We present a method of fractional Fourier transform digital holographic recording by setting an extended Lohmann I-type optical setup in the object arm of a typical off-axis digital holographic recording system. In the experiment, the resolution and the field of view for the reconstructed images of a fractional Fourier transform holograms and an image plane hologram are investigated. When the lens focal length meets a certain condition, the reconstruction imaging resolution of fractional Fourier transform holograms is higher than that of conventional image-plane holograms. The reconstruction results demonstrate that the reconstructed images of the fractional Fourier transform holograms have the higher imaging resolution when the fractional order p is closer to 1.
A method of the zero-order-removal off-axis digital holographic reconstruction by recording three holograms with different beam ratios is presented. The zero-order-removal hologram can be constituted using two fitting coefficients in the combination of three holograms, in which the three off-axis holograms with different intensity ratios of the object and reference beams are recorded via arbitrarily turning a half-wave plate. The validity of the zero-order removal of the resultant hologram is proved by expression. The filtering region for the zero-order-removal hologram can extend to the center of its spatial-frequency spectrum domain, which makes higher spatial frequencies on the positive first-order intercepted. In the experiments, the reconstructed amplitude and phase images demonstrate the effectiveness of this zero-order-removal approach and the improvement on imaging resolution.
KEYWORDS: Phase shifts, Digital holography, 3D image reconstruction, Holography, Holograms, Microscopy, Spatial light modulators, Image resolution, Super resolution, Digital recording
Reconstructing super-resolution images typically requires three or four phase-shift holograms for spectrum decomposing and combination. In the process of recording holograms, the phase shift accuracy of the instrument and environmental vibration will affect the spectrum decomposing results, resulting in reconstruction artifacts. In the simulation, with respect to two kinds of phase shift biases, the digital holographic reconstruction results with three-step and four-step phase-shift methods are compared. All the phase shift methods are much dependent on the accuracy of phase shift values. The four-step phase shift method needs to record more holograms, so its ability to cope with bias is poor, and its spectrum decomposing results is inferior to that with the three-step phase shift method. In a structured-illumination off-axis digital holographic system, the super-resolution imaging for amplitude and phase reconstructions is performed in experiment by using three-step phase shift processing. The results demonstrate that the resolution of the reconstructed images is improved.
We present a spectrum filtering approach for reconstruction of off-axis digital holograms. The high frequency aliasing terms in Fourier spectrum domain of hologram are generated by non-fitting interpolation. After interpolation processing, the zero-order terms in the aliasing spectrum region are obviously suppressed. Accordingly, for the spectrum filtering of high-frequency signal terms, the available range of frequency band will increase significantly, which can result in the improvement of resolution of the reconstructed images. In order to avoid the effect of non-uniform distribution on the aliasing spectrum terms, all the same-order signal terms in the aliasing spectrum (positive or negative first-order spectrum) are intercepted, respectively, and then added together to reconstruct the complex amplitude distribution of the sample. We perform the reconstruction imaging with a conventional method and the presented method from the same hologram in simulation. The imaging results show that such interpolation method can obviously improve the resolution of the reconstructed amplitude and phase images. This method for off-axis digital holographic reconstruction only relies on a single frame acquisition to achieve high resolution reconstruction imaging without additional requirements to the optical setup, which will have a promising application in real-time imaging for living biological cells or moving objects.
In this paper, we present a method to eliminate the zero-order term in Fourier spectrum of off-axis digital holograms based on the linear change of the hologram. In a typical digital holographic setup, the three off-axis holograms are recorded by the object waves and the reference waves with the different intensity ratios. The three holograms were Fourier transformed, and then the spectra were linearly fitted to obtain two fitting coefficients. Then, the second and third holograms are multiplied by the corresponding coefficients, and the first hologram is subtracted from the hologram that is multiplied by the coefficient to obtain the hologram with the zero-order term eliminated. In the procedure of spectrum filtering, more high-frequency information can be intercepted in this spatial-frequency spectrum after removing the zero-order. The experiment results show that this method can obviously eliminate the zero-order term.
