Polychromatic speckle mitigation for improved adaptive-optics system performance

Adaptive-optics (AO) systems correct the distortions caused by atmospheric turbulence for imaging and laser-transmission applications. Given an extended, uncooperative object, the AO system must create a reference wave for wavefront measurement. It does so by focusing a laser beam onto the object; therefore, creating a beacon. Unfortunately, the extended size of the beacon after propagation gives rise to speckle, causing noise in the wavefront measurements which degrades the AO system's correction of the turbulence effects. In this paper, we use polychromatic illumination to create the reference wave, which results in an associated reduction in the speckle noise. To quantify the benefits, we use split-step wave-optics simulations with the spectral-slicing method for polychromatic light. We assume that the AO system uses a Shack Hartmann wavefront sensor. Furthermore, we assume that the speckle decorrelates over short periods of time corresponding to reasonable object motions. We consider a range of conditions for the object size (i.e. the object Fresnel number), object motion, and illuminator coherence length. The results show a reduction in the speckle-induced error with polychromatic light, especially when the object is large. This finding indicates that polychromatic illumination can improve the performance of AO systems when the object is both uncooperative and extended.