There exists a need for adaptive optics systems which can correct for light distortions which occur in the medium of transmission. Conventional adaptive optics (“AO”) systems measure the characteristics of a lens, and correct for distortions using a computer-controlled deformable mirror, where the device which measures the distortions in the incoming wavefront of light is known as a wavefront sensor.
FIG. 1 depicts conventional AO system 100, which is an example of a closed loop AO system commonly used in the astronomical community. Light beam 101 emanating from nominal point source 102, a star, is directed through reflective and refractive elements within telescope 104, toward fast steering mirror 105. Fast steering mirror 105 directs light beam 101 into deformable mirror 106 and the wavefront sensor 112, where wavefront errors are corrected, as described more fully below.
Light beam 101 enters beam splitter 107, where it is split into two beams. Beam 109 enters science camera 110, where the light beam is viewed by the user of conventional AO system 100. Light beam 111 is directed into Shack-Hartmann wavefront sensor 112, which measures phase errors representing the derivative or slope of the wavefront in order to correct beam train or path length aberrations. These errors are communicated to wavefront computer 114, which calculates a reconstruction matrix to determine the phase or integral of the slope and passes these values as correction voltages to be applied to fast steering mirror 105 as X/Y tip/tilt values to stabilize the image, and phase values Φij (representing the phase values of an array of i,j pixels) to the deformable mirror 106, which restores the image sharpness lost to atmospheric turbulence. Wavefront computer 114 transmits these X/Y tip/tilt and Φij phase correction voltages to mirror drivers 115 and 116. Mirror drivers 115 and 116 apply the correction voltages to fast steering mirror 105 and deformable mirror 106, respectively, which apply the X/Y tip/tilt correction and Φij phase correction to the incoming wavefront from the target or star 102.
FIG. 2 depicts the wavefront sensing technique used by Shack-Hartmann wavefront sensor 112. As shown, this approach is completely geometric in nature, and has no dependence on the coherence of the sensed optical beam. The wavefront of light beam 111 is broken into an array of spatial samples, called subapertures of the primary aperture, by two dimensional array of lenslets 202. The subaperture sampled by each lenslet is brought to a focus at a known distance F behind each array, on two-dimensional detector array 204. The lateral position of the focal spot depends on the local tilt of the incoming wavefront. The location of all the subaperture spot positions and corresponding spot deviations ΔX and ΔY are used to measure the gradient of the incoming wavefront. A two-dimensional integration process, known as ‘reconstruction,’ is then used to estimate the shape of the original wavefront, and thus derive the correction signals for the deformable mirror.
To its disadvantage, however, conventional Shack-Hartmann AO sensor 112 requires massive reconstructors, and the sensor is not highly scalable. Other conventional methods, such as heterodyne direct phase methods or active imaging, have high laser power requirements, or have a high minimum pixel count. Accordingly, it is desirable to provide for wavefront sensing which overcomes the deficiencies of conventional approaches. In particular, it is desirable to provide for the closed loop correction of phase aberrations, using a black fringe wavefront sensor, to avoid the use of a reconstructor.