Tadatoshi Tanifuji

Finite difference time domain (FDTD) analysis of optical pulse responses in biological tissues for spectroscopic diffused optical tomography

FDTD analysis has been successfully formulated for predicting optical pulse response in biological tissues. It has been confirmed that suitable boundary conditions give reasonable numerical results.

Monitoring method for axis alignment of single-mode optical fiber and splice-loss estimation.

A new method for core-axis alignment and precise splice-loss estimation for single-mode optical fibers is presented. By using a differential interference contrast microscope, we can achieve core-axis alignment with an offset below 0.3 microm, which results in a butt-joint loss increase of 0.04 dB compared with that obtained by alignment using a conventional optical monitoring method. Furthermore, splice-loss estimation with a precision of 0.05 dB is attained for a low-loss region without using an index-matching liquid.

Pulse circulation measurement of transmission characteristics in long optical fibers

A pulse circulation method to measure the length dependence of transmission characteristics in optical fibers is described. In principle, the method has the advantage that no unnecessary power is branched off in the loop. Using this method, the length dependence of transmission characteristics of step and graded index fibers has been measured and found to be different in both cases. It is concluded that the difference is due to the material dispersion effect, which significantly influences pulse spreading in long length graded index fibers.

Arc-fusion splicing of single-mode fibers. 3: A highly efficient splicing technique

A highly efficient splicing technique for single-mode fibers is investigated. Several techniques are presented for realizing both a highly precise splice loss estimation and excellent working efficiency. The techniques markedly simplify the splice procedure and improve working efficiency. A monitoring length of 60 km, a splice loss measurement accuracy of ±0.01 dB, a fusion splice time of 2.5 min, and an average splice loss of 0.04 dB are satisfactorily achieved in laboratory experiments.

Time-Resolved Noninvasive Optical Parameters Determination in Three-Dimensional Biological Tissue Using Finite Difference Time Domain Analysis with Nonuniform Grids for Diffusion Equations

The finite difference time domain (FDTD) analysis based on nonuniform grids for solving the diffusion equations in biological tissue has been proposed. It has been confirmed that the analysis greatly reduce the time for calculating time-resolved reflectance in three-dimensional scattering medium preserving the numerical accuracies. Based on the analysis, noninvasive optical parameters determination in three-dimensional inhomogeneous medium divided into 192 homogeneous cubic cell 10 (mm) in edge lengths has been studied. Chi-square fitting for theoretical and experimental time-resolved reflectance was optimized by the downhill simplex method. As a result, it has been become clear that absorption coefficients in all cubic cells in scattering medium are estimated within the accuracy of 10% by utilizing only time-resolved reflectance

Baseband frequency response of a graded-index fiber excited by a step-index fiber

Input modal power distribution and baseband frequency response of a graded-index fiber have been investigated theoretically and experimentally, when the fiber was excited by a step-index fiber. It is found that the bandwidth of the graded-index fiber is measured with good reproducibility and accuracy by using the step-index fiber as an exciter. An appropriate choice of step-index fiber parameters is made with respect to the test graded-index fiber.

Time resolved reflectance of an optical pulse from scattering bodies with nonscattering regions utilizing the finite difference time domain analysis

The finite difference time domain (FDTD) analysis has been successfully formulated for predicting time resolved reflectance of an optical pulse from scattering bodies with nonscattering (clear) regions. The nondiffusive light propagation across a clear region has been calculated based on the invariance of the light intensity along the ray in free space. The additional equivalent source functions generated by the nondiffusive light entering the scattering region have been introduced into the diffusion equations and the FDTD analysis has been formulated. Time resolved reflectance of the three-layered slab containing a clear layer was calculated based on the formulation. It has been become clear that received light intensity and mean time of flight estimated from the reflectance agree with previously reported experimental data quite well. It has also been found that the absorption loss of the scattering region beyond the clear layer is estimated from the time resolved reflectance.

Drastic emission-spectra changes in a semiconductor laser owing to optical feedback from an optical connector

The time-averaged longitudinal-mode power of a semiconductor laser coupled to a fiber changes drastically when an external force is applied to a multimode fiber terminated with an optical connector. No emission-spectra changes have been observed in the case of a single-mode fiber. This phenomenon is considered to be induced by the wavelength-dependent change of the reflected field speckle intensity coupled into the laser-active region.

Time-resolved reflectance of an optical pulse from an adult head model utilizing the finite difference time domain (FDTD) analysis

Time-resolved reflectance of an optical pulse from a four-layered adult head model including cerebrospinal fluid (CSF) has been analyzed based on the finite difference time domain analysis (FDTD) which has been formulated by authors. It has been confirmed that the intensity of the reflected light and the mean time of flight dependences on the source-detector separations estimated from the time-resolved reflectance agree quite well with the previously reported experiments and calculations. The sensitivities for detecting optical property changes of gray and white matter beyond CSF in time-resolved reflectance have been evaluated and it has been become clear that they are enhanced compared with the sensitivities in the intensity of the reflected light and the mean time of flight. The derivatives of reflectance with respect to the absorption coefficients of gray and white matter have shown that the presence of CSF improves the capabilities of time-resolved reflectance for detecting optical property changes in the brain in actual noise limited photon detection systems.