Recording several signal period

The classic pile-up effeet is shown in Fig. 7.77. A single TCSPC channel is unable to record a second photon in a single signal period. Consequently, the second photon is lost. (Actually the term pile-up is not quite correct. It comes from nuclear partiele detection and means the detection of several particles within the luminescence lifetime of a scintillator.)... 

Nevertheless, there is a way to reduce the classic pile-up. The light is distributed into several deteetors, or into several channels of a multianode PMT. Two photons arriving within the same signal period are then more likely to hit different detectors than the same one (Fig. 7.80, right). Pulses that appear at the outputs of different deteetors can easily be identified as multiphoton events. The recording of both photons in the TCSPC module can be suppressed and pile-up distortion can be avoided. 

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. 

Transformation — Several approaches are available for transformation of time domain data into the - frequency domain, including - Fourier transformation, the maximum entropy method (MEM) [i], and wavelet analysis [ii]. The latter two methods are particularly useful for nonstationary signals whose spectral composition vary over long periods of time or that exhibit transient or intermittent behavior or for time records with unevenly sampled data. In contrast to Fourier transformation which looks for perfect sine... 

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