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

Abstract

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.

Novel, high-quality surface plasmon resonance microscopy

“…To the best of our knowledge, this technique of time-averaging using an oscillating membrane to reduce laser speckle and the impact of multi-mode laser illumination has not been implemented previously in an SPR microscope, although it has been used in other systems [21][22][23][24]. We successfully reduced the speckle contrast by an order of magnitude using the oscillating diffuser.…”

Line optical tweezers instrument for measuring nanoscale interactions and kinetics

“…20 Because of coherence, light scattered from each microsphere ͑or defects in the optical train͒ interferes with that from all the others. From a simple centroiding point of view, such interference fringes resemble a time-varying nonuniform background illumination which badly biases and correlates the particle positions.…”

“…To increase the mode content of the beam to that required for Kohler illumination, this light is passed through a multiple mode optical fiber. 20 Laser's emitter ͑1 ϫ 100 m 2 in size͒ is imaged onto the 200 m diameter core of a 5 m long fiber having a large enough critical NA to collect the light ͑see Fig. 2͒.…”

Developing Pulsed Fiber Lasers

“…[5] The effect of multiple longitudinal modes reduces the coherence length of the illuminating laser. 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.…”