Review of tradeoffs for quenched avalanche photodiode sensors for imaging turbid media

We evaluate the efficiency of a new optoelectronic quenched avalanche photo diode sensor (QAPD), with the potential of enabling high-resolution imaging through turbid media with femtosecond-lasers. Our target application is fo r imaging cancer in the human breast. We aim to improve the contrast ratio of the unscattered, image bearing phot ons by reducing or removing the large background of multiply scattered phot ons that result in poor spatial resolution. The approach that we are taking is to use temporal discrimination: ultra-short (less than a picosecond) la ser pulses and time gated detection can select only those photons which tra vel almost straight through the tissue. The feasibility of resolving a 1 mm structure in 50 mm turbid tissue sample, using our system, will be discuss ed-there are no results reported to date that approach this performance. We aim to enhance the detection of short pulses through time resolved singl e photon counting technique, in this technique, the earliest arriving photo n of the transmitted pulse is detected with a QAPD. When combined with an e fficient electronic quenching circuit, the QAPD is insensitive to the later arriving diffuse photons. High accuracy and efficient measurement of the a rrival time of the first photons is achieved by time to amplitude conversio n electronics with a temporal resolution of 3 ps. This paper contains a discussion of the laser source, detection circuit, in cluding QAPD, high-speed comparator, critical biasing and intensity counter . (C) 2000 Elsevier Science Ltd. All rights reserved.