In this paper, automatic filtering for amplitude and phase reconstruction in off-axis digital holography is developed. A user-friendly interface for automatic filtering is given via program design with MATLAB. The hologram to be processed is input at the front end, and automatic spectrum filtering in Fourier spectrum domain of digital holograms is realized by using clustering algorithm at the back end. The amplitude and phase images are reconstructed from the intercepted spatial-frequency spectrum by using the reconstruction algorithm. This automatic filtering program has high robustness, which can achieve reconstruction imaging for off-axis holograms correctly and effectively in the case of different off-axis angles or different image sizes. For the user interface, upon inputting an off-axis digital hologram and confirming the operation, the reconstructed amplitude and phase images can be quickly output. This user interface has the advantages of simple operation, adjustable parameters and clear feedback. Since K-means clustering is used, this filtering algorithm increases the efficiency in processing experimental data and the reliability of reconstruction imaging. The digital hologram computer-generated is used to simulate filtering processing. The results show that the quality of reconstructed images by using the presented automatic filtering is not inferior to that by conventional manual filtering.
An optimization algorithm is developed to extend the unambiguous range of DWDH, which is suitable for imaging living onion epidermal cells, and meanwhile to reduce the noise. In the experiments, the holograms with two wavelengths can be acquired in a single shot recording by using off-axis dual-wavelength digital holography, resulting to obtain their two sets of spatial spectrum via Fourier transform for these two holograms. The denoised unwrapping phase image of sample is straightforward reconstructed with the optimization algorithm, instead of increasing the noise due to longer synthetic wavelength. As a result, the unwrapped phase images of onion epidermal cells are achieved.
KEYWORDS: Digital holography, Holograms, Image resolution, 3D image reconstruction, Super resolution, Digital imaging, Holography, Reconstruction algorithms, Spatial resolution, Charge-coupled devices
In this paper, an off-axis digital holography multi-frame image super-resolution reconstruction method is presented. Each low-resolution hologram will have small displacement in different directions. The obtained low-resolution holograms are processed iteratively with the super-resolution algorithm to obtain the super-resolution hologram, and then the amplitude image is reconstructed from the super-resolution hologram. The imaging results show that the resolution of the reconstructed image after super-resolution processing is obviously improved, because of increase of high-frequency information in the high-resolution hologram.
A noise reduction method based on a shorter synthetic-wavelength in DWDH is proposed in this paper. The unwrapped phase at longer synthetic-wavelength is calculated by using the wrapped phases of two individual wavelengths. By comparing the amplified phase of longer synthetic wavelength, of which magnification is equal to that of wavelengths with the wrapped phase of a single wavelength, the difference of phase noises between the longer synthetic-wavelength phase map and the single-wavelength phase map can be calculated, and then accurate height at single-wavelength can be achieved. The proposed method for calculating phase noise is performed by comparing the phases between the single-wavelength with the shorter synthetic wavelength, and then the height at shorter synthetic-wavelength can be obtained. Compared with the existing method, the proposed method can reduce certain noise and benefit the phase reconstruction of fine structures.
The optimization of image resolution for digital holographic scanning imaging of biological cells is investigated. Digital holographic scanning imaging experiments on the upper epidermal cells of onions are performed to demonstrate the validity of resolution optimization algorithm. In the experiments, the holograms of the upper epidermal cells of onion are recorded at a certain scanning rate, and then are processed by using the resolution optimization algorithm. As a result, the phase images of the onion epidermal cells with higher contrast and resolution are obtained. According to the synthetic holograms, the changes of cell nucleus and actin microfilament inside onion’s epidermal cells are displayed. In addition, the dehydration process and plasmolysis phenomenon inside onion epidermal cells are also exhibited by recording longterm scanning holograms of living epidermal cells. The experimental results demonstrate that image quality of living onion epidermal cells can be improved by optimizing the algorithms.
A method for measuring the channel size of microfluidic chip by using optical coherence tomography (OCT) is investigated. Based on spectral-domain OCT imaging and image processing, an OCT system for measurement of the microfluidic channel size is developed, which is suitable for planar two-dimensional measurement. The cross-sectional images of the micro-channel of a microfluidic chip are obtained by using OCT continuous B-scan technology. With image processing for the acquired image in noise elimination and dispersion compensation, the size of the micro-channels in depth is achieved. Comparing with the single-scan OCT imaging mode, back-passing multiple-scans mode has the capability of high image quality to achieve high measurement precision of micro-channel size.
