We analyzed the light spectrum after passing through the encoding template element. Comparing the influence of the ideal coding template elements and coding template elements with rounded errors on the far-field spectrum. The far-field spectrum has a red offset. With the changing of the aperture error value r, the deviation of the far-field spectrum relative to the light source spectrum is slightly different, and the closer the aperture is to the circle, the smaller the deviation of the spectrum, that is, the closer the light source spectrum. Five different constituent units from the two-dimensional random coding template is found, and these five constituent units are randomly distributed. The far-field spectral distribution when the beam passes through two units at the same time is numerically calculated, and the analysis shows that the spectral deviation after passing through different types of coding units are obviously different. The analysis result will provide prior information for the target recovery of compressed-sampling hyperspectral imaging, and make the target recovery more accurate.
As a fundamental components for the integrated microwave photonic links, the nonlinear loss of the Silicon waveguides will significantly affect the links’ performance. We study the effects of the effective mode area (Aeff), two-photon absorption (TPA), free-carriers absorption (FCA), and linear propagation loss on the insertion loss in silicon waveguides. According to our simulations, the results show that it should be an efficient way to reduce the insertion loss of the silicon by combining the large mode area, small linear propagation loss and carriers sweep out techniques. And, we can also know that when the nonlinear loss will dominate with the coupled power becoming larger. Thus, the results can provide guidances on the gain, loss, and linear dynamic range of analog photonics links based on silicon waveguide.
By using the finite difference method, the nonlinear effects of high repetition rate femtosecond pulse trains in silicon nanowire waveguides were analyzed. By numerically modeling the propagation of femtosecond pulse in silicon nanowire waveguides with the generalized nonlinear Schrödinger equation, the temporal and spectral properties of femtosecond pulses propagating are discussed, and the physical mechanisms of pulse evolution are demonstrated. The simulation results indicate that, owing to the remarkable nonlinear effect, the carriers in the silicon waveguide are rapidly excited with the increase of the input laser power, subsequently have significant modulation effects on the high repetition rate pulses train. For various repetition rate and power of the laser pulses train, the output pulsestrain will get different temporal compression and broadening under the dominance of different nonlinear effects. Furthermore, the results demonstrate that the spectrum evolution of pulse train with different transmission distances is similar to the calculated results of single pulse transmission. The results will provide an important reference for the design of optical sampling clock, microwave photonic radar system and the sweep of carriers in waveguide.
The encoding aperture errors with different types and different degrees occurred during the process of encoding aperture by micro-Nano technology. The encoding aperture is a key component of the CSSI, and the analysis of errors in encoding aperture processing provides an important evidence for the CSSI. In this paper, based on the error occurring in the process of encoding aperture, the simulation is established by commercial software FDTD by which the optical field modulation of incident light in the CSSI system is analyzed by comparing the ideal encoding aperture and the error encoding aperture. The simulation results show that there is a significant difference in the optical intensity distribution of incident light modulated by a single error aperture and a single ideal aperture, the optical intensity distribution modulated by the ideal aperture has two distinct peaks at the aperture surface, and the optical intensity distribution modulated by the error aperture is approximately twice as large as by the ideal aperture; the optical intensity distribution modulated by the two type aperture has obvious peaks while leaving the aperture surface, and the optical intensity distribution modulated by the ideal aperture is approximately twice as large as by the error aperture; changing the number of pixels of the encoding aperture, the ideal encoding aperture and the error encoding aperture have little difference in modulation of the incident light; comparing the ideal aperture, as the increasing of the rounded radius of error aperture, the influence of the optical field distribution modulation becomes more obvious.
By iteratively stitching together the series of low-resolution (LR) images captured by either various small-aperture illuminations or angle-varied illuminations, the Fourier ptychography (FP) can recover large space-bandwidth- product (SBP) and high-resolution (HR) object images. The FP has been considered to be promising in various computational imaging fields. However, the illumination-based FP is limited by strict requirements of the objects which must be thin and satisfy the one-to-one mapping relationship in the Fourier plane, and the aperture-scanning Fourier ptychography is also limited by the long-time scanning and stable scanning mechanical structures requirements even though it can achieve super-resolution macroscopic imaging. Furthermore, the position and shape of the scanning aperture must be accurately modeled for the reconstruction, otherwise false object images may be output. Herein, based on the 4-f optical correlator structure, we proposed a novel method, termed variable-aperture Fourier ptychography, for reconstructing HR images from series of LR images. The numerical simulations illustrated that the variable-aperture Fourier ptychography can use a small number of LR images to reconstruct the object images, The experiments demonstrated that a high-quality object image with better resolution and contrast than other schemes, include direct imaging based on 4-f system and aperture scanning FP, can be obtained by our method. Two additional experiments proved that it is almost unaffected by the position and shape of the apertures.
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