Characterization of surface acoustic waves by stroboscopic white-light interferometry.

We present phase-sensitive absolute amplitude measurements of surface acoustic wave fields obtained using a stroboscopic white-light interferometer. The data analysis makes use of the high resolution available in the measured interferometric phase data, enabling the characterization of the out-of-plane surface vibration fields in electrically excited microstructures with better than 100 pm amplitude resolution. The setup uses a supercontinuum light source with tailored spectral properties for obtaining the high amplitude resolution. The duration of the light pulses is less than 300 ps to allow the detection of high frequencies. These capabilities enabled a detailed measurement of the focusing of surface acoustic waves by an annular interdigital transducer structure operating at 74 MHz, featuring a maximum vibration amplitude of 3 nm.

Picosecond supercontinuum light source for stroboscopic white-light interferometry with freely adjustable pulse repetition rate.

We present a picosecond supercontinuum light source designed for stroboscopic white-light interferometry. This source offers a potential for high-resolution characterization of vibrational fields in electromechanical components with frequencies up to the GHz range. The light source concept combines a gain-switched laser diode, the output of which is amplified in a two-stage fiber amplifier, with supercontinuum generation in a microstructured optical fiber. Implemented in our white-light interferometer setup, optical pulses with optimized spectral properties and below 310 ps duration are used for stroboscopic illumination at freely adjustable repetition rates. The performance of the source is demonstrated by characterizing the surface vibration field of a square-plate silicon MEMS resonator at 3.37 MHz. A minimum detectable vibration amplitude of less than 100 pm is reached.

Stroboscopic white-light interferometry of vibrating microstructures.

We describe a LED-based stroboscopic white-light interferometer and a data analysis method that allow mapping out-of-plane surface vibration fields in electrically excited microstructures with sub-nm amplitude resolution for vibration frequencies ranging up to tens of MHz. The data analysis, which is performed entirely in the frequency domain, makes use of the high resolution available in the measured interferometric phase data. For demonstration, we image the surface vibration fields in a square-plate silicon MEMS resonator for three vibration modes ranging in frequency between 3 and 14 MHz. The minimum detectable vibration amplitude in this case was less than 100 pm.

Correlation of visual-evoked hemodynamic responses and potentials in human brain.

The interaction of brain hemodynamics and neuronal activity has been intensively studied in recent years to yield better understanding of brain function. We investigated the relationship between visual-evoked hemodynamic responses (HDRs), measured with near-infrared spectroscopy (NIRS), and neuronal activity in humans, approximated with the stimulus train duration or with visual-evoked potentials (VEPs). Concentration changes of oxyhemoglobin (HbO(2)) and deoxyhemoglobin (HbR) in tissue and VEPs were recorded simultaneously over the occipital lobe of ten healthy subjects to 3, 6, and 12 s pattern-reversing checkerboard stimulus trains having a reversal frequency of 2 Hz. We found that the area-under-the-curves (Sigma) of HbO(2) and HbR were linearly correlated with the stimulus train duration and with the SigmaVEP summed over the 3, 6, and 12 s stimulus train durations. The correlation was stronger between the SigmaHbO(2) or the SigmaHbR and the SigmaVEP than between the SigmaHbO(2) or the SigmaHbR and the stimulus train duration. The SigmaVEPs explained 55% of the SigmaHbO(2) and 74% of the SigmaHbR variance, whereas the stimulus train duration explained only 45% of the SigmaHbO(2) and 51% of the SigmaHbR variance. We used Sigma of the NIRS responses and VEPs because we wanted to incorporate all possible processes (e.g., attention, habituation, etc.) affecting the responses. The results indicate that the relationship between brain HDRs and VEPs is approximately linear for 3-12 s long stimulus trains consisting of checkerboard patterns reversing at 2 Hz. To interpret hemodynamic responses, the measurement of evoked potentials is beneficial compared to the use of indirect parameters such as the stimulus duration. In addition, interindividual differences in the HbO(2) and HbR responses may be partly explained with differences in the VEPs.

Hemodynamic responses to speech and music in newborn infants.

We used near-infrared spectroscopy (NIRS) to study responses to speech and music on the auditory cortices of 13 healthy full-term newborn infants during natural sleep. The purpose of the study was to investigate the lateralization of speech and music responses at this stage of development. NIRS data was recorded from eight positions on both hemispheres simultaneously with electroencephalography, electrooculography, electrocardiography, pulse oximetry, and inclinometry. In 11 subjects, statistically significant (P < 0.02) oxygenated (HbO2) and total hemoglobin (HbT) responses were recorded. Both stimulus types elicited significant HbO2 and HbT responses on both hemispheres in five subjects. Six of the 11 subjects had positive HbO2 and HbT responses to both stimulus types, whereas one subject had negative responses. Mixed positive and negative responses were observed in four neonates. On both hemispheres, speech and music responses were significantly correlated (r = 0.64; P = 0.018 on the left hemisphere (LH) and r = 0.60; P = 0.029 on the right hemisphere (RH)). On the group level, the average response to the speech stimuli was statistically significantly greater than zero in the LH, whereas responses on the RH or to the music stimuli did not differ significantly from zero. This suggests a more coherent response to speech on the LH. However, significant differences in lateralization of the responses or mean response amplitudes of the two stimulus types were not observed on the group level.

Comparison between a time-domain and a frequency-domain system for optical tomography.

The quality of phase and amplitude data from two medical optical tomography systems were compared. The two systems are a 32-channel time-domain system developed at University College London (UCL) and a 16-channel frequency-domain system developed at Helsinki University of Technology (HUT). Difference data measured from an inhomogeneous and a homogeneous phantom were compared with a finite-element method (diffusion equation) and images of scattering and absorption were reconstructed based on it. The measurements were performed at measurement times between 1 and 30 s per source. The mean rms errors in the data measured by the HUT system were 3.4% for amplitude and 0.51 deg for phase, while the corresponding values for the UCL data were 6.0% and 0.46 deg, respectively. The reproducibility of the data measured with the two systems was tested with a measurement time of 5 s per source. It was 0.4% in amplitude for the HUT system and 4% for the UCL system, and 0.08 deg in phase for both systems. The image quality of the reconstructions from the data measured with the two systems were compared with several quantitative criteria. In general a higher contrast was observed in the images calculated from the HUT data.