TCSPC synchronisation signal

TCSPC needs a timing reference signal from the light source. This is no problem for picosecond diode lasers, which deliver a trigger output pulse from the laser diode driver. For free-running solid-state lasers or jet-stream dye lasers, a suitable synchronisation signal can be generated by a photodiode. A simple solution is to use a fast PIN photodiode in one of the circuits shown in Fig. 7.42. 

Time-tag recording means that the TCSPC channels do not build up a photon distribution but store each individual photon with its TAC time ( micro time ) and its time from the start of the experiment ( macro time ). The computer calculates the photon distribution at each location along the delay line and the time in the signal period. Time-tag recording of delay line data requires that the macro time clocks of all TCSPC channels be synchronised. Even then it is difficult to assign the data in the position channel to the correct data in the time channel. Due to slightly different CFD thresholds and different dead times, a photon recorded in the position channel need not necessarily be recorded in the time channel, and vice versa. To avoid misinterpretation of the data, a macro time resolution of 50 ns or finer is required. 

The macro time clock can be started by an external experiment trigger or by a start-measurement command from the operating software. In some TCSPC modules the clock signal source of the macro time clock can be selected. The macro time clock can be an internal quartz oscillator, an external clock source, or the reference signal from the laser. Triggering and external clock synchronisation are absolute requirements for multimodule operation in the time-tag mode, see Sect. 5.11.3, page 189. 

The lasers must then be multiplexed at a rate faster than the changes expected in the sample. One way to multiplex lasers is to synchronise their pulse periods and delay the pulses of different lasers by different fractions of the pulse period. The fluorescence signals are recorded simultaneously in the same TAC range of a TCSPC device. The principle is shown in Fig. 5.26. 

In both cases the data acquisition in the TCSPC channels is synchronised with the scanning by clock pulses from the scan controller. It must, however, be taken into account that the length of the lines of the scan varies since the return points of the scan are controlled by the detector overload signals. Therefore, the scan software must store the positions of the return points and the number of pixels between. These positions are used later to adjust the lines horizontally. 

To build up lifetime images, the TCSPC module needs scan synchronisation pulses from the scanner. These can be either obtained directly from the scanner or separated from the video signal of the reflection channel. 

The TCSPC-FCS technique can also be used in conjunction with a continuous laser. Of course, in this case the measurement does not deliver a meaningful miero time, and no lifetime data are obtained. Because the TCSPC module needs a synchronisation pulse to finish the time measurement for a recorded photon, an artificial stop pulse must be provided. This can be the delayed detector pulse itself or a signal from a pulse generator see Fig. 5.116. 

Correlation down to 100 ns is usually enough to resolve diffusion times and intersystem crossing. Nevertheless, cross-correlation data at a shorter time-scale can be obtained by using two TCSPC modules with synchronised macrotime clocks (see Fig. 5.120). Synchronisation can be achieved by using the Sync signal, i.e. the laser pulse repetition frequency, as a macro time clock for both modules. This synchronisation works up to about 100 MHz, so that times down to 10 ns can be correlated. 

For the discussion above it was assumed that the detected light signal was continuous. However, signals measured by TCSPC are mostly pulsed signals. Moreover, the detection and therefore the dead time is synchronised with the signal period. This synchronisation can lead to a different behaviour than predicted by (7.33). Dead-time-related counting loss in nonreversed start-stop systems is illustrated in Fig. 7.82. 

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