 | This articleneeds additional citations forverification. Please helpimprove this article byadding citations to reliable sources. Unsourced material may be challenged and removed. Find sources: "Adaptive optics" – news ·newspapers ·books ·scholar ·JSTOR(February 2023) (Learn how and when to remove this message) |
Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used inastronomicaltelescopes[1] and laser communication systems to remove the effects ofatmospheric distortion, in microscopy,[2]optical fabrication[3] and inretinal imaging systems (ophthalmoscopy)[4] to reduceoptical aberrations. Adaptive optics works by measuring the distortions in awavefront and compensating for them with a device that corrects those errors such as adeformable mirror or aliquid crystal array.
Adaptive optics should not be confused withactive optics, which work on a longer timescale to correct the primary mirror geometry.
Other methods can achieve resolving power exceeding the limit imposed by atmospheric distortion, such asspeckle imaging,aperture synthesis, andlucky imaging, or by moving outside the atmosphere withspace telescopes, such as theHubble Space Telescope.
Adaptive optics was first envisioned byHorace W. Babcock in 1953,[6][7] and was also considered in science fiction, as inPoul Anderson's novelTau Zero (1970), but it did not come into common usage until advances in computer technology during the 1990s made the technique practical.
Some of the initial development work on adaptive optics was done by the US military during theCold War and was intended for use in trackingSoviet satellites.[8]
Microelectromechanical systems (MEMS)deformable mirrors and magnetics conceptdeformable mirrors are currently the most widely used technology in wavefront shaping applications for adaptive optics given their versatility, stroke, maturity of technology, and the high-resolution wavefront correction that they afford.
The simplest form of adaptive optics istip–tilt correction,[9] which corresponds to correction of thetilts of the wavefront in two dimensions (equivalent to correction of the position offsets for the image). This is performed using a rapidly moving tip–tilt mirror that makes small rotations around two of its axes. A significant fraction of theaberration introduced by theatmosphere can be removed in this way.[10]
Tip–tilt mirrors are effectivelysegmented mirrors having only one segment which can tip and tilt, rather than having an array of multiple segments that can tip and tilt independently. Due to the relative simplicity of such mirrors and having a large stroke, meaning they have large correcting power, most AO systems use these, first, to correct low-order aberrations. Higher-order aberrations may then be corrected with deformable mirrors.[10]
When light from a star or another astronomical object enters the Earth's atmosphere, atmosphericturbulence (introduced, for example, by different temperature layers and different wind speeds interacting) can distort and move the image in various ways.[11] Visual images produced by any telescope larger than approximately 20 centimetres (0.20 m; 7.9 in) are blurred by these distortions.
An adaptive optics system tries to correct thesedistortions, using awavefront sensor which takes some of the astronomical light, adeformable mirror that lies in the optical path, and a computer that receives input from the detector.[12] The wavefront sensor measures the distortions the atmosphere has introduced on the timescale of a fewmilliseconds; the computer calculates the optimal mirror shape to correct thedistortions and the surface of thedeformable mirror is reshaped accordingly. For example, an 8–10-metre (800–1,000 cm; 310–390 in) telescope (like theVLT orKeck) can produce AO-corrected images with anangular resolution of 30–60milliarcsecond (mas)resolution atinfrared wavelengths, while the resolution without correction is of the order of 1arcsecond.
In order to perform adaptive optics correction, the shape of the incoming wavefronts must be measured as a function of position in the telescope aperture plane. Typically the circular telescope aperture is split up into an array ofpixels in a wavefront sensor, either using an array of smalllenslets (aShack–Hartmann wavefront sensor), or using a curvature or pyramid sensor which operates on images of the telescope aperture. The mean wavefront perturbation in each pixel is calculated. This pixelated map of the wavefronts is fed into the deformable mirror and used to correct the wavefront errors introduced by the atmosphere. It is not necessary for the shape or size of theastronomical object to be known – evenSolar System objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of theSun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.
Because a science target is often too faint to be used as a reference star for measuring the shape of the optical wavefronts, a nearby brighterguide star can be used instead. The light from the science target has passed through approximately the same atmospheric turbulence as the reference star's light and so its image is also corrected, although generally to a lower accuracy.
The necessity of a reference star means that an adaptive optics system cannot work everywhere on the sky, but only where a guide star of sufficientluminosity (for current systems, aboutmagnitude 12–15) can be found very near to the object of the observation. This severely limits the application of the technique for astronomical observations. Another major limitation is the small field of view over which the adaptive optics correction is good. As the angular distance from the guide star increases, the image quality degrades. A technique known as "multiconjugate adaptive optics" uses several deformable mirrors to achieve a greater field of view.[13]
An alternative is the use of alaser beam to generate a reference light source (alaser guide star, LGS) in the atmosphere. There are two kinds of LGSs:Rayleigh guide stars andsodium guide stars. Rayleigh guide stars work by propagating alaser, usually at nearultraviolet wavelengths, and detecting the backscatter from air at altitudes between 15 and 25 km (49,000 and 82,000 ft). Sodium guide stars use laser light at 589nm to resonantly excite sodium atoms higher in themesosphere andthermosphere, which then appear to "glow". The LGS can then be used as a wavefrontreference in the same way as a natural guide star – except that (much fainter) natural reference stars are still required for image position (tip/tilt) information. Thelasers are often pulsed, with measurement of theatmosphere being limited to a window occurring a fewmicroseconds after the pulse has been launched. This allows the system to ignore most scattered light at ground level; only light which has travelled for several microseconds high up into the atmosphere and back is actually detected.}
Adaptive optics has applications inophthalmology.Ocular aberrations aredistortions in the wavefront passing through the pupil of theeye. Theseoptical aberrations diminish the quality of the image formed on the retina, sometimes necessitating the wearing of spectacles orcontact lenses. In the case of retinal imaging, light passing out of the eye carries similar wavefront distortions, leading to an inability to resolve the microscopic structure (cells and capillaries) of the retina. Spectacles and contact lenses correct "low-order aberrations", such asdefocus andastigmatism, which tend to be stable in humans for long periods of time (months or years). While correction of these is sufficient for normal visual functioning, it is generally insufficient to achieve microscopic resolution. Additionally, "high-order aberrations", such as coma,spherical aberration, and trefoil, must also be corrected in order to achieve microscopic resolution. High-order aberrations, unlike low-order, are not stable over time, and may change over time scales of 0.1s to 0.01s. The correction of these aberrations requires continuous, high-frequency measurement and compensation.
