Analysis of high-power operation of laser excited stationary phosphor

Abstract

Various high temperature phosphor materials, such as glass phosphor, ceramic phosphor, and crystal phosphor have been under stage of development targeting high power white light generation, which are suitable for various high power, small etendue applications. Stationary phosphor plates are getting into commercial projectors for some lower power projectors mostly limited by the power density limits of the phosphor materials. This paper presents a compact rotating, tilted, planar mirror, such that the output focused laser spot can be made to follow an elliptical path on the phosphor plate, increasing the effective area of the focused spot, and resulting in a higher limit of output optical power of the system. The key to such optical design is that the output of the system maintains the same small etendue of a single focused spot, and not the etendue of the circular path, for efficient coupling of the output to the projection optics. The maximum power capacity is very dependent on heat sinking especially the top surface of the phosphor plate. With the current heat sinking methodology, the maximum power is 89 W focused into a spot size in the range of 0.5 mm, which will further be determined accurately. The estimated power density ranges from about 300 to 600 W/sq. mm. along an elliptical path with axes measures 4.23 mm and 6.23 mm at 7,200 RPM. This has an improvement of power density limit many times compared to the phosphor specification of 45W/sq. mm. Further increase of power density limit is expected with further heat sinking developments. It is believed that the heat transmission between the top and the bottom of the phosphor plate would plan an important role in the power capacity. Phosphor plates with smaller thickness are being prepared for further investigation.