KEYWORDS: Holograms, Digital holography, 3D image reconstruction, Image resolution, Reconstruction algorithms, Digital imaging, Detection and tracking algorithms, Phase shifts, Spatial resolution, Signal to noise ratio
In this paper, the method of recognition and superposition of sub-pixel shifting in digital holograms is investigated. A group of non-scanning holograms is recognized and superposed by using sub-pixel shifting algorithm. Further, spatio-temporal scanning holograms with sub-pixel displacement are recognized and superposed to further improve imaging resolution by combining spatio-temporal scanning digital holography with sub-pixel shifting algorithm. The experimental results verify the feasibility of improving resolution with sub-pixel displacement.
The experiment of volume holographic storage for teaching and training the practical ability of senior students in Applied Physics is introduced. The students can learn to use advanced optoelectronic devices and the automatic control means via this experiment, and further understand the theoretical knowledge of optical information processing and photonics disciplines that have been studied in some courses. In the experiment, multiplexing holographic recording and readout is based on Bragg selectivity of volume holographic grating, in which Bragg diffraction angle is dependent on grating-recording angel. By using different interference angle between reference and object beams, the holograms can be recorded into photorefractive crystal, and then the object images can be read out from these holograms via angular addressing by using the original reference beam. In this system, the experimental data acquisition and the control of the optoelectronic devices, such as the shutter on-off, image loaded in SLM and image acquisition of a CCD sensor, are automatically realized by using LabVIEW programming.
In this paper, a real-time measurement of liquid concentration changing in a Y-type microfluidic chip by digital holography is presented. In the experiments, the different concentrations of salt solution are injected into two channels of the Y-type microfluidic chip as a target object, and then the digital holograms related to the target solutions are recorded. The refractive index of the solution can be obtained from the reconstructed phase image. The experiment results show that the real-time changing of liquid concentration in microfluidic chip can be effectively measured by digital holographic microscopy.
In this paper, a method to measure the flow speed based on microfluidic chip by digital holography in real-time is demonstrated experimentally. The injection pressure of microfluidic device is changed to create different flow rate in microfluidic channels. On this basis, the phase distributions within the microfluidic channels can be reconstructed by using digital holographic microscopy, and then flow rate can be obtain by measuring the phase distributions of cross section. The experiment results show that digital holographic phase image is an effective and real-time detection means for the characteristic parameters of micro-fluid such as flow rate and injection pressure. In addition, the chip calibration is made to ensure the validity of the experimental results.
A method of digital holographic imaging for strong diffuse-reflective metal surface is presented. For a strong diffuse-reflection object, the DC term of Fourier spectrum of the hologram is removed by subtracting the patterns the reference beam and object beam from the hologram, which effectively eliminates the influence of the zero-order of the spectrum image on actual information. In view of the more extensive area of Fourier-spectrum region of the hologram for a diffuse reflection object, the spectrum filtering windows is taken as a half area of the Fourier-spectrum region. According the removal of the zero-order term and the use of the half area of filtering windows, the reconstruction imaging for the surface of an alloy plate is achieved, in which both amplitude and phase images are obtained, respectively.
A segmental dispersion compensation method is proposed to compensate the dispersion in frequency domain optical coherence tomography. Tomographic imaging for epidermal layer of an onion slice is achieved in the experimental setup using optical fiber. The axial resolution of the tomography can be improved by using segmental dispersion compensation, because this dispersion compensation method employs segmental dispersion coefficients for the different lateral positions in one specific layer. Comparing with the traditional dispersion compensation method, segmental dispersion compensation method has the capability of separating the specified layer of sample and eliminating the dispersion broadening effect of specified layer.
In this paper, the simulation experiments both of Abbe-Porter spatial filtering and of optical processing of image addition and subtraction with a grating filter are designed and performed. We realize the design and operation of optical information processing simulation experiments based on information optics theory and the experimental principle by using MATLAB programing language. The spatial filtering of Fourier spectrum, one of the main concepts in information optics, is intuitively described via the simulation experiments, and the experiment process is demonstrated step by step. The results show that the simulation experiments are really helpful for the filter's design and the image processing. These developed virtual experiments have been used in experimental teaching for undergraduate students majored in optics or optical engineering, which effectively assist students to understand concept and principle of optical information processing.
