Lensless imaging is a promising technique with a large field of view and high throughput imaging capacity. Limited by its imaging modulations, it suffers from twin image problems due to missing phases. Hence, it requires phase retrieval to recover the phase information as well as suppress the artifact. As the original algorithm, the Gerchberg-Saxton algorithm (GS) retrieves the phase data from at least two intensity measurements. It has low convergence and is sensitive to the initial guess. In this work, we apply denoising in the object field with a guided filter into the iterative framework of the GS algorithm named Guided filtering GS (GFGS). The approach in this paper requires only a single hologram measurement. To suppress artifact noise, guided filtering is performed on the real and imaginary parts of the complex-valued distribution with an appropriate smoothing parameter at the object field.
Phase matching of backward second harmonic generation (SHG) in a periodic structure of collagen fibrils is investigated through theoretical modeling, simulation, and experiments. The lattice structure of collagen fibrils is considered to provide a virtual momentum for assisting the phase matching of backward SHG. Phase matching over a relatively wide excitation wavelength range is achieved by tilting the angle of the fundamental excitation and SHG wave vectors. The SHG intensity in the periodic structure is simulated to quantify the phase matching effect. The effect of the fundamental excitation and the SHG emission angles on the peak excitation wavelength of the SHG excitation spectrum is further validated in experiments, where the excitation and emission angles are controlled by spatial filtering. It is found that an optimized excitation wavelength exists for a certain collagen fibril structure, which shifts toward a shorter wavelength when the excitation and emission angles are increased. Our results show that the lattice structure of collagen fibrils can assist the phase matching, providing a mechanism for generating backward SHG in multiphoton microscopy.
A confocal multiphoton microscopy system with various detection pinholes was used to differentiate backward scattered second harmonic generation (BS-SHG) from backward generated SHG (BG-SHG) based on the fact that BS-SHG is more scattered and therefore has a much bigger spot size than BG-SHG. BS-SHG is quantified from two types of mouse tissues, such as Achilles tendon, and skin, and at various focal depths. It is found that the BS-SHG contributes less to the total backward SHG for the skin than Achilles tendon with thicknesses of around three hundred micrometers. For tissue with larger F/B intensity ratio such as Achilles tendon, increasing the tissue thickness reduces it tremendously. However, for tissue with smaller F/B intensity ratio, tissue thickness increment does not alter it significantly. In addition, larger F/B intensity ratio might be related with a greater scattering coefficient from our Achilles tendon and skin comparison. When the focal point is moved deeper into tissue, the contribution of BS-SHG is found to decrease due to a reduced pass length of the forward propagated photons. On the contrary, when the tissue thickness increases, the contribution of the BS-SHG is increased. These observations for thicker skin tissues are related with our F/B intensity ratio measurement for thin mouse skin sample in terms of that the magnitude of backward generated SHG are dominant among the total backward SHG in mouse skin tissue. Considering the phase mismatching condition in the forward and backward directions, these results may indicate that quasi-phase matching originating from the regular structure of collagen could help with reducing the phase mismatch especially in the backward direction.
The backward second harmonic generation (SHG) in mouse tissues is studied with a confocal multiphoton microscopy system. The total backward collected SHG (B-SHG) consists of the backward generated SHG and the backward-scattered forward-generated SHG (BS-SHG), which can be modeled by a Gaussian and a uniform distribution, respectively, at the confocal pinhole plane. By varying the pinhole size with a series of collection fibers, the proportion of the BS-SHG to the B-SHG and the proportion of BS-SHG to the forward generated SHG can be obtained. The approach is first validated by Monte Carlo simulation. It is then applied to two types of mouse tissues: mouse tail tendon and Achilles tendon. It is found that the BS-SHG contributes less to the B-SHG for the tail tendon than Achilles tendon with thicknesses of ∼300 μm . With the thickness of the Achilles tendon tissue increased to 1000 μm but the focal plane kept at the same depth, as high as ∼10% of the total forward SHG is backscattered and collected. The results indicate that BS-SHG may not be the major source of B-SHG in the tail tendon, but it may be the major source in the Achilles tendon. These methods and results provide a noninvasive method and supporting information for investigating the generation mechanism of SHG and help with optimizing backward SHG microscopy and spectroscopy measurements.
Although second harmonic generation (SHG) imaging has emerged as a powerful tool for imaging biological tissues with submicron resolution, the excitation wavelength dependence of SHG intensity in biological tissues is an optical property that is not fully understood so far. We first calibrate system factors which may potentially affect the accuracy of the wavelength-dependent SHG measurement. Then our calibration is validated by measuring the wavelength dependence of SHG signal from a BaB 2 O 4 crystal under different focusing conditions and comparing with the theoretical calculations. The good agreement between the experimental results and theoretical calculations demonstrates that we have established a reliable method to validate wavelength-dependent SHG measurement over a broad wavelength range. We also investigate the wavelength dependence of a 10-μm thick mouse tendon tissue in both forward and backward directions. It is found that SHG of mouse tendon tissue decreases monotonically for excitation from 750 to 950 nm.
The spectral dependence of SHG intensity in biological tissues is an optical property that is not fully understood so far.
In this paper, we will investigate this problem in detail through experiments. Through examining different biology
tissues, it is found that SHG intensity drops down from shorter to longer wavelength from 375 nm to 460 nm. By
comparing these curves with 1/λn dependence, the n is found to vary from 4.5 to 8.5. These patterns can not be fully
explained by scattering properties of collagen. Other factors such as direct generation of SHG may have contribution to
these wavelength dependence patterns, which needs further investigation.
KEYWORDS: Signal processing, Eye, Polarization, Dispersion, Signal to noise ratio, Four wave mixing, Switches, Data conversion, Binary data, Nonlinear optics
We have experimentally demonstrated simultaneous 10-Gb/s optical time-slot swapping between return-to-zero (RZ)
signal format and non-return-to-zero (NRZ) signal format based on the parametric wavelength exchange (PWE) in the
highly-nonlinear dispersion shifted fiber (HNL-DSF). Original RZ and NRZ signals located at two different wavelengths
are exchanged after the PWE process. After exchanging between RZ and NRZ signals, the swapping ability involving
two different signal formats within particular time slot has been proved. Clear open eye diagrams of periodic mixed RZ
and NRZ signals are recorded on both wavelengths.
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