Adaptive optics

Adaptive optics is a technology used to enhance an optical system's performance by manipulating the incoming wavefronts to reduce distortions. The performance of an optical system is affected by both internal factors—such as component misalignment and element imperfections— and external factors—such as atmospheric disturbance, air turbulence, and temperature changes. Adaptive optics elements (most commonly deformable mirrors) can reduce the effect of external distortion factors and significantly improve image quality and optical coupling.

Adaptive optics is used in a range of fields, including astronomy and microscopy.

Representation of a spherical and planar wavefront

A wavefront is a surface associated with a propagating wave passing through points at the same phase. Typically undistorted wavefronts are planar (parallel wave) or spherical (wave propagating from a point source). These types of waves can be manipulated using common optical elements.

When the wavefront is distorted, complex adaptive optic components are required. Adaptive optics provides precise and programmable control of the wavefront shape that can dramatically improve the performance of many optical systems.

Adaptive optics corrects a wavefront via an optical element that changes shape when an outside control signal is applied. While many different types of adaptive optics elements are available, the most common include deformable mirrors, actuators, and optical cavities.

Diagram showing a deformable mirror in operation

A deformable mirror is an adaptive element with a controllable reflective surface shape. They can remove wavefront distortions by applying different mirror shapes, improving the performance of an optical system.

The two main systems for controlling the mirror shape are known as open-loop and closed-loop control. Open-loop control applies a set of pre-calculated or stored shapes. In comparison, closed-loop control utilizes a wavefront sensor to characterize the incoming wave and calculate the required mirror shape.

There is a range of optical parameters that affect the performance of adaptive optical elements. For deformable mirrors, key parameters include the following:

• Surface Type—Deformable mirrors are either segmented or continuous. Segmented mirrors are made up of many smaller mirror sections that can be controlled individually, with a greater number of segments improving the precision and control of the mirror shape. Continuous membrane mirrors consist of a single surface deformed at different spots.
• Actuation Technology—Segmented deformable mirrors are positioned by actuators using either piston-tip-tilt values or Zernike coefficients. Continuous surface deformable mirrors use actuators behind the reflective surface to deform, producing the required shape. Several options are available, including mechanical actuator posts behind the reflective membrane or magnets and piezoelectric elements that change the mirror surface profile.
• Number of Actuators—Actuators no. defines the quality and number of unique shapes the mirror can produce. Typically, the number of actuators ranges from several tens to hundreds. As the number of actuators increases, so does the versatility in deformation.
• Dimensions—Deformable mirrors range from a few millimeters in diameter to hundreds of centimeters, making them possible for use in both micro and macro applications.

There are key areas in which adaptive optics are used:

• Lasers—adaptive optics can be used to control a laser's beam shape and size to improve accuracy. Adaptive optics is used in laser cutting and manufacturing, atomic trapping, light-matter interaction research at the atomic level, quantum computing, and free-form metrology.
• Microscopy—adaptive optics can correct aberrations arising from samples in microscopy as well as correcting index mismatching within the microscope itself. This improves the resolution possible. Adaptive optics is widely used in multi-photon microscopy, confocal microscopy, and fluorescence microscopy.
• 3D imaging—uses deformable mirrors to increase depth range.
• Vision applications—adaptive optics are used in robotic vision and surveillance cameras to provide real-time or long-distance imaging.
• Biomedical applications—including ophthalmology, use adaptive optics to overcome aberrations caused by the vitreous humor in the human eye in order to capture high-resolution retinal images or increase the depth of scanning in optical coherence tomography.

Earth-based telescopes detect light that has been affected by the atmosphere. These disturbances cause considerable wavefront distortion, reducing the obtainable image and data quality. Adaptive optics can recover a significant amount of the information lost due to these disturbances.

Before and after astronomical images with adaptive optics.

To correct for atmospheric distortions, systems either rely on a bright reference star within the field of view or they produce an artificial reference star using a laser.

Whilst adaptive optics compensate for atmospheric distortions, active optics is required to correct for the deformation of the large primary mirrors.

The development of adaptive optics contains threads from both the astronomical and military communities. The concept was first proposed in 1953 by the US astronomer Horace Babcock while working at Mt. Wilson and Palomar Observatories, now renamed The Carnegie Observatories. Babcock suggested the use of a wavefront corrector to improve astronomical images containing aberrations induced by atmospheric turbulence. His idea could deblur images in part or entirely. However, he also identified limitations of this technique, such as the need for bright enough stars to act as guides that reduced its use to a small fraction of the sky.

Adaptive optics remained a concept until the required technology became mature enough in the early 1970s and defense-oriented research began (funded by DARPA). The first successful demonstration of adaptive optics technology was made in 1973 with a Real-Time Atmospheric Compensator (RTAC). Closely followed by field tests and second-generation systems such as CIS, installed at AMOS (Air Force Maui Optical Station).

Development continued and by the mid-1980s, adaptive optics components such as detector, deformable mirrors, and analog reconstructors were available under restricted use. Astronomers began using adaptive optics for astronomy, starting with a program at the National Optical Astronomical Observatories (NOAO). However, the program saw little traction and was stopped in 1987.

One of the first astronomy experiments using adaptive optics was performed by Luis Alvarez's research group at the Lawrence Berkeley Laboratory. A simple deformable mirror was used to correct in one dimension, demonstrating the potential to sharpen the image of a star.

Early theoretical work on adaptive optics systems was performed by Freeman Dyson (Institute for Advanced Study), Francois Roddier (University of Hawaii), and John Hardy (Itek Corporation).

Sodium reference laser on the 120-in Shane Telescope at Lick Obervatory

In 1991, Claire Max succeeded in getting adaptive optics technology declassified and began work at Lawrence Livermore National Laboratory, integrating a laser adaptive optics system into the Shane 120-in telescope at Lick Observatory. The system became operational in 1996. Utilizing a laser guide star system meant adaptive optics techniques could be applied to the entire sky. The laser light excited sodium atoms in the atmosphere, providing astronomers the information required to calculate atmospheric distortions affecting their astronomical data.