Adaptive optics

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The wavefront of an aberrated image (left) can be measured using a wavefront sensor (center) and then corrected for using a deformable mirror (right). Adaptive optics.gif
The wavefront of an aberrated image (left) can be measured using a wavefront sensor (center) and then corrected for using a deformable mirror (right).

Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in astronomical telescopes [1] and laser communication systems to remove the effects of atmospheric distortion, in microscopy, [2] optical fabrication [3] and in retinal imaging systems [4] to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.[ citation needed ]

Contents

Adaptive optics should not be confused with active optics, which work on a longer timescale to correct the primary mirror geometry.[ citation needed ]

Other methods can achieve resolving power exceeding the limit imposed by atmospheric distortion, such as speckle imaging, aperture synthesis, and lucky imaging, or by moving outside the atmosphere with space telescopes, such as the Hubble Space Telescope.[ citation needed ]

History

Adaptive thin shell mirror. Second adaptive thin shell mirror delivered to ESO.jpg
Adaptive thin shell mirror.

Adaptive optics was first envisioned by Horace W. Babcock in 1953, [6] [7] and was also considered in science fiction, as in Poul Anderson's novel Tau Zero (1970), but it did not come into common usage until advances in computer technology during the 1990s made the technique practical.[ citation needed ]

Some of the initial development work on adaptive optics was done by the US military during the Cold War and was intended for use in tracking Soviet satellites. [8]

Microelectromechanical systems (MEMS) deformable mirrors and magnetics concept deformable 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.[ citation needed ]

Tip–tilt correction

The simplest form of adaptive optics is tip–tilt correction, [9] which corresponds to correction of the tilts 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 the aberration introduced by the atmosphere can be removed in this way. [10]

Tip–tilt mirrors are effectively segmented 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]

In astronomy

Atmospheric seeing

Negative images of a star through a telescope. The left-hand panel shows the slow-motion movie of a star when the adaptive optics system is switched off. The right-hand panel shows the slow motion movie of the same star when the AO system is switched on. Ao movie.gif
Negative images of a star through a telescope. The left-hand panel shows the slow-motion movie of a star when the adaptive optics system is switched off. The right-hand panel shows the slow motion movie of the same star when the AO system is switched on.

When light from a star or another astronomical object enters the Earth's atmosphere, atmospheric turbulence (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.[ citation needed ]

Wavefront sensing and correction

An adaptive optics system tries to correct these distortions, using a wavefront sensor which takes some of the astronomical light, a deformable 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 few milliseconds; the computer calculates the optimal mirror shape to correct the distortions and the surface of the deformable mirror is reshaped accordingly. For example, an 8–10-metre (800–1,000 cm; 310–390 in) telescope (like the VLT or Keck) can produce AO-corrected images with an angular resolution of 30–60 milliarcsecond (mas) resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.[ citation needed ]

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 of pixels in a wavefront sensor, either using an array of small lenslets (a Shack–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 the astronomical object to be known – even Solar System objects which are not point-like can be used in a Shack–Hartmann wavefront sensor, and time-varying structure on the surface of the Sun is commonly used for adaptive optics at solar telescopes. The deformable mirror corrects incoming light so that the images appear sharp.[ citation needed ]

Using guide stars

Natural guide stars

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 brighter guide 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.[ citation needed ]

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 sufficient luminosity (for current systems, about magnitude 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]

Artificial guide stars

A laser beam directed toward the centre of the Milky Way. This laser beam can then be used as a guide star for the AO. Laser Towards Milky Ways Centre.jpg
A laser beam directed toward the centre of the Milky Way. This laser beam can then be used as a guide star for the AO.

An alternative is the use of a laser beam to generate a reference light source (a laser guide star, LGS) in the atmosphere. There are two kinds of LGSs: Rayleigh guide stars and sodium guide stars. Rayleigh guide stars work by propagating a laser, usually at near ultraviolet wavelengths, and detecting the backscatter from air at altitudes between 15–25 km (49,000–82,000 ft). Sodium guide stars use laser light at 589 nm to resonantly excite sodium atoms higher in the mesosphere and thermosphere, which then appear to "glow". The LGS can then be used as a wavefront reference 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. The lasers are often pulsed, with measurement of the atmosphere being limited to a window occurring a few microseconds 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.[ citation needed ]

In retinal imaging

Illustration of a (simplified) adaptive optics system. The light first hits a tip-tilt (TT) mirror and then a deformable mirror (DM) which corrects the wavefront. Part of the light is tapped off by a beamsplitter (BS) to the wavefront sensor and the control hardware which sends updated signals to the DM and TT mirrors. Adaptive optics system full.svg
Illustration of a (simplified) adaptive optics system. The light first hits a tip–tilt (TT) mirror and then a deformable mirror (DM) which corrects the wavefront. Part of the light is tapped off by a beamsplitter (BS) to the wavefront sensor and the control hardware which sends updated signals to the DM and TT mirrors.

