Deformable mirrors (DM) are mirrors whose surface can be deformed, in order to achieve wavefront control and correction of optical aberrations. Deformable mirrors are used in combination with wavefront sensors and real-time control systems in adaptive optics. In 2006 they found a new use in femtosecond pulse shaping. [1]
The shape of a DM can be controlled with a speed that is appropriate for compensation of dynamic aberrations present in the optical system. In practice the DM shape should be changed much faster than the process to be corrected, as the correction process, even for a static aberration, may take several iterations.
A DM usually has many degrees of freedom. Typically, these degrees of freedom are associated with the mechanical actuators and it can be roughly taken that one actuator corresponds to one degree of freedom.
Number of actuators determines the number of degrees of freedom (wavefront inflections) the mirror can correct. It is very common to compare an arbitrary DM to an ideal device that can perfectly reproduce wavefront modes in the form of Zernike polynomials. For predefined statistics of aberrations a deformable mirror with M actuators can be equivalent to an ideal Zernike corrector with N (usually N < M) degrees of freedom. For correction of the atmospheric turbulence, elimination of low-order Zernike terms usually results in significant improvement of the image quality, while further correction of the higher-order terms introduces less significant improvements. For strong and rapid wavefront error fluctuations such as shocks and wake turbulence typically encountered in high-speed aerodynamic flowfields, the number of actuators, actuator pitch and stroke determine the maximum wavefront gradients that can be compensated for.
Actuator pitch is the distance between actuator centers. Deformable mirrors with large actuator pitch and large number of actuators are bulky and expensive.
Actuator stroke is the maximum possible actuator displacement, typically in positive or negative excursions from some central null position. Stroke typically ranges from ±1 to ±30 micrometres. Free actuator stroke limits the maximum amplitude of the corrected wavefront, while the inter-actuator stroke limits the maximum amplitude and gradients of correctable higher-order aberrations.
Influence function is the characteristic shape corresponding to the mirror response to the action of a single actuator. Different types of deformable mirrors have different influence functions, moreover the influence functions can be different for different actuators of the same mirror. Influence function that covers the whole mirror surface is called a "modal" function, while localized response is called "zonal".
Actuator coupling shows how much the movement of one actuator will displace its neighbors. All "modal" mirrors have large cross-coupling, which in fact is good as it secures the high quality of correction of smooth low-order optical aberrations that usually have the highest statistical weight.
Response time shows how quickly the mirror will react to the control signal. Can vary from microseconds (MEMS and magnetics mirrors) to tens of seconds for thermally controlled DM's.
Hysteresis and creep are nonlinear actuation effects that decrease the precision of the response of the deformable mirror. For different concepts, the hysteresis can vary from zero (electrostatically-actuated mirrors) to tens of percent for mirrors with piezoelectric actuators. Hysteresis is a residual positional error from previous actuator position commands, and limits the mirror ability to work in a feedforward mode, outside of a feedback loop.
Segmented concept mirrors are formed by independent flat mirror segments. Each segment can move a small distance back and forth to approximate the average value of the wavefront over the patch area. Advantageously, these mirrors have little or zero cross-talk between actuators. Stepwise approximation works poorly for smooth continuous wavefronts. Sharp edges of the segments and gaps between the segments contribute to light scattering, limiting the applications to those not sensitive to scattered light. Considerable improvement of the performance of the segmented mirror can be achieved by the introduction of three degrees of freedom per segment: piston, tip and tilt. These mirrors require three times as many actuators compared to piston-segmented mirrors. This concept was used for fabrication of large segmented primary mirrors for the Keck telescopes, the James Webb Space Telescope, and the future E-ELT. Numerous methods exist to accurately co-phase the segments and reduce the diffraction patterns introduced by the segment shapes and gaps. Future large space-based telescopes, such as the NASA Large UV Optical Infrared Surveyor will also possess a segmented primary mirror. The development of robust methods to increase the contrast is key for the direct imaging and characterization of exoplanets.
Continuous faceplate concept mirrors with discrete actuators are formed by the front surface of a thin deformable membrane. The shape of the plate is controlled by a number of discrete actuators that are fixed to its back side. The shape of the mirror depends on the combination of forces applied to the faceplate, boundary conditions (the way the plate is fixed to the mirror) and the geometry and the material of the plate. These mirrors allow smooth wavefront control with very large - up to several thousands - degrees of freedom.
