Wavefront distortions or phase aberrations usually degrade the performance of imaging systems, thus limit the imaging resolution and image quality. Adaptive optics (AO) is proposed and developed in order to remove the effect of optical aberrations. Traditional AO systems use wavefront sensors (such as the Shaker-Hartmann sensor) to measure wavefront errors. Development of wavefront-sensorless adaptive optics in recent years eliminates the need for wavefront sensors, which simplifies system structure, reduces the cost and overcomes some limitations of traditional adaptive optics. In this paper, we focus on the implementation of Image-based wavefront-sensorless adaptive optics in Fresnel Incoherent Correlation Digital Holography (FINCH) to improve the imaging quality. Zernike polynomials are introduced to describe aberrations, sharpness and intensity are employed to evaluate the imaging quality dynamically, and the measurement and compensation are implemented combining with curve fitting algorithm. We implemented this method in FINCH system, which can achieve incoherent holograms by using the correlation between the object information and the image of a pinhole. A phase mask is mounted on a SLM to split the beam and shift phase to suppress the twin images in the reconstruction. Both the aberration and the phase mask were introduced by SLM. The image-based wavefront-sensorless adaptive optics is investigated experimentally in a FINCH imaging system.
We developed compressive self-interference digital holographic approach that allows retrieving three-dimensional information of the spatially incoherent objects from single-shot captured hologram. The Fresnel incoherent correlation holography is combined with parallel phase-shifting technique to instantaneously obtain spatial-multiplexed phase-shifting holograms. The recording scheme is regarded as compressive forward sensing model, thus the compressive-sensing-based reconstruction algorithm is implemented to reconstruct the original object from the under sampled demultiplexed sub-holograms. The concept was verified by simulations and experiments with simulating use of the polarizer array. The proposed technique has great potential to be applied in 3D tracking of spatially incoherent samples.
Self-interference digital holography enables holographic recording of object illuminated with spatially incoherent light. While Fresnel incoherent correlation holography (FINCH) has great potential in super-resolution microscopic imaging, structured illumination can be implemented simultaneously to further improve the imaging resolution. In this paper, the imaging characteristics of FINCH with structured illumination are investigated in detail. The basic principle of FINCH with structured illumination is discussed. Effects of characteristics of structured light pattern, such as the period, orientation and modulation depth on lateral-resolution are investigated. The potential of structured illuminated FINCH in three-dimensional super-resolution imaging was demonstrated in the paper.
Spatial light modulator (SLM) has various of applications in the field of imaging, beam shaping, adaptive optics and so on. While SLM is used as an aberration correction element in super-resolution microscopy, the surface flatness of SLM could affect the imaging performance of the system due to the higher sensitivity to aberrations of these kind microscopic techniques. In this paper, the optical surface flatness of SLM is measured experimentally by employing the image plane digital holography. The topography of SLM is retrieved from the captured hologram. Aiming to the application of SLM as an adaptive correction element in super resolution microscopy, the aberrations introduced by the surface flatness of SLM are further evaluated and corrected in the same optical system.
The details of design and implementation of incoherent digital holographic experiments based on LabVIEW are demonstrated in this work in order to offer a teaching modal by making full use of LabVIEW as an educational tool. Digital incoherent holography enables holograms to be recorded from incoherent light with just a digital camera and spatial light modulator ,and three-dimensional properties of the specimen are revealed after the hologram is reconstructed in the computer. The experiment of phase shifting incoherent digital holography is designed and implemented based on the principle of Fresnel incoherent correlation holography. An automatic control application is developed based on LabVIEW, which combines the functions of major experimental hardware control and digital reconstruction of the holograms. The basic functions of the system are completed and a user-friendly interface is provided for easy operation. The students are encouraged and stimulated to learn and practice the basic principle of incoherent digital holography and other related optics-based technologies during the programming of the application and implementation of the system.
