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Adaptive optics technology

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Adaptive Optics Loop 

There are three principal components in any adaptive optics system, otherwise called an adaptive optics closed loop:

  • a wavefront sensor that measures incoming light
  • a wavefront correction device that compensates for the distortions, or aberrations, present in the light
  • a software package that controls the interaction between these first two elements

wavefront sensor diagramWavefront measurement

Over the past decades, wavefront measurement has been extensively explored because of its wide-ranging applications.  The different measurement techniques that have resulted from this research can be regrouped into four primary categories:

  • interferometers
  • phase derivative sensors (Shack-Hartmann)
  • phase 2nd derivative sensors (Roddier, Pyramid WFS, etc.)
  • other techniques including ray tracing, Talbot and others

Each technology has its own proper strengths and limitations. After years of extensive research, the ophthalmology and scientific communities generally agree that the Shack-Hartmann technique is best suited for ophthalmic applications because of its balance between precision, sensitivity and resolution.

deformable mirror diagramWavefront correction

Dynamic wavefront correction, essential to adaptive optics, is generally accomplished by using deformable mirrors (DM) or liquid crystal phase modulators (LCPM).

As the name suggests, deformable mirrors are based on the concept of using a deformable reflective surface that is manipulated by actuators that reshape wavefronts following the instructions sent from the control software after the incoming wavefront has been analyzed by the wavefront sensor.  The actuators apply force to a specific part of the mirror’s surface resulting in the generation of complex shapes that correct for the aberrations present in the wavefront.  Various principals are used in actuator construction including electric, piezoelectric, electrostatic, and electromagnetic techniques.

SLMs are generally based on a liquid crystal array, sometimes coupled with a photorefractive plate, and offer good resolution but are often perturbed by parasite diffraction orders and polychromatic light.  A key advantage of deformable mirrors in adaptive optics is that they can correct for polychromatic aberrations whereas SLM techniques can only correct monochromatic wavefronts.

mirao softwareCommand and control software

The third part of the adaptive optics loop, these software programs use complex algorithms to transform the wavefront data from the wavefront sensor into control sequences that command the deformable mirror’s actuators by regulating the voltage that is applied to them.

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