Transcript Lecture #1 Basic Concepts and Principles of Adaptive Optics

Adaptive Optics:
basic concepts, principles
and applications
Short course of lectures
Vadim Parfenov
Res.Ctr. “S.I.Vavilov State Optical Institute”
Birzhevaya liniay, 14, St.Petersburg, 199034, Russia
[email protected]
Contents
Lecture #1
Basic Concepts and Principles
of Adaptive Optics
Lecture #2
Applications of Adaptive Optics.
New Technologies. Future of AO.
Lecture #1
Basic Concepts and
Principles of Adaptive
Optics
Outline
• Basic Definitions on Optics
• Some definitions concerned with Adaptive
Optics
• History and basic concepts of Adaptive
Optics
• Principles of Linear and Non-linear
Adaptive Optics
• Conclusions
Basic Definitions on Optics
• The energy propagates along the rays.
• A continuous surface orthogonal to all rays is
called wavefront. Wavefront can be defined for a
non-monochromatic light.
• It can be shown that the phase of monochromatic
light is constant on the wavefront surface.
Light
• Wavelengths (color). Optics deals with the
range of 10 nm to about 500 mm.
• Visible light occupies range of 350 to 700 nm.
• The speed of light is 300000 km/s and
depends on the material. Light propagates
slower in transparent dielectrics.
Point source
• Point source is an infinitely small
source of light.
• Point source produces a bunch of
rays.
• As light propagates with the
speed C in all directions, we can
define a spherical surface
indicating the current position of
light, emitted at “zero” moment.
This surface is a spherical
wavefront of light source radiation.
Geometrical optics

Refraction and reflection
of light rays are treated by
geometrical optics:
'
n
Refraction
n1
1
n sin  = n1 sin 1
Reflection
 
'
Imaging optics
2F
F
a
b
1/ a  1/ b  1/ F
Geometrical aberration
An ideal lens
transforms a spherical
wavefront (WF) into a
spherical WF – image
of a point is a sharp
point.
In case of an
aberration, the WF
deviates from sphere,
resulting in a blurred
image of a point
source.
Aberrations
• Aberration can be described in terms of
ray pointing (homocentric beams).
• Aberration can be described in terms of
wavefront shape.
• Only spherical wavefront can form a
good image of a point.
Diffraction
 /D
l  D2 / 
( / D)l  D  l  D2 /l
Diffraction limits the resolution of optics !
Beam divergency depends on the beam diameter (area)
and on the wavelength.
Another parameter is the far-field distance, defining the
applicability of geometrical optics.
Coherence
For spatial coherence all waves should
be statistically in phase over the cross
section of the beam.
For temporal coherence the waves
should be statistically in phase over
some time delay.
 /D
  l /C
Some definitions concerned
with Adaptive Optics
A Wavefront is an abstract representation of the phase of a
propagating wave. It describes a surface which joins points of
equal phase.
A point light source may produce a spherical wavefront which
travels outward from the source. A flat (or collimated)
wavefront may be focussed down by a telescope or other
image-forming device to a diffraction-limited image.
In case of astronomical observations atmospheric turbulence
introduces distortions into the wavefront arriving from
an astronomical source (guide). Such a distorted wave
cannot be focused into a diffraction-limited image unless
some sort of phase correction is applied.
r0 is a time-dependent parameter which entirely defines the
spatial statistics of phase fluctuations occuring across a
telescope aperture. It was originally defined as the maximum
diameter of telescope which can support diffraction-limited
imaging under the particular conditions of turbulence.
At a good observing site one would expect r0 to vary
between about 1 cm and 20 cm, depending on the how
strong the turbulence is at the time.
The Diffraction Limit refers to the smallest (angular) size of
detail theoretically resolvable by a telescope of a particular
size and shape. This is inversely proportional to the diameter
(D) of the aperture of the telescope, so a 10m telescope
should be able to see details in the science object which are
100 times smaller that a 10 cm telescope can.
In fact, with perfect optics, a telescope should be able to
resolve detail which is (wavelength/D) in angular extent.
Unfortunately atmospheric turbulence blurs the image formed
in a telescope so that the diffraction limit of that telescope
cannot be reached without some sort of correction.
A Reference Star (or guide star) is a light source, which is
used to provide a wave whose phase geometry before
encountering the atmosphere is known sufficiently well in
order to be able to measure distortions introduced by the
atmosphere.
It needs to be bright enough so that sufficient photons are
collected in order to make a good estimate of the
geometry of the wavefront distortions.
