Wenhong Gao

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.

Laser Speckle Reduction Using a Motionless Despeckle Element Based on Random Mie Scattering

A motionless despeckle element based on Mie scattering and Brownian motion of suspended particles is proposed, which is achieved by filling a light pipe with diluted milk in laser projection systems. The Brownian motion of the milk fat particles leads to the superposition of the intensity of the instantaneous scattered light within the integration time of the human eye, which causes a reduction in laser speckle contrast because of temporal averaging intensity. Experimental results show that the speckle contrast mainly depends on the concentration of the suspended fat particles, and can be reduced down to 3.94% at the fat particle concentration of approximately 7.0 ×10-4 μm-3.

Speckle characterization in free space and imaging space

There have two ways to observe speckle, free space and image space. We introduced the factors impact speckle contrast in a speckle characterization system and established an equivalent relation between image space speckle characterization and free space speckle characterization, validating the equivalent relation through experiments. The experimental results show that if the equivalent relation come into existence, the speckle contrast measured in image space match well with it in free space. At last we have discussed the speckle in a projection system with a rough surface inside and given the compound speckle measuring method.

The matrices used to reduce speckle in laser projector system

It has been suggested to use Hadamard matrices H(M) of order M for speckle reduction in laser based projection displays by creating a set of M two-dimensional phase masks from rows or columns of the H(M) and introducing them sequentially into the intermediate image plane of the laser projector. The speckle contrast reduction can reach M-1/2. In this paper, we have analyzed the contrast reduction. The result presents that any matrices can be used to form phase mask as long as its columns are orthogonal to each other, such as the parts of columns of Hadamard matrix. The speckle contrast reduction is determined by the number of projection resolution elements lying in single camera resolution element. To get high quality image with low speckle contrast reduction by Hadamard matrix, its order should be as high as possible. However, it is impossible to implement by vibrating diffuser with high order due to the large vibration amplitude. The motionless time-vary diffuser with Hadamard matrix phase pattern based on MEMS technology and Electro-optical effect can be a good choice.

Influence of a Light Pipe on a Linearly Polarized Light Beam and Its Effect on a Laser Projector System

In a projector, the polarization property of the light source through the different optical components of the projector is necessary to be considered when the light source changes to polarized laser light. A light pipe is just one of the typical optical components and widely used in projection system to homogenize the light field. In this paper, a simulation model is presented to investigate the polarization state after a rectangular light pipe structure. The results of the simulation model are experimentally verified using a polarimeter. The key parameters influencing the polarization state of the linearly polarized laser beam after the light pipe are discussed .

Speckle characteristics of a broad-area laser diode

In this paper, far field speckle contrast from a broad area laser diode is measured. The intensity of the incident laser light onto a diffuser is controlled by using polarizer or adjusting input current. A rectangular aperture close to diffuser is used to select different parts of laser light field as illumination spot. The speckle contrast measured for different illuminationspots has no significant change. When the laser diode operates at its typical power output condition, the speckle contrast measured approximately equals 1, while the speckle contrast is depressed tremendously when the laser diode operates with low driving current.

Illumination Angle Three-Dimensional Model for Speckle Noise Suppression in Laser Projector

Angle diversity is widely used for speckle noise reduction in laser projector system. In this paper, the limitation of the existing model for illumination angles producing uncorrelated speckle patterns is discussed. A three-dimensional model of the variation illumination angles in spherical coordinate system is set up. A new parameter ΔΦ is brought in and the formula of the magnitude of the transverse component of the scattering vector difference is derived. Based on the new derivation, the minimal angle difference is investigated theoretically and numerical calculation in spherical coordinate system.

De-coherence characteristic of laser light caused by Mie scattering

An optical system model has been built up for evaluating de-coherence performance of Mie scattering by using ZEMAX software. The optical system consists of a linearly polarized laser source of the wavelength 0.6328 micrometer, the interferometer configuration with a double-arm beam path, the light pipe with the particles solution of variable parameters including the refractive index, the particle size and the particle concentration, and the detectors. Seven types of particles with different refractive index have been used as scattering medium. The de-coherence performance and the light energy loss have been calculated for solutions with different particle concentration and dimension. The calculated results indicate that the de-coherence performance can be improved by increasing the particle concentration in solution and the particle size. The improvement of the performance is more notable as the particle refractive index becomes higher. The dependence of the light energy loss caused by Mie scattering on the refractive index and size of the particle, and the particle concentration in solution is obtained.

Effect of the projection lens on speckle contrast measurement in laser projection displays

Laser speckle degrades image quality in laser displays. The speckle contrast needs to be correctly evaluated in designing laser displays. In this paper, a moving small diffuser for speckle reduction is set up for laser projection display system. We demonstrate the effect of the projection lens on speckle contrast under the scenarios of free-space and image geometry measurement with different configurations. The results show the speckle contrast in free-space geometry is measured as about 74.16% without projection lens and 60.74% with projection lens for the static diffuser, which are reduced to 2.75% when the diffuser moves for both with and without projection lens. In the imaging geometry, the moving diffuser reduces the speckle contrast from 83.22% to 9.61% without projection lens, and from 98.7% to 36.59% with projection lens.