Abstract

Since the early 1960’s it is known that the illumination of optically rough objects with laser light results in the emergence of a phenomenon called speckle. Speckle is usually quantified using the contrast C of this interference pattern. A so-called fully developed and fully polarized speckle pattern has a contrast and therefore also a signal-to- noise ratio S/N of 1, what results in a highly degraded image. Speckle contrast reduction can be achieved e.g. by the intensity-based superposition of several mutually independent speckle patterns [1]. The minimum achievable speckle contrast C_min of the superposition of M independent patterns is given by $C_{\min }=1/\sqrt{M}$. A human observer will sense speckle down to a contrast of about 4%, so it is desirable but also challenging to achieve such low contrasts when using a laser light source. One possibility to decrease the speckle contrast is to use a laser source with a reduced degree of spatial coherence. Investigations have shown that when a broad-area VCSEL is driven by microsecond current pulses of proper parameters they behave as spatially incoherent quasi-homogeneous sources [2].

© 2007 IEEE