ABSTRACT
When the various disturbances are unavoidable, adaptive optics comes to the rescue. Each adaptive optics system should enhance the performance of its “parent” optical system. There are many ways to approach the problem of compensating for the aberrations. Conventional approaches that use wavefront sensors, electronic controls, and correction optics can be used to overcome the effects of the aberrations.
Some passive methods have been suggested. Other unconventional methods use the non-linear properties of certain optical materials to correct for the effects of aberrations. In any case, the application of correction is generally a closed-loop, real-time process. Non-real-time techniques of image processing, using wavefront information, are usually inadequate. The modulation transfer function of the aberrated image is so low that the signal is corrupted by noise. Adaptive optics are necessary up front to improve the modulation transfer function and the signal-to-noise ratio.
The most common approach to correcting for the various aberrations is the principle of phase conjugation, which is sometimes referred to as phase-only conjugation to distinguish it from systems that also correct for amplitude effects. The term phase conjugation is widely used synonymously with non-linear phase conjugation. However, the principle of applying a conjugate phase to a beam is the basis for most forms of adaptive optics, either linear (conventional) or non-linear. This chapter discusses the capabilities and limitations of phase conjugation and provides a detailed treatment of laser guide stars.
