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 ( 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.