Optical: systems and elements – Holographic system or element – For synthetically generating a hologram
Reexamination Certificate
1999-03-10
2001-07-17
Chang, Audrey (Department: 2872)
Optical: systems and elements
Holographic system or element
For synthetically generating a hologram
C359S010000, C359S011000, C359S029000, C359S032000, C382S210000, C382S254000
Reexamination Certificate
active
06262818
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for simultaneous amplitude and quantitative phase contrast imaging by numerical reconstruction of digitally encoded holograms.
2. Discussion of the Background
The idea of recording a hologram of the specimen and reconstructing the hologram with a numerical method has been reported for the first time in 1967 by J. W. Goodman and R. W. Laurence, “Digital image formation from electronically detected holograms,” Appl. Phys. Lett. 11, 77-79 (1967). who used a vidicon detector for the hologram recording, and in 1971 by M. A. Kronrod et al., “Reconstruction of a hologram with a computer,” Soviet Phys.—Technical Phys. 17, 333-334 (1972) who used a digitized image of a hologram recorded on a photographic plate. In these two references, the holograms were recorded in the holographic Fourier configuration for which amplitude contrast images of the specimen can be numerically reconstructed by simply calculating the modulus of the two-dimensional Fourier transform of the hologram. A recent development using a Charged Coupled Device (CCD) camera as recording device, also in the Fourier configuration, has been patented (U.S. Pat. No. 4,955,974 and U.S. Pat. No. 5,214,581) and reported by W. S. Haddad et al., “Fourier-transform holographic microscope,” Applied Optics 31, 4973-4978 (1992)., and by K. Boyer et al., “Biomedical three-dimensional holographic microimaging at visible, ultraviolet and X-ray wavelength,” Nature Medicine 2, 939-941 (1996)., for X-ray amplitude contrast imaging of biological specimens.
In 1994, on the basis of precedent works about digital holography, L. P. Yaroslavskii and N. S. Merzlyakov, Methods of Digital Holography, (Consultants Bureau. New York, 1980)., Schnars et al., “Direct recording of holograms by a CCD target and numerical reconstruction,” Applied Optics 33, 179-181 (1994). have reported the first numerically reconstructed amplitude contrast image from an off-axis hologram recorded in the Fresnel holographic configuration with a CCD camera. With the same reconstruction algorithm but with a low-coherence light source, E. Cuche et al., “Optical Tomography at the Microscopic scale by means of a Numerical Low Coherence Holographic Technique,” Proceedings SPIE on Optical and Imaging techniques for Bioimaging, Vienna, Vol. 2927, 61-66 (1996) have reported an application of numerical holography for tomographic imaging. An application in micro-endoscopy, using also a Fresnel calculation for the hologram reconstruction, has been reported by O. Coquoz et al., “Performances of endoscopic holography with a multicore optical fiber,” Applied Optics 34, 7186-7193 (1995).
All the above mentioned works concern amplitude contrast imaging for which only the modulus of the numerically reconstructed optical field is considered. The first use of a numerically reconstructed phase distribution, from a Fresnel hologram, has been reported by Schnars et al., “Direct phase determination in hologram interferometry with use of digitally recorded holograms,” J.Opt.Soc.Am.A 11, 2011-2015 (1994). for an application in holographic interferometry. As presented in Ref. Schnars et al., the used reconstruction algorithm is the same as for amplitude contrast imaging and do not really allows phase contrast imaging, because, in this case, the reconstructed optical field is the product of the object wave by the complex conjugate of the reference wave (or the product of the complex conjugate of the object wave by the reference wave). However, this reconstruction algorithm can be used in holographic interferometry, because the subtraction between two reconstructed phase distributions obtained with the same reference wave provides an image which represents only the phase difference between the deformed and undeformed states of the object.
The first example of a numerically reconstructed phase contrast image has been reported in 1997 by E. Cuche et al., “Tomographic optique par une technique d'holographie numérique en faible coherence,” J. Opt. 28, 260-264 (1997). who have used a modified reconstruction algorithm including a multiplication of the digital hologram by a digital replica of the reference wave (digital reference wave). In Ref. Cuche et al., the phase contrast image has been obtained with a plane wave as reference and in direct observation, meaning that no magnification or demagnification optics is introduced along the optical path of the object wave.
SUMMARY OF THE INVENTION
In comparison with existing methods for phase contrast microscopy, such as the Zernicke or the Nomarski methods, the present invention provides a straightforward link between phase contrast imaging and optical metrology.
“Quantitative phase contrast” means that the phase contrast image is free of artifacts or aberrations and can be directly used for quantitative measurements of optical properties (e.g. refractive index) or structural information (e.g. topography, thickness). More precisely, “quantitative phase contrast” means here that the value of each pixel of the phase contrast image is equal, modulo 2&pgr;, to the value of the phase of the object wave at the corresponding area of specimen.
“Simultaneous amplitude and quantitative phase contrast” means that two images of the specimen can be reconstructed from the same hologram. One of these images with an amplitude contrast and the other one with a quantitative phase contrast. These images can be analyzed separately or compared the one with the other. Their information content (or the information content of several pairs of images reconstructed for different orientations of the specimen) can be associated in order to build a computed three-dimensional description of the specimen, for example a three-dimensional tomography of semi-transparent specimens. Spectroscopic measurements are also possible with the present invention by recording two or more holograms of the same specimen at different wavelengths. Such multi-wavelength procedure can also be useful for the precise determination of objects dimensions or refractive index in metrology.
In comparison with standard interference microscopy techniques which also provides simultaneously amplitude and quantitative phase contrasts, the advantage of the present invention is that the recording of only one hologram is necessary while the recording of four or more interferograms is required with interference microscopy. Moreover, no displacement or moving of optical elements is needed with the present invention. As a consequence, the acquisition time is reduced providing a lower sensitivity to thermal and mechanical drifts. Robustness is a great assess of the present invention.
An other important advantage of the present invention in comparison with interference microscopy is that the phase aberrations are corrected digitally. Indeed, a microscope objective which is introduced in an interferometer produces a curvature of the wavefronts which affects the phase of the object wave. Therefore, for phase contrast imaging with the present invention or more generally in any interferometric system, this phase aberration must be corrected. In interference microscopy, this problem is solved experimentally by inserting the same microscope objective in the reference arm, at equal distance from the exit of the interferometer. This arrangement called Linnick interferometer requires that if any change has to be made in the object arm, then the same change must be precisely reproduced in the reference arm in such a way that the interference occurs between similarly deformed wavefronts. As a consequence the experimental configuration requires a very high degree of precision. An other possibility (Mirrau interferometry) consists in magnifying the interference pattern. However, it is difficult to achieve high resolution imaging with this technique because a miniaturized interferometer must be inserted between the sample and the microscope objective. The present invention proposes a purely digital method which allows us to perform the correctio
Cuche Etienne
Depeursinge Christian
Chang Audrey
Institute of Applied Optics, Swiss Federal Institute of Technolo
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
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