Fluorescence microscopy is widely used in various of practical applications now. High resolution optical sectional microscopic imaging utilized by confocal two- or multi-photon fluorescence microscopy has became an essential tool in biological researches. However, optical aberrations introduced by nonhomogeneity refractive index of tissues degraded the resolution and brightness of the images. Here we present the implementation of self-interference digital holographic adaptive optics in fluorescence microscopy. Wavefront sensing and correction is achieved by holographic recording and numerical processing approach, dispenses with Shack-Hartmann sensor and deformable mirror-based complicated system. The operation speed of the system is enhanced using off-axis Fourier triangular holography. Both the influence of the size and axial position of the guide star on the quality of the corrected images are investigated.
KEYWORDS: Holography, Holograms, 3D image reconstruction, Digital holography, 3D image processing, Color imaging, Interferometers, Image fusion, Fourier transforms, RGB color model
A novel method of holographic color imaging for 3D spatially incoherent objects is proposed in this paper. Off-axis incoherent Fourier holographic recording of quasi-monochromatic illuminated objects is implemented by modifying the optical configuration of radial shearing triangular interferometer. Three holograms are recorded sequentially when the object is illuminated by red, green and blue color extended light and then synthesized to generate a RGB fashion color image. Holograms of different view of the object are recorded and reconstructed sequentially to achieve a holographic color imaging. Some image fusion skills are employed during the holographic reconstruction to compensate the chromatism and aberrations of the wavelength sensitively system. The method holds great potential in some applications such as high speed 3D spectral imaging.
Compressive sensing has been successfully applied in digital holography (DH), which formulated holography as a compressive sensing problem, thus the reconstruction of hologram is inverted as the decompress and solving the minimization problem. The original information can be reconstructed accurately when the reconstruction conditions are guaranteed in different physical scheme and optical recording set-up. In this paper, the reconstruction conditions are investigated both theoretically and experimentally in near-field Fresnel propagation regime. The effect of recording distance on the physical properties of Fresnel wave propagation is demonstrated, and then show their effect on reconstructed image quality.
Incoherent digital holographic adaptive optics (IDHAO) is a new technology of wavefront sensing and correction. However, the process of the holographic wavefront sensing needs to record digital holograms of object and a guide star independently which reduces the speed of sensing. We present a method for enhancing the operation speed of the IDHAO in this paper. The effects of the size of guide star hologram and the minimum resolvable size of discrete object on compensation of wavefront aberration are discussed. We find that for a discrete object, the hologram of guide star can be extracted from the hologram of the incoherent object. The operation speed of IDHAO system can be enhanced for discrete object by this method. This study improves the potential applications of IDHAO in the field of astronomical observation and so on.
We present a new method of realizing the spatial-identification image encryption based on digital holography in order to improve the encryption degree and increase encryption freedom. Both the object beam and the reference beam are modulated with random phase respectively. The random phase displayed on the spatial light modulator (SLM) in the reference arm is being refreshed synchronously when the aperture is scanning over the original image. The original image is divided into many subareas by the scanning aperture, and the object beam from each subarea interferes with a reference beam with unique random phase. Therefore, many sub-holograms are captured and each sub-hologram is encrypted with its own sub-key. The spatial position of the aperture becomes additional secret key because it is necessary that each sub-key should match with each encrypted sub-hologram for obtaining a completely decrypted image. This method greatly improves the image encryption degree and guarantees the security of the information. The whole original image is retrieved by superposition of all decrypted subareas. The encryption and decryption are demonstrated by simulations.
We propose and experimentally demonstrate a shift-multiplexing complex spectral-domain optical coherence tomography (shift-multiplexing CSD-OCT) method, in which the maximum detection depth of SD-OCT can be greatly extended by incorporating the shift-multiplexing of detection positions with CSD-OCT. The tomographic imaging with twofold or threefold microscopic slides as the target sample is performed. The experimental results show that the tomographic imaging with more uniform brightness and clarity for the different depth regions in a thick sample can be achieved by the shift-multiplexing CSD-OCT system. In particular, even while the sample’s depth is beyond the maximum imaging depth of CSD-OCT system, the tomographic imaging of this sample can still be realized by using the shift-multiplexing CSD-OCT method without the need for any replacement of the equipment, such as high spectral capacity grating or high resolution of CCD. The shift-multiplexing CSD-OCT system can perform the imaging with the optimization and less reduction of sensitivity for the deeper detection position in the sample.