In the Optics of University Physics, two most important concepts are interference and diffraction, which reflect the wave nature of the light. The corresponding content is relatively abundant, such as Young’s Double-slit Interference, One-slit Diffraction, Grating Diffraction. But they are not easy for undergraduates to comprehend. So in order to show the two phenomenon visually, the related demonstrative experiment equipment are well-developed, but they are independent with each other. The students can accept them separately, but in the same time ignore the connection between them two. Actually, interference and diffraction are consistent in essence from the view of coherent superposition and redistribution of the light intensity. The difference only depends on the specific parameters. The objective of our paper is to analyze the condition for two slits to produce interference and diffraction phenomenon. Also, apply the knowledge of Fourier Optics to analyze the Fresnel diffraction. This would be a good example for the theory of grand unification in physics. Firstly, the intensity distribution is deduced for two-slit interference, one-slit Fraunhofer diffraction, two-slit Fraunhofer and Fresnel diffraction applying complex-amplitude integration method. In the same time, the simulated experimental results by MATLAB are shown. Secondly, the experimental results are given to verify the analysis. Finally, a new idea is presented to realize an equipment of demonstrative experiment for teaching.
KEYWORDS: Digital holography, Optical filters, Holograms, Reconstruction algorithms, Digital imaging, Digital filtering, Holography, 3D image reconstruction, Distortion, Image registration
An automatic angular-spectrum filtering for the phase reconstruction of dual-wavelength digital holograms in a common-path configuration is presented. The major procedure of this automatic angular-spectrum filtering consists of excluding the zero-order region of Fourier spectrums and locating the center of order +1 region of the angular spectrums for two individual wavelengths. The phase map of the object is retrieved with the automatic angular spectrum-filtering algorithm in dual-wavelength digital holographic system, which demonstrates that the automatic angular spectrum-filtering algorithm is feasible and effective. It provides an efficient solution for angular-spectrum filtering in real time dual wavelength digital holographic microscopy.
A polarization-multiplexing off-axis dual-wavelength digital holography is presented. The structure of a groove grating is measured by using the polarization-multiplexing dual-wavelength digital holographic system with a co-path optical configuration. Two holograms of the grating is recorded at the wavelengths 671nm and 656nm by using a pair of CCDs, and then their two phase maps are reconstructed by numerical simulation of diffraction process of the holograms, respectively. The synthetic phase image is obtained by subtracting two phase images directly. Thus, the unwrapping phase map of the grating is achieved by using the polarization-multiplexing dual-wavelength digital holography in the off-axis optical setup.
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.
Optical Coherence Tomography (OCT) was successfully applied in the microstructure imaging of biological tissue
after being proposed firstly in 1991 by the researchers of MIT. As a novel optical imaging technology, it mainly uses
interference principles to achieve noninvasive and high resolution visualization of samples. OCT works analogously to
an ultrasound scanner, the major difference is that ultrasound pulses are replaced by broadband light. According to
whether need for mechanical axial scan in the depth direction, it can be classified into the time-domain OCT (TD-OCT)
and frequency-domain OCT (FD-OCT). The FD-OCT system overmatches the TD-OCT in imaging speed because of its
depth collection advantage. But in the reconstructive image of FD-OCT detection, the complex-conjugate ambiguity will
seriously deteriorate the imaging effect of tomogram. So the technique of removing the complex-conjugate image is
employed that is called complex FD-OCT. The complex FD-OCT has widely application in many fields, especially in
the refractive index measurement. The refractive index is an important parameter characterizing light propagation in the
medium. In the paper, we present a method to measure the average refractive index of the sample with substrate
calibration by using complex FD-OCT method, in which we can calculate it without depending on the parameters of
system such as spectral width of light source. Due to the measurement of average refractive index relative to the actual
thickness and optical length, it is necessary to obtain them of the sample experimentally. The complex FD-OCT method
can easily achieved the optical length via measuring the positions of the sample’s front and rear surfaces. In the
experiment, the coverslip (the borosilicate glass) is chosen as the sample and the calibration substrate. We make use of
the substrate to load the sample on it, and then the tomogram of the sample can be achieved by means of OCT’s lateral
scan in the edge of the sample and complex FD-OCT method. According to the experimental results, we can acquire the
sample’s tomographic information and position of the substrate. The ratio of actual thickness and optical length can be
indirectly calculated out with the pixel number obtained by analyzing the image data. So with only one time scan, we can
complete the measurement of average refractive index of the sample without aid of other instruments.
