Journal of Fluorescence, VoL 4, No. 1, 1994 Multiplexed Time-Correlated Single-Photon Counting D. J. S. Birch, 1 D. McLoskey, 1 A. Sanderson, 1 K. Suhling, 1 and A. S. Holmes x Received October 18, 1993 We review the techniqueof multiplexed time-correlatedsingle-photon counting whereby multiple fluorescence decay curves are recorded in parallel by statistically time-sharing a single time-to- amplitude converter. Application of the multiplexing technique to measuring the fluorescence lifetime topographyof a self-absorbingsample is demonstrated.Further possibilities are discussed for multiplexed optical fiber sensor networks with built-in intelligencefor detecting and discrim- inating between different metal ions in solution. KEY WORDS: Single-photon timing; multiplex; self-absorption; fluorescence sensor. INTRODUCTION The time-correlated single-photon counting (TCSPC) technique is the most widely used method of measuring fluorescence decay times [1,2]. Advantages of TCSPC include a wide dynamic range, known data statistics (Poisson) and data weights, a conveniently digitized data format, visual display of kinetics, and high time reso- lution. However, TCSPC is quite inefficient insofar as the fluorescence count rate detected must be kept to a few percent of the excitation repetition rate to avoid data pileup [1,2]. Although this pileup limitation is incon- venient in principle, it is rarely a problem in practice due to the high repetition rate of present day flashlamp and laser sources. A much more fundamental limitation concerns the implementation of TCSPC, which, by and large, addresses only one small fraction of the available fluorescence contour. The fluorescence decay signature of a sample, F, can be expressed generally in terms of a data surface, e.g., F =F(I, kv, t, p,r_) (1) where I is the emission intensity, hF the fluorescence wave- length, t time, 1~ polarization, and r the spatial location. t Department of Physics and Applied Physics, University of Strath- clyde, Glasgow G40NG, Scotland, UK. 91 The larger the fraction of the available data surface which can be addressed in a single measurement, the higher is the measurement specificity. Most present-day TCSPC fluorometers are simplexed instruments, e.g., decay curves are recorded at one fluorescence wave- length at a time which is clearly inefficient given the much greater data surface available. In recent years we have developed parallel processing technology which en- ables a much greater fraction of the fluorescence data surface to be recorded simultaneously using TCSPC and which overcomes the usual pileup limitation. In multiplexed TCSPC (MUX-TCSPC) each detec- tion channel is associated with its own timing discrimi- nator and memory segment of a multichannel analyzer (MCA) just as in the usual simplexed approach. In MUX- TCSPC the "stop" signals from the multiple detection channels are combined to provide an unsorted mixture of signals to the stop input of the time-to-amplitude con- verter (TAC). The TAC conversion events which then follow are sorted into different segments of the MCA memory by using the timing discriminator output from each channel to initiate logic levels which address the appropriate MCA memory segment for each channel. Coincident stop events in more than one channel are rejected by inhibiting the TAC conversion if they occur within an enable time window determined by the TAC range. Consequently each channel can be operated at the 1053-0509/94/0300-0091507.00/0 9 1994 Plenum Publishing Corporation