Speckle characteristics of laser diodes for SWIR and NIR active imaging

Laser illumination makes it possible to perform high resolution imaging when ambient light level is insufficient to overcome camera noise. The relatively long coherence length of most lasers, however, causes coherent speckle in the camera image plane, which can result in a significant decrease of the image quality and the maximum achievable target identification range. We characterized several types of NIR and SWIR laser diode illumination sources, with emphasis placed on measuring the properties of coherent speckle observed in the camera image plane. Image plane speckle contrast was measured by illuminating the imaged Lambertian surface with single-mode laser, multi-mode laser, wide-stripe laser with two active junctions and broad-band emission, and NIR and SWIR vertical cavity surface emitting laser (VCSEL) arrays. The impact of various imaging system parameters, including pixel size, imaging lens focal length, F-number, and IFOV on the contrast and characteristic size of the speckle intensity distribution were determined. Speckle contrast dependence on the polarization properties of various reflecting surfaces was measured. The reduction of speckle contrast with increasing source spectral width, and increasing size of spatially incoherent VCSEL emitter arrays will be described. We show that a speckle contrast of 5-10% is achievable for a typical long range SWIR imaging system.