RESUMO
The quasi-spiral 2D diffractive optical element (DOE) based on M-sequence of length N=15 is designed and manufactured. The speckle suppression efficiency by the DOE rotation is measured. The speckle suppression coefficients of 10.5, 6, and 4 are obtained for green, violet, and red laser beams, respectively. The results of numerical simulation and experimental data show that the quasi-spiral binary DOE structure can be as effective in speckle reduction as a periodic 2D DOE structure. The numerical simulation and experimental results show that the speckle suppression efficiency of the 2D DOE structure decreases approximately twice at the boundaries of the visible range. It is shown that a replacement of this structure with the bilateral 1D DOE allows obtaining the maximum speckle suppression efficiency in the entire visible range of light.
RESUMO
This paper reports the findings from an experimental evaluation of speckle suppression efficiency using a method based on a moving 2D Barker code diffractive optical element (DOE). The optical setup and the optical scheme parameters of the method are presented. A speckle contrast of ~4.4-5.3% and speckle suppression coefficient (coefficient of speckle contrast reduction) of k>8 was obtained in experiments. However, the experimentally obtained speckle suppression coefficient was approximately 1.5 times smaller than the theoretical prediction. It is speculated that the discrepancy between the theoretical and the experimental data is due to an inexact match between the optical setup and the optimal optical parameters of the method. Analysis of the experimental data revealed that once the optical scheme is optimized, it will be possible to obtain a speckle suppression that is closer to the theoretical prediction.
RESUMO
A method for speckle suppression based on Barker code and M-sequence code diffractive optical elements (DOEs) is analyzed. An analytical formula for the dependence of speckle contrast on the wavelength of the laser illumination is derived. It is shown that speckle contrast has a wide maximum around the optimal wavelength that makes it possible to obtain large speckle suppression by using only one DOE for red, green, and blue laser illumination. Optical schemes for implementing this method are analyzed. It is shown that the method can use a simple liquid-crystal panel for phase rotation instead of a moving DOE; however, this approach requires a high frequency of liquid-crystal switching. A simple optical scheme is proposed using a 1D Barker code DOE and a simple 1D liquid-crystal panel, which does not require a high frequency of liquid-crystal switching or high-accuracy DOE movement.
RESUMO
A new type of probe for the near-field optical microscope incorporating metallic strips on the surface of a dielectric cone (microstrip probe) is proposed. The numerical simulation has shown a significant improvement in optical efficiency for this type of probe compared with the conventional probe. It was found theoretically that scanning near-field optics with the microstrip probe is promising for applications in optical information recording and can be employed as optical heating elements in magnetic information recording. Application of the microstrip probe in nonlinear optical research of nanoscaled medium is possible due to strong electric and magnetic field near the aperture and weak dependence of its transmission parameters on the frequency of the incident beam. A model for the near-field strip probe with cleaved apex is proposed. A method is presented to control the distance between the probe apex and sample surface based on mechanical vibrations of the cleaved apex excited by applying voltage pulses. The oscillation amplitude and attenuation can be determined by measuring the amplitude of microwave radiation excited by oscillations of charges with opposite sign at the probe apex. The investigation was based on mathematical models and experiments necessary to confirm the theoretical prediction.
