What is Adaptive Optics?

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- GoPhotonics

Dec 19, 2023

Adaptive Optics (AO) is a method used to improve the performance of optical systems. It is designed to reduce the effects of distortions and aberrations caused by the Earth's atmosphere or other optical elements in the system to attain diffraction-limited imaging. This method is important for optimizing the signal-to-noise ratio (S/N) and increasing the resolution and clarity of the obtained images. 

Adaptive optics serves as a technology based on manipulating the optical wavefront and results in an improved final output. A wavefront is a surface associated with a propagating wave passing through points sharing the same phase. It is typically planar or spherical in its undistorted state and can be altered using common optical elements.

The active manipulation of a wavefront using adaptive optical elements, such as a deformable mirror, enables precise control over its shape. This level of precision and "programmable" control leads to significant enhancement in the performance of various optical systems. Adaptive optics find widespread application in imaging and non-imaging scenarios to eliminate aberrations, enhance image quality, and shape laser beams.

Experimental Setup

A simple adaptive optical imaging system is shown in the figure above. The system comprises of a deformable mirror, mirror control electronics, imaging sensor, and conventional optical elements. 

A deformable mirror is an adaptive element with a controllable reflective surface shape. They can modify their surface profile to best reflect incoming light and are used for wavefront manipulation or system enhancement applications. By introducing the correct mirror shape, a distorted input wavefront can be improved.

The control system can apply predetermined mirror shapes to correct different types of aberrations. This same system can also be used to introduce known optical aberration into a system to gain an understanding of their impact on system performance. 

A beam splitter is placed in the optical path and used to reflect part of the light onto a wavefront sensor. The information derived from this sensor is used to calculate the shape of the mirror needed to correct any wavefront distortion. This data is then fed back into the control system of the mirror which in turn changes the shape of the mirror. It is a closed loop system that can continuously sample and measure the wavefront quality, which can then feedback this information to control the mirror shape.

A wavefront sensor is capable of characterizing the shape of an incident wavefront. This sensor is fabricated with a microlens array mounted in front of a CCD or CMOS detector array

As shown in Figure above, when a plane wavefront is incident on the sensor, each lenslet focuses the light in the center of a predefined set of pixels in the array that is placed at the focal plane of the lenslet array. Similarly, a distorted wavefront is incident on the microlens array, the focus spots are in different locations within the pixels associated with each lenslet. By analyzing the locations of the individual spots on the detector array, it is possible to characterize the shape of the wavefront incident on the wavefront sensor. This information can be used to determine the shape of the deformable mirror surface needed to correct the distortion.

Working of Adaptive Optics System

The fundamental concept behind AO involves the initial measurement of atmospheric distortions, followed by corrective adjustments before the light's arrival at the camera. A visual representation of this process is illustrated in the figure below.

The light from the telescope is first collimated and directed towards an adaptive or deformable mirror. Under ideal conditions, with no atmospheric turbulence, the light's wavefront would remain perfectly straight and parallel. Next, the light is directed to a beam splitter, dividing it into two parts. One part goes to the wavefront sensor, which measures the distortions in the wavefront and then sends a corrective signal to the adaptive mirror. This adaptive mirror has the capability to alter its shape, thereby eliminating the distortions introduced by atmospheric turbulence within the light wave. As a result of this correction process, the light with a refined wavefront proceeds to the high-resolution camera, where a diffraction-limited image is formed.

Wavefront Sensing and Correction

An adaptive optics system tries to fix the distortions by using a wavefront sensor that captures a portion of the incoming light, a deformable mirror positioned within the optical path, and a computer linked to the sensor. The wavefront sensor measures the atmospheric distortions within a few milliseconds, and the computer calculates the precise adjustments needed to reshape the deformable mirror's surface, effectively correcting the distortions.

To perform adaptive optics correction, the incoming wavefronts' shapes are measured across the optical aperture. This aperture is divided into pixels on a wavefront sensor, achieved through lenslets or curvature/pyramid sensors. The average wavefront distortion in each pixel is determined. This pixelated wavefront map guides the deformable mirror in correcting atmospheric-induced errors. The deformable mirror ensures incoming light is corrected, resulting in sharp images.

Reconstruction Algorithms

The success of Adaptive Optics depends on sophisticated algorithms known as reconstruction algorithms. These algorithms take the wavefront measurements and determine how the deformable mirror should be adjusted to correct the incoming light. This process involves complex mathematical computations that account for various factors, including the telescope's optical design, the atmospheric conditions, and the target object's characteristics. The outcome is a reconstructed wavefront that nearly matches the ideal, undistorted wavefront.

Applications of Adaptive Optics

Adaptive Optics has different applications across various fields such as astronomy, medical field, laser communication systems, defense, etc. In astronomy, AO is instrumental in capturing high-resolution images of celestial objects, allowing astronomers to study distant galaxies, exoplanets, and stars with unparalleled detail. It has also enabled the discovery of exoplanets by enhancing the accuracy of measurements during planetary transits.

In the medical field, Adaptive Optics has been used in ophthalmology to improve the quality of retinal imaging, aiding in the early detection and diagnosis of eye conditions like glaucoma and macular degeneration. Also, this method has made significant contributions to laser eye surgery, enhancing the precision of procedures like LASIK. 

Adaptive Optics’ applications extend to laser communication systems, where it compensates for atmospheric turbulence to ensure reliable data transmission over long distances. In defense and national security, this method is employed in imaging and targeting systems to enhance the performance of surveillance and reconnaissance equipment.

Also, this technology has been used in microscopy, enabling scientists to capture detailed images of biological samples, such as living cells and tissues, with greater clarity. This has advanced our understanding of cellular processes and facilitated breakthroughs in medical research.

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