Adaptive optics

Functional principle of adaptive optics in astronomy.

Adaptive optics (AO for short) is a technique that improves the quality of optical systems by eliminating existing wavefront interference, e.g. B. by air turbulence , reduced or compensated as best as possible, usually by movement or deformation of mirrors.

The technology of adaptive optics was developed in the military field in the 1970s and almost twenty years later it was first used in the civilian field in earth-based observational astronomy .

On the technology of adaptive optics

An AO usually consists of three components. (1) A wavefront sensor - for example a Hartmann-Shack sensor - measures the optical interference, (2) a control computer (real-time computer in the picture above) calculates correction signals with which (3) correction elements can be controlled in such a way that corrected wavefronts are generated as a result become. The three components form a control loop (a closed control loop in the picture above), which in astronomical applications is typically run through a few hundred times per second.

In the simplest case, a 2-axis tilting mirror can be used as a correction element, with the aid of which the image movement caused by the atmosphere can be compensated. In this case, the image movement can be measured with a position sensitive device, for example .

The compensation of optical errors of a higher order, such as astigmatism , coma , etc., (see also Zernike polynomials ) generally requires mirrors whose reflective surface can be deformed with the help of actuators, so-called deformable mirrors . Adaptive optics with liquid crystal correction elements (often referred to as liquid crystal spatial light modulators, LC SLM; or also as liquid crystal on silicon, LCOS) can change wavefronts both in reflection and in transmission.

Another technique - active optics - is used to compensate for mirror curvatures, e.g. B. arises when pivoting the telescope. In astronomy, the two correction methods differ in the speed of control: with active optics, control is in the order of 1 time per second, with adaptive optics, on the other hand, it is significantly faster, in the order of 100 times per second.

Use in astronomy and microscopy

An AO can be used, for example, to compensate for the wavefront interference (more precisely phase interference ) generated when starlight passes through turbulent layers of the atmosphere . A guide star or an artificial guide star generated by a laser is measured .

Without AO, all earthbound , large astronomical telescopes in the optical field work far below their theoretical capabilities. This means e.g. For example, for a reflecting telescope with a 10 m aperture, its resolution or image sharpness is poorer by a factor of 10-50 (depending on the wavelength) than specified by the telescope optics. The limitation of the image quality is therefore not due to the telescope, but to the thermo-optically turbulent air layers.

Large telescopes are equipped with AO to correct these atmospheric disturbances ( seeing ). This technology is also used in solar telescopes . AO also plays an important role in laser communication or laser beam guidance through the atmosphere or in military reconnaissance.

In recent years the use of these methods for microscopy and in ophthalmology has been increasingly researched in order to compensate for the aberrations of the human eye and either enable diagnostic methods to achieve better resolution or to improve human visual performance.

Use in laser cutting systems

Another area of application of adaptive optics are laser cutting systems with carbon dioxide lasers . Since the beam is guided via movable mirrors and not via glass fibers , the length of the beam path between the laser and the workpiece changes depending on the position of the cutting head. In order to always stay in focus , adaptive optics are used. These are, for example, hollow copper mirrors. The surface can be curved by applying water pressure, which shifts the focus position.

Use with high-power lasers

Adaptive optics are used in high-power lasers to compensate, among other things, optical imaging errors caused by "heated" optics. Examples are the PHELIX laser system, the Vulcan laser, the European Extreme Light Infrastructure , or lasers that are used in inertial fusion research (USA: National Ignition Facility , China: Joint Laboratory on High Power Laser and Physics).

literature

HW Babcock : The Possibility of Compensating Astronomical Seeing , Publications of the Astronomical Society of the Pacific, Vol. 65, No. 386, p.229, 1953, doi : 10.1086 / 126606 , bibcode : 1953PASP ... 65..229B
John W. Hardy: Adaptive Optics for Astronomical Telescopes , Oxford University Press, 1998, ISBN 978-0195090192
François Roddier: Adaptive optics in astronomy , Cambridge Univ. Press, Cambridge 2004, ISBN 0-521-55375-X
Ulrich Wittrock: Adaptive optics for industry and medicine , Springer, Berlin 2005, ISBN 3-540-23978-2
Robert K. Tyson: Principles of adaptive optics , CRC Press, Third Edition, 2011, ISBN 978-1-4398-0858-0 (Hardback)
Martin J Booth: Adaptive optical microscopy: the ongoing quest for a perfect image , Light: Science & Applications (2014) 3, e165; doi : 10.1038 / lsa.2014.46
Austin Roorda and Jacque L. Duncan: Adaptive Optics Ophthalmoscopy , Annual Review of Vision Science, Vol. 1: 19-50 (Volume publication date November 2015), doi : 10.1146 / annurev-vision-082114-035357

Web links

Individual evidence

↑ Adaptive Optics - The sharp look into space and into the eye , accessed on July 6, 2016

↑ Quanquan Mu, Zhaoliang Cao, Lifa Hu, Dayu Li, and Li Xuan, doi : 10.1364 / OE.14.008013 , Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device, Opt. Express 14, 8013-8018 ( 2006)

↑ Shirai, Tomohiro; Takeno, Kohei; Arimoto, Hidenobu; Furukawa, Hiromitsu, doi : 10.1143 / JJAP.48.070213 , Adaptive Optics with a Liquid-Crystal-on-Silicon Spatial Light Modulator and Its Behavior in Retinal Imaging, Japanese Journal of Applied Physics, Volume 48, Issue 7R, article id. 070213 (2009)

↑ Kainan Yao, Jianli Wang, Xinyue Liu, and Wei Liu, doi : 10.1364 / OE.22.017216 , Closed-loop adaptive optics system with a single liquid crystal spatial light modulator, Opt. Express 22, 17216-17226 (2014)

↑ Solar Adaptive Optics Project ( Memento of the original from February 18, 2013 in the Internet Archive ) Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice. nsosp.nso.edu, accessed March 8, 2013 @1@ 2

[1] Adaptive Optics - The sharp look into space and into the eye , accessed on July 6, 2016

[2] Quanquan Mu, Zhaoliang Cao, Lifa Hu, Dayu Li, and Li Xuan, doi : 10.1364 / OE.14.008013 , Adaptive optics imaging system based on a high-resolution liquid crystal on silicon device, Opt. Express 14, 8013-8018 ( 2006)

[3] Shirai, Tomohiro; Takeno, Kohei; Arimoto, Hidenobu; Furukawa, Hiromitsu, doi : 10.1143 / JJAP.48.070213 , Adaptive Optics with a Liquid-Crystal-on-Silicon Spatial Light Modulator and Its Behavior in Retinal Imaging, Japanese Journal of Applied Physics, Volume 48, Issue 7R, article id. 070213 (2009)

[4] Kainan Yao, Jianli Wang, Xinyue Liu, and Wei Liu, doi : 10.1364 / OE.22.017216 , Closed-loop adaptive optics system with a single liquid crystal spatial light modulator, Opt. Express 22, 17216-17226 (2014)