25 ns software correlator for photon and fluorescence correlation spectroscopy

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Abstract
A 25 ns time resolution, multi-tau software correlator developed in LABVIEW based on the use of a standard photon counting unit, a fast timer/counter board (6602-PCI National Instrument) and a personal computer (PC) (1.5 GHz Pentium 4) is presented and quantitatively discussed. The correlator works by processing the stream of incoming data in parallel according to two different algorithms: For large lag times (τ⩾100 μ s ), a classical time-mode (TM) scheme, based on the measure of the number of pulses per time interval, is used; differently, for τ⩽100 μ s a photon-mode (PM) scheme is adopted and the time sequence of the arrival times of the photon pulses is measured. By combining the two methods, we developed a system capable of working out correlation functions on line, in full real time for the TM correlator and partially in batch processing for the PM correlator. For the latter one, the duty cycle depends on the count rate of the incoming pulses, being ∼100% for count rates ⩽3×10 4 Hz , ∼15% at 10 5 Hz , and ∼1% at 10 6 Hz . For limitations imposed by the fairly small first-in, first-out (FIFO) buffer available on the counter board, the maximum count rate permissible for a proper functioning of the PM correlator is limited to ∼10 5 Hz . However, this limit can be removed by using a board with a deeper FIFO. Similarly, the 25 ns time resolution is only limited by maximum clock frequency available on the 6602-PCI and can be easily improved by using a faster clock. When tested on dilute solutions of calibrated latex spheres, the overall performances of the correlator appear to be comparable with those of commercial hardware correlators, but with several nontrivial advantages related to its flexibility, low cost, and easy adaptability to future developments of PC and data acquisition technology.