Digital holography for quantitative phase-contrast imaging

We present a new application of digital holography for phase- contrast imaging and optical metrology. This holographic imaging technique uses a CCD camera for recording of a digital Fresnel off-axis hologram and a numerical method for hologram reconstruction. The method simultaneously provides an amplitude- contrast image and a quantitative phase-contrast image. An application to surface profilometry is presented and shows excellent agreement with contact- stylus probe measurements. (C) 1999 Optical Society of America.

Polarization imaging by use of digital holography

Tristan Colomb, Etienne Cuche, Christian Depeursinge, Pierre Marquet

We present what we believe to be a new digital holographic imaging method that is able to determine simultaneously the distributions of intensity, phase, and polarization state at the surface of a specimen on the basis of a single image acquisition. Two reference waves with orthogonal polarization states interfere with the object wave to create a hologram that is recorded on a CCD camera. Two wave fronts, one for each perpendicular polarization state, are numerically reconstructed in intensity and phase. Combining the intensity and the phase distributions of these two wave fronts permits the determination of all the components of the Jones vector of the object-wave front. We show that this method can be used to image and measure the distribution of the polarization state at the surface of a specimen, and the obtained results indicate that precise quantitative measurements of the polarization state can be achieved. An application of the method to image the birefringence of a stressed polymethyl methacrylate sample is presented.

2002

Numerical reconstruction of digital holograms

Etienne Cuche

This thesis presents a new method for the numerical reconstruction of digital holograms recorded in the off-axis configuration using a CCD camera. The propagation of the reconstructed wave front, from the hologram plane to an observation plane, is achieved by a calculation of scalar diffraction in the Fresnel approximation. An image representing the distribution of intensity at the surface of the object can be obtained by calculating the intensity of the reconstructed wave front. The focusing of this image is performed digitally by adjusting a parameter which defines the distance between the hologram and the observation plane. A second image representing the phase distribution at the surface of the object can be obtained from the same hologram by calculating the argument of the reconstructed wave front. We show here that for getting a phase distribution that represents only the contribution of the object wave, the hologram or the reconstructed wave front must be multiplied by a computed replica of the complex conjugate of the wave used as reference during the recording. For a plane reference wave, the reconstruction of the phase distribution requires a precise adjustment of two digital parameters defining its propagation direction. An application to surface profilometry has demonstrated the quantitative nature of the obtained phase distribution which gives access to the topography of the object. A vertical resolution of λ/100 has been estimated on the basis of a preliminary study. The introduction of a microscope objective along the optical path of the object wave enhances the transverse resolution of the method. In this case, the wave front deformation produced by the microscope objective can be digitally compensated. Under the assumption of a paraboloidal deformation, we show that the wave front curvature can be numerically corrected by multiplication with an array of complex numbers computed using the complex conjugate of the analytical expression describing the defocusing aberration. A preliminary study indicates that the transverse resolution of the method approaches that of classical optical microscopy (diffraction limit). The application of a digital technique of spatial filtering allows for the elimination of the zero order of diffraction and of the twin image. This operation enhances the signal to background ratio. We show also that the influence of diffraction effects associated to the numerical reconstruction method can be reduced by apodization of the hologram aperture. This operation is performed digitally by means of a procedure which defines the transmission of the hologram aperture using a cubic spline interpolation.

EPFL2000

3D imaging of surfaces and cells by numerical reconstruction of wavefronts in Digital Holography applied to transmission and reflection microscopy

Tristan Colomb, Etienne Cuche, Christian Depeursinge, Pierre Magistretti, Mihaela Marian, Pierre Marquet, Frédéric Montfort

We report on a numerical method for the reconstruction of wavefronts in Digital Holographic Microscopy (DHM). It yields simultaneous amplitude and quantitative phase contrast imaging by acquiring a single hologram. The reconstruction method computes the propagation of the complex optical wavefront diffracted by the object and is used to determine the refractive index and/or shape of the object with accuracies in the nanometer range along the optical axis. The technique comprises the recording of a digital hologram of the specimen by means of a standard CCD camera at the exit of a Mach-Zender or Michelson type interferometer and an adjustment procedure to determine precisely the reconstruction parameters. The quantitative nature of the reconstructed phase distribution has been demonstrated by an application to surface profilometry where step height differences of a few nanometers have been measured. Another application takes place in biology for transmission phase-contrast imaging of living cells in culture with resolution reaching 30 nanometers in height.

IEEE2002

Numerical reconstruction of digital holograms

Etienne Cuche

EPFL2000