1、Introduction to Adaptive Optics and Deformable MirrorsIn optical systems, misalignment of components, quality imperfections of elements, or aberrations can internally reduce performance, while heat and atmosphere can externally reduce performance. Although design considerations and care during use o

2、f an optical system can reduce these problems, they may be too severe to solve with conventional methods. For example, in astrophotography, a field in which it is difficult to control external forces such as atmospheric disturbance, active methods are used for correcting performance. The key in both

3、 macro astronomy applications and micro optical applications is adaptive optics (AO).UNDERSTANDING WAVEFRONTSAdaptive optics (AO) is a technology used to enhance the quality of an optical system through manipulating the wavefront. This improves the final output, increasing performance compared to a

4、non-corrected system. A wavefront is defined as a line or wave consisting of points that have the same phase. Wavefronts are typically planar (flat) or spherical, and can change with the use of common optical elements. For example, a positive lens focuses collimated light to a point, changing plane

5、waves to spherical waves (Figure 1). For more information on aberrations and how to correct for them, please read Chromatic and Monochromatic Optical Aberrations.Introducing a flat wavefront to a standard optical element modifies the wavefront with a general degree of control. Employing an active me

6、ans of manipulating a flat wavefront with adaptive optics and deformable mirrors provides precise, measurable control. This precision control, unobtainable by non-adaptive elements, is why an adaptive optics system is employed in a broad range of imaging and non-imaging applications, typically to im

7、prove image quality or shape laser beams and reduce noise.Figure 1: Plano-Convex (PCX) Lens Changing Collimated Light (Plane Waves) to Spherical WavesTHE THEORY BEHIND ADAPTIVE OPTICSAdaptive optics (AO) correct a wavefront by comparing a test wave to a perfect, ideal wave, and then modifying the te

8、st wave in order to reach the ideal. A simple setup includes a beamsplitter, wavefront sensor, deformable mirror, and embedded control electronics (Figures 4a 4b). A common type of wavefront sensor is the Shack Hartmann Wavefront Sensor, which is made of small identical lenses placed in an array, ca

9、lled a lenslet array. When a perfectly flat wavefront is incident on the sensor, each lenslet focuses exactly in the center of the lens, at a distance equal to the focal length and where the CCD chip is placed (Figure 2a). When a distorted wavefront is incident on the lenslet array, the focus spots

10、are in different locations (Figure 2b). The deviation between the perfect and distorted spot patterns can be analyzed to reconstruct the wavefront incident on the sensor. In Figures 2a 2b, blue lines and spots represent the ideal model; green lines and spot the distorted wavefront which is observed

11、in real-world applications.Figure 2a: Wavefront Sensor Lenslets Focusing Flat WavefrontsFigure 2b: Wavefront Sensor Lenslets Focusing Distorted Wavefronts (Blue Spots are Ideal, Green are Observed)Figure 3: Deformable Mirror Correcting Distorted WavefrontsOnce the distorted test wavefront is measure

12、d, the wavefront sensor communicates to a deformable mirror to correct the wavefront. This is done by matching the deformation in the wavefront to the deformation in the mirror, which creates a flat wavefront (Figure 3). Please note that Figure 3 is an exaggerated view of the surface of a deformable

13、 mirror.A complete adaptive optics system is iterative in nature. First, a test wave is measured, either before or after it encounters the deformable mirror (the primary optical element in the system). This is typically done in a closed-loop system, where a portion of the beam is sent to and measure

14、d by a wavefront sensor. A closed-loop system offers the feedback necessary to correct for aberrations in real-time, a key factor when designing and optimizing an optical system. Figures 2a, 2b, and 3 illustrate the contribution of each component of an adaptive optics system. Figures 4a 4b illustrat

15、e a complete system where incoming light is manipulated by a deformable mirror, and then measured by a wavefront sensor to determine what changes to make in the deformable mirror to further correct the beam. To simplify the illustration, yellow arrows indicate only reflected light; incident light (e

16、manating from the left) is not depicted.Figure 4a: Sample Adaptive Optics System Before Feedback from Wavefront SensorFigure 4b: Sample Adaptive Optics System After Feedback from Wavefront SensorKEY ADAPTIVE OPTICS PARAMETERSSurface quality, actuation technology, number of actuators, and dimensions

17、are key adaptive optics (AO) parameters. Understanding these parameters helps to pick the best combination for the application at hand. Each combination has advantages and disadvantages concerning cost, power consumption, optical load, lag, and repeatability. Membrane electrostatic mirrors, for exam

18、ple, are advantageous because they lack response lag, are low cost, are insensitive to changes in wavelength, and have low power consumption, large dynamic behavior, and relatively high optical load. In addition, they offer high performance in aberration generation. Yet, membrane electrostatic mirro

19、rs also have a limited number of maximum strokes (maximum deformation) and have high correlations within the electrodes. Solutions exist to limit the disadvantages, such as placing an actuator on the top of the element to either pull or push the membrane.Most adaptive optics use wavefront sensors, a

20、s illustrated in Figures 4a 4b, but sensorless techniques have developed in order to circumvent complex software and instrumentation. These systems correct deformation by computing mirror deformation with a search algorithm to find the optimum combination to maximize image quality. Table 1 summarize

21、s the importance of four key adaptive optics parameters.Table 1: Importance of Key Adaptive Optics ParametersSurface QualityDeformable mirrors can either be segmented or continuous. Segmented mirrors, though far less common, have many smaller sections of mirror that can be controlled individually. C

22、ontinuous mirrors consist of a single surface that can be deformed at different spots.Actuation TechnologyContinuous surface deformable mirrors require actuators behind the reflective surface to deform it into the necessary shape. There are several options for this, ranging from mechanical pins behi

23、nd the reflective membrane that push the membrane out, to magnets that attract the surface in different directions. Another option is piezoelectric, where an applied voltage can change the mirror shape.Number of ActuatorsThe number of actuators defines the quality and the quantity of unique shapes t

24、he mirror can produce. As the number of actuators increases, so does the versatility in deformation. Typically, the number of actuators ranges from 32 - 64.DimensionDeformable mirrors can range from a few millimeters to hundreds of centimeters. Their range in size makes them ideal for micro and macr

25、o applications.APPLICATION EXAMPLESAdaptive optics (AO) are used in many imaging and non-imaging applications, including 3D imaging, vision, and biomedical applications. 3D imaging uses deformable mirrors to increase depth range, creating truer 3D. In vision applications, adaptive optics are used in

26、 robotic vision and surveillance cameras to provide real time or long-distance imaging.In microscopy, adaptive optics correct for aberrations in static lenses, decreasing cost when high-quality optical objectives are needed. Biomedical applications, including ophthalmology, use adaptive optics to ov

27、ercome aberrations in the human eye in order to capture high resolution retinal images, or to increase depth of scanning in optical coherence tomography. A major advancement in the adaptive imaging of the retina occurred with the invention of the scanning laser ophthalmoscope (SLO). When using a SLO, a laser projects a thin point of light on the retina, and the reflected beam is captured by a sensor. A SLO creates high contrast, superior l