The three-step or many steps phase shifting method is usually employed to resolve the complex-conjugate ambiguity in Spectral-domain optical coherence tomography (SD-OCT). However it reduces the image quality and also the imaging speed is slow. In this paper two steps phase-shifting is used in digital image processing to resolve the complex-conjugate ambiguity and improves the quality of reconstructed image in SD-OCT. In the two-step phase shifting method the phase shifting operation is used only once which simplified the experiment and also the effect of relative error in SD-OCT on image quality is eliminated.
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.
In this paper,the phase-type grating recorded in a Fe:Cu:LiNbO3 crystal is measured by dual-wavelength digital holography. In the experiment, a volume hologram, which is recorded in a 3-mm-thick Fe:Cu:LiNbO3 crystal by interference of two recording beams at the wavelength of 532 nm, is reconstructed to be imaging by dual-wavelength digital holography. Two lasers of the different wavelengths 660 nm and 671 nm are used to obtain a larger beat wavelength. Each laser output, which is spatially-filtered and collimated, is split into a reference and object beams in an interferometer setup based on the Mach-Zehnder configuration. In dual-wavelength phase unwrapping, two individual phase images are obtained by using each wavelength, respectively, and the phase image of beat wavelength is obtained by subtracting one single wavelength phase image from the other and then adding 2π whenever the resultant value is less than zero. In the final synthetic image, the discontinuities are removed after reduce the noise of the beat wavelength phase image. Thus, a 3D surface profile of the phase grating is obtained.
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.
The fixing phase of the two-center holographic recording is not consistent with the mono-exponential formula, and the
charge-transfer mechanism of the fixing phase is studied theoretically. The electron gratings of the shallower traps center
have a great influence on the fixed space-charge-field, the stronger electrons grating of the shallower traps center would
enhance the saturation space-charge-field of the deeper traps center in the fixing phase. The effects of the Fe
concentration, Cu concentration and the oxidization-reduction state of the crystals on the fixed space-charge-field are
investigated by using the Runge-Kutta method. The results show that the higher Fe concentration, the lower Cu
concentration and the weakly oxidized crystal are available for enhancing the saturation space-charge-field of the deeper
traps center.
The multiplexed holographic gratings can achieve simultaneously three-dimensional imaging of an object with the Bragg
selectivity. The recording conditions of multiplexed holograms for imaging different depths of the object space are
investigated experimentally. We find that the cross talk can be ignored when ΔΖ is approximately more than 2ΔΖFWHM, and the cross talk is degenerate with the increase of the depth separation. Further we adopt the point-spread function
(PSF) of the imaging system to estimate depth resolution of the volume holographic imaging system. Then to avoid
image overlap we examine multiplexed holograms recorded with different Δθ. We also calculate Δθ theoretically, and find that the theoretical value is consistent with the experimental result. The optimized holographic gratings are used to image the resolution target which is located at the different longitudinal positions to simulate a 3D object. The results
demonstrate that the resolution target located at the two different depths can be reconstructed and clearly imaged on
CCD.
The single wavelength off-axis reflection digital holographic microscopy (DHM) can be applied in micron optical
tomography measurement through the pre-magnification method. The pre-magnifying surface profile of the object is
imaged by the lens placed in front of the object to be measured, and then is recorded by the CCD camera. The
Reconstructed image can be got through Fresnel Diffraction calculation, the 3D surface shape information of the object
was shown after the phase filtering and unwrapping process to the reconstruction image. The result in this experiment
proves that off-axis reflection digital holographic microscopy can be applied to actual measurement of micron object, to
achieve the tomography without touching or breaking the object.
