Pulse Amplitude Jitter

The secondary emission coefficient at a particular dynode depends on the dynode material and energy of the primary electrons. For typical interdynode voltages used in PMTs, the secondary emission coefficient, n, is between 4 and 10. Because the secondary emission is a random process the number of the generated secondary electrons varies from electron to electron. The width of the distribution can be expected at least of the size of the standard deviation, n, of a poissonian distribution of the secondary emission coefficient, n. Therefore the single-photon pulses obtained from a PMT have a considerable amplitude jitter. For TCSPC applications it is important that the pulse amplitudes of the majority of the pulses are well above the unavoidable noise background. 

If the discriminator threshold is set low enough an extremely high peak appears at very low amplitudes. This peak does not originate in the PMT but is caused by electronic noise of the preamplifier and noise pickup from the environment. Some typical pulse amplitude distributions are shown in Fig. 6.13. 

The narrow peak at the left is the electronic noise. In the upper two tubes the main peak and the low-amplitude peak are clearly separated from the electronic noise peak. In the lower left tube, the peaks can be separated at maximum gain in the lower right tube the gain is insufficient to separate the pulse distribution from the electronic noise background. 

The pulse height distribution at high detector gain often shows a structure, probably due to the discrete numbers of secondary electrons emitted at the first dynode. An example for an H7422P-40 module is shown in Fig. 6.14. 

It is often believed that the width of the pulse amplitude distribution depends on the cathode material. Certainly there are differences in the peak ratios of the main 

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Due to the random nature of the amplification process in a photomultiplier tube or avalanche photodiode, the single-photon pulses have a considerable amplitude jitter (see Fig. 1.5). For analog processing, the amplitude jitter contributes to the noise... 

Fig. 1.6 Effectof amplitude jitter (gain noise) of the single-photon pulses of a PMT on the recording of an optical pulse recorded by an analog oscilloscope left) and by photon counting (right)...

Of course, a simple leading edge discriminator cannot be used to trigger on such pulses. The amplitude jitter would introduce a timing jitter of the order of the pulse rise time (Fig. 4.1, left). In practice the timing jitter is even larger because any discriminator has an intrinsic delay that depends on the signal slope speed and the amount of overdrive. 

Theoretically the temporal position of the baseline-eross point is independent of the pulse amplitude. In praetiee there is a residual jitter due to the dependenee of the intrinsic delay of the diseriminator on the slope speed and the amount of overdrive. The residual jitter ean be minimised by selecting a trigger threshold slightly different from zero. 

Fig. 6.11 Amplitude jitter of SER pulses. R5900 PMT at -900 V. Scale 5 mV and 1 ns per division...

The transit time between the absorption of a photon at the photocathode and the output pulse from the anode of a PMT varies from photon to photon. The effeet is called transit time spread", or TTS. There are three major TTS components in conventional PMTs and MCP PMTs - the emission at the photoeathode, the transfer of the photoelectron to the multiplieation system, and the multiplication process in the dynode system or mieroehannel plate. The total transit time jitter in a TCSPC system also contains jitter indueed by amplifier noise and amplitude jitter of the SER. 

Synehronously pumped jet-stream dye lasers are sometimes used to obtain light in the gap between the TiiSapphire fundamental and the SHG. The systems need permanent alignment and supervision. The pulses of synchronously pumped dye lasers have a eonsiderable amplitude jitter and amplitude drift, which makes aceu-rate synchronisation of a TCSPC device difficult. Moreover, even the cleanest systems release some laser dye into the environment, and contamination of samples with laser dyes is a problem. Dye-flow systems become an even less attractive option when a hose breaks and spills liters of dye solution on the floor. 

Detector modules with internal diseriminators, sueh as the Hamamatsu H7421 PMT modules or the Perkin Elmer SPCM-AQR single photon APD modules, deliver stable output pulses without amplitude jitter. The timing performance is defined by the internal diseriminator of the deteetor module, not by the CFD of the TCSPC device. Thus changing the CFD eonfiguration does not improve the time resolution of these detectors. 

The CFD threshold determines the minimum amplitude of the input pulses that trigger the CFD. The threshold of the CFD in the deteetor channel has a considerable influence on the efficieney of a TCSPC system. As described under Sect. 6.2, page 222, the single-photon pulses of a PMT have a strong amplitude jitter. The general shape of the amplitude distribution is shown in Fig. 7.60, left. 

Higher pulse amplitudes in general give lower timing jitter because the influence of the background noise is smaller and the influence of the amplitude jitter on the timing is reduced. Therefore TCSPC users often increase the CFD threshold... 

The zero cross level adjustment minimises the timing jitter induced by amplitude jitter of the detector pulses. The zero cross level is therefore often called walk adjust". In early TCSPC systems the walk adjust had an enormous influenee on the shape of the instrument response function (IRF). In newer, more advaneed systems the influence is smaller. The reason is probably that detectors with shorter single electron response are used and the discriminators in the newer CFDs are faster. Therefore, the effective slope of the zero cross transition is steeper, with a correspondingly smaller influence of the zero eross level. Figure 7.63 shows the IRF for an XP2020UR linear-focused PMT and an H5773-20 photosensor module for different zero cross levels. 

In most TCSPC applications the pulses in the reference channel have less amplitude jitter than the PMT pulses. Therefore the delay lines and the zero eross and threshold settings of the referenee CFD have little influence on the time resolution. 

A fiber laser may behave as a driven second-order system by introducing a time-dependent parameter into the laser cavity, such as loss modulation, gain (or pump) modulation, phase modulation, etc. For example, driving the mode locking element in a mode locked fiber laser at slightly below the cavity fundamental frequency has been observed to result in chaotic behavior, characterized by severe amplitude jitter on the optical pulses generated. 

Another important property of PMTs is the pulse height distribution. The amplification of individual photoelectrons by the PMT is a stochastic process that causes variations in the gain of individual photoelectrons. As a result significant jitter in the amplitude of the output pulses is observed, see Fig. 3.6. These pulse height variations can be more than a factor of 10. The lowest pulse heights mainly consist of (thermal) noise, indicated by the dashed line in Fig. 3.6. The pulse height distribution exhibits a peak corresponding to detected photons. The threshold level of the... 

From the standpoint of time domain (e.g., time-correlated single photon counting) experiments the method of modelocking is not too crucial as long as the pulse jitter is modest (some picoseconds), and the pulse intensity doesn t vary too much if the time-to-amplitude converter is being started instead of stopped by the excitation pulse, it may be immaterial. From the standpoint of the frequency domain, however, the... 

In most TCSPC modules, rate counters complement the CFDs in the photon pulse and reference charmels of TCSPC devices. The CFDs of the photon channel and the reference charmel are often different. The photon ehannel is designed for lowest amplitude-induced timing jitter and the reference ehannel for highest trigger rate. Some TCSPC devices do not use a CFD in the referenee charmel at all and rely instead on the stability of the reference pulses. 

The new generation of digital sampling oscilloscopes (36,45) and specially designed time-domain measuring setups (TDMS) (38) offer comprehensive, high precision, and automatic measuring systems for TDS hardware support. They usually have a small jitter faetor (< 1.5 ps), important for rise time, a small flatness of ineident pulse (< 0.5% for all amplitudes), and in some systems a unique option for... 

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