Adaptive optics

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

Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effects of rapidly changing optical distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the impact of ocular aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a spatial phase modulator such as a deformable mirror or liquid crystal array. AO was first envisioned by Horace W. Babcock in 1953, but did not come into common usage until advances in computer technology during the 1990s made the technique practical. Adaptive optics should not be confused with active optics, which works on a longer timescale to correct the primary mirror geometry itself. 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 which 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 widely used in night time and solar telescopes, to correct the aberration introduced by the atmosphere on the light path and improve image quality over what would be possible according to the atmospheric seeing. Tip-tilt mirrors are effectively segmented adaptive optics mirrors having only one segment which can tip and tilt, rather than having an array of multiple segments which can tip and tilt independently.

The Shack-Hartmann sensor is one type of wavefront sensor used for adaptive optics. The pupil plane aperture is subdivided using a lenslet array into a number of sub-apertures. The lenslets focus light into an array of spots on the CCD, and the displacements of these spots from known reference positions indicate the local slopes of the wavefront. These local slopes, in turn, are used to reconstruct the overall shape of the wavefront (in wavefront sensing) or may be used directly to compute the instructions for the deformable mirror or phase modulator.

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Adaptive optics in astronomy

When light from a star or another astronomical object enters the Earth's atmosphere, turbulence (introduced, for example, by different temperature layers and different wind speeds interacting) distort and move the image in various ways[1] (see astronomical seeing for a full discussion). Images produced by any telescope larger than a few metres are blurred by these distortions. For example, an 8-10 m telescope (like the VLT or Keck) can produce AO-corrected images with a angular resolution of 30-60 milli-arcsecond (mas) resolution at infrared wavelengths, while the resolution without correction is of the order of 1 arcsecond.

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 adaptive optics system is switched on. The image of the star is much more compact, and breaks up less with adaptive optics switched on. However, the image of the star also changes shape much more quickly when the adaptive optics system is switched on.

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.

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 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 pixellated 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. 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.

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 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.

An alternative is the use of a laser beam to generate a reference light source (a Laser guide star, LGS) in the atmosphere. LGSs come in two flavors: 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. Sodium guide stars use laser light at 589 nm to excite sodium atoms 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.

Other approaches that can yield resolving power exceeding the limits of atmospheric seeing include speckle imaging, aperture synthesis, lucky imaging and space telescopes such as NASA's Hubble Space Telescope.

Adaptive optics in retinal imaging

Ocular aberrations are distortions in the wavefront passing through the pupil of the eye. These 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 with frequencies between 10 Hz and 100 Hz. 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--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 once and corrected once, followed by the acquisition of retinal images, operation is said to be in "open loop". If the residual wavefront error (i.e. the wavefront error which remains after correction) is continually measured and used to reshape the mirror, operation is said to be "closed loop". The latter is necessary in the case of rapidly changing aberrations, and as such is generally used in retinal imaging systems.

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.

Other uses of adaptive optics

Besides its obvious 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 Solar Telescope. 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 Boeing Airborne Laser programme is the principal example of this.

Adaptive optics has been used to enhance the performance of free space optical communication systems [1]. Medical applications include imaging of the retina, where it has been combined with optical coherence tomography [2]. Development of an Adaptive Optics Scanning 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[3].

After propagation of a wavefront parts of it may overlap leading to interference and prevent 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.

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.

See also

References

  1. ↑ Max, Claire. "Introduction to Adaptive Optics and its History" (pdf) in American Astronomical Society 197th Meeting.. 

External links