What is adaptive opticsused for

A central component of an adaptive optics system is the wavefront correction device, which physically alters the optical wavefront to counteract the measured distortions. It is essentially a phase modulator with some spatial resolution – a spatial light modulator.

How doesadaptive opticswork

The requirements for wavefront correction can vary greatly from one application to another. The following aspects must be considered:

The phase excursions for the reflected light are directly related to the spatial displacements according to <$\delta \varphi = 2 \: k \: \delta z = (4\pi / \lambda) \: \delta z$> (where the factor 2 is due to the double passage of light). The included wavelength dependence is ideal if the aberrations are caused by refractive index variations without any wavelength dependence – which is at least a reasonable approximation in the case of atmospheric turbulence, for example. A hypothetical spatial light modulator that produces wavelength-independent phase changes would be substantially less suitable for various applications.

In adaptive optics, one can design a system to directly address Zernike modes (modal control), compensating for them up to a certain maximum order. Alternatively, one can use the Zernike model of aberrations for analysis purposes, but not directly in the manner mentioned above. For instance, it can be employed to work with spatial zones of the modulator (zonal control). Of course, this decision has a profound impact on signal processing.

Adaptive optics has important applications in systems where laser beams need to be sent through the atmosphere, where atmospheric distortions have detrimental effects. Some examples:

Please do not enter personal data here. (See also our privacy declaration.) If you wish to receive personal feedback or consultancy from the author, please contact him, e.g. via e-mail.

The control system in an adaptive optics setup is responsible for generating control signals for the correction device based on the wavefront measurements obtained by the wavefront sensor. Ideally, it should provide accurate compensation while fully utilizing the speed offered by the correction device. This task is not trivial for several reasons:

These components may be combined into an open-loop or closed-loop control system. Here, an open loop means that the sensor detects the uncorrected wavefronts, i.e., cannot “see” how well the correction works. A closed loop means that the sensor detects the corrected wavefronts, thus having the chance to eventually achieve near-perfect compensation despite adverse aspects such as, for example, a possible nonlinearity of the phase corrector system. However, one then needs to take care that dynamic instabilities are avoided. Often, one then applies an integrating control system, exhibiting a response function which decreases substantially with increasing frequency.

What is adaptive opticsin telescopes

One way to overcome this obstacle was to develop large space-based telescopes, such as the Hubble Telescope and the James Webb Space Telescope. For several reasons, however, it remained highly desirable to solve this problem by significantly reducing the effects of atmospheric distortion in ground-based telescopes. For example, ground-based telescopes can have much larger mirrors than space-based telescopes, and they can be optimized incrementally due to their greater accessibility during operation.

The most common type of wavefront corrector is the deformable mirror (Figure 2). Typically, such a mirror has a substantial number of actuators that can deform the surfaces in various ways to correct a wide range of wavefront distortions. Deformable mirrors used in adaptive optics systems come in several forms, including continuous-faceplate deformable mirrors, segmented mirrors, and micro-electromechanical system (MEMS) mirrors. Each of these types has its specific advantages and limitations, and the choice of a deformable mirror depends on the specific requirements of the application.

Different terms in that equation correspond to various types (“modes”) of aberrations such as piston, tilt, defocus, astigmatism or coma, as explained in the article on optical aberrations.

Wavefront sensors are devices that measure the shape of an optical wavefront. The most commonly used type of wavefront sensor in adaptive optics systems is the Shack–Hartmann wavefront sensor. This sensor consists of an array of microlenses that focus the incoming light onto a detector plane (a focal plane array). Each microlens creates a focal spot on the detector, and the local wavefront direction can be inferred from the position of such a spot, i.e., from the deviation of its position from the nominal position.

Another variant are deformable phase plates, which can be operated in transmission rather than in reflection. They contain a deformable membrane above a liquid, which allows for modulation of the optical path length in the liquid:

Using our advertising package, you can display your logo, further below your product description, and these will been seen by many photonics professionals.

Nikon Instruments is a division of Nikon Corporation, which is headquartered in Tokyo. Its US operations are based in Melville, New York and its European operations in Amstelveen, Netherlands. Nikon Instruments is a specialist in optical instrumentation.

What is adaptive opticsin astronomy

In ophthalmology, it is often necessary to capture precise high-resolution images of the retina, which is the light-sensitive organ in the eye. For example, this technology can help improve the diagnosis and treatment of various retinal diseases, including retinal detachment, glaucoma, diabetic retinopathy, age-related macular degeneration, and retinitis pigmentosa. Furthermore, high image resolution, which enables imaging at the cellular level, is crucial for fundamental vision research. For instance, it has played a significant role in accurately identifying various types of photoreceptors.

