Transient functional blood flow change in the human brain measured noninvasively by diffusing-wave spectroscopy.

Multispeckle diffusing-wave spectroscopy (DWS) is used to measure blood flow transients in the human visual cortex following stimulation by 7.5 Hz full-field and checkerboard flickering. The average decay time tau(d) characterizing the decay of the DWS autocorrelation function shows a biphasic behavior; within about 2 s after stimulation onset, tau(d) increases rapidly to about 6% above the baseline value. At later times, tau(d) slowly decreases and reaches a steady-state value about 5% below the baseline value after about 15 s. The initial increase of the DWS signal suggests a transient reduction of the cortical blood flow velocity shortly after stimulation onset. Measurements of this transient response at different positions over the primary visual cortex show a spatial pattern different from the one measured by electroencephalography.

Fiber-based multispeckle detection for time-resolved diffusing-wave spectroscopy: characterization and application to blood flow detection in deep tissue.

We present a technique for the measurement of temporal field autocorrelation functions of multiply scattered light with subsecond acquisition time. The setup is based on the parallel detection and autocorrelation of intensity fluctuations from statistically equivalent but independent speckles using a fiber bundle, an array of avalanche photodiodes, and a multichannel autocorrelator with variable integration times between 6.5 and 104 ms. Averaging the autocorrelation functions from the different speckles reduces the integration time in diffusing-wave spectroscopy experiments drastically, thus allowing us to resolve nonstationary scatterer dynamics with single-trial measurements. We present applications of the technique to the measurement of arterial and venous blood flow in deep tissue. We find strong deviations both of the shape and characteristic decay time of autocorrelation functions recorded at different phases of the pulsation cycle from time-averaged autocorrelation functions.

Programmable common-path vector field synthesizer for femtosecond pulses.

We demonstrate a novel design for a femtosecond vector field synthesizer. Pulse shaping of all four degrees of freedom of the electric field (amplitude, phase, ellipticity, and orientation angle) is achieved with a single 1D double-layer spatial light modulator in a zero-dispersion compressor by modulating the amplitude and phase of the two transverse polarization components in separate halves of the modulator. Being a common-path arrangement, it is interferometrically stable and therefore usable for long-term measurements. The method can be broadly applied in coherent control and nonlinear spectroscopy.

Shaping a femtosecond pulse with a programmable thermo-optically driven phase modulator.

A femtosecond pulse shaping apparatus based on a thermo-optically driven spatial phase modulator is presented. The modulator cell is illuminated by a standard projector and is easily controllable with a computer. In principle the setup allows for two dimensional pulse shaping, however, all initial demonstrations reported here, such as feedback controlled dispersion compensation or pulse train generation, are performed in a one dimensional phase-only shaping geometry.