M.A. Webster

Spectral and temporal speckle field measurements of a random medium

The zero-mean circular complex Gaussian field statistics of a random medium are experimentally demonstrated in the optical domain, thus verifying this key assumption of statistical optics. Using a frequency-tunable laser source in a fixed-path-length interferometer, we obtain optical field fluctuations in the time and frequency domains that clearly show that the ensemble-averaged temporal intensity converges to the photon transit time distribution, which for the samples used is in excellent agreement with a diffusion model.

Study of scattering media with polarized coherent light

Speckle patterns measured as a function of frequency are used to describe the polarization properties for scattering samples. We demonstrate differences in the degree of polarization and frequency resolved correlations for differing amounts of scatter

Time-domain response of a diffusive medium from speckle pattern frequency correlations

Summary form only given. The optical characterization of scattering media has many important biological and environmental sensing applications. Useful spectroscopic or imaging data from within a scattering domain requires some form of measured data inversion with a model for the scattering domain. Laser speckle data using light of appropriate coherence, coupled with a diffusion equation model, can be used to extract the scattering domain parameters. An external-cavity laser diode whose center frequency could be tuned over a range of approximately 60 GHz was used to illuminate several diffusive media slab samples. Using suitable imaging optics consisting of an aperture and lens, the intensity speckle pattern from the output face of the sample was measured using a CCD camera.

Variable coherence in determining the scattering parameters of diffuse media using laser speckle

Summary form only given. We demonstrate the application of a variable-coherence laser source for determining the scattering parameters of a diffuse medium and the potential for imaging spatially-dependent scatter. A key concept in this work in the ability to synthesize a variable-coherence source by frequency modulating a tunable laser diode (with a center wavelength of 850 nm) at a rate much faster than the integration time of the detector. This allows for a rapid measurement and the adaption of laser coherence to the degree of scatter, which we show is critical in obtaining the necessary sensitivity.

Obtaining photon transit time distribution of diffuse media using laser speckle statistics for imaging applicatlons

Summary form only given.Optical diffusion tomography typically uses a low coherence source, such as an LED, whose output intensity is sinusoidally modulated, and involves a coherent detection scheme at this modulation frequency. These measurements yield the frequency domain representation of the photon transit time distribution for travel through a scattering medium. Inversion of this data, based upon a diffusion equation model for the medium, is performed to reconstruct the imaging domain.

Characterization of thick scattering media via speckle measurements using a tunable coherence source

Summary form only given. Non-intrusive characterization of thick optically scattering media and detection of embedded inhomogeneities is of interest for various industrial and biomedical applications. We recently demonstrated that laser speckle statistics, which previously had been used for characterization of surface roughness, could also be used for characterization of thick, optically scattering media. Our measurements are based on the dependence of the speckle contrast on the laser coherence time relative to the variance in photon travel times through the medium. Therefore, for an appropriate coherence time, material parameters can be inferred from the observed speckle contrast. We have previously determined material thickness and scattering parameters and detected inhomogeneities embedded within turbid media, using a fixed coherence length (HeNe) laser. In our current experiments, we perform the speckle contrast measurements as a function of coherence length, which we vary by frequency modulating a diode laser, and demonstrate good agreement with a theoretical model based on photon diffusion. By varying the laser bandwidth, we can extract material parameters without adjusting material thickness or we can optimize the coherence time over a wide range as needed to characterize materials with different scattering parameters. We note also that our method is conceptually similar to photon diffusion imaging, where one measures variation in an output RF modulation as a function of modulation frequency relative to variances in photon travel times, but the implementation is completely different.