Development of speckle reduction technology in laser projection displays

Abstract

Lasers as illumination sources in displays have excellent features compared to lamps and Light Emitting Diodes (LEDs). Lasers have the following advantages: high luminance, small etendue, long lifetime, linear polarization, low power consumption and mercury free, and they can enable a wider color gamut. However, the highly temporal and spatial coherence of the laser introduces a grainy light intensity distribution known as speckle when the laser beam illuminates a rough screen, and the image quality on the screen, thus, will be degraded.

In line-scan and full-frame laser projectors, a widely used method to reduce speckle is by the temporal averaging of different speckle patterns during the human eye integration time (50 ms, typically). Spatially, this must be carried out when the size of the projection lens resolution element is smaller than that of one human eye resolution element. In general, a fast changing random diffuser in light propagations is introduced, which requires a motor working at high frequency to actuate the random diffuser. The research goal of this thesis is to develop novel speckle reduction methods and devices to replace the motor and to make the laser projectors more compact. We carried out the research in the following four parts:

In the method of polarization diversity, we built an experiment to obtain arbitrary rotation of the laser polarization, thereby finding the correlation between speckle intensities. We derived a generalized speckle contrast formula for the superposed speckle patterns. This formula further explored the theory of speckle contrast for the sum of correlated speckle intensities as it related to polarization diversity, and could be used to optimize speckle reduction.

In the method of a changing diffuser with random phase cells, several switchable diffraction gratings were positioned differently (by changing both orientation and frequency). The diffraction gratings worked together to produce dynamic diffraction laser beams onto a static random phase diffuser. Simulations were conducted in ZEMAX to maximize the speckle reduction efficiency.

In the method of a changing diffuser with deterministic orthogonal phase codes, we proposed using a two levels (-1 and +1 as variables) Orthogonal Array (OA) to generate a binary phase diffuser for speckle reduction in laser projection displays. Compared to Hadamard Matrix (HM), the diffuser generated from OA was more flexible. Speckle contrast when introducing the binary phase diffuser at an intermediate image plane within the projector was calculated, and the minimum speckle Contrast Ratio (CR) could be achieved by finite steps change of the diffuser’s patterns. By Kronecker algebra, the two-dimensional (2D) diffuser was replaced by two one-dimensional (1D) ones with the same function and they could be implemented into the laser projector electronically and easily. A MEMS mirror array actuated by the electrostatic force was designed and fabricated, and an Electro-optic (EO) Binary Phase Modulator (BPM) was proposed.

In the method of angle diversity, a Microelectromechanical Systems (MEMS) scanning mirror and a Liquid Crystal (LC) Spatial Light Modulator (SLM), respectively, served to temporally change laser beams. Because of the introduction of a static diffuser, compound speckle formed on the projection screen. A characterization method to evaluate the compound speckle reduction efficiency was demonstrated for analysis and discussion, and the speckle contrast was reduced from 0.38 to 0.14 in a commercial laser projector.