A natural guide star (i.e. an astronomical source), or even
the science object itself, may be used as a reference star;
however, if one is not available close to the science object
then a laser beacon may be manufactured.
A Laser Beacon is an artificial reference star which may be
manufactured if a sufficiently bright natural star is not
available close to the science object. It is made by shining a
very high-power laser into a small region of the atmosphere
and collecting and observing light which is scattered back to
the ground.
The Science Object is the astronomical object which is under
study; this may or may not be the same as the reference
source used in adaptive optics.
Phase Corrector is used in an adaptive optics system to take out
some of the distortions introduced by the atmosphere (or any
other similar phenomena). It often consists of a mirror (or other
optical element) which can be rapidly deformed to the equivalent
shape of the wavefront which must be subtracted out.
A Wavefront Sensor is used to measure the phase distortions
introduced by the atmosphere into a reference wave (which are
similar to those introduced into the wave arriving from the
science object in the case of an adaptive optics system). Such a
device may also be used to perform seeing measurements.
Open-loop is the mode of operation of an adaptive optics
system in which the phase aberrations are measured but not
corrected. In this situation, the wavefront sensor observes the
full aberrations introduced by the atmosphere during each cycle.
The open-loop frequency is simply the inverse of the time taken
to detect the phase distortions, calculate the signals required for
correction and drive the phase corrector.
Closed-loop is the mode of operation of an adaptive optics
system in which the phase corrector is used to correct
measured phase aberrations. Once the loop is closed, the
wavefront sensor will only measure residual, or uncorrected,
phase distortions, i.e. the difference between the actual phase
aberrations and the most recent position of the phase corrector.
A Pupil image is the intensity pattern which is formed on the
telescope pupil; in other words it might be what would be observed on
a flat screen placed just above the telescope, over a region which
corresponds to the shape of the telescope aperture (usually an
annular ring). Normally, one might expect the pattern to be uniform,
but atmospheric turbulence diffracts the light from a source above the
atmosphere and causes fluctuations in the pattern, rather like the
ripples seen on the bottom of a swimming pool.
Zernike Polynomials are a convenient set of circular or annular basis
functions which may be used to represent an arbitary phase
distribution over a telescope pupil in a mathematical form. A sum of a
number of polynomials, each with its own weighting, may be used to
reconstruct an atmospherically degraded wavefront; the higher the
number of polynomials used, the better, in general, will be the fit to
the actual physical phase distribution.
History and Basic Concepts
of Adaptive Optics
1953 - Horace Babcock,
then director of the Mount Wilson
and Palomar Observatories, was first
who suggested how one can build an astronomical
telescope with compensation of atmospheric seeing
(H.Babcock, Publ. Astron.Soc.Pac., 65, 229(1953)).
1957 – Vladimir Linnik, Russian Academician
from the Vavilov State Optical Institute, independently
described the same concept in Soviet journal
Optika i Spektoskopiya
( V.Linnik, (USSR), Opt.Spectosk., 3, 401 (1957)).
Linnik was first who suggested to use
an artificial “beacon” to compensate aberrations
of optical imaging systems.
(He proposed to place a portable light source
at 8- to 10-km altitude at the airplane or dirigible to provide
the reference wavefront).
In 1950s realization of adaptive optical systems
was beyond the technological capabilities…
For this reason…
…first fully operational adaptive optics system
was built and installed by American military
scientists only in the mid of 1970s on
a surveillance telescope at Haleakala Observatory
in Maui, Hawaii, USA, where it imaged
satellites launched by the Soviet Union.
1980 – Nick Woolf & Roger Angel
recognized the “polychromatic” nature
of adaptive optics:
Because the atmosphere is only weakly dispersive,
natural stars measured at optical wavelengths
can be used to correct wavefront errors
at infrared wavelenghts.
(N.Woolf, R.Angel, in Optical and Infrared
Telescopes for the 1980s, A.Hewitt, ed., Kitt Peak Natl.
Observarvatory, Tucson, Ariz. (1980), p..1062)
1968 – M.Kogelnik & K.S.Pennington proposed a concept
of holographic correction of aberration of optical systems.
(M.Kogelnik, K.S.Pennington, JOSA, 58, 273-274 (1968))
1971 – Russian scientists Yu.Denisyuk & S.Soskin
carried out first experiments on an aberrated telescope
primary mirror holographic correction
(Yu.Denisjuk, S.Soskin (USSR), Optika I Spektroskopiya, 33, 992-993 (1971) ).
Works of M.Kogelnik, K.S.Pennington, Yu.Denisyuk, & S.Soskin –
beginning of Non-linear Adaptive Optics.
