Multidetector TCSPC

Fig. 3.2 Principle of TCSPC multidetector operation. The detectors are receiving different signals originating from the same excitation laser. The photon pulses from both detectors are combined, and the times of the pulses are measured in a single TAC. A routing signal indicates which of the detectors detected the currently processed photon. The TCSPC module puts the photons from different detectors into different memory segments...

Fig. 3.4 TCSPC multidetector operation. By the. .channel signal from the ronter, the photons of the individual detectors are routed into separate memory blocks...

It has been mentioned above that the efficiency of TCSPC is near-ideal over a wide range of sample-response times and intensities. As will be shown, advaneed TCSPC is also able to record in several detection channels simultaneously. If a signal has to be resolved not only in time but also in wavelength, spatial eoordi-nates, or polarisation, the multidetector capability yields an enormous inerease in efficiency. As long as the detected photon rate does not exceed the eounting eapa-bility of the TCSPC device, the acquisition time can be considerably shorter than for any analog recording technique. 

Now consider an array of detectors over which the same photons flux is spread. Because it is unlikely that the complete array detects several photons per period it is also unlikely that several detectors of the array will detect a photon in one signal period. This is the basic idea behind multidetector TCSPC. Although several detectors are active simultaneously they are unlikely to deliver a photon pulse in the same signal period. The times of the photons detected in all detectors can therefore be measured in a single TAC. 

Fig. 3.3 Routing module for multidetector TCSPC. For each photon, the routing module delivers the photon pulse and a. .channel signal that indicates in which detector the photon was detected...

The. .disable count signal is activated if several photons are detected in different detectors within the response time of the router. It suppresses the storing of a detected photon in the memory of the TCSPC module. Thus the multidetector technique elegantly uses the. .disable count signal to reduce pile-up effects at low pulse repetition rates. If several photons appear within the same signal period, they are more likely to be detected in different detectors than in the same one. Therefore. the multidetector technique is able to detect a large fraction of the mul-... 

Of course, the multidetector technique does not increase the maximum throughput rate of a TCSPC system. In any TCSPC device there is a small but noticeable loss of photons due to the dead time" of the processing electronics. The dead time of advanced TCSPC devices is of the order of 100 ns, and for count rates above 1 MHz the counting loss becomes noticeable (see Sect 7.9, page 332). The counting loss for a multidetector TCSPC system is the same as for a single detector system operated at the total count rate of the detectors of a multidetector system. 

An important and sometimes confusing feature of the multidetector technique is that the relative counting loss is the same for all channels, independent of the distribution of the rates over the detectors. The reason is that the photons detected by all detectors are processed by the same TCSPC channel so that the counting loss depends on the overall count rate. However, the photons appear randomly in the particular detector channels. Therefore the dead time caused by a detection event in one detector on average causes the same relative loss for all detector... 

An alternative to the multidetector technique is parallel operation of several independent TCSPC channels, which increases the total counting capability at the expense of higher system cost. Please see Sect. 3.7, page 45. 

With a dead time of 100 ns per TCSPC channel, total useful count rates of the order of 20 MHz can be achieved. All four channels can be used for multidetector operation. The high count rate and the high number of channels make multimodule TCSPC systems exceptionally useful for diffuse optical tomography [34], and high count rate applications in laser scanning microscopy [39]. Details are described under Sect. 5.5, page 97 and Sect. 5.7, page 129. 

Often a long-pass filter must be inserted in the polychromator to improve the blocking of scattered laser light. The spechnm at the output of the polychromator is detected by a multianode PMT, and the decay curves in the spectral channels are recorded by multidetector TCSPC. 

Of course, multidetector TCSPC is unable to correlate the photons between the individual start detectors. More flexibility is achieved by using multimodule TCSPC systems. Multimodule systems can be used to obtain antibunching and FCS results simultaneously or even to correlate photons on a continuous time scale from the picosecond to the millisecond range (see Sect. 5.11.3, page 189). 

Multidetector TCSPC can be used to obtain several spectroseopie parameters simultaneously from a single molecule [108, 154, 155, 295, 419, 500]. The optical setup used in [419] is shown in Fig. 5.129. 

To obtain position sensitivity, the single anode can be replaced with an array of individual anode elements [297, 298] see Fig. 6.4. The position of the corresponding photon on the photocathode can be determined by individually detecting the pulses from the anode elements. Multianode PMTs are particularly interesting in conjunction with the multidetector capability of advanced TCSPC techniques. 

TCSPC techniques also have enormous potential in drug screening and in DNA analysis by spectroscopic techniques. It is commonly believed that the data throughput of TCSPC is insufficient for these applications. However, as with many other applications, it is likely that the bottleneck is the photostability of the sample rather than the throughput of TCSPC. Thus advanced TCSPC techniques, especially combinations of multidetector and multimodule techniques, appear likely to be used in this field. 

W. Becker, A. Bergmann, G. Biscotti, Fluorescence lifetime imaging by multidetector TCSPC, In OSA Biomedical Optics Topical Meetings on CD ROM (The Optical Sciety of America, Washington, DC) WDl (2004)... 

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