Reversed start-stop

Importantly, the dead-time of TACs and TDCs is comparatively long, typically 125-350 ns. When a photon arrives within this time interval after the detection of a photon, it will not be observed. Therefore, care must be taken that the count rate of the experiment is sufficiently low to prevent this pulse-pileup. TACs and TDCs usually operate in reversed start-stop geometry. Here, the TAC is started by the fluorescence signal and stopped by the laser trigger. 

The setup shown in Fig. 2.14 is based on the presumption that the period of the excitation pulses is constant and free of jitter down to the order of 1 ps. This is certainly correct for a titanium-sapphire laser or other mode-loeked laser systems using low-loss cavities. For pulsed diode lasers the pulse period jitter ean be eon-siderably higher. These lasers are eontrolled by quartz oseillators whieh ean have a pulse period jitter of the order of some 10 ps. The reversed start-stop eonfiguration can easily be made insensitive to pulse period jitter by introdueing a passive delay line in the referenee ehannel. The effeet of the delay is shown in Fig. 2.15. 

Fig. 2.15 Reversed start-stop. Left undelayed reference signal, right delayed reference signal. The laser pulse which released the photon is marked black. With an appropriate delay in the reference channel the time of the photon is measured against the correct laser pulse...

Fig. 4.3 Frequency divider in the reference path of the reversed-start-stop configuration. With an n-to-1 divider n signal periods are recorded...

Reversed start-stop systems require a stop pulse at the end of the recorded time interval. It is therefore often necessary to delay the reference pulses from the laser. The best way to delay the signal is to use a cable, since this does not introduce a noticeable jitter. It is, however, not commonly known that the transit time in a cable depends on the temperature. Figure 7.49 shows the delay change in 8 m of a standard RG 174 cable and RG 316 high-quality cable. 

Fig. 7.67 Reversed start-stop requires a delay in the reference channel to stop the TAC with the correct laser pulse...

The TAC parameters determine the time scale and the part of the signal that is reeorded. The available parameters may differ for different TCSPC deviees. Espe-eially deviees based on direct time-to-digital conversion (TDC) or sine-wave eon-version may differ eonsiderably from devices using the TAC/ADC prineiple and reversed start-stop, which will be considered below. 

The time measurement block of a TCSPC device working in the reversed start-stop mode is shown in Fig. 7.69. 

Fig. 7.71 Effect of different TAC Offset in a reversed start-stop system. Left Increased offset shifts the curve right, i.e. shifts the recorded time interval to an earlier part of the signal. Right For very small and very large offsets the beginning and the end of the TAC characteristic come into view. A Offset 0, B Offset 50% of maximum TAC core voltage...

Fig. 7.74 Recording of a signal within a 400 ns pulse period in the reversed start stop mode. Left Stop with the next laser pulse. TAC range = 500 ns, TAC gain = 10. Right Stop with the delayed laser pulse. TAC range = 50 ns, TAC gain = 1. The FWHM of the peak is reduced from 216 ps to 87 ps...

Fig. 7.83 Dead-time-related couQting loss in a reversed start-stop system. The stop is with a delayed laser pulse. The signal period is longer than the sum of the stop delay and the TAC/ADC dead time...

Figure 7.84 shows what happens if a reversed start-stop system is operated at low repetition rate without a delay line in the stop line of the TAC. 

The situation for reversed start-stop and high repetition rate signals is shown in Fig. 7.85. The TAG is started when a photon is detected and stopped with the next laser pulse. Within the time between the start and the stop, the TAG is unable to record a second photon. The resulting loss is the classic pile-up effect. 

Gounting loss in reversed start-stop systems operated at high signal repetition rate can be compensated for by a dead time compensation in the acquisition time. The idea behind the dead time compensation is to increase the acquisition time by the sum of all dead time intervals that occurred during the measurement. The principle is shown in Fig. 7.86. 

For TCSPC with nonreversed start-stop, the end of the dead-time interval is automatically synchronised with the laser pulses. For reversed start-stop this in not the case. The situation is shown in Fig. 7.87. 

Fig. 7.87 Dead-time-related signal distortion in high-repetition-rate reversed-start-stop systems. The dead time ends anywhere in one of the subsequent signal periods, which causes a step in the detection probability...

The simplest and most accurate way to calibrate a TCSPC system is to use the pulse period of a high repetition rate laser as a time standard. The pulse period of Ti Sapphire lasers is between 78 and 90 MHz and accurately known. Diode lasers are usually controlled by a quartz oscillator and have an absolute frequency accuracy of the order of several tens of ppm. The signal is recorded in the reversed start-stop mode with a frequency divider in the reference path. The recorded waveform covers several laser periods, and the time between the pulses can be measured and compared with the known pulse period. 

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