A picosecond optoelectronic cross correlator using a gain modulated avalanche photodiode for measuring the impulse response of tissue

摘要: Human tissue is relatively transparent to light between 700 and 1000 nm in the near infrared (NIR). NIR spectroscopy a technique that can measure non-invasively safely, optical properties of tissue. Several different types spectroscopic instrumentation have currently been developed, ranging from simple continuous intensity systems, through complex time frequency resolved techniques. This thesis describes development infra-red time-resolved system, using an inexpensive avalanche photodiode (APD) detector microwave step recovery diode (SRD) novel way implement totally electronic crosscorrelator, with no moving parts. The aim work was develop instrument monitor scattering changes during laser induced thermal therapy. APD gain-modulated by rapidly varying bias voltage electrical pulses generated SRD (120 ps full width half maximum (FWHM) 8 V amplitude). resulting cross-correlator had temporal resolution 275 FWHM significantly faster than 750 when operating conventional fixed voltage. Spurious responses caused were observed, which removed addition Schottky diodes on SRD’s output, although this slightly degraded system 380 FWHM. ability tested Intralipid phantom containing absorbing dye. An 800 fibre coupled mode-locked (2 pulse width) source used measuring point spread function (TPSF) at 5 30 mm away fibre. Five numerical algorithms derive coefficient measured TPSF compared. optimum choice algorithm found depend whether absolute accuracy or minimum computation most important consideration.