Phase-shifting digital holography in image reconstruction

A phase-shifting digital holography scheme developed to investigate internal defects in artworks is described. Phase-shifting is utilized to obtain a clear reconstructed object wave from a rough surface texture. A reverse-transform algorithm is employed to reconstruct the object wave on its original position of unknown distance or the imaging position from the object wave information on the holographic plane. To get the clearest reconstruction the exact registration of the unknown distance is determined by applying the intensity sum as the auto-focusing function, The spatial resolution of the reconstruction image is also investigated for a variety of affecting factors. Laboratory results of reconstruction images under deformation are presented.

Optical encryption of multiple three-dimensional objects based on multiple interferences and single-pixel digital holography

We present an optical encryption method of multiple three-dimensional objects based on multiple interferences and single-pixel digital holography. By modifying the Mach-Zehnder interferometer, the interference of the multiple objects beams and the one reference beam is used to simultaneously encrypt multiple objects into a ciphertext. During decryption, each three-dimensional object can be decrypted independently without having to decrypt other objects. Since the single- pixel digital holography based on compressive sensing theory is introduced, the encrypted data of this method is effectively reduced. In addition, recording fewer encrypted data can greatly reduce the bandwidth of network transmission. Moreover, the compressive sensing essentially serves as a secret key that makes an intruder attack invalid, which means that the system is more secure than the conventional encryption method. Simulation results demonstrate the feasibility of the proposed method and show that the system has good security performance.

Possibility to break through limitation of measurement range in dual-wavelength digital holography

By using the beat frequency technique,the dual-wavelength digital holography(DWDH)can greatly increase the measurement range of the system.However,the beat frequency technique has a limitation in measurement range.The measurement range is not larger than a synthetic wavelength.Here,to break through this limitation,we propose a novel DWDH method based on the constrained underdetermined equations,which consists of three parts:(i)prove that the constrained underdetermined equation has a unique integer solution,(ii)design an algorithm to search for the unique integer solution,(iii)introduce a third wavelength into the DWDH system,and design a corresponding algorithm to enhance the anti-noise performance of DWDH.As far as we know,it is the first time that we have discovered that the problem of DWDH can belong in a problem of contained underdetermined equations,and it is also the first time that we have given the mathematical proof for breaking through the limitation of the measurement range.A series of results is shown to test the theory and the corresponding algorithms.More importantly,since the principle of proposed DWDH is based on basic mathematical principles,it can be further extended to various fields,such as dual-wavelength microwave imaging and dual-wavelength coherent diffraction imaging.

A new method of aperture synthetizing in digital holography

This paper proposes a new method of aperture synthetizing in digital holography based on the principle of holography.In the new method aperture synthetizing is achieved by reconstructing each sub-hologram respectively,firstly,moving each reconstructed wave field referred to the benchmark reconstructed wave field according to the relationship between spacial motion and frequency shift,and finally splicing them by using superposition. Two different recording ways,using plane wave to record and using spherical wave to record,are analyzed,and their moving formula is deduced,too.Simulation and experiment are done. The results show that in comparison with the traditional method of aperture synthetizing in digital holography,the new method can decrease calculation and save reconstructed time obviously which has better applicability.

Speckle reduction by selective spatial-domain mask in digital holography

An improved method of using a selective spatial-domain mask to reduce speckle noise in digital holography is proposed.The sub-holograms are obtained from the original hologram filtered by the binary masks including a shifting aperture for being reconstructed. Normally, the speckle patterns of these sub-reconstructed images are different. The speckle intensity of the final reconstructed image is suppressed by averaging the favorable sub-reconstructed images which are selected based on the most optimal pixel intensity sub-range in the sub-holograms. Compared with the conventional spatial-domain mask method, the proposed method not only reduces the speckle noise more effectively with fewer sub-reconstructed images,but also reduces the redundant information used in the reconstruction process.

Characteristic of femtosecond laser-pulsed digital holography

Digital holography can be applied to ultrafast detection when a femtosecond laser pulse is used. In this paper, the interference process of two femtosecond laser pulses is studied and the recording process of the femtosecond laser pulsed digital hologram is simulated. Holograms at different recording angles are generated by integrating the instantaneous interference field. By analyzing the distribution of the reconstructed phase error, the characteristics of femtosecond laser pulsed digital holography are discussed.

Quasi noise-free digital holography

One of the main drawbacks of Digital Holography(DH)is the coherent nature of the light source,which severely corrupts the quality of holographic reconstructions.Although numerous techniques to reduce noise in DH have provided good results,holographic noise suppression remains a challenging task.We propose a novel framework that combines the concepts of encoding multiple uncorrelated digital holograms,block grouping and collaborative filtering to achieve quasi noise-free DH reconstructions.The optimized joint action of these different image-denoising methods permits the removal of up to 98%of the noise while preserving the image contrast.The resulting quality of the hologram reconstructions is comparable to the quality achievable with non-coherent techniques and far beyond the current state of art in DH.Experimental validation is provided for both singlewavelength and multi-wavelength DH,and a comparison with the most used holographic denoising methods is performed.