1980s – beginning of the era of artificial laser guide stars.
1983 – Russian scientists V.Lukin & V.Matyukhin,
from the Institute of Atmospheric Optics (Siberian
branch of the Russian Academy of Sciences), renewed
an idea of V.Linnik on the use of reference light beacon
for adaptive image correction
(V.Lukin, Vmatyukhin, (USSR), Kvantovaya elektronika, 10, 2465-2473(1983) )
1985 – French astronomers Renaud Foy & Antonie Laberyie
suggested that backscattered light from a laser could be used
to produce what now is called a laser guide star
( R.Foy, A.Laberyie, Astron.Astrophys., 152, L29 (1985) ).
1991 – Declassification of the US military research programs
concerned with Star Wars –
US military research groups stepped forward to announce
that they too had been investing in both adaptive optics and
laser-guide-star (LGS) researches and independently devised
LGS concept approximately four years before Foy and
Laberyie, but it was not known in open literature.
ADAPTIVE OPTICS
SYSTEM
ESO/VLT
CFHT
Mt.WILSON
GEMINI
SOR (FUGATE)
MMT
ANGLO-AUSTR.
TELESCOPE
NATIONAL
SOLAR OBS.
YERKES
(U.CHICAGO)
LA PALMA
(U.DURHAM)
KECK
CIS
SUBARU
AEOS
Mt.HALEAKATA
AOA
RODDIER
ITEC
HAPPER
ASTRONOMY SYSTEM
MERKLE
UTRC
FOY&LABEYRIE
ESO/ONERA
FEINLIEB
BECKERS
HUDGIN
HUGNES
TYLER
TTC
TECNOLOGY
PERKIN-ELMER
BABCOCK
MIT/LL
LUKIN
LINNIK
FRAED
GREENWOOD
THEORY
LASER GUIDE STARS
What’s now ?
- Era of building a number of large adaptive
astronomical telescopes.
- There are a number of non-astronomical
applications of adaptive optics
Principles of Adaptive Optics
An Adaptive optics system is optical system
which automatically corrects for light
distortions in the medium of transmission in
real time.
An adaptive optics system measures the
characteristics of the aberrated wavefront of
incoming light and corrects for it either by
means of a deformable mirror controlled by a
computer or by means of nonlinear optics.
Two alternative approaches:
Linear Adaptive Optics
and
Non-linear Adaptive Optics
What is difference ?
Linear Adaptive Optics
Real-time compensation of wavefront
aberrations by means of adjustable optical
elements.
Non-linear Adaptive Optics
Compensation of wavefront aberrations by
means of non-linear optics phenomena.
Linear Adaptive Optics
deals with optical systems operating in wide
spectral range
(astronomical telescopes, imaging systems, etc.).
Non-linear Adaptive Optics
deals with coherent optical systems
(high-power lasers, laser communication systems,
coherent optical imaging systems, etc.).
Principles of Linear
Adaptive Optics
Linear Adaptive Optics
includes Active and just Adaptive Optics
Active Optics
deals with wavefront errors of rather low temporal (less than 0.01 Hz)
and spatial frequencies.
Adaptive Optics
deals with rapidly varying wavefront distortions (up to 1000 Hz).
ADAPTIVE OPTICS
ACTIVE OPTICS
SPATIAL
FREQUENCY
10/D
1/D
F
I
G
U
R
I
N
G
LOCAL AIR
GRAVITY THERMAL
ATMOSPHERE
WIND
0.1/D
dc
0.01
0.1
1
10
TEMPORAL FREQUENCY (Hz)
100
FREQUENCY DOMAIN OF WAVEFRONT ABERRATIONS GENERATED BY VARIOSLY SOURCES. THE
SPATIAL FREQUENCY IS MEASURED IN TERMS OF
D, THE DIAMETER OF THE TELESCOPE.
The simplest example for considering
principles of Adaptive Optics is case
of astronomical observations through
distorted Earth athmosphere…
STELLAR
OBJECT
ATMOSPHERIC
EFFECTS AND
OPTICAL SYSTEM
ABERRATIONS
LIGHT BEAM
SPLITTER
ASTRONOMICAL
DETECTOR
ACTIVE
ELEMENT
CONTROL
SYSTEM
WAVEFRONT
SENSOR
THE PRINCIPLE OF ACTIVE AND ADAPTIVE OPTICS
Linear Adaptive Optics Systems work in a
conceptually very simple manner:
Light arriving from a distant optical source (star or any extended
object) is essentially a plane (flat) wave until atmospheric
turbulence (or other aberrant medium) deforms the wavefront’s
shape or, equivalently, induces local phase delays across the
wavefront.
