Diffuse optical tomography using near infrared light has been developed for the purpose of imaging oxygenation states in human body. The technology is classified into three groups by measuring techniques; CW, frequency and time domain techniques. This review describes light propagation in biological tissues and their optical properties for solving the forward problem in the image reconstruction algorithm which resorts to the technique of inverse problems. Recent development of the algorithms are also reviewed and future applications are described.

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Near infrared spectroscopy and imaging have been applied to measure brain activities. The light scattering in the tissue causes ambiguity in the volume of tissue sampled with a source-detector pair of near infrared instruments and hence theoretical modelling to obtain light propagation in the head is very important for the brain activity measurements. In this paper two theoretical methods, Monte Carlo simulation and hybrid Monte Carlo-diffusion method, for calculation of light propagation in the brain are reviewed. The light propagation in the brain is strongly affected by the heterogeneity of tissue, especially the presence of low scattering cerebrospinal fluid (CSF). The results indicate that the presence of the CSF layer improves the sensitivity of the optical signal to brain activities.

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Optical topography, which visualizes brain activation, has been developed over the last several years. It uses near-infrared light to measure the blood-volume change associated with the brain activation in the cortex. The changes are measured at plural points, and topographic images, which show the distributions of the blood volume change, are obtained. In the current study, such optical topography was used to measure brain-functions. To improve spatial resolution of the topographical image, high-density optical fiber arrangement was proposed. Phantom experiment shows that the spatial resolution is improved by the high-density measurement.

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Near-infrared spectroscopy (NIRS) measures changes in the hemoglobin oxygenation state in the human brain. NIRS has recently started to be used for neuroimaging as well as for clinical monitoring. CW-type NIRS instruments have high temporal resolution and allow prolonged-time and continuous measurements, but they do not provide absolute values of changes in hemoglobin concentrations. In contrast, time-resolved spectroscopy (TRS), which uses short-pulse laser diodes as light sources, makes quantification possible. Quantification is necessary for imaging of brain activity. Recently, a 64-channel time-resolved optical tomographical imaging system (optical CT) and a single-channel TRS instrument have been developed. By the use of this optical CT or combining the single-channel TRS instrument with the multichannel CW-type NIRS instrument, we can obtain topographical images. NIRS is completely noninvasive and does not require strict motion restriction during measurements, unlike PET and fMRI. NIRS will provide a new direction for functional mapping studies.

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We review the application of Optical coherence tomography (OCT) for visualizing a depth resolved functional structure of cat brain in vivo. The OCT system is based on the known fact that neural activation induces structural changes such as capillary dilation and cellular swelling. Detecting these changes as an amplitude change of the scattered light, an OCT signal reflecting neural activity i.e., fOCT (functional OCT) could be obtained. Experiments have been done to obtain a depth resolved stimulus-specific profile of activation in cat visual cortex.

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Basic methodology and recent technology of real-time imaging systems to observe brain activities by using voltage sensitive dyes (VSD) were described. By the pioneer studies that were done in 1970-80, it was shown that some dyes change their optical properties, like fluorescence, by neural activity. This finding enables us to obtain a multi site recording without any electrode. Recent image sensor technologies can provide high spatiotemporal resolution for brain activity recordings. We developed a CMOS imaging system which has 100x100 resolution and 0.1 msec scanning time for this purpose.

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A table-top sized hybrid fluorescence spectroscopy system with picosecond, 3-MeV electron-beam from a photocathode and picosecond optical pulses was successfully demonstrated. This system has shown that the properties of an electron-beam pumped Ce:LiCAF: ultraviolet laser medium significantly differ from those of an optically pumped medium. The case of electron-beam pumped, the longer wavelength side of the spectrum is significantly enhanced and the fluorescence lifetime of this medium became slightly longer compared to the optically pumped case. This experimental results suggest that additional potential advantages of electron-beam pumping of Ce:LiCAF for an ultrashort pulse amplifier can be derived from its modified fluorescence spectral shape and longer lifetime.

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We used a laser-induced fluorescence (LIF) method to monitor plant damage caused by environmental changes. A time-resolved LIF measurement system which can monitor rapid fluorescence phenomena was developed with a 200-fs and 660-nm laser source and a spectroscopic detection system with a time resolution of 30 ps. Leaf samples of a morning glory irradiated by UV-B were prepared for investigating the capability of the system for monitoring the damage process. The chlorophyll fluorescence lifetime of the leaves was monitored at 685 nm and 735 nm. At those wavelengths, the LIF lifetimes became shorter during UV-B irradiation, tending to recover to the control values after quitting the irradiation, and then finally exhibiting shorter lifetimes again. Relationships between the lifetimes and the plant's activities are discussed.

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In order to increase the plasma production efficiency in laser triggered lightning experiments, we propose that a hyper-velocity micro-particle flow produced by a chemical explosion is used along the laser beam path. A preliminary experiment shows the feasibility of the proposed method in the laboratory by using a 100-J, 10-ns CO₂ laser.

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We demonstrate highly efficient frequency doubling of a 160 mW-AlGaAs three-section tunable distributed Bragg reflector (DBR) laser diode with real refractive-index-guided structure in a quasi-phase-matched second harmonic generation (QPM-SHG) ridge-type waveguide device. We realized high coupling efficiency of the DBR laser diode and the QPM-SHG waveguide device using a planar-type butt-coupling configuration. High-power SHG blue violet laser was fabricated, whose volume is 0.3 cc. This SHG blue-violet laser satisfied the several specifications for optical disk systems, and the pulsed blue-violet light (410 nm) power of 62 mW was generated.

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When a continuously oscillating CO₂ laser beam is irradiated to the surface of water, an impact sound is produced; in addition, a crater is formed on the water surface. The process by which an impact sound is produced, and a crater is formed, when a CO₂ laser beam of 7.5-21 W is irradiated to the surface of water was investigated using a high-speed video camera (2000 frames/second) and a high-sensitivity condenser microphone having a reaction time of 0.5 × 10^-6 second in the 10-35 kHz frequency range. As a result, the following was determined. (1) The source of the impact sound is created on the water surface. (2) An impact sound is produced from the water surface, and a crater is formed on the water surface, when the power density of the laser beam irradiated to the water surface is in the range, 8 × 10⁶ - 4 × 10⁷ W/m². (3) As time passes, the crater grows to a hemispherical shape with a diameter of 2 × 10^-3 m, and then collapses and disappears. The time required for this is approximately 5 × 10^-3 second.

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