Correlation of Delayed Detector Signals

A single TCSPC module detecting via several deteetors and a router is unable to reeord several photons within the dead time of the signal processing electronies. The signals of the detectors therefore eannot be eorrelated at a time scale shorter than the dead time. The problem ean be solved by routing of delayed deteetor signals [538]. The principle is shown in Pig. 5.119. 

Micro Time Start-Stop Time Macro Time Time from start of experiment Detector Channel 1 or 2 

Several deteetors are eonneeted via a router to the same TCSPC module. The photon pulses from the seeond deteetor are delayed by more than the dead time of the TCSPC module. More than two detectors ean be used if their delay lines are different by more than the module dead time. The stop pulses for the TCSPC module come from the pulsed laser, or, if a CW laser is used, from an external eloek generator. Due to the different delay of the deteetor signals, photons deteeted simultaneously do not arrive simultaneously at the router inputs. Therefore, photons detected in the same laser pulse period are reeorded at different times and stored in the FIFO data file with a macro time offset. The differenees in the maero times caused by the delay lines in front of the router are known and ean easily be corrected when the photons are correlated. 

The setup was used to track the intensity fluetuations, the lifetime and the number of molecules in the laser focus simultaneously [538]. An applieation to single-moleeule FRET is described in [237]. 

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