Abstract: Investigations in performing an optical tomography of biological specimen have been carried out by using the phase-correlated single photon counting method. This method is capable of selecting non-interacted, on-beam-axis forward scattered and snake-line photons, i.e. the straight-forward propagating photons. The phase correlated single photon counting unit performs the mutual monitoring of a high-speed photomultiplier tube and adjacent gain by a broadband preamplifier. The detection occurs in phase correlation to the difference frequency of the optical modulation of the two arms of a Mach-Zehender interferometer arrangement. The object arm is crossing the scattering test medium. To prevent strong intensity ratios between the two interferometer arms the reference arm crosses an optically equivalent phantom medium, but without introduction of inhomogeneities. The correlation between the phase of the optical modulation and the electrically derived phase can be varied by 2$pi phase shifting of the electrical signal. This allows the user to set a phase range comparable to the time window in time-resolved detection. Two different principle methods can be used for generating tomographic images: the confocal scanning and conventional (object rotation and translation) tomographic setups. !9
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