Magnetic concept mirrors consist of a thin flexible continuous membrane actuated by voicecoils and magnets. This technology allows great design flexibility to achieve very different performances. Depending on the design choices made, they can achieve unrivaled stroke - up to a hundred microns of deformation - or very high speed - <ms timeframe. As the membrane is a single sheet of material, very high optical quality is also achievable. This technology can exhibit good stability and keep its shape almost unchanged for weeks. The actuator count can range from several tens of actuators to several thousand actuators.
MEMS concept mirrors are fabricated using bulk and surface micromachining technologies. They consist of a thin reflective membrane controlled by a multitude of actuators. [4] MEMS mirrors could break the high price threshold of conventional adaptive optics. They enable a higher actuator count at a more cost-effective price allowing for accurate wave-front correction. [4] MEMS mirrors offer fast response times from the actuators [5] with limited hysteresis. An additional benefit is that micromachining technologies allow for the benefit of economies of scale to create cheaper and lighter deformable mirrors with a greater number of actuators. [6]
Membrane concept mirrors are formed by a thin conductive and reflective membrane stretched over a solid flat frame. The membrane can be deformed electrostatically by applying control voltages to electrostatic electrode actuators that can be positioned under or over the membrane. If there are any electrodes positioned over the membrane, they are transparent. It is possible to operate the mirror with only one group of electrodes positioned under the mirror. In this case, a bias voltage is applied to all electrodes, to make the membrane initially spherical. The membrane can move back and forth with respect to the reference sphere.
Bimorph concept mirrors are formed by two or more layers of different materials. One or more of (active) layers are fabricated from a piezoelectric or electrostrictive material. Electrode structure is patterned on the active layer to facilitate local response. The mirror is deformed when a voltage is applied to one or more of its electrodes, causing them to extend laterally, which results in local mirror curvature. Bimorph mirrors are rarely made with more than 100 electrodes.
Ferrofluid concept mirrors are liquid deformable mirrors made with a suspension of small (about 10 nm in diameter) ferromagnetic nanoparticles dispersed in a liquid carrier. In the presence of an external magnetic field, the ferromagnetic particles align with the field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Using proper magnetic field geometries, any desired shape can be produced at the surface of the ferrofluid. This new concept offers a potential alternative for low-cost, high stroke and large number of actuators deformable mirrors. [7] [8] [9]
Active optics is a technology used with reflecting telescopes developed in the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, and mechanical stress. Without active optics, the construction of 8 metre class telescopes is not possible, nor would telescopes with segmented mirrors be feasible.
Adaptive optics (AO) is a technique of precisely deforming a mirror in order to compensate for light distortion. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, in microscopy, optical fabrication and in retinal imaging systems to reduce optical aberrations. Adaptive optics works by measuring the distortions in a wavefront and compensating for them with a device that corrects those errors such as a deformable mirror or a liquid crystal array.
In optics, the coma, or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration.
Scanning laser ophthalmoscopy (SLO) is a method of examination of the eye. It uses the technique of confocal laser scanning microscopy for diagnostic imaging of the retina or cornea of the human eye.
The Giant Magellan Telescope is a 25.4-meter, ground-based, extremely large telescope under construction at Las Campanas Observatory in Chile's Atacama Desert. Commissioning is anticipated in the early 2030s. Once complete, the Giant Magellan will be the largest Gregorian telescope ever built observing in optical and mid-infrared light. The telescope uses seven of the world’s largest mirrors to form a light collecting area of 368 square meters.
In physics, the wavefront of a time-varying wave field is the set (locus) of all points having the same phase. The term is generally meaningful only for fields that, at each point, vary sinusoidally in time with a single temporal frequency.
Electron optics is a mathematical framework for the calculation of electron trajectories in the presence of electromagnetic fields. The term optics is used because magnetic and electrostatic lenses act upon a charged particle beam similarly to optical lenses upon a light beam.
In optics, tilt is a deviation in the direction a beam of light propagates.
Dielectric elastomers (DEs) are smart material systems that produce large strains and are promising for Soft robotics, Artificial muscle, etc. They belong to the group of electroactive polymers (EAP). DE actuators (DEA) transform electric energy into mechanical work and vice versa. Thus, they can be used as both actuators, sensors, and energy-harvesting devices. They have high elastic energy density and fast response due to being lightweight, highly stretchable, and operating under the electrostatic principle. They have been investigated since the late 1990s. Many prototype applications exist. Every year, conferences are held in the US and Europe.
The digital micromirror device, or DMD, is the microoptoelectromechanical system (MOEMS) that is the core of the trademarked Digital Light Processing (DLP) projection technology from Texas Instruments (TI). Texas Instrument's DMD was created by solid-state physicist and TI Fellow Emeritus Dr. Larry Hornbeck in 1987. However, the technology goes back to 1973 with Harvey C. Nathanson's use of millions of microscopically small moving mirrors to create a video display of the type now found in digital projectors.
