Generating the 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. 

The resistor, R, protects the diode against overload and accidental reversal of the supply voltage. The capacitor, C, provides a low-impedance RF-retum path to ground. The capacitor must be a low-impedance surface-mount type. The resistor is not critical. The connection length in the path GND-C-Photodiode-Connector must be kept to a minimum, or a 50 Ohm strip line on a printed circuit board must be used. Please note that the circuit gives a fast response only if the output is terminated with 50 Ohm. 

The requirements for the diode depend on the amplitude fluctuations and the intensity drift of the laser. Good TiiSapphire lasers have negligible amplitude flue- 

The pulse current, obtained for a given average power, and vice versa, can be estimated by 

Si photodiodes have a poor sensitivity in the UV range. UV-enhanced Si PIN photodiodes are available, but usually do not perform well at short pulses. In fre- 

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