TCSPC multiplexed

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. 

The routing capability of TCSPC can be used to multiplex several light signals and record them quasisimultaneously. The principle of multiplexed TCSPC is 

Several optical signals are multiplexed on the microsecond or millisecond time scale. Multiplexing of signals can be accomplished by switching several diode lasers, either electronically or by fibre switches, or by rotating elements in an optical system. The channel signal indicates the current state of the multiplexing 

The system records the photon distribution over the time in the signal period, the detector channel number, the multiplex-ehannel number, and one or two additional coordinates determined by the sequeneer. The result can be interpreted as a sequence of photon distributions for all combinations of detector and multiplexing eharmels. 

An important application of multiplexed multidetector systems is diffuse opti-eal tomography (DOT). In DOT several pieoseeond diode lasers are multiplexed into the input of a fibre switeh. The multiplexed lasers are switched conseeutively into a large number of optieal fibres whieh deliver the light to the sample. The 

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Fig. 3.5 Multiplexed TCSPC operation. Several signals are actively multiplexed into the detector. The destination in the TCSPC memory is controlled by a multiplexing signal at the channel input. For each multiplexing channel a separate photon distribution is built up over the signal time period and the sequencer coordinates...

In principle, parallel imaging ean be eombined with multiplexing and sequencing. Of course, the size (or pixel number) of the images, sequeneer steps, and multiplexing channels that can be used simultaneously are limited by the memory space in the TCSPC module. 

The lasers must then be multiplexed at a rate faster than the changes expected in the sample. One way to multiplex lasers is to synchronise their pulse periods and delay the pulses of different lasers by different fractions of the pulse period. The fluorescence signals are recorded simultaneously in the same TAC range of a TCSPC device. The principle is shown in Fig. 5.26. 

For pulse-by-pulse multiplexing, the timing reference signal for the TCSPC channels comes from one of the lasers. For pulse group multiplexing, the trigger output signals of the lasers are combined in a reversed power splitter. 

Recently Liebert et al. have demonstrated that advanced TCSPC is able to record effects of brain activity with 50 ms time resolution, clear separation of scattering and absorption, and probably better depth resolution than CW or frequency-domain techniques [324, 327, 328]. A system of four parallel TCSPC modules with four individual detectors and several multiplexed laser diode lasers is used. A fast sequence of time-of-flight distributions is recorded in consecutive time intervals of 50 to 100 ms. Variations of the optical properties in the brain are derived from the intensity and the first and second moments of the time-of-flight distributions [325]. 

The setup shown in Fig. 5.49 can, in principle, be used to record fast changes in the brain at 4 laser wavelengths and 32 detector positions. However, the limited speed of the fibre switch normally allows one to record sequences only for one or two source positions at a time. The result is a total number of 128 to 256 waveforms each 50 to 100 ms or 32 to 64 per TCSPC module. The corresponding readout rate in the memory swapping mode is well within the range of currently used TCSPC modules. However, improved fibre switches may allow one to multiplex a larger number of source positions at a rate of 100 s" or faster. The data transfer rate then exceeds 10 Mbyte/s, and precautions have to be taken to sustain this rate over a longer time. 

Technical Aspects of TCSPC-Based DOT Multiplexing of Lasers... 

Figure 5.60 illustrates the situation. Multiplexing different wavelengths on a pulse-by-pulse basis is shown left. Two or more lasers are multiplexed, and a reference pulse from one of the lasers is used as a stop signal for the TCSPC channel(s). The TAC range is increased in order to record the time-of-flight distributions for all lasers as a single waveform within the TAC window. 

Of course, a TCSPC system works effieiently only with a high-repetition-rate excitation source. Diode lasers ean be built with any repetition rate up to about 100 MHz and are available with 375 nm, 405 nm, 440 nm, and 473 nm emission wavelength. Diode lasers are cost-effieient and ean be multiplexed at ps rates see Excitation Wavelength Multiplexing, page 87. For shorter wavelengths, frequency-doubled or frequency-tripled titanium-sapphire or neodymium-YAG lasers can be used. 

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