Speckle Reduction by Driving Current Modulation for Red Semiconductor Laser Diodes

Abstract

Speckle reduction method for laser diode based displays was newly proposed. The method was based on temporal change of the active layer temperature which could be controlled by optimizing the driving condition of the laser diode under pulse. The red LD consisted of AlGaInP has low thermal conductivity and small energy difference between the active layer and the cladding one in the conduction band, resulting in the large temporal change of the lasing spectrum under the adequate driving condition in pulse. It indicates the speckle contrast could be controlled by changing the condition. The speckle contrast dependence on the driving condition of the high-power 638-nm lateral single mode laser diode was experimentally studied. In the experiment, not only the conventional square waveform but also the saw one were used. The speckle contrast under the adequate conditions for both waveforms was reduced approximately 25% compared with that under CW. In particular, the saw one might be expected as a very robust driving condition for the speckle reduction. This can be used with other speckle reduction method such as moving screen, moving diffuser and so on. In this sense, the newly proposal may be the speckle reduction method for “last one mile”.