A deformable mirror can be used to correct wavefront errors in an astronomical telescope.
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.
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 technology used to improve the performance of optical systems by reducing the effect of incoming wavefront distortions by deforming a mirror in order to compensate for the distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems 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.
Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry.
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.
History
Adaptive thin shell mirror.
Adaptive optics was first envisioned by Horace W. Babcock in 1953, 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.
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.
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.
Tip–tilt correction
The simplest form of adaptive optics is tip-tilt correction, 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.
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.
In astronomy
Astronomers at the Very Large Telescope site in Chile use adaptive optics.
Atmospheric seeing
When light from a star passes through the Earth's atmosphere, the wavefront is perturbed.
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. Visual images produced by any telescope larger
than approximately 20 centimeters are blurred by these distortions.
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. 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 m 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.
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.
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.
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.
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.
Artificial guide stars
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.
In retinal imaging
Artist's impression of the European Extremely Large Telescope deploying lasers for adaptive optics
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.
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.
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.
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".
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.
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.
In microscopy
In microscopy, adaptive optics is used to correct for sample-induced aberrations.
The required wavefront correction is either measured directly using
wavefront sensor or estimated by using sensorless AO techniques.
Other uses
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 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.
Adaptive optics has been used to enhance the performance of free-space optical communication systems
and to control the spatial output of optical fibers.
Medical applications include imaging of the retina, where it has been combined with optical coherence tomography.
Also the development of Adaptive Optics Scanning Laser Opthalmoscope
(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. Development of an Adaptive Scanning Optical Microscope (ASOM) was announced by Thorlabs in April 2007. Adaptive and active optics are also being developed for use in glasses to achieve better than 20/20 vision, initially for military applications.
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.
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 in optical trapping applications to multiplex and dynamically reconfigure laser foci that are used to micro-manipulate biological specimens.
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 some Analog-to-digital converters and a microcontroller runs a PID controller algorithm. The controller drives some digital-to-analog converters which drive stepper motors attached to mirror mounts.
If the beam is to be centered onto 4-quadrant diodes, no Analog-to-digital converter is needed. Operational amplifiers are sufficient.
