Laser-diode microscope with fiber illumination

Abstract

Two new fiber-illuminated laser diode microscopes which generate weak speckle images are proposed. In the first microscope, the output of a current-modulated laser diode is passed through a single multimode fiber whose exit face illuminates a conventional microscope with a continuously changing radiation pattern. This type of illumination generates time-accumulated, uncorrelated or partially correlated speckle image patterns that are averaged by a video detector to reduce speckle noise. In the second microscope, the exit face of a modified fiber bundle which functions as an extended secondary source consisting of N uncorrelated point sources illuminates the microscope. These point sources generate their own respective speckle image patterns that are ensemble-averaged by the detector to eliminate speckle noise. The modified fiber bundle is designed to ensure that the N point sources are mutually uncorrelated by making the length difference of any two fiber guides among the N fiber guides making up the bundle to be greater than the coherence length of the light source. Since the averaging of N speckle patterns in this arrangement is done in space domain (ensemble averaging), laser diode needs not be modulated. Both arrangements eliminate speckle noise without employing any moving mechanical parts, offer flexibility, compactness, and ease in adapting to conventional microscope.