Yaron Hefetz

Experimental tests of a simple diffusion model for the estimation of scattering and absorption coefficients of turbid media from time-resolved diffuse reflectance measurements

When a picosecond light pulse is incident upon a turbid medium such as tissue, the temporal distribution of diffusely reflected and transmitted photons depends on the optical absorption and scattering properties of the medium. From diffusion theory it is possible to derive analytic expressions for the pulse shape in terms of the optical interaction coefficients of a homogeneous semi-infinite medium. Experimental tests of this simple model in tissue-simulating liquid phantoms of different geometries are presented here. The results of these tests show that, in a semi-infinite phantom, the application of the diffusion model provides estimates of the absorption and transport-scattering coefficients that are accurate to better than 10%. Comparable accuracy was also obtained with this simple model for finite slab, cylindrical, and spherical volumes as long as the objects were of sufficient size. For smaller volumes the absorption coefficient was overestimated because of the significant loss of photons at the bounda ries of the object.

The potential of time-resolved reflectance measurements for the noninvasive determination of tissue optical properties

The time spectrum of photons diffusely reflected from a turbid medium such as tissue following a short incident light pulse depends on the optical absorption and scattering properties of the medium. The results of diffusion model calculations are presented, together with preliminary experimental tests of the model using liquid phantoms to simulate tissue optical properties.

Time-resolved diffuse reflectance and transmittance studies in tissue simulating phantoms: a comparison between theory and experiment

When a picosecond light pulse is incident on an optically turbid medium such as tissue, the temporal distribution of diffusely reflected and transmitted photons depends on the optical absorption and scattering properties of the medium. From diffusion theory it is possible to derive analytic expressions for the pulse shape in terms of the optical interaction coefficients of a homogeneous semi-infinite medium. Experimental tests of this model in tissue-simulating liquid phantoms of different geometries are presented here.

Grid-based simulation program for gravitational wave interferometers with realistically imperfect optics

We describe an optical simulation program that models a complete, coupled-cavity interferometer such as those used by the Laser Interferometer Gravitational-Wave Observatory (LIGO) Project. A wide variety of interferometer deformations can be modeled, including general surface roughness and substrate inhomogeneities, with no a priori symmetry assumptions about the nature of interferometer imperfections. Several important interferometer parameters are optimized automatically to achieve the best possible sensitivity for each new set of perturbed mirrors. The simulation output dataset includes the circulating powers and electric fields at various points in the interferometer, both for the main carrier beam and for its signal-sideband auxiliary beams, allowing an explicit calculation of the shot-noise-limited gravitational-wave sensitivity of the interferometric detector to be performed. Here we present an overview of the physics simulated by the program, and demonstrate its use with a series of runs showing the degradation of LIGO performance caused by realistically deformed mirror profiles. We then estimate the effect of this performance degradation upon the detectability of astrophysical sources of gravitational waves. We conclude by describing applications of the simulation program to LIGO research and development efforts.

Evaluation of the performance of polished mirror surfaces for the TAMA gravitational wave detector by use of a wave-front tracing simulation

We evaluated the performance of polished mirror surfaces for the TAMA interferometric gravitational wave detector by comparing the experimental results with a wave-front tracing simulation. The TAMA mirror surfaces were polished to a roughness of a few nanometer rms. We confirmed that these polished mirrors do not limit the present TAMA sensitivity and that the target shot-noise sensitivity will be achieved with these mirrors, even if a power-recycling technique is introduced in the next stage of the TAMA.

The Potential Of Time-Resolved Reflectance Measurements For The Noninvasive Determination Of Tissue Optical Properties

The time spectrum of photons diffusely reflected from a turbid medium such as tissue following a short incident light pulse depends on the optical absorption and scattering properties of the medium. The results of diffusion model calculations are presented, together with preliminary experimental tests of the model using liquid phantoms to simulate tissue optical properties.