Holograms captured by digital holographic microscopic systems that use high-coherence lasers as light sources are heavily influenced because of coherent noise. The contrast of interferograms will be affected although coherent noise can be suppressed by using short coherence light sources. This paper develops a reflective common-path digital holographic microscopy (DHM) combined with partially coherent illumination. The light is scattered by a rotating diffuser and coupled into a multimode fiber, which form into an extended light source. After passing through an imaging system, the designed blazed grating generated by spatial light modulator (SLM) works as a polarization point diffraction plate at the Fourier plane, which has a circular area playing the role of a reflective pinhole at the position corresponded to the image of the multimode fiber. Being benefit from the polarization-selective modulation property of the particular blazed grating, polarization directions of diffracted component and non-diffracted component are modulated to be orthogonal. Because of the common-path configuration, the length of the object and reference path is equal and high contrast polarization phase-shifted interferograms are captured by polarization camera. Quantitative phase images are further obtained. A resolution target and a standard step plate are measured with the proposed system. The result of the resolution target shows that the lateral resolution is 715nm at least, which is higher than the reference method and close to the diffraction limit. Otherwise, the measured height of the standard step plate is 92nm and has only 4.76% deviation from the actual height, which represents high vertical resolution.
KEYWORDS: Phase shifts, Holograms, Holography, Background noise, Digital holography, Deep learning, Phase unwrapping, Optical simulations, Network architectures, Signal to noise ratio
In-line holography has been widely used in various fields because of its advantages such as simple optical path, low requirement of light source coherence and low utilization of the camera spatial bandwidth product, but it is difficult for in-line holography to restore object information from a single in-line hologram. The traditional phase-shifting algorithm requires at least three phase-shifting holograms, moreover the error caused by the intermediate phase-shifting may be accumulated and amplified. In recent years, deep learning has been widely used in the optical field due to its advantages in data analysis. Deep learning has provided new solutions for in-line holographic reconstruction, which can solve the problems that are difficult to avoid in traditional methods. In this paper, a method for generating phase-shifting holograms based on a modified Y-4net is proposed to reduce the experimental workload in data collection and the noise in phase-shifting image, which is referred to as Ps-4net. The proposed Ps-4net can generate four virtual phase-shifting fringe patterns from a single frame hologram and calculate the phase from virtual phase-shifting holograms. Simulation and experimental results show that the Ps-4net can effectively reduce the workload of data collection and the phase-shifting hologram is generated and the noise is removed at the same time.
The orbital angular momentum (OAM) owning to the special superiority with the inherent orthogonality, has been identified as an information carrier. In this paper, we proposed a simple and direct method for measurement the OAM of the vortex beam by using vortex grating encrypted OAM holography. When the detected beam illuminates the designed OAM holography, the topological charge (TC)value will be directly read from the reconstruct results which appear as a series of solid Gaussian or ring spots at the output surface of the system. Thus, according to the reconstruct results, we can not only easily detect the modulus but also the sign of the incident beam. Furthermore, it can simultaneously identify OAM modes of multiple incident vortex beams. It's demonstrated that the proposed method is an efficient and flexible method.
Quantitative phase information which can reflect the internal structure and refractive index distribution of the object is able to be obtained by diffractive and interferometry techniques. However, the phase resolution achieved by the diffraction method is lower than that of interferometry method; while the setup for interferometry method is more complex. To obtain high-resolution phase images without reference beam path, we propose an end-to-end DL based super resolved quantitative phase imaging method (AF-SRQPI) based on generative adversarial network (GAN) to transform low-resolution amplitude images into super-resolved phase images. Meanwhile, considering the inevitable out-focusing during the long hours of observing, autofocusing function is also included by the network. In the training process, out-of-focus low-resolution amplitude images are used as the inputs and corresponding super-resolved phase images obtained by structured illumination digital holographic microscopy (SI-DHM) are used as the ground truth labels. The well-trained network can reconstruct the high-resolution phase image at high speed (20fps) from a single-shot out-of-focus amplitude image. Comparing with other DL-based reconstruction schemes, the proposed method can perform autofocusing and superresolution phase imaging simultaneously. The simulation results verify that the high-resolution quantitative phase images of different biological samples can be reconstructed by using AF-SRQPI .
Perfect optical vortices (POVs), consists of a single bright ring structure, has been widely studied owing to its radius independent of orbital angular momentum (OAM). However, most of the existing works about POVs are limited to single ring structure. Flexible shaping of intensity distribution of POVs is vital for multiple applications. In this paper, we propose a method generate phase tunable multi-ring perfect optical vortices (MR-POVs) where each ring size is independent of its OAM. The scheme is based on the radical discontinuous spiral phase plate (RD-SPP) which introduces controllable phase jumps along radial direction. It is experimentally demonstrated that the vortex nature of the MR-POVs through an interferometric method, showing that each ring of MR-POVs possesses same topological charge value (magnitude and sign), and the intensity ratio between each ring can be freely regulated by adjusting phase distribution, which could offer more flexible optical gradient force for guiding and transporting particles. In addition, simulation and experimental results show that the integer and fractional MR-POV can generated by the independent regulation of angular and radial factors. This work expands our understanding of multi-ring POV and may provide a new idea for optical tweezers and OAM communications.
The measurement of large aperture optical components becomes much more critical as they are increasingly being used in high power systems and astronomical systems. Large aperture transmissive optical elements always suffer from stressinduced birefringence, which leads to the difference of profiles for linearly polarized beams with different orientations. A spatially varying optical path difference is introduced into the measurement result as wavefront aberration in a dynamic interferometer based on the polarization phase-shifting method. This paper proposed a method to measure and correct the birefringence effect for a 600mm aperture dynamic interferometer based on the rotation of the incident polarized beam, needing no more additional elements. The interferometer includes a Kepler beam expander system consisting of two collimators with 100mm and 600mm aperture. The 600mm aperture collimator is viewed as a transmitting element between the interference cavity of a 100mm TF and a 600mm RF. The polarization state of the incident beam can be switched between P light and S light, where a phase difference can be obtained between two measurements. The difference between the two distributions can be viewed as the birefringence effect on the dynamic interferometer. Besides, the system error can be removed during the data subtraction. Experiments are conducted on a 600mm dynamic interferometer and the correction result is compared with a wavelength tuning method which is free of polarization errors brought by the stress-induced birefringence stress. A comparison is also conducted with the correction result realized via a wave plate model proposed before.
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