This paper discusses ptychography, a coherent diffraction imaging technique. Advantages of ptychography with
respect conventional imaging techniques for cell visualisation are highlighted and demonstrated using unstained healthy
and tumorous mouse cells as the object under investigation. A novel procedure to automatically refocus a possible
slightly out of focus ptychographic data set will be discussed, by which an improved quantitative analysis and
discrimination is enabled.
A hybrid phase retrieval approach is proposed to address the twin image problem in the reconstruction of in-line digital holograms. The approach is a variant iterative transform algorithm and exploits two mostly natural constraints of a sample, namely, the finite transmission and the finite support. Here, the initial sample support estimate is first refined by applying the finite transmission constraint with phase flipping. The approach provides better reconstruction than if only the finite transmission constraint is used and improve the convergence rate of Fienup's algorithm owing to a better estimate of support especially for strong samples with complex structures. Both simulation and experimental results are presented.
In the field of diffraction microscopy, a coherent illuminating beam of finite extent impinges on a specimen and the
resulting diffraction pattern is recorded. The complex transmission function of the specimen is recovered using iterative
algorithms that exploit redundancies in the measured data. This is normally oversampling of the diffraction pattern when
it is known the object or illumination is of the finite size. In the case of curved illumination, there is no direct relationship
between the collection angle and the resolution of the recovered image. The result is a recovered image with varying
resolution over the field of view as different parts of the object are illuminated by different wave-vectors. An extension
of the Coherent Diffractive Imaging (CDI) technique (employing a single diffraction pattern) is to use multiple
diffraction patterns collected from adjacent parts of the object and is called ptychography. In ptychography, translation of
the illuminating wave across the specimen introduces translational diversity that leads to faster convergence of iterative
phase retrieval as well as extending the field of view. In this paper we investigate the expression of resolution
information in the diffraction pattern using curved illumination in order to facilitate specimen recovery with uniformly
improved resolution over the entire field of view.
In this paper, we present a phase retrieval method where a sequence of diffraction speckle intensities, recorded by tuning
the illumination wavelength, is used. These recordings, combined with an iterative calculation method, allow the
reconstruction of the amplitude and the phase of the wavefront. The main advantages of this method are: simple optical
setup and high immunity to noise and environmental disturbance, since no reference beam or additional moving parts are
needed. Furthermore, this method allows for an extended wrap-free phase measurement range by using synthetic
wavelengths. The technique shows great potential in some fields of micro-metrology, such as lensless phase contrast
imaging and wavefront sensing.
A practical method is proposed for the wavefront measurement of arbitrary complex-valued fields. A mask having
random phase is placed in the path between the object and the image sensor. Three or more diffraction patterns are
collected, as the mask translated in the direction parallel to the sensor. Phase retrieval is performed by propagating the
wave field back and forth between the sensor and the mask plane and making the following change on the calculated
wavefront: at the sensor plane, the modulus of calculated wavefront is replaced with the square root of recorded
intensity; while at the mask plane, the modulation phase is updated to the one corresponding to the next mask position
for next iteration. This process starts from a random estimate of the object field falling on the mask and ends when the
change of the amplitude of two successively retrieved object fields before the mask is below a given threshold. Further
propagation of the retrieved field from mask to object plane yields the original object field. Results from both simulated
data and experimental data show that this method works quite well in terms of its absence of stagnation, suitability for
complex-valued field, and high immunity to the noise in recordings. The technique is believed to find wide applications,
such as aspherical lens testing, and diffraction imaging of micro-objects.
In digital holographic microscopy, a high numerical aperture object lens of good quality is required in order to achieve high lateral resolution. As well known, such lenses usually have large aberrations and are difficult to fabricate, especially in the ultra-violet and infrared spectral regions. In these circumstances, a system without objective lens is highly preferred. According to imaging theory, this means that the hologram should be recorded with a high numerical aperture (NA). For the reconstruction of high NA holograms, the Rayleigh-Sommerfeld diffraction integral without approximation must be evaluated. However the current mostly used three algorithms, namely, the Fresnel algorithm, the angular spectrum algorithm, and the convolution algorithm are not suitable. In this paper, the properties of these algorithms are presented. Then a modified convolution algorithm is proposed. In this method, a shift parameter is introduced in the discrete representation of diffraction kernel and then reconstructions with different shift values are combined. The modified convolution method is able to give samplings of diffraction-limited resolution for the full field of view. The simulation results of point field with different reconstruction algorithm are presented. Experimental results of a test dot array are also given.