These deformations or phase delays in the wavefront can be
monitored by wavefront sensor and compensated by deformable
mirror in real time.
Incoming light from an astronomical source reflected off the
deformed mirror leaves the mirror’s surface in its original pristine
state, as it had never encountered any atmospheric distortions.
Principle 3. Wavefront sensing
The wavefront sensor (WFS) measures the distortions
in the wavefront of light incoming into adaptive optical
system.
Different approaches:
- Hartmann method;
- Interferometric methods (phase-shifting, shearing, two-wavelength
interferometry);
- Curvature sensing;
- Scene-based sensing (WFS operating on image of extended object).
Hartmann Sensor
Y
plane wavefronts
X
turbulent medium
aberrated wavefronts
disturbed wavefront
hartmann mask
detector plane
The Shack-Hartmann
method is the most
commonly used approach.
This approach is completely geometric in nature and so has no dependence on
the coherence of the sensed optical beam.
The incoming wavefront is broken into an array of spatial samples, called
subapertures of the primary aperture, by a two dimensional array of lenslets.
The subaperture sampled by each lenslet is brought to a focus at a known distance
F behind each array. The lateral position of the focal spot depends on the local tilt of
the incoming wavefront; a measurement of all the subaperture spot positions is
therefore a measure of the gradient of the incoming wavefront.
A two-dimensional integration process called reconstruction can then be used
to estimate the shape of the original wavefront, and from there derive
the correction signals for the deformable mirror.
unknown wavefront
imager (CCD, CMOS imager)
Hartmann mask
 limited frame rate
light spots
 data-reduction algorithm
CCD
or CMOS imager
matrix of
PSD's
bitmap
image
processing
spot positions
position-sensitive detector matrix (PSD)
wavefront
reconstruction
 fast readout
 direct spot position output
wavefront shape
Main elements and typical parameters of
Shack-Hartmann Wavefront Sensor
CCD- Camera
Lenslet Array
Spectral bandwidth
200-..1600 nm
Dynamic phase range of the beam
Up to 15
aperture
mm
Accuracy of measurements
0.08mm
Data acquisition and analysis frequencyUp to 30 Hz
CCD image input
Using Matrix
MeteorII/Standard
frame grabber
The number of Zernike terms
apertures
The number of working- sub
36
Up to400
Principle 4. Deformable mirrors
and wavefront correctors
Deformable mirrors and wavefront correctors are
optical elements which are used to compensate
distortions of wavefront.
Two types of deformable mirrors:
- Large-size adjustable mirrors (used to correct slow
changes of WF)
- Small-size wavefront correctors (used to compensate
fast changes of WF)
Large-size adjustable mirrors
Large primary mirror of astronomical (or imaging telescope) is
flexible enough for mechanical devices (so-called actuators) to
provide constant adjustment of its figure in accordance with the
wavefront measurements.
Three types of adjustable primary mirrors are in the use or
under development of present technology:
- Continuous mirrors (which maybe moderately thin menisci);
- Segmented mirrors (typically – structured honeycomb separate mirrors);
- Membrane-type (inflatable) mirrors manufacturing from thin polymer foil.
Segmented mirror
Segmented mirror. General view
3-meter segmented mirror of adaptive imaging telescope
developed at the Vavilov State Optical Institute
(St.Petersburg, Russia)
MEMBRANE
MIRROR
1-m prototype of membrane mirror
developed at the U.S. Air Force
Research Laboratory
0.3-m prototype of membrane
mirror developed at the Vavilov
State Optical Institite, St.Peterburg,
Russia
Small-size wavefront correctors
Wavefront correctors are small-size deformable mirrors
(or other types of adjustable optical elements), which are
used for compensation of high-frequency (up to 1000 Hz)
distortions of wavefront.
Because of the high bandwidth and the small field, usually
wavefront correctors have diameter of 10 to 20 cm and
are located within the optical train of the telescope or in
separate box behind the focus of the telescope at an
image of the pupil.
1. Segmented wavefront corrector
developed at the Vavilov State Optical Institute (St.Petersburg, Russia) for the space-based NASA/CSA lidar ORACLE
2. Bimorph Mirrors
2. Bimorph mirrors
Design of the bimorph mirror
General view of copper bimorph mirror
(developed by the Adaptive Optics group of Russian Academy of
Sciences)
3. Micromachined Membrane Deformable Mirrors
The shape of tensed membrane is
controlled by the electrostatic attraction
to the grid of electrodes
(developed by G.Vdovin, Delft Technical University, Netherlands)
4. Liquid-crystal spatial light modulators (LC SLM)
First application of LC SPMs in Adaptive Optics D.Es’kov, A.Onokhov, V.Reznichenko, V.Sidorov, the Vavilov State Optical Institute, Russia
(reported at the SPIE Symposium on Astronomical Telescopes for 21 century, Hawaii, USA, March, 1994)
Principle 5. Artificial Laser Guide Star
For what Laser Guide Star is needed ?
