

• For military applications
• Starfire Optical Range, Albuquerque, late 1980s
• Used copper vapor laser (hard to work with, very
  inefficient)
• Got good wavefront correction closed-loop
• Thermotrex, San Diego, late 1980s

18
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

23
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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Lick Observatory
Photo by Dave Whysong, UCSB
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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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First Keck LGS Results (9/19/03)
Extraordinary seeing
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Keck laser guide star performance required guide
star brightness

• Keck NGS Keck LGS
• LGS has lower peak Strehl ratio, but works for
  fainter guide stars

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Keck laser guide star performanceStrehl vs.
angular distance from guide star

• Keck NGS Keck LGS
• Strehl falls to 0.3 at 12 arc sec Strehl falls
  to 0.3 at 25 arc sec

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Galactic Center with Keck laser guide star AO
Keck laser guide star AO
Best natural guide star AO
Andrea Ghez, UCLA group
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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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(No Transcript)
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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

36
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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Questions about lasers?
39
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
Deformable mirror
Tip-tilt mirror
Beam splitter
Wavefront sensor
Tip-tilt sensor
Beam splitter
Imaging camera
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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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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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Compare NGS and LGS performance

• Predictions ESO VLT

• Measurements Keck LGS

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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