Ocular aberrations are generally measured using awavefront sensor, and the most commonly used type of wavefront sensor is theShack–Hartmann. Ocular aberrations are caused by spatial phase nonuniformities in the wavefront exiting the eye. In a Shack-Hartmann wavefront sensor, these are measured by placing a two-dimensional array of small lenses (lenslets) in a pupil plane conjugate to the eye's pupil, and a CCD chip at the back focal plane of the lenslets. The lenslets cause spots to be focused onto the CCD chip, and the positions of these spots are calculated using a centroiding algorithm. The positions of these spots are compared with the positions of reference spots, and the displacements between the two are used to determine the local curvature of the wavefront allowing one to numerically reconstruct the wavefront information—an estimate of the phase nonuniformities causingaberration.
Once the local phase errors in the wavefront are known, they can be corrected by placing a phase modulator such as a deformable mirror at yet another plane in the system conjugate to the eye's pupil. The phase errors can be used to reconstruct the wavefront, which can then be used to control the deformable mirror. Alternatively, the local phase errors can be used directly to calculate the deformable mirror instructions.
If the wavefront error is measured before it has been corrected by the wavefront corrector, then operation is said to be "open loop".
If the wavefront error is measured after it has been corrected by the wavefront corrector, then operation is said to be "closed loop". In the latter case then the wavefront errors measured will be small, and errors in the measurement and correction are more likely to be removed. Closed loop correction is the norm.
Adaptive optics was first applied to flood-illumination retinal imaging to produce images of single cones in the living human eye. It has also been used in conjunction withscanning laser ophthalmoscopy to produce (also in living human eyes) the first images of retinal microvasculature and associated blood flow and retinal pigment epithelium cells in addition to single cones. Combined withoptical coherence tomography, adaptive optics has allowed the firstthree-dimensional images of living conephotoreceptors to be collected.[14]
In microscopy, adaptive optics is used to correct for sample-induced aberrations.[15] The required wavefront correction is either measured directly using wavefront sensor or estimated by using sensorless AO techniques.
Besides its use for improving nighttime astronomical imaging and retinal imaging, adaptive optics technology has also been used in other settings. Adaptive optics is used for solar astronomy at observatories such as theSwedish 1-m Solar Telescope,Dunn Solar Telescope, andBig Bear Solar Observatory. It is also expected to play a military role by allowing ground-based and airbornelaser weapons to reach and destroy targets at a distance includingsatellites in orbit. TheMissile Defense AgencyAirborne Laser program is the principal example of this.
Adaptive optics has been used to enhance the performance of classical[17][18] and quantum[19][20]free-space optical communication systems, and to control the spatial output of optical fibers.[21]
Medical applications include imaging of theretina, where it has been combined withoptical coherence tomography.[22] Also the development of Adaptive Optics Scanning Laser Ophthalmoscope (AOSLO) has enabled correcting for the aberrations of the wavefront that is reflected from the human retina and to take diffraction limited images of the human rods and cones.[23] Adaptive andactive optics are also being developed for use in glasses to achieve better than20/20 vision, initially for military applications.[24]
After propagation of a wavefront, parts of it may overlap leading to interference and preventing adaptive optics from correcting it. Propagation of a curved wavefront always leads to amplitude variation. This needs to be considered if a good beam profile is to be achieved in laser applications. In material processing using lasers, adjustments can be made on the fly to allow for variation of focus-depth during piercing for changes in focal length across the working surface. Beam width can also be adjusted to switch between piercing and cutting mode.[25] This eliminates the need for optic of the laser head to be switched, cutting down on overall processing time for more dynamic modifications.
Adaptive optics, especially wavefront-coding spatial light modulators, are frequently used inoptical trapping applications to multiplex and dynamically reconfigure laser foci that are used to micro-manipulate biological specimens.
A rather simple example is the stabilization of the position and direction of laser beam between modules in a large free space optical communication system.Fourier optics is used to control both direction and position. The actual beam is measured byphoto diodes. This signal is fed intoanalog-to-digital converters and then amicrocontroller which runs aPID controller algorithm. The controller then drivesdigital-to-analog converters which drivestepper motors attached tomirror mounts.
If the beam is to be centered onto 4-quadrant diodes, noanalog-to-digital converter is needed.Operational amplifiers are sufficient.
{{cite book}}:|journal= ignored (help)
