Tsuneyuki Urakami

Photon Migration Model for Turbid Biological Medium Having Various Shapes

Fundamental, yet novel, formulas representing diffuse re-emission of short-light-pulse incidence on turbid media independent of their shapes are deduced from a simple photon migration model in which the survival probability of a photon is determined by the zigzag photon path length and the absorption coefficient within the medium. The results can be applied to derive the absolute concentration of the absorptive constituent in variously shaped media. The coincidence between our model and the diffusion equation is seen if we adopt a new diffusion coefficient independent of the absorption coefficient.

Quantitation of absorbing substances in turbid media such as human tissues based on the microscopic Beer-Lambert law

Abstract We derive analytic expressions in the form of an implicit function for the system size, volume shape, refractive-index-mismatched boundary, and source-detector separation, to determine the concentrations of absorbing substances in highly scattering media such as human tissue. The basis of our derivation is the microscopic Beer-Lambert law that holds true when we trace a zigzag photon path within the medium. The validity of our prediction is evaluated by Monte Carlo simulations for transmission and reflection from an infinitely wide, 20-mm-thick slab. Quantitative spectroscopies are compared by measuring a tissue-like, liquid phantom using photon density waves modulated at 100 MHz, where the absorption of the medium is changed (the absorption coefficient μ a ≈ 0.002–0.02mm −1 at 786 nm).

Frequency Domain Analysis of Photon Migration Based on the Microscopic Beer-Lambert Law

Exact analytic solutions for frequency domain responses of photon migration in variously shaped turbid media are derived based on a model in which the microscopic Beer-Lambert law holds. We show that the system function specified by the Fourier transform of the impulse response is a regular function, and that the temporal or spatial changes in the macroscopic absorption coefficient and the concentration of the absorber can therefore be determined from macroscopic observables such as amplitude, phase and modulation frequency of the probe light. The feasibility of using this technique in spectroscopy to determine the absolute concentration of an absorber in turbid media is also discussed. The advantage of these methods is that we do not need to take the boundary conditions into account. Similar approaches may also yield simple techniques to determine physical properties in various scientific fields.

Optimization of resonant two-photon absorption with adaptive quantum control

Adaptive quantum control is applied to the resonant two-photon absorption of Rb atoms, which includes an intermediate state between the initial and final states. The phase distribution of incident laser pulses in the frequency domain is decided adaptively free from a priori knowledge on the phase distribution with the help of a simulated annealing algorithm to achieve the maximum luminescence intensity from the state associated with the excited state. The optimized phase pattern acquired in our system matches a theoretical prediction based on the perturbative treatment of the semiclassical model on the absorption of light.

Simple Subtraction Method for Determining the Mean Path Length Traveled by Photons in Turbid Media.

The mean path length is a key parameter in the study of light propagation in turbid media such as living tissues. In this paper, we propose a simple subtraction method for determining the mean path length traveled by photons in turbid media. The method is based on the fact that the mean time delay (i.e., the center of gravity) of the measured re-emission profile is the sum of the mean time delays of the instrumental function (i.e., the impulse response of the measuring system) and the impulse response of a highly scattering medium. Using this method, the mean path length can be calculated quickly and accurately without the need for performing deconvolution. The theory, computer simulation and experimental demonstration are described.

Femtosecond Pulse Delivery through Long Multimode Fiber Using Adaptive Pulse Synthesis

We report on a delivery technique for 164-fs optical pulses with a peak power of 1.1 kW through a long multimode optical fiber and a glass block using an adaptive pulse-shaping feedback loop. We used two devices to optimize the input pulse; a pulse stretcher and a pulse shaper. 382-ps chirped pulses are compressed to 370-fs pulses at the output end of a glass block joined to a standard graded-index multimode fiber, 96 m in length and with a core diameter of 50 µm (the output end of the system). The adaptive pulse shaper compensates for the remaining high-order phase dispersion, which results in 164-fs pulses at the output end of the system. Our work shows that an adaptive pulse synthesis technique provides a powerful and convenient technique for programmable fiber dispersion compensation over a broad optical bandwidth.

Picosecond fluorescence spectroscopy of purple membrane in Halobacterium halobium with a photon-counting streak camera

Abstract Fluorescence lifetimes and spectra of native and deionized purple membranes of Halobacterium halobium at 22°C were measured to be blue-shifted transient previously found by absorption spectroscopy is attributed to bacteriorhodopsin in the lowest excited-singlet state. Ultraweak fluorescence of the light-adapted purple membrane with 2.5 × 10 −4 quantum yield could be detected even though the excitation pulse energy at 570 nm was reduced to 0.88 pJ (72 μW average power).

Time Integrated Spectroscopy of Turbid Media Based on the Microscopic Beer–Lambert Law: Application to Small-Size Phantoms Having Different Boundary Conditions

Continued work on time-integrated spectroscopy (TIS) is presented to quantify absorber concentrations in turbid media. We investigated the applicability of the TIS method to small-size media that have different boundary conditions by measuring two 20×20×50 mm3 cuboid liquid tissue-like phantoms at various absorption levels (absorption coefficients of the phantom from 2.5×10-3 to 4.4×10-2 mm-1 at 782 nm and from 3.1×10-3 to 2.7×10-2 mm-1 at 831 nm). The scattering and absorbing solution was filled into ordinary and black-anodized aluminum containers to provide different boundary conditions. By means of a single equation, the absorber concentrations have been recovered within errors of a few percent in both cases. This demonstrates that the TIS method can quantify absorbers in small-size media having different boundary conditions.

Time integrated spectroscopy of turbid media based on the microscopic Beer–Lambert law: consideration of the wavelength dependence of scattering properties

Abstract A simple expression is proposed to describe the dependence of the mean pathlength on scattering properties. Based on this expression and the microscopic Beer–Lambert law, a dual-wavelength time integrated spectroscopy (TIS) method in which the influence of wavelength dependence of scattering properties on both mean pathlength and intensity is taken into consideration, is developed to determine the absolute concentration of an absorber in highly scattering media. The validity and the accurate performance of the method are well demonstrated by measuring the transmission through a slab-like phantom and reflection from a semi-infinite phantom. In both cases, with a single equation, the absorber concentrations were determined within errors of a few percent.

Isotropic Photon Injection for Noninvasive Tissue Spectroscopy

Isotropic photon injection is proposed as a practical technique for noninvasive tissue spectroscopy, where the photon migration is almost plane symmetric including that at photon incidence and detection points. In addition, the reciprocity theorem holds for the photon migration process within the medium. This condition is easily achieved using simple optical components. A boundary approximation for isotropic photon injection is developed to solve the photon diffusion equation. This approximation is applied to determine the diffuse photon emission from the surface of the medium for diffuse reflection and transmission in both the time and the frequency domains, as well as for the steady state. The predictions for temporal response and intensity of diffuse photon emission are in good agreement with the results of a Monte Carlo simulation.