Adaptive optics research at Lincoln Laboratory

Adaptive optics is a technique for measuring and correcting optical aberrations in real time. It is particularly useful for atmospheric compensation-the correction of aberrations incurred by light propagating through the atmosphere. For more than two decades Lincoln Laboratory has been a leader in adaptive optics research and has performed seminal experiments in atmospheric compensation, including the first thermal-blooming compensation of a high-energy laser beam, the first compensation of a laser beam propagating from ground to space, and the first compensation using a synthetic beacon. In this article we describe the fundamental concepts of adaptive optics for atmospheric compensation and briefly review more than 20 years of Lincoln Laboratory work in the field. A DAPTIVE OPTIcs-the term almost defines it-self~is a technique for performing real-time. cancellation of optical aberrations. Adaptive optics can be used to correct various aberrations, including laser-device aberrations, aberrations resulting from optical fabrication errors, and thermally induced aberrations in telescopes, but the most common use for adaptive optics is in correction for atmospheric distortions. Light propagating through the atmosphere can be severely aberrated by atmospheric turbulence. This turbulence-induced aberration is an easily observed, everyday phenomenon. It is what causes objects to appear distorted when viewed from across a black-topped runway on a hot, sunny day; it is what causes stars to twinkle and dance. For many people the twinkling of stars is a romantic phenomenon, but for astronomers the atmospheric-induced distortion of starlight means that the new generation of 8-to-1 O-m telescopes-although having impressive light-collecting capacity-will have visible-light resolution no better than that of an amateur astronomer's 1O-to-15-cm backyard telescope. With adaptive optics it is possible to compensate for atmosphere turbulence and, thus, to achieve the full resolution capabilities oflarge telescopes. Figure 1 outlines the basic arrangement for an imaging atmospheric compensation system. Light from a star is collected by a telescope and is reflected offa deformable mirror. Part of the light is sent to an imaging camera, and part to a wavefront sensor. The wavefront sensor measures an array oflocal phase gradients, or wavefront tilts, which are processed in a wavefront reconstructor to derive a phase map ofthe incoming wavefront. The phase values are then used in a multichannel servo loop to drive the deformable mirror so as to flatten the incoming wavefront. If the incoming wavefront is flattened in this way, .the image resolution will be near diffraction limited. The required size and speed of the …