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A special electronic circuit for phase-resolved imaging of scattering media like tissues has been developed. Two contrarily arranged acousto-optic modulators provide a frequency shift of about 1 MHz between an object beam, which crosses the medium, and a reference beam. These two beams have been derived from one laser diode source (670 nm). After passing the medium an interferometric heterodyning of the two beams is provided. That gives a periodic signal determined by the shift frequency. This signal has been optical-electrically transformed by a photomultiplier and adjacent photon-counting and compared to the electrically provided shift frequency, generated by the two oscillation frequencies of the acousto-optic modulators and an electronic circuit. So it is possible to select the photons with the undisturbed phase (in- phase-photons). That amount consists of the number of unscattered, directly forward scattered, and random phased background. The single and multiple scattered photons with various randomly distributed pathways together with the background noise should contribute a constant (dc) value. Therefore by phase-gated imaging a decrease of background noise is achieved. The electronic circuit performs an rf-mixer, analog phase-shifting unit, high-speed digital gate, photon-detecting unit including preamplifier and discriminator, dual-channel high-speed counter and a control logic. The counter readout is provided by an external PC via I/O-card. This instrumentation allows a high-resolution cw coherence imaging and performs in combination with conventional tomographic arrangements (object rotation and translation) or confocal scanning a new approach to optical tomography for biological specimen in the range up to 5 mm thickness.
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