Adaptive Optics: basic concepts, principles and applications Short course of lectures

1. Adaptive Optics:basic concepts, principles and applicationsShort course of lectures Vadim Parfenov Res.Ctr. “S.I.Vavilov State Optical Institute” Birzhevaya liniay, 14, St.Petersburg, 199034, Russia vadim@optilas.spb.ru

2. Contents Lecture #1 Basic Concepts and Principles of AdaptiveOptics Lecture #2 Applicationsof Adaptive Optics. New Technologies. Future of AO.

3. Lecture #1 Basic Concepts and Principles of Adaptive Optics

6. Light • Wavelengths (color). Optics deals with the range of 10nm to about 500 mm. • Visible light occupies range of 350 to 700nm. • The speed of light is 300000 km/s and depends on the material. Light propagates slower in transparent dielectrics.

7. 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.

8. Geometrical optics Refraction and reflection of light rays are treated by geometrical optics: Refraction Reflection

9. Imaging optics

10. 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.

11. 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.

12. Diffraction  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.

13. 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.

14. Some definitions concerned with Adaptive OpticsA 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.

15. In case of astronomical observations atmospheric turbulence introduces distortions into the wavefront arriving froman astronomical source (guide).Such a distorted wave cannot befocusedinto a diffraction-limited image unless some sort of phase correction is applied.r0is 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 r0to vary between about 1cmand20cm, depending on the how strong the turbulence is at the time.

16. 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 10cm 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 reachedwithout some sort of correction.

17. 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.

18. 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.

19. 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.

20. 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.

21. 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.

22. History and Basic Concepts of Adaptive Optics1953 - 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)).

23. 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).

24. In 1950s realization of adaptive optical systems was beyond the technological capabilities… For this reason…  …first fully operationaladaptive 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.

25. 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)

26. 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.

27. 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) )

28. 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.

29. 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 ASTRONOMY SYSTEM ITEC HAPPER UTRC MERKLE FOY&LABEYRIE BECKERS ESO/ONERA FEINLIEB HUDGIN HUGNES TTC TYLER LINNIK LASER GUIDE STARS PERKIN-ELMER TECNOLOGY BABCOCK LUKIN FRAED MIT/LL GREENWOOD THEORY

30. What’s now ?- Era of building a number of large adaptive astronomical telescopes.- There are a number of non-astronomical applications of adaptive optics

31. 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.

32. Two alternative approaches: Linear Adaptive Optics and Non-linear Adaptive Optics What is difference ?

33. Linear Adaptive Optics Real-time compensation of wavefrontaberrations by means of adjustable optical elements.

34. Non-linear Adaptive Optics Compensation of wavefront aberrationsby means of non-linear optics phenomena.

35. 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.).

36. Principles of Linear Adaptive Optics

37. Linear Adaptive Optics includesActive 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).

38. ADAPTIVE OPTICS ACTIVE OPTICS FIGURING LOCAL AIR 10/D ATMOSPHERE SPATIAL FREQUENCY 1/D WIND GRAVITY THERMAL 0.1/D dc 0.01 0.1 1 10 100 TEMPORAL FREQUENCY (Hz) FREQUENCY DOMAIN OF WAVEFRONT ABERRATIONS GENERATED BY VARIOSLY SOURCES. THE SPATIAL FREQUENCY IS MEASURED IN TERMS OF D, THE DIAMETER OF THE TELESCOPE.

39. The simplest example for considering principles of Adaptive Optics is case of astronomical observations through distorted Earth athmosphere…

41. STELLAR OBJECT PERFECT WAVEFRONT ATMOSPHERIC EFFECTS AND OPTICAL SYSTEM ABERRATIONS LIGHT BEAM SPLITTER PERTURBED WAVEFRONT ASTRONOMICAL DETECTOR ACTIVE ELEMENT CORRECTEDWAVEFRONT CONTROL SYSTEM WAVEFRONT SENSOR THE PRINCIPLE OF ACTIVE AND ADAPTIVE OPTICS

42. Linear Adaptive Optics Systemswork 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.

44. Principle3. 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).

45. Hartmann Sensor

46. The Shack-Hartmann methodisthe 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 subaperturesof 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 positionsis 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.

47. imager (CCD, CMOS imager) • limited frame rate • data-reduction algorithm • position-sensitive detector matrix (PSD) • fast readout • direct spot position output

49. 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)

50. 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.