A time-resolved optical imaging system using near-infrared light has been developed. The system had three pulsed light sources and total 64 channels of detection, working simultaneously for acquisition of the time-resolved data of the pulsed light transmitted through scattering media like biological tissues. The light sources were provided by high power picosecond pulsed diode lasers, and optical switches directed one of the light sources to the object through an optical fiber. The light signals reemitted from the surface of the object were collected by optical fibers, and transmitted to a time-resolved detecting system. Each of the detecting channels consisted of an optical attenuator, a fast photomultiplier, and a time-correlated single photon counting circuit which contained a miniaturized constant fraction discriminator/time-to-amplitude converter module, and a signal acquisition unit with an A/D converter. The performance and potentiality of the imaging system have been examined by the image reconstruction from the measured data using solid phantoms.

We developed a three wavelength time-resolved spectroscopy system cadled the TRS-1 0 for use as a tissue oxygenation monitor. The TRS-1 0 achieved a higher data acquisition rate and a system miniaturization maintaining high sensitivity and time resolution. The TRS-1O consists of a three wavelength picosecond light pulser (PLP) with auto power control as a pulsed light source, a photomultiplier tube (PMT) having high speed and high sensitivity and miniaturized signal processing circuits for time-resolved measurement (CFD, TAC, A/D converter and histogram memory). The TRS-1O system was used to measure the time course of oxygen metabolism in a human forearm with a pneumatic cuff attached to occlude blood flow. Our system succeeded in accurately measuring concentrations of Oxy-(Hb02) and deoxyhemoglobin (Hb) in the human forearm based on diffusion theory by means of TRS data observation having an accumulation time of one second per point.

Pulse shapes in a dispersive transparent material modulated by group-velocity dispersion, self-phase modulation, and self-focusing induced by a femtosecond laser light were observed directly with femtosecond timeresolved optical polarigraphy probing the induced instantaneous birefringence. The first observation of the state of femtosecond laser pulses about the interaction region inside the transparent bulk material indicated that the pulse propagation was accomplished with a multiple conelike structure that was hypothesized from a numerical simulation with an extended nonlinear Schrödinger equation.

A time-resolved imaging technique for visualizing ultrafast propagation dynamics of intense light pulses in a medium has been demonstrated. The method probes the instantaneous birefringence induced by a pulse in the medium. Through consecutive femtosecond snapshot images of intense femtosecond laser pulses propagating in air, ultrafast temporal changes in the two-dimensional spatial distribution of the optical pulse intensity were clearly seen.

A concise theoretical treatment has been developed to describe the optical responses of a highly scattering inhomogeneous medium using functions of the photon path distribution (PPD). The treatment is based on the microscopic Beer-Lambert law and has been found to yield a complete set of optical responses by time- and frequency-domain measurements. The PPD is defined for possible photons having a total zigzag pathlength of l between the points of light input and detection. Such a distribution is independent of the absorption properties of the medium and can be uniquely determined for the medium under quantification. Therefore, the PPD can be calculated with an imaginary reference medium having the same optical properties as the medium under quantification except for the absence of absorption. One of the advantages of this method is that the optical responses, the total attenuation, the mean pathlength, etc are expressed by functions of the PPD and the absorption distribution.