Adaptive opticsOphthalmology

For these reasons, the design of a control system often involves the use of sophisticated algorithms and high-speed signal processors to achieve rapid and precise correction of wavefront distortion. Modern digital technology provides powerful components for such systems, which can also be fine-tuned during operation. This allows for achieving maximum control speed and accuracy in a closed-loop system while avoiding any dynamic instability.

Nikon launched the Optiphot and Labophot microscopes in the 1970's and established the CF Optical system. In the 1980's, they released 80 new products, including the inverted Diaphot microscope, which was used in IVF techniques. The introduction of the Eclipse range of infinity optics in the 1990's was a departure from traditional microscope design. CFI60 optics have high numerical apertures (N.A.s) and long working distances, increased resolution and light-gathering capability and high performance in confocal imaging. The infinity optical system also has the benefit that the distance between the objective and the eyepiece tube is not fixed, allowing a variety of imaging modules to be inserted into the light path without any compromise in optical quality. This has allowed microscopes to be used as versatile imaging workstations that provide users with instant access to several imaging methods. Using a Nikon Diaphot microscope, in 1996, Dolly the sheep was the first mammal to be successfully cloned from an adult cell. Nikon has introduced a series of digital camera systems optimized for microscopy-based applications enabling the digital transfer of images.

Adaptive optics is a technological solution to a fundamental problem in optical science: the distortion (aberration) of wavefronts. These are typically caused by random inhomogeneities (spatial variations) in optical systems or in the atmosphere; in the latter case, the distortions can also be time-dependent due to air motion (turbulence). Adaptive optics attempts to solve this problem by correcting these distortions, often in real time. Such techniques are applied in advanced imaging methods to provide clearer, sharper images, obtaining the native resolution of the instrument, as is limited e.g. by diffraction). Also, there are some other application areas, e.g. where optimum beam focusing is required.

The following considerations are necessary mainly because optical distortions do not simply remain unchanged along the optical path in the system, but rather evolve in a complicated manner. In particular, phase distortions are eventually transformed into intensity variations in the image plane, which is exactly the problem that AO aims to address.

Due to the challenge of accurately measuring the necessary wavefront corrections, sensorless techniques are frequently employed. However, the effectiveness of these methods largely depends on the objects being imaged. A helpful aspect is that the aberrations in microscopy samples are usually not time-dependent, which allows sensorless methods to work.

A simple measure of the strength of aberrations is the root mean square (r.m.s.) deviation of the optical path length. This usually needs to be compensated to a level well below <$\lambda / 10$>.

What is adaptive opticspdf

Other types of wavefront sensors, in particular pyramid wavefront sensors, are also used in certain applications. Pyramid sensors offer advantages in terms of sensitivity and dynamic range, making them suitable for demanding applications with strong wavefront distortion.

Several types of wavefront correctors have been optimized for use in astronomical telescopes [6]. Although large telescopes are often equipped with active optics based on a multi-segment primary telescope mirror, the associated positioning control is usually far too slow to be used for adaptive optics. Therefore, an additional fast wavefront corrector with a large number of actuators (hundreds or even thousands) and high speed is required.

Adaptive optics operates on the principle of measuring wavefront distortions and compensating for these distortions to deliver a corrected optical wavefront. While the optical setups using that principle can differ in many respects, a system typically contains the following key components:

Furthermore, AO has been used in research to investigate the finer aspects of ocular optics, offering valuable insights into the individual contributions of various eye structures to overall visual acuity. For example, AO has been used to investigate the effect of the eye's tear film on visual quality, to study the impact of age and disease on the optical properties of the cornea and lens, and to identify photoreceptors and their defects in patients with limited color vision.

Nikon has developed two advanced microscopy systems that can produce significantly higher resolution than conventional optical microscopes. The N-SIM microscopy system combines Structured Illumination Microscopy technology, licensed from UCSF, with Nikon's Eclipse Ti research inverted microscope to produce images with twice the resolution of conventional microscopes. The N-STORM super-resolution microscope system combines “Stochastic Optical Reconstruction Microscopy” technology, licensed from Harvard University, with Nikon's Eclipse Ti to provide a resolution that is 10 times greater than that of conventional optical microscopes. that is 10 times greater than that of conventional optical microscopes.

Traditional AO systems with a single laser guide star can only correct for distortions within a limited area of the sky. Multi-conjugate adaptive optics (MCAO) systems, however, have been developed which use multiple guide stars and deformable mirrors to correct distortions over a wider area. As a result, they enable the study of larger astronomical regions.

In principle, even better performance can be achieved with multiconjugate adaptive optics (MCAO) (see e.g. Ref. [8]), which essentially means using wavefront sensors (and correctors) in at least two different planes that are not mutually conjugate. This way, one can better compensate for aberrations originating from different places – for example, from atmospheric turbulence in a wider range of directions, as well as imperfect components of an imaging system. Of course, this approach greatly increases system complexity and cost, and is therefore rarely employed.

