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.
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.
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.
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 decreased down to 13% has been achieved. In this article, Barker-like
binary phase codes of length longer than 13 are used at an intermediate image plane. It is shown by theoretical
calculation that much better speckle reduction with speckle contrast factor up to 6% can be achieved by using
longer binary phase codes other than the Barker code. Preliminary experimental results are also presented
indictaing good speckle reduction.
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.
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.
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.