The resolution optimization and sampling strategies in practical digital in-line holography (DIH) with spherical wave illumination are analytically studied by optimizing system parameters. Optimal parameters of holographic recording, including illumination wavelength, numerical aperture of the spherical illumination wave, source-sensor distance, object–sensor distance, and sampling parameters of the sensor, for achievable resolution are worked out. A formula is derived to guide the DIH system design in general cases. Different sampling strategies associated with corresponding reconstruction waves (plane wave and spherical waves with various curvatures) are analyzed. The reason why recording with spherical wave and reconstructing with plane wave works well in practice is explained in detail. We also present how to determine the reconstruction distance and magnification to reconcile the curvature difference. The analysis is carried out mathematically and verified by simulated holograms. Optical experiments with U.S. Air Force resolution target are carried out based on the analysis for further verification.
Metasurface is used to manipulate the optical field recently. In holography, the complex amplitude computer generated hologram can improve the quality of the reconstructed image. However, the current devices limit the application of complex amplitude modulation. Several works have been done for complex amplitude modulation by metasurface. In this work, a novel metasurface structure has been proposed to realize complex amplitude modulation. This kind of metasurface can modulate arbitrary complex amplitude. Furthermore, it has a thinner thickness, making it easier to fabricate.
The photonic crystals possess complex dispersion relation and the unusual dispersion leads to negative refraction. Based on negative refraction, three main physical phenomena, negative Goos–Hänchen displacement, inverse Doppler effect and abnormal Cherenkov radiation have been proposed. In this work, two abnormal physical phenomena are discussed. Firstly, the negative Goos–Hänchen shift displacement is simulated by using common FDTD method. The negative displacement is measured experimentally at the wavelength of 10.6 μm. Secondly, a novel phenomenon, the dual Doppler effect, in the simultaneous occurrence of both normal and inverse Doppler effect in one moving two-dimensional wedge-type photonic crystal with negative index is investigated by using spatial Fourier Transformation. This phenomenon is also simulated by an improved FDTD method. The results have potential applications in precision measurement, cloaking, sensor, radar deception and so on.
The inverse Doppler effect in photonic crystal with negative refractive index had been proofed experimentally in our previous research. In this paper, we studied the spatial harmonics of Bloch wave propagating in such PhCs by FFT method. The lagging and front phase evolutions reveal that both backward wave and forward wave exist in these harmonics. Subsequently, we studied the double Doppler effect phenomenon that both the normal and inverse Doppler exist in one photonic crystal simultaneously by using the improved dynamic FDTD method which we made it suitable for dealing with moving objects. The simulative Doppler frequency shifts were consistent with the theoretical values. Our study provides a potential technology in measurement area.
The analysis of the temperature property of sensors based on symmetrical metal-cladding optical waveguide (SMCOW) is focused on analyzing the temperature property of reflectivity of SMCOW sensors, which is theoretically studied with single-factor investigation under spectral and angular interrogation scheme. There are mainly four factors influencing the temperature dependence of reflectivity, it is the temperature dependence of refractive index and thickness of guiding layer, along with the temperature dependence of the metal film thickness and metal-dielectric function. The simulation result shows that the effect of temperature on the reflectivity of SMCOW is mainly attributed to the temperature dependence of refractive index and thickness of guiding layeron the contrary, the temperature properties of metal film hardly contributes to the influence of temperature on the reflectivity. Based on the analysis, the sensitivities of SMCOW with guiding layer of different optical glasses are computed under both spectral and angular interrogation. This paper is supposed to provide direction in designing temperature-sensitive SMCOW structure sensors.