KEYWORDS: Digital holography, 3D image reconstruction, Holograms, Image segmentation, Fourier transforms, Microscopy, Holography, Digital recording, Reconstruction algorithms, Cervical cancer
The number of cells is commonly employed to describe the cell viability and the status of cell culture in a certain extent.
An automatic and non-invasive detecting method for the status analysis of cell culture is developed based on digital
holography microscopy (DHM) technology. Digital holographic imaging can retrieve quantitative information of object
wavefront by the numerical reconstruction from a single digital hologram recorded by a detector such as CCD or CMOS
camera, which is especially suitable for the morphology detection of the transparent or semi-transparent cells. In this
contribution, the lensless Fourier transform (LFT) based holography configuration is designed for cell imaging without
prestaining, and the amplitude and phase of living cells can be reconstructed by digital reconstruction and phase
unwrapped algorithms. Then the image filtering and segmentation are combined for the automatic evaluation of the level
of confluency. In imaging experiments, the culture status of the cervical cancer cell TZMbl is detected, and the results
demonstrate that digital holography microscopy provides a feasible non-invasive method for monitoring the living cell
culture.
Information Optics (i.e. Fourier Optics) is a compulsory professional course in the teaching program for juniors in the field of applied physics at Beijing University of Technology. Various methods are applied to information optics teaching in order to obtain satisfying teaching effect. Active and interactive teaching method based on exploring forefront topics was proposed and put into practice, especially for teaching “holography and holographic technology application” section of the coursein which the teaching activity was not restricted to classroom any more. A visiting to the exhibit of forefront production of holographic display was introduced as an episode in the teaching. The process of teaching was designed elaborately to an interactive activity between the teacher and students, and to stimulate students to cooperate. The teaching practice proves that the active and interactive teaching method is much favorable by students and successful in information optics teaching.
Two-center holographic recordings of angle-multiplexed holographic patterns are experimentally performed in
LiNbO3:Ce:Cu crystals with gating light at 365nm and recording light at 633nm. The noise characteristics of the
holograms are analyzed. 50 holograms with almost equal diffraction efficiencies are multiplexed in a LiNbO3:Ce:Cu
crystal by using equal-time exposure schedule .The average efficiency reaches 7.64×10-6.
KEYWORDS: Charge-coupled devices, Imaging systems, Data storage, Holographic data storage systems, Binary data, Spatial light modulators, Holography, Signal to noise ratio, Data processing, Holograms
We use the conception of pixel-matched spread function (PMSF) to analyze the physical process of intra-page cross talk
in a holographically imaged data page, and design an arithmetic to suppress the intra-page cross talk generated in the
reconstructed data page due to limited aperture of imaging optical system. By applying this arithmetic to a data page of
512×512 pixels captured from our CCD, the raw bit error rate (BER) decreased from 1.1×10-3 to 2.4×10-4.
In recent years, doped KLTN (K1-xLixTa1-yNbyO3) crystal in paraelectric state was reported to be a new type of
holographic storage medium owning to its great promotion for diffraction efficiency, through quadratic electro-optical
effect generated by external electrical field. In this paper, we describe the preparation method of the KLTN crystal in our
laboratory and report our preliminary experimental research on electrically controllable holography in a KLTN: Cu
crystal. It was found that the writing intensity and polarization of the readout beam affect the diffraction efficiency
remarkably, and the voltage selectivity of holograms stored in the crystal was observed, which is potential to increase the
storage capacity.
The Applied Physics program at Beijing University of Technology was designed to nurture innovative talent in modern applied physics, providing students both solid theoretical grounding and training for practical scientific research skills by offering 4-year BS degree. In order to fit in with the needs of the fast developing of our society, the education objectives and the program curriculum need to be correspondingly adjusted. This paper reviews the two revises of Applied Physics program, launched in 2003 and 2007 respectively.