Such single quantities carry very little information; they do not provide any details about the spatial shape or the spatial variation of aberrations. Therefore, more sophisticated methods have been developed to indicate the extent of aberration present in a system that requires compensation. The most common form is a decomposition into aberration modes based on Zernike polynomials. Here, the wavefront distortion is characterized by a function <$W(\rho, \varphi)$> which specifies the deviation of the optical path length as a function of the radial coordinate <$\rho$> (normalized to the pupil radius) and the azimuthal angle <$\varphi$>. This is then viewed as a linear superposition of Zernike polynomials <$Z_n^m(\rho, \varphi)$> with amplitude coefficients <$a_n^m$>:

Well before the year 2000, large ground-based astronomical telescopes reached a performance that was no longer diffraction-limited, but severely limited by the turbulent nature of the Earth's atmosphere. In other words, it was much worse than what would be theoretically possible for a large telescope in the absence of atmospheric distortions. This is true even for telescopes placed on high mountains in regions with more favorable atmospheric conditions. The main problem is variations in refractive index caused by temperature inhomogeneities, which affect the density of the air. These effects are most pronounced for telescopes that operate at short wavelengths of light. The problem is compounded by turbulence, which causes rapid variations in optical distortions.

What is adaptive opticsexamples

In addition to retinal imaging, AO has also been applied in the field of refractive surgery, such as Laser-Assisted In Situ Keratomileusis (LASIK), and other vision correction methods. These procedures aim to reshape the cornea in order to correct refractive errors such as myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. AO systems can be used to measure the patient's wavefront aberration with high precision before surgery, which helps guide the reshaping of the cornea. After surgery, AO can be used to evaluate the effectiveness of the procedure and identify any remaining aberrations.

Note also that there are also sensorless imaging systems, i.e., not requiring a wavefront sensor. Here, the required wavefront correction is obtained by varying the effects of the correction device (with some set of settings) on the obtained images themselves – using some preexisting knowledge of the image, e.g. that it should contain well localized features.

Given the importance and high cost of astronomical installations, substantial investments were made to implement this technology with high precision. Using highly advanced AO technology, all of these observatories now provide significantly improved images of galaxies, planets, and other celestial bodies compared to previous telescopes. This has allowed for a much closer approach to the final limit of diffraction-limited resolution for large telescopes. This has facilitated the detailed study of a wide range of astronomical phenomena. For instance, AO has enabled astronomers to examine the surfaces of planets in our solar system, observe the formation of stars in distant galaxies, and study the supermassive black hole at the center of our galaxy, the Milky Way.

how doesadaptive opticscompensate for atmospheric distortions?

Note: the article keyword search field and some other of the site's functionality would require Javascript, which however is turned off in your browser.

Alongside developments in inter-disciplinary, clinical and confocal microscope solutions[buzzword], Nikon's digital imaging technologies and software solutions[buzzword] support all microscopy and imaging functions, image management and analysis. The Eclipse Ti series of inverted microscopes allow the simultaneous mounting of confocal, TIRF module and multiple stacked epi-fluorescence filter turrets to make this microscope function as a live cell imaging platform. Long term timelapse imaging of living cells is possible in the BioStation series. Nikon partnered with OptraSCAN for accelerating adoption of digital pathology systems and solutions[buzzword] in North America.

The potential of adaptive optics is being utilized in a wide range of fields, each presenting its own distinct challenges and requirements. Some of these are discussed below.

Typically, an adaptive optical system contains a single wavefront sensor and a single wavefront corrector. This configuration is called single-conjugate adaptive optics (SCAO). The correct placement of these components in the optical system is critical to achieving good performance. This can be understood by considering Fourier optics principles. In general, one will try to position both the wavefront sensor and the wavefront corrector in planes that are conjugate to those where the aberrations are largely generated. This positioning offers the best chance of properly compensating for the aberrations. For a telescope, for example, one often (but not always) uses pupil-conjugate planes. This is because atmospheric propagation mainly causes phase changes in that plane, which in turn lead to intensity changes in the image plane and distort the image. Trying to compensate for this with a phase modulator in the image plane would not work. However, sometimes a deformable secondary mirror is used, which, unfortunately, is not in the pupil conjugate plane.

Similar to astronomy, one faces the problem of (also somewhat time-dependent) image aberrations, but in this case, the aberrations are primarily caused by the cornea and the lens of the eye. Of course, such aberrations also limit the eye's vision, but here we are focusing on the challenge of diagnosis. Adaptive optics is used to achieve significant improvements in the quality of retinal images by compensating for aberrations. AO retinal imaging can reveal pathological changes in the retina at an early stage, enabling early intervention and enhancing patient outcomes. Besides, the measurements quickly and reliably determine the necessary details for prescription glasses.

