Understanding Wavefronts | Adaptive Optics Theory | Key Parameters| Applications
In 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 of 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 macro astronomy applications and micro optical applications is adaptive optics (AO).
UNDERSTANDING WAVEFRONTS
Adaptive 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 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 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 means 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 improve image quality or shape laser beams and reduce noise.
Figure 1: Plano-Convex (PCX) Lens Changing Collimated Light (Plane Waves) to Spherical Waves [View Larger Image]
THE THEORY BEHIND ADAPTIVE OPTICS
Adaptive optics (AO) correct a wavefront by comparing a test wave to a perfect, ideal wave, and then modifying the test 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, called 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 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 in real-world applications.
Figure 2a: Wavefront Sensor Lenslets Focusing Flat Wavefronts [View Larger Image]
Figure 2b: Wavefront Sensor Lenslets Focusing Distorted Wavefronts (Blue Spots are Ideal, Green are Observed) [View Larger Image]
)
Figure 3: Deformable Mirror Correcting Distorted Wavefronts [View Larger Image]
Once the distorted test wavefront is measured, 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 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 measured 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 illustrate 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 (emanating from the left) is not depicted.
Figure 4a: Sample Adaptive Optics System Before Feedback from Wavefront Sensor [View Larger Image]
){kind=link}
){kind=link}
){kind=link}
){kind=link}
){kind=link}