ABSTRACT
This chapter discusses requirements of wavefront sensing for adaptive optics and optical and mechanical components that comprise a wavefront sensor. An inertial adaptive optics system must have a way to sense the wavefront with enough spatial resolution and speed to apply a real-time correction. The direct approach employs an explicit determination of the phase or optical path difference (OPD) of the wave. After the wavefront is reconstructed, the information is used as feedback to correct for the unwanted components of the phase. The indirect approach never reduces the information to an explicit indication of wavefront but translates information related to the phase into signals used to compensate for the wavefront.
Wavefront sensors in an adaptive optics system are used for real-time operation. Since many disturbances can be on the order of hundreds of hertz, the sensors must operate much faster. The spatial resolution requirement of adaptive optics wavefront sensors is usually very high as well. It is not uncommon to require resolution to 1/100 of an aperture diameter, which requires many channels of parallel sensing. Wavefront sensors in adaptive optics are often required to deal with random wavefronts like atmospheric turbulence. This randomness requires a greater number of degrees of freedom. Adaptive optics wavefront sensing requires a large dynamic range (many wavelengths) to account for the vast OPD over the pupil of interest.
The final, but fundamental, requirement of adaptive optics wavefront sensing is its ability to determine OPD independent of intensity. For many applications, imaging resolved objects through the atmosphere for instance, the adaptive optics system will sense vast differences of intensity. Since the adaptive optics system is only normally capable of varying OPD over the aperture, the OPD must be determined without the confusing variation in the amplitude, manifested by the intensity non-uniformity. The use of broadband (white) light is required when the absolute OPD cannot be determined at a single wavelength.