High-end applications, such as astronomical telescopes sometimes require the compensation of hundreds or even thousands of Zernike terms. This, of course, necessitates wavefront sensors and correctors with a correspondingly large number of channels. Nevertheless, the r.m.s. deviations can be moderate, for example, limited to a few micrometers. In retinal imaging, the corrections are an order of magnitude stronger but require only a moderate number of Zernike terms (e.g., a few dozen terms).

In some cases, the peak-to-valley aberrations are given, which are more directly related to the required dynamic range of a wavefront corrector, although they are in some ways less representative than the r.m.s. deviation.

Microscopy is an area where a significant portion of image imperfections can be attributed to aberrations, either in the optical system or in the samples themselves. The latter becomes relevant for microscopy techniques where images are taken at some (often variable) depth inside a sample. This is the case in confocal scanning microscopes and in two-photon excitation fluorescence microscopes. There are several technical variants of adaptive optics designed for specific use cases, including both traditional methods of microscopy and advanced techniques such as confocal scanning microscopy and fluorescence microscopy.

One of the challenges in achieving high-quality image correction is accurately measuring time-dependent wavefront distortions. A fundamental principle involves utilizing distant stars as virtual point sources, which deliver fairly flat wavefronts that are then distorted by our atmosphere. For accurate wavefront correction, a guide star must be (a) sufficiently close to the direction of observation (so that the corresponding isoplanatic patch contains the objects of interest) and (b) sufficiently bright for accurate wavefront measurements. Since a star may not always be available for observation, an alternative option is to use an artificial guide star, which is created by sending an appropriate laser beam into the sky (→ laser guide stars). One possibility is to create a sodium beacon, where the laser beam's wavelength is precisely tuned to an absorption resonance of sodium atoms at 589.2 nm. These atoms are found at about 90 km altitude in the atmosphere and emit fluorescent light after absorbing the laser light. There are also Rayleigh beacons, which use Rayleigh scattering of light from a pulsed laser, typically at a shorter wavelength, such as in the green spectral range. Laser guide stars have greatly improved the sky coverage, although they are not ideal, primarily due to the cone effect.

Here you can submit questions and comments. As far as they get accepted by the author, they will appear above this paragraph together with the author’s answer. The author will decide on acceptance based on certain criteria. Essentially, the issue must be of sufficiently broad interest.

The design and implementation of adaptive optics involves a complex interplay of physics, engineering, and computational algorithms. This multidisciplinary technology relies on advances in fields such as optics, control systems, signal processing, and more. It is a dynamic and vibrant field that continues to evolve and find new applications.

A critical detail is the number of sensor elements, which limits the number of degrees of freedom. Obviously, this number must be at least equal to the number of aberration modes to be corrected (see below), and is often actually chosen several times higher.

By submitting the information, you give your consent to the potential publication of your inputs on our website according to our rules. (If you later retract your consent, we will delete those inputs.) As your inputs are first reviewed by the author, they may be published with some delay.

Continued research and development in this field should lead to the creation of more effective and economically viable adaptive optics systems. Further advances in wavefront sensors, wavefront correctors, optical setups, and control strategies can be expected.

There are also liquid crystal spatial light modulators (LCSLM), which are based on the liquid crystal modulator technology commonly used in displays. Note, however, that in this case, the wavelength dependence of the achieved phase changes may be problematic, and such modulators typically operate with polarized light. On the other hand, they offer high spatial resolution, and some of them operate in transmission rather than reflection, which can be advantageous. However, many modulators are based on liquid crystal on silicon (LCOS) technology, which is also reflective.

Note: this box searches only for keywords in the titles of articles, and for acronyms. For full-text searches on the whole website, use our search page.

There are several common control algorithms used in adaptive optics systems, including integrator-based controllers, adaptive filters, and model-based controllers. The selection of a control algorithm depends on the specific requirements of the application, including the level of wavefront distortion, the desired speed of correction, and the limitations of the correction device.

Despite the significant advancements and widespread adoption of adaptive optics over the years, there are still various challenges that need to be addressed. Some examples:

The company, Nippon Kōgaku Kōgyō Kabushikigaisha (日本光学工業株式会社 "Japan Optical Industries Co., Ltd."), was formed in 1917 with the merger of three Japanese optical manufacturers to produce precision optical glass. In 1925, the brand expanded to produce the first microscope with a revolving nosepiece and interchangeable objectives – the Joico microscope. Over the next few decades, the microscopy division introduced new polarising and stereomicroscopes, and metrology products for measuring and inspection.