Zhaomin Tong

Laser speckle reduction due to spatial and angular diversity introduced by fast scanning micromirror

We utilize spatial and angular diversity to achieve speckle reduction in laser illumination. Both free-space and imaging geometry configurations are considered. A fast two-dimensional scanning micromirror is employed to steer the laser beam. A simple experimental setup is built to demonstrate the application of our technique in a two-dimensional laser picture projection. Experimental results show that the speckle contrast factor can be reduced down to 5% within the integration time of the detector.

Speckle reduction in line-scan laser projectors using binary phase codes

A Barker binary phase code of maximum length 13 has previously been used for speckle reduction in line-scan laser projectors, and a speckle contrast factor decrease down to 13% has been achieved. In this Letter, Barker-like binary phase codes of lengths longer than 13 are used at an intermediate image plane. It is shown by theoretical calculation that a much better speckle reduction with a speckle contrast factor up to 6% can be achieved by using longer binary phase codes other than the Barker code.

Speckle reduction using a motionless diffractive optical element

Speckle reduction by moving diffuser has been previously studied in display systems with coherent light sources, such as lasers. In this Letter, we propose a motionless diffractive optical element (DOE) for speckle reduction. The DOE was designed based on finite-element method simulations, fabricated using micromachining technology, and characterized for despeckle efficiency. Experiments using a DOE with two gratings have indicated that the speckle was suppressed to 50%, which shows fair agreement with theoretical analysis. With some modification of this DOE, the speckle noise can be reduced to 10% according to the theory.

Compound Speckle Characterization Method and Reduction by Optical Design

Speckle and the compound speckle can be reduced by angle diversity. In laser projection displays, simple, low cost and efficient speckle reduction techniques require smart optical design . Using a MEMS scanner together with a condenser lens, laser beams with different illumination angles are obtained on the diffuser surface with low speckle contrast ratio (CR). After homogenizing within a rod integrator, the speckle field illuminates a display panel, and is projected onto the screen which forms the compound speckle. Characterization method to evaluate the compound speckle reduction efficiency is analyzed and discussed in a simplified optical system. The preliminary speckle reduction is demonstrated in a commercial projector where a 600 mW green laser has been used as the illumination source, and the compound speckle CR is brought down from 0.38 to 0.14.

Replacing Two-Dimensional Binary Phase Matrix by a Pair of One-Dimensional Dynamic Phase Matrices for Laser Speckle Reduction

In this paper, a method is proposed to create a pair of one dimensional binary phase modulator matrices 1D-BPMs, which is mathematically equivalent to a single two-dimensional binary phase modulator matrix 2D-BPM based on Sylvester constructed Hadamard matrices. An advantage of 1D-BPM over 2D-BPM is that the number of drive electrodes can be reduced from N2 to 2N for a N×N Hadamard phase matrix, thus simplifying the drive interconnects and drive electronics. 1D-BPM can be based on electro-optical material and hence can be driven electrically, which is an advantage over the 2D-BPM which is usually etched on a glass substrate and actuated mechanically to implement the speckle reduction in laser projectors. We have discussed the algorithm for constructing the pair of dynamic 1D-BPMs to replace the 2D-BPM. Preliminary experiments are performed to demonstrate the validity of our concept.

Speckle reduction using orthogonal arrays in laser projectors.

We propose using a two-level (-1 and +1 as variables) orthogonal array (OA) to generate a binary phase diffuser for speckle reduction in laser projection displays. Compared with the Hadamard matrix, the diffuser generated from OA is more flexible. The speckle contrast ratio (CR) when introducing the binary phase diffuser at an intermediate image plane within the projector is calculated, and the minimum speckle CR can be achieved by finite step change of the diffuser patterns. With Kronecker algebra, the two-dimensional diffuser can also be replaced by two one-dimensional diffusers with the same function, and it can be implemented into the laser projector electronically and easily.

Speckle contrast for superposed speckle patterns created by rotating the orientation of laser polarization

For a rough diffuser that fully depolarizes a linearly polarized laser, the speckle contrast of a fully developed speckle image can be reduced from 1 to 0.5 by polarization diversity. This reduction is achieved by rotating an x-polarized laser to the y-polarized orientation to form four independent speckle patterns with equal intensities during the detector exposure time. For the case of arbitrary rotation of the polarization, we derived a generalized speckle contrast formula for the superposed speckle patterns. This formula completes the theory of speckle contrast for the sum of correlated speckle intensities as it relates to polarization diversity.

Sinusoidal rotating grating for speckle reduction in laser projectors: feasibility study

This paper describes a novel idea for reduction of speckle contrast in laser display projectors using the rotation of a diffraction pattern whose zeroth order has been canceled out without loosing power. The feasibility of the proposed method was investigated by illuminating gratings with a sinusoidal phase on two spatial light modulators (SLMs) in series for minimal intensity modulation, where the phase grating pattern was rotated with respect to the previous one on both SLMs. Two series of measurements were done with different periods of the sinusoidal grating. For each series, an image of the speckle pattern was recorded at discrete rotation angles of the phase grating, and then an average image was calculated. Experimental results were compared with a new theoretical model for speckle contrast of N partially correlated speckle patterns. The experimental measurement results compare well with the theoretical predictions resulting in a minimum speckle contrast of 0.36, with further reduction possible. Parameters necessary to achieve target contrast (0.08 or less) are discussed.

Polymer-based multiple diffraction modulator for speckle reduction

We report a polymer based multiple diffraction modulator, in which PDMS (polydimethylsiloxane) is utilized as the actuation material, for speckle reduction. The properties of the PDMS are characterized based on its response time and deformability, which are the key properties concerned in this work. The structure dependent properties of PDMS are discussed. Using the described technique, the PDMS satisfy the system demand. The modulator is used to create real-time diffraction patterns by dynamic gratings formed by flexible PDMS. The diffracted light passes through a diffuser, which is placed after the modulator, and induces speckle patterns on the screen. Speckle-reduction is achieved by adding the time-varying speckle patterns in the integration time of the detector. It is observed that using the modulator which has two gratings, the speckle contrast ratio reaches to 50%, which shows fair agreement with the simulation.

Simulation of laser speckle reduction by using an array of diffraction gratings

An array of diffraction gratings and a Random Phase Plate (RPP) are used to suppress laser speckle effect. Dynamic diffraction spots are generated on the surface of the RPP, after which the scattering lights are perceived by a detector. Speckle Contrast Ratio (CR) and Number of Independent Speckle Patterns (NISP) with different gratings rotation orientations (θ), gratings frequencies (grooves per millimeter: f), diameters of laser beam (D), and distances between the array of diffraction gratings and the RPP (Z) are calculated based on ZEMAX simulations, and an optimized model is proposed.