Anatoly S. Lapchuk

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 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.

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.

Mode propagation in optical nanowaveguides with dielectric cores and surrounding metal layers

The mode spectrum in an optical nanowaveguide consisting of a dielectric-core layer surrounded by two identical metal layers is investigated. A simple model based on mode matching to predict the properties of mode propagation in such optical nanowaveguides is proposed. It is shown that quasi-TM00 and quasi-TM10 modes supported by an optical microstrip line do not have a cutoff frequency, regardless of the size of the metal strips, the thickness of the dielectric slab, and the cross-sectional shape. The transverse size of the TM00 mode supported by a nanosized microstrip line was found to be approximately equal to the transverse dimension of the microstrip line. In closed rectangular and elliptical nanowaveguides, i.e., in which all dielectric surfaces are covered with metal films, the cross-sectional shape of the waveguide should be stretched along one side to produce propagation conditions for the fundamental mode.

Local plasmon resonance at metal wedge

A local plasmon resonance on a metal wedge is studied by using the Meixner approach [J. Meixner, IEEE Trans. Antennas Propag.AP-20, 442 (1972)]. It is found that the singular field behavior of a local plasmon resonance as a function of the distance from the edge of the wedge is sensitive to the wavelength and wedge angle, and ranges from a dramatic increase in amplitude close to its theoretical limit to pure oscillatory behavior with only minor amplitude variation. Field singularities for gold, silver, and aluminum wedges are calculated. It is shown that, unlike an ideal-conductor wedge, the real part of the power index of the electric field singularity does not decrease monotonically as a function of the wedge angle, but has a minimum for some angle depending on the wavelength and material parameters. If the dielectric surrounding the wedge has a positive permittivity equal to the absolute value of that of the metal, and hence satisfies the plasmon resonance condition, then the electric field has a peculiar behavior for a wedge whose shape is close to the flat surface.

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.

Fine-pitch high-efficiency spatial optical modulator for mobile display applications

Diffractive spatial optical modulators (SOM) employing a fine-pitch pixel array were introduced. The micromechanical designs of the lead zirconate titanate (PZT) actuator and mirror ribbon structure were optimized for a small volume while maintaining the same level of performance. The same design rule and fabrication equipment were used for a new 10-µm-pitch and conventional 16-µm-pitch SOM. The optical efficiency of the new SOM was 78% (zeroth-order diffraction), which is an improvement over that of the 16-µm-pitch SOM (73%). The full on/off contrast ratio showed no differences, and a high displacement of 500 nm was achieved. The stress of the Pt/PZT/Pt actuating layer was the main parameter affecting the initial gap height and displacement of the ribbon. The required ribbon flatness could be achieved by applying a stress gradient on the SiN layer. The temperature-sensitive characteristics, which degrade image quality, could be minimized by a mechanical compensation method that takes advantage of the thermal expansion effect of Si substrates. The estimated lifetime of the device is >4000 h. The developed fine-pitch SOM device has sufficient response time and ribbon displacement to be suitable for high-quality embedded laser-projection displays. The VGA optical module was successfully demonstrated in a mobile laser projection display.

Near-Field Optical Microprobe Array of Waveguide Mode for the Optical Data Storage Application

To develop new high density optical data storage, we have studied near field optical microprobe array of waveguide mode, including the design optimization of microprobe structure, fabrication using micro-fabrication processing and evaluation of optical efficiency.