Jianglei Di

High resolution digital holographic microscopy with a wide field of view based on a synthetic aperture technique and use of linear CCD scanning

Theoretical analysis shows that, to improve the resolution and the range of the field of view of the reconstructed image in digital lensless Fourier transform holography, an effective solution is to increase the area and the pixel number of the recorded digital hologram. A new approach based on the synthetic aperture technique and use of linear CCD scanning is presented to obtain digital holographic images with high resolution and a wide field of view. By using a synthetic aperture technique and linear CCD scanning, we obtained digital lensless Fourier transform holograms with a large area of 3.5 cm x 3.5 cm (5000 x 5000 pixels). The numerical reconstruction of a 4 mm object at a distance of 14 cm by use of a Rayleigh-Sommerfeld integral shows that a theoretically minimum resolvable distance of 2.57 microm can be achieved at a wavelength of 632.8 nm. The experimental results are consistent with the theoretical analysis.

Recording and reconstruction of a color holographic image by using digital lensless Fourier transform holography

The numerical recording and reconstruction of a color holographic image are achieved by using digital lensless Fourier transform holography. Firstly, for a color object, three monochromatic digital holograms with different wavelengths (red, green, blue) are recorded by a black-white CCD, respectively. Then the reconstructed monochromatic holographic images (red, green, blue) are adjusted to be same in size through padding digital holograms with zeros, and the corresponding digital color holographic image is acquired by accurately syncretizing the resized reconstructed monochromatic images. One of the advantages using lensless Fourier transform holography is that it can well assure the precise superposition of the reconstructed images. By applying median filtering technique and superposing the speckle fields with different distributions, the speckle noises are well suppressed and the quality of the digital color holographic image is greatly improved. This digital color holography with high quality of reconstruction effect would have potential applications on digital holographic display of color objects.

Real-time visualization of Karman vortex street in water flow field by using digital holography

The Karman vortex street generated behind a circular cylinder in water flow field is displayed and analyzed in real time by means of digital holography. Using a modified Mach-Zehnder interferometer, a digital hologram of the flow field in still state and then a video of continuous digital holograms in flowing state are recorded at 14.6 frames per second by a CCD camera, respectively. A series of sequential phase maps of the flow field are numerically reconstructed from the holograms in different states above based on double-exposure holographic interferometry. By seriating these phase maps, the shape and evolution of Karman vortex street can be displayed in real time in the form of a movie. For comparison, numerical simulation of the Karman vortex street under the boundary conditions adopted in the experiment is also presented, and the consistent results indicate that the experimental observation of Karman vortex street by using digital holography is successful and feasible.

Real-time monitoring of the solution concentration variation during the crystallization process of protein-lysozyme by using digital holographic interferometry

We report a real-time measurement method of the solution concentration variation during the growth of protein-lysozyme crystals based on digital holographic interferometry. A series of holograms containing the information of the solution concentration variation in the whole crystallization process is recorded by CCD. Based on the principle of double-exposure holographic interferometry and the relationship between the phase difference of the reconstructed object wave and the solution concentration, the solution concentration variation with time for arbitrary point in the solution can be obtained, and then the two-dimensional concentration distribution of the solution during crystallization process can also be figured out under the precondition which the refractive index is constant through the light propagation direction. The experimental results turns out that it is feasible to in situ, full-field and real-time monitor the crystal growth process by using this method.

Resolution improvement of digital holographic images based on angular multiplexing with incoherent beams in orthogonal polarization states.

We present a method to improve the resolution of digital holographic images based on angular multiplexing with incoherent beams from two orthogonal polarized components of natural light. Two incoherent subholograms are synchronously recorded by two pairs of incoherent object waves and reference waves with orthogonal polarization states, in which the object is illuminated by two incoherent beams from different directions. The increase in resolution is obtained through phase correction and superposition of two reconstructed object waves. Experimental results show that the resolution and quality of the reconstructed image can be effectively improved.

Digital holographic interferometry based on wavelength and angular multiplexing for measuring the ternary diffusion

We present a novel method for dynamical measurement of the ternary diffusion by using digital holographic interferometry based on wavelength and angular multiplexing techniques. Two laser beams with different wavelengths (λ1=532 nm and λ2=473 nm) are used for the ternary diffusion measurement, and they are adjusted to make corresponding interference fringes in orthogonal directions on the CCD target. The orthogonal splitting of the fringes enables the spatial frequencies of each wavelength to be filtered separately in the Fourier spectrum of the hologram. Finally, they are reconstructed to obtain the molar concentration. This method is also suitable for two-phase diffusion measurement.

Transmission and total internal reflection integrated digital holographic microscopy.

We develop a transmission and total internal reflection (TIR) integrated digital holographic microscopy (DHM) by introducing a home-made Dove prism with a polished short side. With the help of angular and polarization multiplexing techniques, the 2D refractive index distribution of a specimen adhered on the prism surface is determined using TIR-DHM. Meanwhile, the thickness profile is unambiguously calculated from the phase information using transmission DHM. This integrated microscopy is nondestructive and dynamic and can be used to simultaneously measure the index distribution and thickness profile of transparent or semi-transparent liquid or solid samples.

Dual-wavelength common-path digital holographic microscopy for quantitative phase imaging based on lateral shearing interferometry

A dual-wavelength common-path digital holographic microscopy based on a single parallel glass plate is presented to achieve quantitative phase imaging, which combines the dual-wavelength technique with lateral shearing interferometry. Two illumination laser beams with different wavelengths (λ1=532  nm and λ2=632.8  nm) are reflected by the front and back surfaces of the parallel glass plate to create the lateral shear and form the digital hologram, and then the hologram is reconstructed to obtain the phase distribution with a synthetic wavelength Λ=3339.8  nm. The experimental configuration is very compact, with the advantages of vibration resistance and measurement range extension. The experimental results of the laser-ablated pit, groove, and staircase specimens show the feasibility of the proposed configuration.

Dynamical measurement of refractive index distribution using digital holographic interferometry based on total internal reflection

We present a method for dynamically measuring the refractive index distribution in a large range based on the combination of digital holographic interferometry and total internal reflection. A series of holograms, carrying the index information of mixed liquids adhered on a total reflection prism surface, are recorded with CCD during the diffusion process. Phase shift differences of the reflected light are reconstructed exploiting the principle of double-exposure holographic interferometry. According to the relationship between the reflection phase shift difference and the liquid index, two dimensional index distributions can be directly figured out, assuming that the index of air near the prism surface is constant. The proposed method can also be applied to measure the index of solid media and monitor the index variation during some chemical reaction processes.

Visual and quantitative measurement of the temperature distribution of heat conduction process in glass based on digital holographic interferometry

The temperature distribution of heat conduction process in transparent solid medium is visually and quantitatively measured based on digital holographic interferometry. A series of phase maps reflecting the temperature distribution of a glass sample during the heat conduction process are numerically reconstructed from the digital holograms. Then, based on the derived relationship between temperature variation and phase change, we obtained the full field temperature distribution of the glass sample by the method of sample point calibration. By seriating maps of the temperature distribution, a movie is produced to show the heat conduction phenomenon more vividly. What is more, based on the thermodynamic model of heat conduction, we numerically figured out the temperature distribution of the glass sample using the finite element algorithm. It turns out that the experimental results are consistent with the numerical simulation results very well.