Not just any star can be used as reference beacon !
1. It must be bright enough, that is, generate enough light to serve as a reference
light source.
When observing at visible wavelengths, astronomers using adaptive optics
require a 5-magnitude star, one that is just bright enough to be seen unaided.
(For near-infrared observations, only a 10-magnitude star is needed, which is 100
times fainter.)
2. Even though there may be hundreds of thousands or even a million stars bright
enough to be guide stars, they only cover a small fraction of the sky.
There just isn't a natural guide star in the area you want to observed !
First civil Laser Guide Star System
for Astronomy
ALFA
ALFA is the Adaptive Optics with a Laser For Astronomy system
for the Calar Alto 3.5-m telescope.
This is a joint project between the MPIA in Heidelberg and the
MPE, Germany.
ALFA is based on a Shack-Hartmann sensor with a high-speed lownoise CCD camera, a 97-actuator deformable mirror, a tip-tilt
sensor (CCD camera), a tip-tilt mirror and a continuous Ar-Ion
laser pumped dye laser which generates the laser beacon in the
sodium layer of the mesosphere.
Lick Observatory
of the University of California on Mount
Hamilton near San Jose, C.A., U.S.A
Laser Guide Star Laser Projection
System of the University of
California's Lick Observatory
Three views of the satellite Seasat from the U.S. Air Force
Starfire Optical Range 3.5 m adaptive optical telescope
(AF Kirtland Airbase, NM):
(a) through the turbulence,
(b) real time correction using adaptive optical system,
(c) post-processed with the blind deconvolution algorithm.
Principle 7. Multi-modular telescopes
The main idea – using the set of separate small-size telescopic modules one
can achive the same resolution, which can be obtained with large-aperture
primary mirror.
Design of multi-modular telescopic system:
1,2, 5 – aspherical mirrors;
3,4,6 – flat mirrors;
7 - spherical mirror;
8 – receiver of image
Preliminary conclusions
• Linear AO is applicable to the optical imaging systems
and can increase the resolution of its optics;
• Key components of any AO systems include:
- a wavefront sensor to measure the distortions of the
wavefront coming from a star or science object;
- a wavefront correction device (deformable mirror or
wavefront corrector);
- a control computer, which can be relatively slow for
active optics, but must be extremely fast for adaptive
optics.
Principles of Non-linear
Adaptive Optics
Compensation of wavefront aberrations by means of nonlinear optics phenomena (phase conjugation, four-wave
mixing, dynamic holography, etc.)
You have to know everything about it from
course of lectures of Prof. Andreoni !
PHASE CONJUGATION OF CO2 LASER RADIATION
DUE TO FOUR-WAVE MIXING IN SF6 (i 1 mcs)
[1] R.C. Lind et al.,1979 - FIRST EXPERIMENTS WITH TEA CO2 LASERS
[2] N.G.Basov, V.P.Kovalev et al.,1982 - DEMONSTRATION OF HIGH PC FIDELITY
[3] D.A.Goryachkin, V.P.Kalinin,N.A.Romanov et al.,1983 -PCM REFLECTIVITY 100%
[4] D.A.Goryachkin, V.P.Kalinin, I.M.Kozlovskaya, V.E.Sherstobitov, 1987 - FIRST
EXPERIMENTS WITH SF ON THE P20 LINE - REFLECTED BEAM PULSE ENERGY ABOVE 2.5J
5 mm
aberrator
2·10-3 rad
initial beam
aberrated beam
restored beam
Preliminary conclusions
• Non-linear AO is applicable to the coherent optical
systems (including high-power lasers and laser optical
communication systems);
• Researches in the field of non-linear AO also include its
application in imaging optical systems;
• Non-linear AO technology does not require investments
comparable with linear AO (Lower costs) !
Conclusions
• AO is very promising optical technology which can be applicable to
the astronomical telescopes and optical imaging systems and can
increase the resolution of its optics;
• There are two alternative approaches to compensation of wavefront
aberrations based on the use of Linear AO and Non-linear AO;
• AO is a very research-intensive area;
• Huge investments in AO technology are still necessary to get
scientific results and to bring financial returns.