Victor Yurlov

Speckle suppression in scanning laser display

The theory of speckle noise in a scanning beam is presented. The general formulas for the calculation of speckle contrast, which apply to any scanning display, are obtained. It is shown that the main requirement for successful speckle suppression in a scanning display is a narrow autocorrelation peak and low sidelobe level in the autocorrelation function of the complex amplitude distribution across a scanning light beam. The simple formulas for speckle contrast for a beam with a narrow autocorrelation function peak were obtained. It was shown that application of a diffractive optical element (DOE) with a Barker code phase shape could use only natural display scanning motion for speckle suppression. DOE with a Barker code phase shape has a small size and may be deposited on the light modulator inside the depth of the focus of the reflected beam area, and therefore, it does not need an additional image plane and complicated relay optics.

Speckle suppression in laser display using several partially coherent beams.

An optical scheme for speckle suppression using two or three partially coherent beams in a projection system is proposed. Diffractive optical elements (DOE) placed in the intermediate image plane create several beams carrying the image to a screen. Transparent plates of different thicknesses are placed in the Fourier plane of the projective lens and used for beam decorrelation. The coherence matrix algorithm for speckle suppression is used to calculate the speckle contrast ratio. It is shown that for a small decorrelation length and using the same maximum thickness of the transparent plates, two partially coherent beams would provide better suppression than three beams with different diffraction orders. However, for a large decorrelation length, the three beam setup provides better speckle suppression for all three colors examined with a suppression coefficient close to theoretical limits. Verification of speckle suppression using three-beam decorrelation is reported.

Speckle suppression in scanning laser displays: aberration and defocusing of the projection system.

Aberration and defocusing effects in the mechanism of speckle suppression in laser projection displays were studied using the Fresnel approximation and the thin lens model. The analysis was performed with the assumption that aberrations change only the phase (and not the amplitude) in the rear principal plane of the display projection system. The analysis showed that aberrations should not have any influence on speckle contrast. It also showed that a screen shift relative to the image plane (defocusing) results only in a rescaling of the scanning beam autocorrelation function, which is equivalent to refocusing the objective lens to a new position of the screen. The optimal beam shape for optimal speckle suppression was also studied. A homogeneous field intensity distribution in the spatial frequency domain was found to provide close to the best speckle suppression.

Spatial optical modulator (SOM): high-density diffractive laser projection display

A new type of diffractive spatial optical modulators, named SOM, has been developed by Samsung Electro-Mechanics for laser projection display. It exhibit inherent advantages of fast response time and high-performance light modulation, suitable for high quality embedded laser projection displays. The calculated efficiency and contrast ratio are 75 % and 800:1 respectively in case of 0th order, 67 % and 1000:1 respectively in case of ±1st order. The response time is as fast as 0.7 &mgr;s. Also we get the displacement of 400 nm enough to display full color with single panel in VGA format, as being 10 V driven. Optical module with VGA was successfully demonstrated for its potential applications in mobile laser projection display such as cellular phone, digital still camera and note PC product. Electrical power consumption is less than 2 W, volume is less than 13 cc. Brightness is enough to watch TV and movie in the open air, being variable up to 6 lm. Even if its optimal diagonal image size is 10 inch, image quality does not deteriorate in the range of 5 to 50 inch because of the merit of focus-free. Due to 100 % fill factor, the image is seamless so as to be unpleasant to see the every pixels partition. High speed of response time can make full color display with 24-bit gray scale and cause no scan line artifact, better than any other devices.

A novel diffractive micro-optical modulator for mobile display applications

A diffractive optical modulator has been fabricated based on a micromachining process. Novel properties of its fast response time and dynamics were fully understood and demonstrated for the strong potentials in embedded mobile laser display. Bridged thin film piezo-actuators with so called open mirror diffraction structure has been designed. Optical level package also was achieved to successfully prove its display application qualities. Display circuits and driving logic were developed to finally confirm the single-panel laser display at a 240Hz VGA (640×480). With its efficiency of more than 75% and 13cc volume optical engine with the MEMS-based VGA resolution SOM showed 7 lm brightness at a 1.5W electrical power consumption. Detailed design principle, fabrication, packaging and performances of the invented SOM are described.

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%.

Impact of speed, direction, and accuracy of diffractive optical element shift on efficiency of speckle suppression

The impact of the precision and direction of motion of a diffractive optical element (DOE) placed inside an optical system on the efficiency of speckle suppression is analyzed. A simple model for calculating the speckle suppression efficiency in systems having a moving DOE and a random diffuser is developed. Simulation results show that at the optimal inclination angles of a regular 2D DOE and at sufficiently rapid displacement (at least N DOE periods), the speckle contrast exhibits a broad minimum. At angles that differ significantly from the optimal values, the speckle contrast varies rapidly and has peaks that exceed the minimum value by several tens of percent. For certain DOE inclination angles, the speckle contrast is several times higher than the minimum value.

Impact of aberrations on speckle suppression efficiency on moving a DOE inside the optical system

Abstract The impact of aberration on the speckle suppression efficiency is investigated in a laser projector system containing a moving diffractive optical element (DOE). The results of a qualitative analysis based on the number of diffraction orders passed through the optical system are presented, along with a quantitative analysis built upon the Fresnel approximation and the thin lens model. It is shown that the speckle contrast in the paraxial area of the screen is practically insensitive to aberrations — limited to a few percent at most, due to the change in angle between diffraction orders. However, the speckle contrast in peripheral areas changes stepwise if aberrations change the number of diffraction orders that illuminate the area.

A study of image contrast restriction in displays using diffractive spatial light modulators

The peculiarities of 1D projection displays using spatial light modulators (SLM1) are considered. The conditions for high contrast images in SLM are investigated. It is shown that an amplitude imbalance between the light reflected from the passive and active parts of SLM gratings imposes a limitation on the achievable contrast ratio. This imbalance may come from errors in SLM structure sizes and from diffraction effects inside SLM layers. The key point is the relationship between ridge and hole sizes in SLM diffraction structures. It is shown that odd diffraction orders (especially 1st order) have a significantly decreased sensitivity to ridge/hole tolerances. This phenomenon affects the SLM’s contrast ratio. The optimal ridge/hole ratio for different SLM diffraction orders has been found. To test the theory, experimental samples were manufactured and contrast ratio measurements were carried out. The experimental data are in good agreement with the theoretical results.

Numerical simulation of characteristics of near-field microstrip probe having pyramidal shape

A pyramid-type microstrip probe (PTMP) with metal tips is proposed for scanning near-field microscopes to obtain high spatial resolution of a few nanometers and high optical efficiency. Properties of an ordinary PTMP and the PTMP with a single metal tip are investigated by using a rigorous finite-integral technique simulation (MICROWAVE STUDIO package) and analyzing characteristics of working modes of the probe. Numerical simulation has demonstrated that an ordinary PTMP and the PTMT with a single metal tip exhibit large far- and near-transmission coefficients, field enhancement, and high spatial resolution. These high parameters imply that both types of microstrip probe may be utilized for optical and magnetic data storage, nanolithography, and other types of nanotechnology that use light for modification of a thin surface layer.