The Effect of Laser Temporal Coherence on Active Speckle Imaging

Abstract

Speckle imaging denotes a class of high-resolution astronomical imaging techniques; this class includes speckle interferometry, Knox-Thompson imaging, and the bispectrum technique.1,2,3 These methods are typically used under conditions of natural illumination, in which the light emanating from the object is temporally incoherent in the sense that the spread of the wavelength of the radiated light is on the order the mean wavelength. On the other hand, these techniques may be applied to conditions in which the object is laser-illuminated. Laser radiation is characterized by its intense spectral brightness and directionality. The directionality of laser radiation is especially useful for illumination of distant objects for which the high resolution offered by speckle imaging is most needed. However, the spectral brightness of lasers, i.e., temporal coherence, may result in laser speckle. This laser speckle can interefere with the speckle imaging process. Laser speckle appears most prominently in the pupil plane of the telescope, unlike the atmospherically-induced speckle evident in the focal plane; this latter speckle is that for which the speckle imaging techniques are named. Atmospherically-induced speckle appears in the focal plane of the telescope, but nonetheless may be affected by laser speckle. Natural questions then arise as to whether the laser speckle causes degradation, and if so, what can be done to mitigate its effect.