Speckle characteristics of a laser projector using nonmodal laser emission of a semiconductor laser

Abstract

In this contribution we show how spatially incoherent emission from a broad-area vertical-cavity surface-emitting laser (BAVCSEL) can be used for low-speckle laser projection. In our projection setup, we use a microlens beam homogenizer in order to homogenize the intensity distribution and to exploit the low spatial coherence of the VCSEL. In order to investigate the speckle in a projection setup using a BA-VCSEL as light source, we compare speckle values in case of modal and nonmodal emission of the BA-VCSEL. Furthermore, the microlens beam homogenizer can either be illuminated with the laser's near field or far field, leading to comparable results. Speckle contrast values as low as 3.5% in case of nearfield illumination, and 2.5% in case of farfield illumination, are measured without using any additional or mechanically moving components to destroy the coherence of the laser beam. The microlens array in the setup is essential in order to obtain speckle reduction, since it generates an overlap of mutually independent speckle patterns, thus reducing the overall speckle in the projected image. We successfully model the speckle contrast reduction, taking into account all contributing speckle reducing factors.