speckle contrasts

Speckle patterns are intensity patterns produced by coherent waves interfering with each other. They typically occur due to reflections of coherent laser light in rough surfaces or in media with scattering particles on the scale of the wavelength. Speckle is often undesired in imaging because of the grainy image produced. On the other hand, the spatial correlations present in the speckle pattern contain information that can be used to reconstruct the object that generates the speckle. Low-cost vibration sensors have been demonstrated, which measure the frequency of the vibrations by monitoring the speckle pattern that changes in time. As speckle patterns are wavelength-dependent, after calibration they can also be exploited for implementing low-cost high precision wavemeter. In this contribution we study experimentally how the statistical properties of the speckle pattern at the output of a multimode fiber, generated by using as light source a diode laser in the visible range, depend on the exposure time of the CCD camera and on the degree of coherence of the light, which is controlled by varying the laser pump current from below to above the threshold. Using the standard speckle contrast measure (the mean intensity of the pattern normalized to the standard deviation), we determine under which conditions the speckle pattern can be either minimized or maximized.