The detection of ballistic and diffuse light in confocal and heterodyne imaging systems in transillumination is studied experimentally and theoretically. We find an optimum pinhole size for ballistic light detection and diffuse light rejection for confocal imaging. The ratio of ballistic and diffuse light is found to be determined primarily by sample parameters and aberrations introduced by the sample. For sample and illumination characteristics that are typical for biomedical imaging, the limits of ballistic light detection in confocal imaging are close to the noise limits of standard detectors. Heterodyne detection with narrow-bandwidth light can extend these limits, depending on the spatial and the temporal coherence properties of the transmitted scattered light.
We demonstrate autocorrelation measurements of 85-fs Ti:sapphire laser pulses, using a 32-pixel ZnSe detector array in a single-shot geometry. The two-photon photoconductor is fabricated by deposition of an array of interdigitated gold fingers on a single-crystal ZnSe substrate.
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