Ocular aberrations are distortions in the wavefront passing through the pupil of the eye. These optical aberrations diminish the quality of the image formed on the retina, sometimes necessitating the wearing of spectacles or contact 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 as defocus and astigmatism, 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.[ citation needed ]

Measurement of ocular aberrations

Ocular aberrations are generally measured using a wavefront sensor, and the most commonly used type of wavefront sensor is the Shack–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 causing aberration.[ citation needed ]

Correction of ocular aberrations

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.[ citation needed ]

Open loop vs. closed loop operation

If the wavefront error is measured before it has been corrected by the wavefront corrector, then operation is said to be "open loop".[ citation needed ]

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.[ citation needed ]

Applications

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 with scanning 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 with optical coherence tomography, adaptive optics has allowed the first three-dimensional images of living cone photoreceptors to be collected. [14]

In microscopy

A deformable mirror can be used to correct wavefront errors in an astronomical telescope. Deformable mirror correction.svg
A deformable mirror can be used to correct wavefront errors in an astronomical telescope.

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.[ citation needed ]

Other uses

GRAAL is a ground layer adaptive optics instrument assisted by lasers. GRAAL instrument.jpg
GRAAL is a ground layer adaptive optics instrument assisted by lasers.

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 the Swedish 1-m Solar Telescope, Dunn Solar Telescope, and Big Bear Solar Observatory. It is also expected to play a military role by allowing ground-based and airborne laser weapons to reach and destroy targets at a distance including satellites in orbit. The Missile Defense Agency Airborne Laser program is the principal example of this.[ citation needed ]

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 the retina, where it has been combined with optical 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 and active optics are also being developed for use in glasses to achieve better than 20/20 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.[ citation needed ]

Adaptive optics, especially wavefront-coding spatial light modulators, are frequently used in optical trapping applications to multiplex and dynamically reconfigure laser foci that are used to micro-manipulate biological specimens.[ citation needed ]

Beam stabilization

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 by photo diodes. This signal is fed into analog-to-digital converters and then a microcontroller which runs a PID controller algorithm. The controller then drives digital-to-analog converters which drive stepper motors attached to mirror mounts.[ citation needed ]

If the beam is to be centered onto 4-quadrant diodes, no analog-to-digital converter is needed. Operational amplifiers are sufficient.[ citation needed ]

See also

Related Research Articles

<span class="mw-page-title-main">Chromatic aberration</span> Failure of a lens to focus all colors on the same point

In optics, chromatic aberration (CA), also called chromatic distortion and spherochromatism, is a failure of a lens to focus all colors to the same point. It is caused by dispersion: the refractive index of the lens elements varies with the wavelength of light. The refractive index of most transparent materials decreases with increasing wavelength. Since the focal length of a lens depends on the refractive index, this variation in refractive index affects focusing. Chromatic aberration manifests itself as "fringes" of color along boundaries that separate dark and bright parts of the image.

<span class="mw-page-title-main">Active optics</span> Shaping technology for reflecting telescopes

Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, and mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.

<span class="mw-page-title-main">Reflecting telescope</span> Telescopes which utilize curved mirrors to form an image

A reflecting telescope is a telescope that uses a single or a combination of curved mirrors that reflect light and form an image. The reflecting telescope was invented in the 17th century by Isaac Newton as an alternative to the refracting telescope which, at that time, was a design that suffered from severe chromatic aberration. Although reflecting telescopes produce other types of optical aberrations, it is a design that allows for very large diameter objectives. Almost all of the major telescopes used in astronomy research are reflectors. Many variant forms are in use and some employ extra optical elements to improve image quality or place the image in a mechanically advantageous position. Since reflecting telescopes use mirrors, the design is sometimes referred to as a catoptric telescope.

