For example, turbulence in the Earth's atmosphere causes the twinkling effect that we see when we look up at the stars.  Astronomers use adaptive optics to compensate for this distortion in order to get a clearer view of the heavens.

There are three principal components in any adaptive optics system, otherwise called a loop.  A wavefront sensor that measures light, a device that corrects the distortions present in the light and a software package that controls the interaction between these two elements.

Given that this technology is based on the measurement and calculation of wavefronts, it is important to understand what this term means.  Light is made up of waves, like the ones you see when you throw a rock into a still pool of water.  The ripples, or waves, that you see protruding from the center are a good example of what light waves resemble.  When scientists talk about measuring the wavefront, they measure the particularities of the curve, or front, of one individual wave rather than measuring the ensemble of all the waves in the light beam as a whole.

Wavefront correction is accomplished by using two different types of technology -Liquid Crystal Phase Modulators or Deformable Mirrors.   In this part of the website, we will concentrate on the correction technologies that are at the heart of adaptive optics.

Deformable mirrors (DM) use a continuous reflective surface that is manipulated by actuators to modify the totality of the wavefront.  Contrarily, liquid crystal phase modulators (LCPM) manipulate wavefronts in a local manner.  DMs offer the advantage of being achromatic (they do not effect color) and offer an exceptional range of use.  LCPMs offer the advantage of being able to correct wavefronts presenting high levels of spatial frequency, or distortions.

Within the domain of DMs, there are several families of technology currently in use.

The first is called a bimorph mirror.  These devices use two extremely thin layers of conductive material called piezo electric plates that are sandwiched together to create a mirror surface between 50mm to 125mm in diameter. An electric current is applied to devices called actuators that, when exposed to this current, act upon the plates locally (where they are positioned) to modify the mirror surface, thereby correcting the wavefront.  Even with the exceptional reflective quality of the mirror's surface, the large size of the actuators used in this technology limits the number that can be applied to a given surface to between 30 and 60, depending on the pupil size (the useful diameter of the mirror’s surface).  This restriction limits the mirrors ability to perform precise distortions.  The advantages of this technique include the ability to work with strong optical deformations, also called aberrations, and high-energy light sources like lasers.  Although these DMs provide good linearity  and offer the possibility of a large pupil diameter for wider light sources, their relatively high level of hystereris, between 10 and 25%, limits their performance.

Another type of DM uses what are called stacked-up piezo electric plates.  These devices act in a similar manner to bimorph mirrors, but the effect of stacking the actuators allows for a higher actuator density and sensitivity, which in turn offers more control over the level of distortion applied to the mirrors surface when the current is applied.  Like bimorph mirrors, they allow for local correction of wavefronts but are equally reserved for larger reflective surfaces between 40mm and 100mm.  Though the quality of the results offered by these devices is inferior to that of bimorph mirrors, their advantages include the capacity for greater actuator density (between 30 and 200) as well as the ability to work with highly aberrated wavefronts and high-energy light sources like lasers.  Even with its increased actuator density and improved linearity, these mirrors still present a relatively high level of hysteresis ranging from 5 to 15%.

Next, there are electrostatic mirrors.  These DMs are based on a technique that uses an extremely thin membrane, measured in microns, as its reflective surface.  The mirror surface is controlled by a matrix of electrostatic actuators that react individually to the applied electric current to deform the mirror's surface.  The density of these actuators can be very high, which enables them to be compatible with exceptionally small pupil diameters ranging from 10mm to 20mm.  Even though these devices present a high level of linearity (up to 30%) and the quality of the results is considered mediocre, these DMs offer a low stroke, ranging from 2m to 5m   The advantages of these DMs include their small size and low cost.

Finally, there are deformable membrane mirrors, like the mirao™ 52-e from our sister company Imagine Eyes.  Based on a new approach to DM technology that enables high actuator density and precise manipulation on a small pupil size (15mm), these devices allow users to generate high-quality results even when correcting highly aberrated wavefronts.

There are two types of LQPM, pixilated and non-pixilated, each offering their own advantages and shortcomings.

The pixilated LQPM technology is the simpler and easier to implement of the two.  These devices work when a liquid crystal array (LCA), mounted on a photorefractive plate, is exposed to a wavefront.  Each pixel on the LCA has a unique electronic address and transmits the data it collects for processing.  Once processed, the corrected wavefront is displayed on a liquid crystal display (LCD) mounted behind the first array and retransmitted across the photorefractive plate.  These devices offer high spatial resolution because of each pixel's ability to manipulate itself independently of the others, which, in turn, allows for things like the generation of holograms.  Like any method, this technique has its inconveniences including a diffraction pattern caused by the LQPM's pixilation, high chromaticity of the corrected wavefront and low correction capacities.

Non-pixilated technology is somewhat more complex and therefore more difficult to implement.  These phase modulators function by using a reflection of the wavefront.  The correcting element is a LCPM mounted so that the LCA interacts with a plate of photorefractive material.  The reaction from the light on the LCA mounted on the plate is transformed into an electric current that is interpreted by the device, then the correcting element corrects the wavefront.  Even though this technique is more advanced, it is still imperfect because a part of the wavefront remains uncorrected   Thanks to the use of a reflected wavefront, there is no diffraction pattern and therefore no pixilization.  Although it still has high chromaticity, low correction dynamics and inferior spatial resolution to pixilated models, the spatial resolution of these devices is remains good.

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