In this paper, it is proposed to record the matched filter in the photorefractive crystal by using the synthetic discriminant function as the input. The synthetic discriminant function is generated by training a set of rotated object images. Multiple matched filters are stored by angle-multiplexing in the same volume. Due to the large difference among the optical throughouts of the images, a wavelet transform filter is directly introduced into the experimental setup to improve the object discrimination largely. Computer simulation and experimental results demonstrate that it is possible to achieve the rotation-invariant pattern recognition with the proposed method
"Specialized Physics Experiments" is a significant laboratory-experimental course for Bachelor Degree in the Applied Physics discipline. In this paper, the introduction of advanced research in volume holographic storage to an integrated experiment of the course is presented. The results show that by introducing sophisticated experiment, which contains contemporary optics, mechanics, electronics, and computing, to the course experiments, the knowledge and technical abilities of undergraduate students can be effectively extended. This presents a model of combining scientific research projects with undergraduate teaching.
The recent research on high-density, large-capacity, nonvolatile holographic storage in photorefractive crystals, carried out at Beijing University of Technology, is reviewed in this paper. A batch thermal fixing technique was developed to overcome the volatility of a large number of multiplexed holograms, which has also proved to increase the effective dynamic range of crystals, hence to increase the storage density. The key of this technique is determination of the inter-batch optical erasure time constant that is much longer than the convenient (intra-batch) erasure time constant of the crystal. Disk-type storage is proposed to increase the degree of spatial multiplexing, so that the whole volume of the material can be used for storage, and the capacity increases remarkably. Incorporating batch thermal fixing into holographic disk storage yields to a track-division thermal fixing (TDTF) scheme for holographic disk storage to overcome the volatility of information and increase the storage density and capacity as well. Analyses and experimental results on a high density of 50 bits/μm2 (10 Gbits/cm3) show that high-density huge-capacity mass storage, indicated by storing 1 tera-bits or more data in a single crystal of reasonable size, can be achieved by means of TDTF holographic disk storage scheme.
Thermal-fixing holographic storage in photorefractive crystals is an effective nonvolatile storage technique for stabilizing holograms against the optical erasure. The developing characteristics of fixed holograms in iron-doped lithium niobate dominate their final diffraction efficiencies after developing, provided that the ionic compensation in the fixing stage is nearly complete. Developing kinetics of fixed holograms under homogeneous illumination is studied. The effect of developing light intensity on developing efficiency has been demonstrated experimentally. The fixed holograms with the same initial recording grating strength are developed under illumination of the different light intensities, such as 200mW/cm2, 400mW/cm2, 600mW/cm2, and 800mW/cm2, respectively. The fixed holograms with the different initial recording grating strengths are developed under illumination of the same light intensities as well. The overdeveloping characteristics of fixed holograms are described. The developing efficiency of the hologram with a given initial recording grating strength is found to depend on the intensity of developing light. These features can be explained with the joint effect of the resulting space-charge holographic field of the fixed hologram and the photovoltaic field under homogeneous illumination.
The batch-thermal fixing scheme was combined into holographic disk storage, for the first time to our knowledge, yielding to a track-division thermal fixing scheme (TDTF), in order to increase the storage density and overcome the volatility of the holographic memories. As a specific batch-thermal fixing scheme, the concept of TDTF scheme was described in this paper. Three principal holographic time-constants of a photorefractive crystal for TDTF scheme were measured, based on which an appropriate exposure-time sequence was calculated and applied to record multiple holograms. The disk mount has been designed and finely manufactured, consequently, the reposition accuracy of the crystal disk can meet the requirement after the crystal experienced off-line thermal fixing. 5000 images, each contains 768×768 pixels, divided into 4 tracks , have been stored in a disk-shaped 0.03wt% Fe doped LiNbO3 crystal using TDTF scheme. The samples of retrieved holograms showed approximately equal diffraction efficiency and good fidelity. The Experiment resulted in a high areal density of 50 bits/μm2 and volume density of 10 Gbits/cm3.