<span class="mw-page-title-main">Astronomical seeing</span> Atmospheric distortions of light

In astronomy, seeing is the degradation of the image of an astronomical object due to turbulence in the atmosphere of Earth that may become visible as blurring, twinkling or variable distortion. The origin of this effect is rapidly changing variations of the optical refractive index along the light path from the object to the detector. Seeing is a major limitation to the angular resolution in astronomical observations with telescopes that would otherwise be limited through diffraction by the size of the telescope aperture. Today, many large scientific ground-based optical telescopes include adaptive optics to overcome seeing.

<span class="mw-page-title-main">Scanning laser ophthalmoscopy</span>

Scanning laser ophthalmoscopy (SLO) is a method of examination of the eye. It uses the technique of confocal laser scanning microscopy for diagnostic imaging of the retina or cornea of the human eye.

<span class="mw-page-title-main">Laser guide star</span> Artificial star image used by telescopes

A laser guide star is an artificial star image created for use in astronomical adaptive optics systems, which are employed in large telescopes in order to correct atmospheric distortion of light. Adaptive optics (AO) systems require a wavefront reference source of light called a guide star. Natural stars can serve as point sources for this purpose, but sufficiently bright stars are not available in all parts of the sky, which greatly limits the usefulness of natural guide star adaptive optics. Instead, one can create an artificial guide star by shining a laser into the atmosphere. Light from the beam is reflected by components in the upper atmosphere back into the telescope. This star can be positioned anywhere the telescope desires to point, opening up much greater amounts of the sky to adaptive optics.

<span class="mw-page-title-main">Wavefront</span> Locus of points at equal phase in a wave

In physics, the wavefront of a time-varying wave field is the set (locus) of all points having the same phase. The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal frequency.

<span class="mw-page-title-main">Extremely Large Telescope</span> Major astronomical facility in Chile

The Extremely Large Telescope (ELT) is an astronomical observatory currently under construction. When completed, it will be the world's largest optical/near-infrared extremely large telescope. Part of the European Southern Observatory (ESO) agency, it is located on top of Cerro Armazones in the Atacama Desert of northern Chile.

<span class="mw-page-title-main">C. Donald Shane telescope</span>

The C. Donald Shane telescope is a 120-inch (3.05-meter) reflecting telescope located at the Lick Observatory in San Jose, California. It was named after astronomer C. Donald Shane in 1978, who led the effort to acquire the necessary funds from the California Legislature, and who then oversaw the telescope's construction. It is the largest and most powerful telescope at the Lick Observatory, and was the second-largest optical telescope in the world when it was commissioned in 1959.

<span class="mw-page-title-main">Defocus aberration</span> Quality of an image being out of focus

In optics, defocus is the aberration in which an image is simply out of focus. This aberration is familiar to anyone who has used a camera, videocamera, microscope, telescope, or binoculars. Optically, defocus refers to a translation of the focus along the optical axis away from the detection surface. In general, defocus reduces the sharpness and contrast of the image. What should be sharp, high-contrast edges in a scene become gradual transitions. Fine detail in the scene is blurred or even becomes invisible. Nearly all image-forming optical devices incorporate some form of focus adjustment to minimize defocus and maximize image quality.

In optics, tilt is a deviation in the direction a beam of light propagates.

<span class="mw-page-title-main">Deformable mirror</span>

Deformable mirrors (DM) are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics. In 2006 they found a new use in femtosecond pulse shaping.

<span class="mw-page-title-main">Shack–Hartmann wavefront sensor</span>

A Shack–Hartmannwavefront sensor (SHWFS) is an optical instrument used for characterizing an imaging system. It is a wavefront sensor commonly used in adaptive optics systems. It consists of an array of lenses of the same focal length. Each is focused onto a photon sensor. If the sensor is placed at the geometric focal plane of the lenslet, and is uniformly illuminated, then, the integrated gradient of the wavefront across the lenslet is proportional to the displacement of the centroid. Consequently, any phase aberration can be approximated by a set of discrete tilts. By sampling the wavefront with an array of lenslets, all of these local tilts can be measured and the whole wavefront reconstructed. Since only tilts are measured the Shack–Hartmann cannot detect discontinuous steps in the wavefront.

A wavefront curvature sensor is a device for measuring the aberrations of an optical wavefront. Like a Shack–Hartmann wavefront sensor it uses an array of small lenses to focus the wavefront into an array of spots. Unlike the Shack-Hartmann, which measures the position of the spots, the curvature sensor measures the intensity on either side of the focal plane. If a wavefront has a phase curvature, it will alter the position of the focal spot along the axis of the beam, thus by measuring the relative intensities in two places the curvature can be deduced.

