Light-scattering-induced retardation as a high-sensitivity image contrast revealing collagen fibers.

Retardation induced by media can be used as an image contrast to depict the cumulative birefringent features and local variations of the sample, respectively. It is commonly assumed that the retardation is induced by the light propagation; however, the light scattering would generate the retardation as well. In our work, the scattering-induced retardation as a high-sensitivity image contrast for revealing collagen fibers is presented. First, it is shown that the retardation induced by fiber scattering is equal to π when modeled as cylinders. Using the data for the chicken breast and the palm measured by the polarization-sensitive optical coherence tomography system as an example, the scattering-induced retardation is calculated. The measured value of π is in complete agreement with the theory, and the corresponding retardation per unit distance is two orders of magnitude greater than the light-propagation-induced retardation, demonstrating its predominant role on the overall retardation and providing a possibility for highly sensitive displays. Compared with the accumulated retardation image and the differential retardation image, the scattering-induced retardation images could exhibit sharper fiber structures even in deeper regions. This work might be helpful for the early diagnosis of collagen-related diseases.

The wavenumber linearisation without calibration device for spectral-domain optical coherence tomography.

The wavenumber nonlinearity leads to blurred reconstructed images in spectral-domain optical coherence tomography (SDOCT). In this work, a wavenumber-linearisation method without calibration devices is presented, based on the fact that the difference between the phases of adjacent peak and valley points is equal to π $\pi $ . The theoretical model is derived, and the efficacy of the method was proven by acquiring SDOCT data from TiO2 phantom and zebrafish. The results exhibit the superior performance of our method. Compared with the linear phase-based method, the resolution could be improved at least a factor of 2. Compared with the polynomial fitting method, the resolution could also be improved by nearly half.

Self-denoising method for OCT images with single spectrogram-based deep learning.

The presence of noise in images reconstructed with optical coherence tomography (OCT) is a key issue which limits the further improvement of the image quality. In this Letter, for the first time, to the best of our knowledge, a self-denoising method for OCT images is presented with single spectrogram-based deep learning. Different noises in different images could be customized with an extremely low computation. The deep-learning model consists of two fully connected layers, two convolution layers, and one deconvolution layer, with the input being the raw interference spectrogram and the label being its reconstructed image using the Fourier transform. The denoising image could be calculated by subtracting the noise predicted by our model from the label image. The OCT images of the TiO2 phantom, the orange, and the zebrafish obtained with our spectral-domain OCT system are used as examples to demonstrate the capability of our method. The results demonstrate its effectiveness in reducing noises such as speckle patterns and horizontal and vertical stripes. Compared with the label image, the signal-to-noise ratio could be improved by 35.0 dB, and the image contrast could be improved by a factor of two. Compared with the results denoised by the average method, the mean peak signal-to-noise ratio is 26.2 dB.

Estimation of parameters for evaluating subsurface microcracks in glass with in-line digital holographic microscopy.

In this work, a new method for imaging subsurface damage (SSD) is proposed, which is, to the best of our knowledge, the first application of the in-line digital holographic microscopy (IDHM) to the reconstruction of the subsurface damage in glass. By combination of the in-line arrangement and an objective lens to image the hologram on the CCD surface, the method is characterized by its high resolution in both the lateral and depth directions. Then the three-dimensional reconstruction of the microcracks within the glass was realized by numerically focusing en-face images at different depths, and the sizes of SSD along the transversal and depth directions were estimated. Based on the experimental results, the cracks can be divided into two categories: one is that the cracks begin from the surface of optical elements, the other is totally within the components. To indicate the propagation or development trajectory of the cracks and predict the magnitude of the laser-induced damage threshold, the relative intensity distributions of the light scattered by the cracks compared with the ones without cracks were also reconstructed. In this case all the required parameters for evaluating SSD are obtained with our IDHM system, so that the SSD produced in the manufacturing process can be reduced or removed more easily to optimize the performance of the optical component and extend its lifetime. These results provide the guidance for the optical system design of precision measurements.

A general model for resolution of digital holographic microscopy.

For digital holographic microscopic imaging, the resolution in the reconstructed image is one of the most important parameters. To optimize the lateral resolution, a general model for the resolution of digital holographic microscopy (DHM) is proposed in this work, in which the effects of the sizes of each pixel, total area of the charge coupled device (CCD) and the microscopic objective lens are taken into account. Comparison between our model and others was carried out by calculating the point spread function (PSF) of DHM at different reconstruction distances and with different fill factors. It is shown that the effect of fill factors on the resolution of DHM becomes significant when the reconstruction distance is long. For high resolution DHM imaging the influence of fill factors must be taken into account when estimating the resolution of the reconstructed image. Furthermore, It is also demonstrated that the sidelobe of PSF can be cut effectively choosing appropriate values of the fill factors. Finally, the reconstructions of polyethylene microspheres have been implemented to demonstrate the theoretical analysis. These results obtained are helpful for estimation of the resolution and design of the DHM systems.