It is increasingly common to use laser light as an illumination source for optical microscopy. Among the advantages of many lasers are superb brightness, narrow spectrum, long instrument life, and low heat production. The finite coherence length of laser light, a singular advantage for many laser applications, is a detriment in wide field microscopy because of laser speckle. Speckle is observed as a spatially fluctuating pattern of light and dark that is familiar to users of laser pointers. It is caused by the random coherent superposition of light that has traveled different paths from a laser light source to a sample illuminated by the laser light, and can be a formidable obstacle to uniform illumination of a field within an imaging instrument. The use of lasers in wide field microscopy is often forsaken because of speckle.
Where random amplitudes interfere constructively, the speckle field is bright. Where random amplitudes interfere destructively, the speckle field is dark. While the electric fields of the light oscillate at high frequencies (on the order 1014 Hz), for a fixed optical propagation path, the time-averaged spatial phase relationships remain fixed.
One commonly used strategy to cope with speckle is to vary the optical propagation path on a time short compared to the response of the eye (on the order of tens of milliseconds) or short microscope camera exposures (on the order of one millisecond). The time-averaged speckle can be acceptably smooth when the speckle modulation is both sufficiently deep and sufficiently rapid.
It is also common to couple the laser illumination source to the microscope via a multimode fiber optic cable. The coherent light travels through the fiber optic cable in multiple electromagnetic modes, which can be thought of as independent solutions of Maxwell's Equations for propagation in an optical waveguide. By varying the geometry of the cable, phase relationships between the independent modes change, and thus their coherent geometric superposition changes as well.
Currently-used approaches include a vibrating fiber coil, piezo-electric fiber-stretcher, holographic diffuser (spinning disk), fiber optic elements for reducing speckle noise, and gross mechanical manipulators. However, each of these approaches suffers from significant shortcomings and limitations. For example, vibrating coils exhibit low frequency motion and do not take advantage of the natural vibration modes of the cables and also display poor speckle abatement. Piezo fiber stretchers offer limited fiber types, and display lifetime fatigue issues and poor speckle abatement. In addition, they are inefficient optically. Fiber optic elements also exhibit poor speckle abatement and are complicated to manufacture. Gross mechanical manipulators are clumsy, noisy, and develop fiber fatigue.
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