Boston Micromachines Corporation is a US company operating out of Cambridge, Massachusetts. Boston Micromachines manufactures and develops instruments based on MEMS technology to perform open and closed-loop adaptive optics. The technology is applied in astronomy, beam shaping, vision science, retinal imaging, microscopy, laser communications, and national defense. The instruments developed at Boston Micromachines include deformable mirrors, optical modulators, and retinal imaging systems, all of which utilize adaptive optics technology to enable wavefront manipulation capabilities which enhance the quality of the final image.
A ferrofluid mirror is a type of deformable mirror with a reflective liquid surface, commonly used in adaptive optics. It is made of ferrofluid and magnetic iron particles in ethylene glycol, the basis of automotive antifreeze. The ferrofluid mirror changes shape instantly when a magnetic field is applied. As the ferromagnetic particles align with the magnetic field, the liquid becomes magnetized and its surface acquires a shape governed by the equilibrium between the magnetic, gravitational and surface tension forces. Since any shapes can be produced by changing the magnetic field geometries, wavefront control and correction can be achieved.
A MEMSmagnetic field sensor is a small-scale microelectromechanical systems (MEMS) device for detecting and measuring magnetic fields (Magnetometer). Many of these operate by detecting effects of the Lorentz force: a change in voltage or resonant frequency may be measured electronically, or a mechanical displacement may be measured optically. Compensation for temperature effects is necessary. Its use as a miniaturized compass may be one such simple example application.
A microscanner, or micro scanning mirror, is a microoptoelectromechanical system (MOEMS) in the category of micromirror actuators for dynamic light modulation. Depending upon the type of microscanner, the modulatory movement of a single mirror can be either translatory or rotational, on one or two axes. In the first case, a phase shifting effect takes place. In the second case, the incident light wave is deflected.
ALPAO is a company which manufactures a range of adaptive optics products for use in research and industry, including deformable mirrors with large strokes, wavefront sensors, and adaptive optics loops. These products are designed for astronomy, vision science, microscopy, wireless optical communications, and laser applications.
GREGOR is a solar telescope, equipped with a 1.5 m primary mirror, located at 2,390 m altitude at the Teide Observatory on Tenerife in the Canary Islands. It replaces the older Gregory Coudé Telescope and was inaugurated on May 21, 2012. First light, using a 1 metre test mirror, was on March 12, 2009.
Electrostatic–pneumatic activation is an actuation method for shaping thin membranes for microelectromechanical and microoptoelectromechanical systems. This method benefits from operation at high speed and low power consumption. It can also cause large deflection on thin membranes. Electrostatic-pneumatic MEMS devices usually consist of two membranes with a sealed cavity in between. One membrane-calling actuator deflects into the cavity by electrostatic pressure to compress air and increase air pressure. Elevated pressure pushes the other membrane and causes a dome shape. With direct electrostatic actuation on the membrane, a concave shape is achieved.
The 3.6m DevasthalOptical Telescope is a clear-aperture Ritchey–Chrétien telescope built by Aryabhatta Research Institute of Observational Sciences (ARIES) and is located at the Devasthal Observatory site near Nainital, Kumaon, India. ARIES operates another 1.3m telescope at the same location. The telescope was activated remotely on 31 March 2016 by Indian Prime Minister Narendra Modi and Belgian Prime Minister Charles Michel from Brussels. The telescope optics were built in collaboration with the Belgian firm Advanced Mechanical and Optical System (AMOS).
Optical Telescope Element (OTE) is a sub-section of the James Webb Space Telescope, a large infrared space telescope launched on 25 December 2021, consisting of its main mirror, secondary mirrors, the framework and controls to support the mirrors, and various thermal and other systems.
The Goode Solar Telescope (GST) is a scientific facility for studies of the Sun named after Philip R. Goode. It was the solar telescope with the world's largest aperture in operation for more than a decade. Located in Big Bear Lake; California, the Goode Solar Telescope is the main telescope of the Big Bear Solar Observatory operated by the New Jersey Institute of Technology (NJIT). Initially named New Solar Telescope (NST), first engineering light was obtained in December 2008, and scientific observations of the Sun began in January 2009. On July 17, 2017, the NST was renamed in honor of Goode, a former, and founding director of NJIT's Center for Solar-Terrestrial Research and the principal investigator of the facility. Goode conceived, raised the funds, and assembled the team that built and commissioned the telescope, and it was the highest resolution solar telescope in the world (until the end of 2019) and the first facility class solar telescope built in the U.S. in a generation.
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