Speckle reduction in laser projections with ultrasonic waves

In laser front projection display, the observer in the far field of the image pixels on the screen will observe serious speckle noise. For the first time we show a way to eliminate speckles in the far field by modulating a laser beam with traveling ultrasonic waves in a liquid cell. The resulting running interference fringes in a pixel on the screen will produce a boiling speckle pattern, which will be time-averaged in the eye of the observer. By superposition of ultrasonic waves of different frequencies, the speckle contrast can be reduced even further. We optimized the modulation frequencies, as well as the scattering properties of different screen materials. The intensity modulation depth of the speckle pattern could be reduced to a few percent without noticeable influence on the beam quality.