KEYWORDS: Crystals, Holography, Data storage, Signal to noise ratio, Holograms, Laser crystals, Multiplexing, Image quality, Scattering, 3D image reconstruction
Ten thousand data pages, each containing 768×768 pixels, have been stored in a single section of a disk-shaped, iron-doped LiNbO3 crystal using spatioangular multiplexing with a convergent spherical reference beam, leading to an areal density of 33.7bits/μm2 and a volumetric density of 6.7Gbits/cm3. The system design considerations for the achievement of the goals ensure the success of the experiment. Customer-designed Fourier transform and imaging optics with short focal length provide tightly confined object beam at the crystal and good iamge quality in the detector array. An optimized reflection configuration avoids the detrimental scattering from the crystal surface to enter the detector array. An optimzied reflection configuration avoids the detrimetnal scattering from the crystal surface to enter the detector. The images were reconstructed with good fidelity. The signal to noise ratio (SNR) was measured to be 3.6 for the worst-case in the sampled retrieved images, from which, a raw bit error rate of 1.6×10-4 before error correction could be estimated.
We have investigated the characteristics of thermal fixing holograms in batch. The loss of signal-to-noise ratio (LSNR) is used in comparing the noise performance of hologram before and after each batch of thermal fixing. The experiment results on batch fixing show that the deterioration in hologram fidelity is mainly caused in recording phase at the room temperature, and the repeated fixing processes won't further produce significant noise to degrade holograms.
In this paper a Cu:Fe:LiNbO3 crystal is chosen to be the recording material with the gating light of 355 nm pulsed laser and the writing light of 633 nm He-Ne laser. We investigate the effect of the pulsed UV pre-exposure on the writing time constant, the achieved maximum refractive-index modulation and writing sensitivity of the crystal.
Volume holographic storage has attracted intensive attention during the past decades, owing to mainly the potential high storage capacity and fast data transfer rate. Thick media such as photorefractive materials allow a large number of holograms to be multiplexed in a common volume of the material, resulting in high storage capacity.
The steady-state differential equations of two waves coupling in photorefractive materials for reflection geometry have been numerically solved, and the Bragg-shift are calculated for a range of the coupling intensity and the phase shift. By means of a quasi-steady state approximation, the Bragg-shift as a function of the coupling intensity has been found, and hence as a function of the recording time. There is a good agreement between experimental measurements and numerical calculation.
The diffraction efficiency and angular selectivity of photorefractive volume holograms, the important performances for multiplexed holograms, are strongly affected by material properties and recording conditions. In this paper the diffraction efficiency and angular selectivity of reflection gratings in photorefractive crystals are studied by means of the coupled wave theory, with respect to the recording geometry and polarization states of recording beams. The result shows that, although the angular selectivity of reflection gratings recorded with the s- polarized beams is slightly worse than that written with p-polarized ones, the diffraction efficiency of s-polarized beams is generally higher and more uniform than that of p-polarized beams over all accessible angular ranges. The theoretical results are also verified by experimental measurements. The s-polarized beams would be more suitable for recording and readout of reflection-type holograms in photorefractive crystals.
In this paper, we describe an image recognition system using a multi-channel optical correlator as the computational element and a volume holographic memory as a template database. By recording the holographic matched filters in the disk-type Ce:Fe:LiNbO3 crystal, the multi-channel optical correlator is constructed. The system performs the multi-category pattern recognition. Experimental results are presented and discussed.
In this paper the gray scale fidelity of images retrieved from photorefractive volume holographic memories is studied in detail. We propose, for the first time to our knowledge, to use the correlation coefficient between the original and the retrieved images as one of the measures for proper assessment of the gray-scale fidelity. The influence of recording object-reference beam ratio on the fidelity is investigated by using these quantities, and the dynamic evolution of the gray scale fidelity during hologram recording is calculated and experimentally measured.
In this paper, a new kind of random phase shifter is proposed which is made of photopolymer plate by recording speckle patterns in it. The spectrum distribution of this phase shifter in 2-dimensions is measured and compared with that of random phase shifter made of silver halide plate. The result shows that the spectral distribution of the former is more intense and uniform over a more round area than the latter. An image storage experiment demonstrates that a high-quality image can be retrieved if this speckle random phase shifter is used for Fourier transform hologram recording.
The vertical angular selectivity of volume holographic gratings in photorefractive crystals is investigated both theoretically and experimentally. The vertical selective angle and the shape of the grating degeneracy lines are derived by using a simplified geometrical model relating the k-vector sphere to the reference point plane. The study shows that volume gratings have finite selectivity in vertical direction, so that grating degeneracy does not occur along vertical lines.
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