Speckle characteristics of laser diodes

Abstract

Laser, as the illumination source in the projector, has an intrinsic problem for high quality image. The speckle noise caused by laser coherence masks the details when the image projected on the screen. Therefore, to characterize the speckle noise of the different laser light sources becomes an important issue in developing the laser display technology. The cheapest laser light sources, red and blue broad-area edge-emission laser diodes (BAEEL) invented in the last decades, will be characterized for future high brightness laser projector design. In this thesis, the laser emission spectrum has been measured at different operating temperatures and driving conditions. The speckle images from the laser diodes are formed on the glass diffuser screens and recorded by the CCD camera. For measuring the speckle characteristics of laser diodes, two typical measurement plans were designed, 1) the laser diodes with collimation lenses to form a small light spot in which all the light rays are roughly parallel, 2) the laser diodes without collimation lenses to get a large illumination area in the far field measurement. For the first measurement plan, we find that the speckle contrast strongly depends on the operating temperature and driving current, which can be explained by the temperature and current dependence of the emission spectrum. For the second measurement plan, three different optical-pass configurations have been setup in IMST optical lab. Under a certain measurement conditions, at specific temperature and driving current, we investigate the speckle images of the blue laser diodes in the far field, where the illumination field is a bar-shape with a large area. The speckle contrast of the different areas in the large illumination field is measured by moving the diffuser screen, by moving the diffuser screen with optical axis alignment, and by rotating the diffuser screen with optical axis alignment. The correlation of the speckle images measured in the different illumination areas is analyzed. We have observed the speckle contrasts depending on the measurement configuration. By changing the operating temperature and driving current, the speckle characteristics of the semiconductor laser diodes can be modulated; It is possible to design high brightness and low speckle noise projectors by engineering the driving scheme, temperature control, and integration configuration of the semiconductor laser diodes.