Ultrafast, cross-correlated harmonic imaging through scattering media.

A simple upconversion scheme utilizing 40-fs pulses is shown to permit high-contrast imaging of objects obscured by a highly scattering medium when no ballistic component is evident in the scattered light and imaging is performed with any portion of the scattered light pulse. We present a time-gated, inherently low-pass spatially filtered imaging method that minimizes signal-averaging requirements and greatly facilitates imaging under severe scattering (turbid) conditions.

Investigation of ultrafast time gating by spatial filtering.

With a spatial-filtering method of gating, we explore image formation through scattering media using first-arriving light. Gating times of a few femtoseconds and less are produced, and the resolution at these extremely short gating times is investigated.

Comparison of field correlations in multiply scattered quasi-monochromatic light.

An experiment is described that directly compares the degradations, with the number of scattering mean free paths, of two field correlations that may be used to form gates for imaging techniques in scattered light: the correlation of the scattered wave with an unscattered reference wave and the correlation of two wave-vector components of the scattered wave itself. Results for 20-microm polymer spheres show that the latter correlation is consistently larger well into the multiple-scattering regime (up to 10 mean free paths) for wave-vector separations less than at least 50 mm(-1) and that the two correlations tend to merge in this scattering regime for larger wave-vector separations.

Realization of time gating by use of spatial filtering.

A method for simulating conventional time gating in low-coherence optical imaging processes in highly scattering media is given. The method uses monochromatic instead of broadband light, and spatial filtering is substituted for time gating. The process enables the study of imaging techniques in scattering media to be carried out in an easy and highly controllable way. Experimental results are given.

Ensemble-averaged Shack-Hartmann wave-front sensing for imaging through turbid media.

A technique is described for ensemble-averaging the light wave emerging from a turbid medium, enabling the recovery of optical information that is otherwise lost in a speckle pattern. The technique recovers both an amplitude and a phase function for a wave that has been corrupted by severe scattering, without the use of holography. With the phase estimated, an ensemble-averaged field is constructed that can be backprojected to form an image of the object obscured by the scattering medium. Experimental results suggest that the technique can resolve two object points whose signals are unresolved on the exit surface of a diffuser.