Speckle-free image in a laser-diode microscope by using the optical feedback effect.

Besides its desirable power level, the use of a laser as a light source in microscopes opens the possibility of observing high signal-to-noise ratio images of dynamic phenomena that are sensitive to certain polarizations, excitation wavelengths, and phase shifts. However, the image quality is degraded by speckle noise. We propose a new fiber-illuminated laser-diode microscope that generates speckle-free images. The feedback effect in the laser diode is employed to transform a single-mode free-running laser into a multimode laser and to generate an output light whose multimode spectrum changes with time. The output of the laser diode is then passed through a multimode fiber whose exit face illuminates a conventional microscope with a continuously changing speckle pattern. These uncorrelated speckle patterns are averaged by a video detector to reduce speckle noise. The technique eliminates speckle noise without employing any moving mechanical parts and requires no additional electronics or extra optical elemen ts except one mirror and one beam splitter. An experimental result showing excellent reduction of speckle is presented.