Measurement of tissue temporal point spread function (TPSF) by use of a gain-modulated avalanche photodiode detector

Abstract

This paper describes an opto-electronic cross-correlator designed for measurement of the temporal point spread function (TPSF) of light at the bedside. Ultra-short (< 2 ps) pulses of light from a mode-locked laser were used to illuminate a tissue phantom. The light exiting from the tissue phantom was coupled by an optical fibre to a small-area (200 microns diameter, Hamamatsu S2381) avalanche photodiode (APD). The gain of the photodiode was modulated at the repetition rate of the pulsed laser (82 MHz). Usually the gain was approximately 100, but for a period of approximately 130 ps (FWHM) the gain was increased to approximately 105. A lock-in amplifier, which sampled the integrated APD current, gave an output proportional to the difference between the current in the low- and high-gain states. Hence a small section of the TPSF was selectively sampled. An overall temporal resolution of 275 ps FWHM was achieved. As the timing of the gain modulation was controlled by an all-electronic variable-time-delay system, the whole of the TPSF could be sampled without requiring any moving prism or mirror which is typical of many cross-correlators. Hence the system is mechanically very rugged, which enhances its durability in a portable instrument.

Image analysis methods for diffuse optical tomography

“…These distributions have been studied by many investigators, and specifically quantified by papers in the early 1990's. 30,51,52,56,64,65 Imaging of edges has not proven all that useful, as the wide spread of photons really limits the ability to visualize the edge of objects clearly, and the spatial variation in the resolution ultimately complicates the analysis. In diffuse tomography imaging, it is easier to resolve a smooth circular heterogeneity embedded in a field than step changes.…”

Photoelectric Property Modulation by Nanoconfinement in the Longitude Direction of Short Semiconducting Nanorods

“…Figure , panel d is the simulated free electron density in the cross-section of a 0.5D ZnO nanomaterial based on eq , in which electron density is not uniform but higher in the core and lower close to the outer surface. While the excited electron density can be written as where N C is total electrons number generated per square meter, V is the material volume, A is the photo illuminated area, η is the quantum efficiency, I d is the light illumination intensity, λ is the light wavelength, h is the Boltzmann constant, and c is the light speed in vacuum. Therefore, by combining eqs and , in which only nanomaterial’s height h rod and illumination light intensity I d are variables, a 3D contour plot of the photoinduced electron density as a function of light intensity and materials height can be calculated as shown in Figure , panel e. Obviously, the result based on the new model fits the C-AFM characterization well.…”

Development of a time‐resolved optical tomography system for neonatal brain imaging

“…They usually employ PMT or MCP-PMT detectors, which have the additional advantage of a large collection area. An alternative type of detector is the avalanche photodiode (APD) [Kirkby 1996], which is very compact and low cost, but unfortunately suffers from an inherently small detection area. The main drawback of TCSPC systems is a comparatively low temporal resolution, which is typically in the range of tens to hundreds of picoseconds.…”