In this study we investigate the imaging mechanism of digital holography. The imaging process is separated into three steps: hologram recording, phase retrieval, and object field reconstruction. For hologram recording, the average effect due to the sensor pixel aperture and the role of the physical reference beam are addressed particularly. The average effect of pixel aperture is equivalent to a low pass filter, which acts on the interference term between the object field and the reference wavefront. An optimal physical reference beam is then to minimize the bandwidth of the interference term so that more object information can pass through the filter. For the reconstruction of object field, emphasis is paid on the correspondence between the underlying physical process and the discrete system represented by the reconstruction algorithms. The implication of sampling theory on each reconstruction algorithm is discussed in detail. The sampling
requirement imposes a limitation only on the maximum extension of object field. Our analysis indicates that the achievable spatial resolution by digital holography is determined by the recording numerical aperture and wavelength of light, the same as the conventional microscopy. The independent analysis of each part illumines the way to optimize the system performance.
This paper demonstrates the recent achievements in the field of Brillouin based distributed optical fiber temperature sensing. When a dispersion-shifted fiber was subjected to a temperature cycle between 20 and 820 °C, the Brillouin shift exhibited an undesired hysteresis with a maximum frequency discrepancy of larger than 48 MHz between heating and cooling processes. After the fiber was annealed for 9 h at 850 °C, however, the hysteresis almost disappeared for repeated temperature cycles in the ranges of 20-820 °C and of 500-800 °C with deviations of the measurements from the best-fit curve of less than ±12.5MHz. The temperature dependence of Brillouin shift in the range of 20-820 °C in the annealed fiber was well expressed by a second order function of temperature. A sensing scheme that utilizes both output signals of the fiber Mach-Zehnder interferometer used as an optical frequency discriminator has been proposed. The scheme that has the advantages of less system adjustment and fast measurement, combined with a suitably annealed fiber, offers a reliable means for the Brillouin shift-based distributed sensing over the wide temperature range.
Digital reconstruction of samples of the object wave front amplitude from samples of its hologram is addressed and is treated as a process of sampling the object wave front. Signal sampling is a linear transformation that is fully specified by its point spread function. Point spread functions of the hologram reconstruction algorithms are derived that explicitly show how the reconstruction results depend on the holographic setups and photographic camera physical parameters such as object-to-camera distance, radiation wave length, camera size, pitch, fill factor and alike. Three reconstruction algorithms are introduced and analyzed: a general algorithm and more commonly known Fourier and Convolution ones extended to enable reconstruction with arbitrary scale factor. For convolution algorithm, it is shown additionally that reconstruction results contain, in general, certain extra distortions as compared to general and Fourier reconstructions.
In digital holography, holograms are recorded by a CCD-array, and the complex amplitude of the object wave is numerically reconstructed via computer. For different recording conditions and different properties of objects, different reconstruction algorithms are required. The conventional reconstruction algorithms were conceived directly by replacing the diffraction integral with summation. Each method has its limitation in the valid range for correctly calculating the diffraction integral. The Single Fourier Transform method is valid for far Fresnel zone hologram, whereas the convolution method is appropriate for near Fresnel holograms. Here, we present a general reconstruction model from the perspective of “Generalized sampling theory”. Given that the function space in which the unknown complex amplitude lies, an approximation of the continuous complex amplitude at the CCD can be synthesized from a set of basis functions with the recorded samples as weights. Back-propagation of the approximated complex amplitude to the original object plane yields an expression relating the continuous complex amplitude of the object with the recorded samples. By adopting different basis functions and different formulas for describing the diffraction process, an optimal reconstruction algorithm can be developed for various recording conditions and different diffraction characteristics of the object. Contrary to the conventional algorithms where values are available only at specific grid, complex amplitude at any position of the object can be obtained using this model. In addition, the effect due to the non-zero fill factor of the CCD can also be incorporated into the reconstruction algorithm to be further compensated by over-weighting the high frequency components. Two basis functions: Dirac delta- and Sinc-, are studied in detail.
Temperature dependence of spontaneous Brillouin intensity in a dispersion-shifted fiber has been investigated in a wide temperature range theoretically and experimentally. It has been found that Brillouin intensity varied with temperature linearly in the range —27 °C to 819 °C by a coefficient of(0.26±0.02)%/°C after the fiber coating was pyrolyzed totally. Temperature measurement in the range above has been realized with a spatial resolution of 13 m at the end of 4 km long test fiber, which demonstrates the feasibility of the present system for the distributed sensing of wide-range temperature.