Fresnel Incoherent Correlation Holography (FINCH) enables holograms to be created from incoherent light illumination of 3D objects. The optical setup of FINCH is usually simple and compact owe to its in-line geometry while the reconstruction of hologram suffers from the obstruction of zero-order item and twin image. Phase-shift technology is combined with FINCH in order to obtain zero-order-free and twin-image-free reconstruction. Three-step phase-shifting is adopted in all the publications of FINCH and the application of other multi-step phase-shift technology in FINCH are not investigated yet. The Fresnel holograms are sequentially recorded with different multi-step phase-shifting (including four, three, and two-step) to form the complex hologram and the quality of the reconstructed images are compared by simulations and experiments respectively in this study. Several parameters including resolution, SNR and normalized cross-correlation are applied to evaluate the quality of reconstruction images. Although various noises would be introduced by the optical elements and the experimental environment in practice, four-step phase-shifting provides the best quality of the reconstructed image but the system resolution is not different from others. In addition, the influence of different phase shift plus to the quality of reconstruction images in the three-step phase-shifting FINCH is investigated and the results show that the quality of reconstruction images which use the π/2 is better than that 2π/3.
Fresnel incoherent correlation holography (FINCH) is one of the methods for recording holograms of 3D samples under incoherent illumination. The FINCH combines the theory of spatial self-coherence and the in-line phase-shift technology together to form a complex hologram. A spatial phase light modulator (SPLM) plays important roles as the dynamic diffraction optical element (DOE) and phase shifter. When the incoherent light generated from each object point of the 3D samples incidents to a SPLM, it can be split into two spatial self-coherent beams with different curvatures. The hologram caused by these two beams can then be captured by an image detector. Three holograms with different phase shift are recorded sequentially for eliminating the zero-order and twin image, and then a complex valued hologram is obtained by superposing the three holograms. In this paper, the modulation characteristics of SPLM and phase shift error in FINCH are investigated. Based on digital holography, phase modulation characteristics of SPLM are measured under coherent and narrow-bandwidth incoherent illumination respectively. Phase shift error due to quasi monochromatic light illumination is then analyzed in FINCH. The effect of phase shift error on the quality of reconstructed image is also investigated. It is demonstrated the FINCH setup has a smaller phase shift error by experiment.
Imaging fiber bundle is a necessary element in a conventional endomicroscopy imaging system. The combination of a
proximal spatial light modulator as a means of achieving beam scanning and an imaging fiber bundle for light delivery
and collection enables the wavefront at the distal end of the fiber bundle to be synthesized, controlled and scanned. In
this way the endomicroscope is very different from conventional systems which use proximal scanning mirrors or distal
scanning heads. Thus, it is necessary to investigate the effect of primary parameters, such as diameter of each core, core-core
separation and phase mask applied to the face of the imaging fiber bundle on the characteristics of focusing spot.
These effects were simulated by numerically generating distal wavefronts and propagating them using the method of
angular spectrum of plane-wave. The axial and lateral resolution and SNR were introduced to evaluate the characteristics
of the focus. The imaging system could be optimized and reduced constraints on the imaging fiber bundle used based on
these results.
In the paraelectric phase photorefractive crystal, the quadratic electro-optic effect can result in Bragg gratings in the
volume of the crystal. The reconstruction process and the diffractive properties of the grating can be governed by the
externally applied electric field. The measurement of the diffraction efficiency as a function of the applied electric
field during recording is described. Dependence of electrically controlled Bragg grating in paraelectric phase KLTN:
Cu crystals on readout illumination beam polarization were experimentally investigated. The results of experiments
show that the diffraction efficiency of electric-controlled volume grating can also be controlled by the polarization of
readout light. All these results would offer an possibility for a photorefractive optical switch by adjusting the
polarization of the readout beam to control the diffraction efficiency.
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