Investigation of speckle suppression beyond human eye sensitivity by using a passive multimode fiber and a multimode fiber bundle.

A method of speckle suppression without any active device is expected for pico-projectors. The effectiveness of the passive method of speckle reduction using a single multimode fiber and a multimode fiber bundle was actually measured and theoretically analyzed. The dependences of the speckle contrast and speckle suppression coefficient on the parameters of multimode fiber and projection systems were investigated. Our results shown that the efficiency of speckle suppression was limited because only the radial direction of the objective lens aperture was used. An improvement using both of the radial and azimuthal directions of the objective lens aperture is required.

Dispersion of speckle suppression efficiency for binary DOE structures: spectral domain and coherent matrix approaches.

We present the first general theoretical description of speckle suppression efficiency based on an active diffractive optical element (DOE). The approach is based on spectral analysis of diffracted beams and a coherent matrix. Analytical formulae are obtained for the dispersion of speckle suppression efficiency using different DOE structures and different DOE activation methods. We show that a one-sided 2D DOE structure has smaller speckle suppression range than a two-sided 1D DOE structure. Both DOE structures have sufficient speckle suppression range to suppress low-order speckles in the entire visible range, but only the two-sided 1D DOE can suppress higher-order speckles. We also show that a linear shift 2D DOE in a laser projector with a large numerical aperture has higher effective speckle suppression efficiency than the method using switching or step-wise shift DOE structures. The generalized theoretical models elucidate the mechanism and practical realization of speckle suppression.

Full speckle suppression in laser projectors using two Barker code-type diffractive optical elements.

The mathematical model of a speckle-suppression method based on two Barker code-type diffractive optical elements (DOEs) moving in orthogonal directions is developed. The analytic formulae for speckle suppression efficiency are obtained. The model indicates that the one pair of DOEs can be used for laser beams of different colors. The speckle contrast is not dependent on the distance from the viewer to the screen until the distance decreases below the distance where the spatial resolution of the eye on the screen is less than the length of the image of the DOE structure period on the screen. The analysis of the simulated results demonstrates that the method can decrease the speckle contrast to less than 5%, which is below human eye sensitivity, with an optical efficiency greater than 90%.

Optimal speckle suppression in laser projectors using a single two-dimensional Barker code diffractive optical element.

An effective method of speckle suppression using one 2D diffractive optical element (DOE) moving with constant velocity based on the periodic Barker code sequence is developed. We prove that this method has the same optical parameters as the method based on two 1D Barker code DOEs stretched and moving in orthogonal directions. We also show that DOE movement in a special direction allows the full numerical aperture of the objective lens to be used for speckle averaging by angle diversity. It is found that the 2D DOE based on a Barker code of length of 13 allows the speckle contrast to be decreased below the sensitivity of the human eye with optical losses of less than 10%.