Adaptive Optics Tutorial

Adaptive Optics Tutorial

An adaptive optics system automatically corrects for light distortions in the medium of transmission. For example, if you look far down a road on a very hot and sunny day, you will often see what is usually called a mirage. What you are seeing is the rapidly changing temperature in the air causing it to act like a thick, constantly bending lens.

An adaptive optics system measures the characteristics of the lens and corrects for it by means of a deformable mirror controlled by a computer. The device that measures the distortions in the incoming wavefront of light is called a wavefront sensor.

Light from a nominal point source above the atmosphere enters the primary aperture and is split between a camera and a wavefront sensor ( See Fig. 1). The sensor measures the wavefront distortion and controls a tilt mirror to stabilize the image and a deformable mirror which restores the image sharpness lost to atmospheric turbulence. The adaptive optics system technologies developed and delivered by AOA include adaptive wavefront compensation for optical systems and wavefront measurement. In recent years, the technology and practice of adaptive optics have become, if not commonplace, at least well-known in the astronomical community.

Figure 1

A key technology supplied by AOA is the wavefront sensor. The most commonly used approach is the Shack-Hartmann method. As shown in Figure 2, this approach is completely geometric in nature and so has no dependence on the coherence of the sensed optical beam. The incoming wavefront is broken into an array of spatial samples, called subapertures of the primary aperture, by a two dimensional array of lenslets. The subaperture sampled by each lenslet is brought to a focus at a known distance F behind each array. The lateral position of the focal spot depends on the local tilt of the incoming wavefront; a measurement of all the subaperture spot positions is therefore a measure of the gradient of the incoming wavefront. A two-dimensional integration process called reconstruction can then be used to estimate the shape of the original wavefront, and from there derive the correction signals for the deformable mirror.

Figure 2

The incoming wavefront sample is analyzed into spatial subapertures by a miniature lens array which creates a pattern of spots on a two-dimensional array. The deviation of each spot from its nominal center is proportional to the input tilt at the corresponding subaperture.

The transformation from spot array to wavefront output is illustrated in figure 3, below. The processing steps are shown clockwise from upper left, digitized spot pattern, vector representation of the spot deviations from nominal, reconstructed mirror profile, and Zernike decomposition. At center is the simple optical arrangement that makes the measurement possible.

Wavefront Analysis

Figure 3

References:

1.R. Q. Fugate and W. J. Wild, "Untwinkiling the Stars Part 1", Sky Telescope,
24-31 (May 1994).
2.B. M. Welsh, B. L. Ellerbroek, M. C. Roggemann, and T. L. Pennington,
"Fundamental performance comparison of a Hartmann and a shearing
interferometer wavefront sensor", Applied Optics 34:4186-4195 (July 1995).
3.G. P. Collins, "Making stars to see stars: DOD adaptive optics work is
declassified", Physics Today, 17-21, (February 1992).