Speckle noise reduction in digital holography with spatial light modulator and nonlocal means algorithm

An integrated method based on optical and digital image processing is presented to suppress speckle in digital holography. A spatial light modulator is adopted to introduce random phases to the illuminating beam. Multiple holograms are reconstructed and superimposed, and the intensity is averaged to smooth the noise. The adaptive algorithm based on the nonlocal means is designed to further suppress the speckle. The presented method is compared with other methods reduction is improved, and the proposed method is effective The experimental results show that speckle and feasible.

Monitoring the Diffusion Layer During Passive Film Breakdown on Alloy 800 with Digital Holography

The effects of chloride and thiosulfate ions on localized corrosion of alloy 800 are investigated through dynamical observation of the change in phase image of the diffusion layer during passive film breakdown using digital holography. The results indicate that solution chemistry has a significant effect on film breakdown and diffusion layer. The phase distribution changes at different applied potentials show that in the process of film breakdown, dissolution of metal ions from pitting is not remarkable in chloride-only solution, whereas dissolution of metal ions is significantly high in thiosulfate and chloride solution. Thiosulfate has a combined effect with chloride ions in passive film degradation.

Twin-image problem in digital holography—a survey(Invited Paper)

Zero-order and twin images are a serious obstacle in achieving a high-quality output in in-line digital holography （DH）. They decrease the useful bandwidth of the off-axis DH. Over the years the twin image removal problem was approached both by instrumental and numerical means. The paper provides an extended survey of the proposed solutions with their pros and cons as a guide for further advance in this field. Processing of a single spatial carrier fringe pattern involves spatial filtering in the frequency domain, spatial phase-shifting （PS） or wavelet transform. A point source digital holographic microscopy （DHM）, introduction of calibration measurements or various modifications of PS technique are instrumental solutions to the twin image problem for in-line DH. Numerical solutions to the same problem include iterative and non-iterative approaches, diffraction-based and inverse problem solutions, reconstruction of purely real or phase objects and of complex objects, reconstruction of plane and volume objects. Elimination only of the zero-order image relies on non-linear filtering or additional calibration measurements.

Surface roughness measurement and analysis of mechanical parts based on digital holography

We measure the surface roughness of the mechanical parts based on digital holography. A digital off- axis hologram recording setup for reflective samples is built. Firstly, the height reconstruction error 2.3% of the setup is calibrated by using the quartz step height standard （VLSI- SHS-880QC）. Then, the standard scribed-line model and the grinding roughness specimen are selected as the test samples and their surface roughness are 0.095 6 jam and 0.025 3 jam, with errors 6.3%, 0.9%, respectively. The results are in good agreement with the given roughness parameters. At last, we also analyze the window effect of the filter on the roughness measurement value based on digital holography. In conclu- sion, the paper demonstrated effectively that the digital holography could provide the surface feature for the rough- ness measurement with high accuracy.

Holography and speckle in phase-shifting digital holography(Invited Paper)

The correlation properties of the optical field diffusely reflected from a rough surface under coherent illumination are analyzed numerically. The cross-correlations of the complex amplitudes and the intensities before and after translation and/or tilt of the surface are calculated at an arbitrary observation plane in three-dimensional （3D） space. The results provide us with the 3D distributions of phase changes and speckle displacement that lead to the distributions measured by holographic interferometry and speckle relationships and physical interpretation of them are and signal-to-noise ratio of the displacement to be correlation technique. Comparisons with analytical also discussed.

Measurement of in-plane deformations of microsystems by digital holography and speckle interferometry(Invited Paper)

The reliability of microsystems is an important issue and for their quality inspection, it is necessary to know the displacements or deformations due to the applied mechanical, thermal, or electrostatic loads. We show how interferometrical techniques like digital holography and speckle interferometry can be used for the measurement of in plane deformations of microsystems with nanometric accuracy and we give a description of the measurement uncertainties.

Aperture synthesis based solely on phase images in digital holography

Aperture synthesis is an important approach to improve the lateral resolution of digital holography(DH) techniques.The limitation of the accuracy of registration positions between sub-holograms affects the quality of the synthesized image and even causes the failure of aperture synthesis.It is a major issue in aperture synthesis of DH.Currently intensity images are utilized to find the registration positions of sub-holograms in aperture synthesis.To improve the accuracy of registration positions, we proposed a method based on similarity calculations of the phase images between sub-holograms instead of intensity images.Furthermore, a quantitative indicator, degree of image distortion, was applied to evaluate the synthetic results.Experiments are performed and the results verify that the proposed phase-image-based method is better than the state-of-the-art intensity-image-based techniques in the estimation of registration positions and provides a better synthesized final three-dimensional shape image.