Boston Micromachines Corporation is a US company operating out of Cambridge, Massachusetts. Boston Micromachines manufactures and develops instruments based on MEMS technology to perform open and closed-loop adaptive optics. The technology is applied in astronomy, beam shaping, vision science, retinal imaging, microscopy, laser communications, and national defense. The instruments developed at Boston Micromachines include deformable mirrors, optical modulators, and retinal imaging systems, all of which utilize adaptive optics technology to enable wavefront manipulation capabilities which enhance the quality of the final image.

<span class="mw-page-title-main">Ferrofluid mirror</span>

A ferrofluid mirror is a type of deformable mirror with a reflective liquid surface, commonly used in adaptive optics. It is made of ferrofluid and magnetic iron particles in ethylene glycol, the basis of automotive antifreeze. The ferrofluid mirror changes shape instantly when a magnetic field is applied. As the ferromagnetic particles align with the magnetic field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Since any shapes can be produced by changing the magnetic field geometries, wavefront control and correction can be achieved.

ALPAO is a company which manufactures a range of adaptive optics products for use in research and industry, including deformable mirrors with large strokes, wavefront sensors, and adaptive optics loops. These products are designed for astronomy, vision science, microscopy, wireless optical communications, and laser applications.

<span class="mw-page-title-main">Goode Solar Telescope</span>

The Goode Solar Telescope (GST) is a scientific facility for studies of the Sun named after Philip R. Goode. It was the solar telescope with the world's largest aperture in operation for more than a decade. Located in Big Bear Lake; California, the Goode Solar Telescope is the main telescope of the Big Bear Solar Observatory operated by the New Jersey Institute of Technology (NJIT). Initially named New Solar Telescope (NST), first engineering light was obtained in December 2008, and scientific observations of the Sun began in January 2009. On July 17, 2017, the NST was renamed in honor of Goode, a former, and founding director of NJIT's Center for Solar-Terrestrial Research and the principal investigator of the facility. Goode conceived, raised the funds, and assembled the team that built and commissioned the telescope, and it was the highest resolution solar telescope in the world (until the end of 2019) and the first facility class solar telescope built in the U.S. in a generation.

A pyramid wavefront sensor is a type of a wavefront sensor. It measures the optical aberrations of an optical wavefront. This wavefront sensor uses a pyramidal prism with a large apex angle to split the beam into multiple parts at the geometric focus of a lens. A four-faceted prism, with its tip centered at the peak of the point spread function, will generate four identical pupil images in the absence of optical aberrations. In the presence of optical aberrations, the intensity distribution among the pupils will change. The local wavefront gradients can be obtained by recording the distribution of intensity in the pupil images. The wavefront aberrations can be evaluated from the estimated wavefront gradients.

References

  1. ↑ Beckers, J.M. (1993). "Adaptive Optics for Astronomy: Principles, Performance, and Applications". Annual Review of Astronomy and Astrophysics. 31 (1): 13–62. Bibcode:1993ARA&A..31...13B. doi:10.1146/annurev.aa.31.090193.000305.
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  10. 1 2 "Adaptive Optics without trouble | Technical articles | Technical documents". www.okotech.com. Retrieved 10 June 2023.
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  14. ↑ Zhang, Yan; Cense, Barry; Rha, Jungtae; Jonnal, Ravi S.; Gao, Weihua; Zawadzki, Robert J.; Werner, John S.; Jones, Steve; Olivier, Scot; Miller, Donald T. (2006). "High-speed volumetric imaging of cone photoreceptors with adaptive optics spectral-domain optical coherence tomography". Optics Express. 14 (10): 4380–94. Bibcode:2006OExpr..14.4380Z. doi:10.1364/OE.14.004380. PMC   2605071 . PMID   19096730.
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  22. ↑ "Retinal OCT Imaging System to Incorporate Adaptive Optics". adaptiveoptics.org. 10 April 2006. Retrieved 28 June 2010.
  23. ↑ Roorda, Austin; Romero-Borja, Fernando; Iii, William J. Donnelly; Queener, Hope; Hebert, Thomas J.; Campbell, Melanie C. W. (6 May 2002). "Adaptive optics scanning laser ophthalmoscopy". Optics Express. 10 (9): 405–412. Bibcode:2002OExpr..10..405R. doi: 10.1364/OE.10.000405 . ISSN   1094-4087. PMID   19436374.
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Bibliography

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