Laser Guide Stars, Part 2 Lecture 11 - PowerPoint PPT Presentation

1 / 55
About This Presentation
Title:

Laser Guide Stars, Part 2 Lecture 11

Description:

Saturation effects in the Na layer, from Kibblewhite's paper ... Saturation, continued ... Disadvantages: potential saturation, less efficient excitation of ... – PowerPoint PPT presentation

Number of Views:344
Avg rating:4.0/5.0
Slides: 56
Provided by: clair97
Category:

less

Transcript and Presenter's Notes

Title: 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

8
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
11
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

12
Overall layout (any kind of laser)
13
Principles of laser action
  • Stimulated emission

Mirror
14
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.

15
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.

16
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.

19
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

20
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)

21
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

22
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

24
Lick Observatory
Photo by Dave Whysong, UCSB
25
Keck laser guide star
26
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

27
PARSEC dye laser at the VLT, Chile
  • Under the Nasmyth platform
  • More compact than Lick and Keck lasers (I
    think...)

28
First Keck LGS Results (9/19/03)
Extraordinary seeing
29
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

30
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

31
Galactic Center with Keck laser guide star AO
Keck laser guide star AO
Best natural guide star AO
Andrea Ghez, UCLA group
32
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
33
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
34
(No Transcript)
35
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

37
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

38
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
40
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

41
Tip-tilt mirror and sensor configuration

Telescope
Deformable mirror
Tip-tilt mirror
Beam splitter
Wavefront sensor
Tip-tilt sensor
Beam splitter
Imaging camera
42
Sky coverage is determined by distribution of
(faint) tip-tilt stars
  • Keck gt18th magnitude

From Keck AO book
43
LGS Hartmann spots are elongated
Sodium layer
Laser projector
Telescope
Image of beam as it lights up sodium layer
elongated spot
44
Elongation in the shape of the LGS Hartmann spots
Representative elongated Hartmann spots
Off-axis laser projector
Keck pupil
45
Keck Subapertures farthest from laser launch
telescope show laser spot elongation
Image Peter Wizinowich, Keck
46
LGS spot elongation due to off-axis projection
hurts system performance
From Keck AO book
Ten meter telescope
47
New CCD geometry for WFS being developed to deal
with spot elongation
CW Laser
Pulsed Laser
Sean Adkins, Keck
48
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

49
Pixel Island Concept
50
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
51
Cone effect, continued
  • Characterized by parameter d0
  • Hardy Sect. 7.3.3 (cone effect focal
    anisoplanatism)
  • ?sFA2 ( D / d0)5/3

52
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

53
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

54
Compare NGS and LGS performance
  • Predictions ESO VLT
  • Measurements Keck LGS

55
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
Write a Comment
User Comments (0)
About PowerShow.com