Crack Detection by Optical Voice Recorder Based on Digital Holography

The detection of crack on materials is an important issue in industry.On the contrary to conventional methods,such as manual inspection,sensor detection,and image processing techniques,a new simple method to detect the crack is proposed with optical voice recorder based on digital holography in this paper.Holograms obtained with sound waves passing through the materials are recorded by using the digital holography technique.Temporal behavior of the sound wave passing through the material,which is obtained from these holograms,gives image of crack.In this article,cracks in various materials are determined by the proposed new method,and crack images obtained with this new system are presented.

Digital holography as metrology tool at micronanoscale for soft matter

The appearance of the first laser approximately 12 years after the invention of holography by Gabor(1948)revolutionized the field of optical metrology.In fact,the invention of holographic interferometry enabled the exploitation of interferometry on non-mirror surfaces and full-scale objects.The holography-based measurement methods has been implemented to several industrial systems or in support of R&D with the aim of improving new products in many fields(automotive,aerospace,electronics,etc.).To date,holography has been considered an important measurement tool for non-destructive inspection(NDI),strain-stress measurement,and vibration analysis at various engineering sites.Recently,the new paradigm of Industry4.0 has seen the introduction of new technologies and methods of processing materials as well as the development of manufacturing approaches for the realization of innovative products.For example,direct printing,additive,and bottom-up manufacturing processes are expected to involve new ways of making products in future,and most innovative fabrication processes will be based on the manipulation of soft matter(e.g.,starting from the liquid phase)that will be shaped at the nanoscale.The inherent characteristics of digital holography(DH)make it a powerful and accurate tool for the visualization and testing of final products,as well as for in situ and real-time monitoring and quantitative characterization of the processes involved during the fabrication cycle.This review aims to report on the most useful applications of soft matter,where the capabilities offered by DH,such as three-dimensional(3D)imaging,extended focus,3D tracking,full-field analysis,high sensitivity,and a wide range of measurements from nanometers to centimeters,permit completely non-invasive characterizations on a full-scale.Several holographic experimental results of typical samples are reported and discussed where DH plays a primary role as a tool gauge for soft matter.

Parallax limitations in digital holography: a phase space approach

The viewing direction in Digital Holography can be varied if different parts of a hologram are reconstructed.In this article parallax limitations are discussed using the phase space formalism.An equation for the parallax angle is derived with this formalism from simple geometric quantities.The result is discussed in terms of pixel size and pixel number of the image sensor.Change of perspective is demonstrated experimentally by two numerical hologram reconstructions from different parts of one single digital hologram.

The application of digital holography for accurate three-dimensional localisation of mosquito-bednet interaction

Understanding mosquito interaction with long-lasting insecticidal bednets is crucial in the development of more effective intervention methods to protect humans from malaria transmission.As such,a 240×240×1000 mm laboratory setup for the in-line recording of digital holograms and subsequent in-focus reconstruction and 3D localisation of mosquitoes is presented.Simple bednet background removal methods are used to accurately localise a mosquito obscured by a bednet in 3D coordinates.Simulations and physical data demonstrate that this method is suitable for mosquitoes positioned 3−1000 mm behind a bednet.A novel post-processing technique,involving a cascade-correlation of a Tamura of Intensity focus metric extracted from digitally reconstructed scenes,accurately localises mosquitoes positioned 35−100 mm behind a bednet from a single digital hologram.The result of this study is a scalable digital holographic methodology to examine mosquito-bednet interaction in 3D at a level of accuracy previously only seen in 2D imaging of mosquitoes in a much smaller volume.

High-precision method for measuring the photothermal properties of transparent media with digital holography

Quantitative phase microscopy by digital holography provides direct access to the phase profile of a transparent subject with high precision. This is useful for observing phenomena that modulate phase, but are otherwise difficult or impossible to detect. In this letter, a carefully constructed digital holographic apparatus is used to measure optically induced thermal lensing with an optical path difference precision of less than 1 nm. Furthermore, by taking advantage of the radial symmetry of a thermal lens, such data are processed to determine the absorption coefficient of transparent media with precisions as low as 1 × 10-5cm-1 using low power （30 mW） continuous wave （CW） excitation.

Incoherent digital holographic spectral imaging with high accuracy of image pixel registration

Fresnel incoherent correlation holography(FINCH) is a unique three-dimensional(3D) imaging technique which has the advantages of scanning-free,high resolution,and easy matching with existing mature optical systems.In this article,an incoherent digital holographic spectral imaging method with high accuracy of spectral reconstruction based on liquid crystal tunable filter(LCTF) and FINCH is proposed.Using the programmable characteristics of spatial light modulator(SLM),a series of phase masks,none of whose focal lengths changes with wavelength,is designed and made.For each wavelength of LCTF output,SLM calls three phase masks with different phase constants at the corresponding wavelength,and CCD records three holograms.The spectral images obtained by this method have a constant magnification,which can achieve pixel-level image registration,restrain image registration errors,and improve spectral reconstruction accuracy.The results show that this method can not only obtain the 3D spatial information and spectral information of the object simultaneously,but also have high accuracy of spectral reconstruction and excellent color reproducibility.