PicoQuant GmbH HydraHarp 400 Software V. 3.0.0.1
throughput can be as high as 40 Mcps. This total transfer rate must be shared by the input channels. For all
practically relevant photon detection applications the effective rate per channel is more than sufficient.
For maximum throughput, T2 mode data streams are normally written directly to disk, without preview other
than count rate and progress display. However, it is also possible to analyze incoming data ”on the fly”. The
HydraHarp software provides a real-time correlator for preview during a T2 mode measurement (see section
5.3.7). Other types of real-time processing must be implemented by custom software. The HydraHarp software
installation CD contains demo programs to show how T2 mode files can be read by custom software. The
implementation of custom measurement programs requires the HydraHarp programming library, which is
available as a separate option.
5.3.3. T3 Mode
In T3 mode the SYNC input is dedicated to a periodic sync signal, typically from a laser. As far as the
experimental setup is concerned, this is similar to histogramming mode. The main objective is to allow high
sync rates (up to 150 MHz) which could not be handled in T2 mode. Accommodating the high sync rates in
T3 mode is achieved as follows: First, the sync divider is employed as in histogramming mode. This reduces
the sync rate so that the channel dead time is no longer a problem. The remaining problem is now that even
with a divider of 16, the sync rate is still too high for collecting all individual sync events like ordinary T2 mode
events. Considering that sync events are not of primary interest, the solution is to record them only if they arrive
in the context of a photon event on any of the input channels. This is the same as the original TTTR mode
concept as it was first conceived for the TimeHarp system. The event record is then composed of two timing
figures: 1) the start–stop timing difference between the photon event and the last sync event, and 2) the arrival
time of the event pair on the overall experiment time scale (the time tag). The latter is obtained by simply
counting sync pulses. From the T3 mode event records it is therefore possible to precisely determine which
sync period a photon event belongs to. Since the sync period is also known precisely, this furthermore allows
to reconstruct the arrival time of the photon with respect to the overall experiment time.
Each T3 mode event record consists of 32 bits. There are 6 bits for the channel number, 15 bits for the start–
stop time and 10 bits for the sync counter. If the counter overflows, a special overflow record is inserted in the
data stream, so that upon processing of the data stream a theoretically infinite time span can be recovered.
The 15 bits for the start–stop time difference cover a time span of 32,768×R where R is the chosen
resolution. At the highest possible resolution of 1 ps this results in a span of 32 ns. If the time difference
between photon and the last sync event is larger, the photon event cannot be recorded. This is the same as in
histogramming mode, where the number of bins is larger but also finite. However, by choosing a suitable sync
rate and a compatible resolution R, it should be possible to reasonably accommodate all relevant experiment
scenarios. R can be chosen in doubling steps between 1 ps and 33.5 µs.
Dead time in T3 mode is the same as in the other modes (80 ns typ.) Within each photon channel,
autocorrelations can be calculated meaningfully only starting from lag times larger than the dead time. Across
channels dead time does not affect the correlation so that meaningful results can be obtained at the chosen
resolution, all the way down down to zero lag time. This requires dedicated software.
The 32 bit event records are queued in a FIFO (First In First Out) buffer capable of holding up to 2 M event
records. The FIFO input is fast enough to accept records at the full speed of the time–to–digital converters (up
to 12.5 Mcps each). This means, even during a fast burst no events will be dropped except those lost in the
dead time anyhow. The FIFO output is continuously read by the host PC, thereby making room for new
incoming events. Even if the average read rate of the host PC is limited, bursts with much higher rate can be
recorded for some time. Only if the average count rate over a long period of time exceeds the readout speed of
the PC, a FIFO overrun could occur. In case of a FIFO overrun the measurement must be aborted because
data integrity cannot be maintained. However, on a modern and well configured PC a sustained average count
rate of 9 Mcps is possible. If your device supports USB 3.0 and is connected appropriately, the throughput can
be as high as 40 Mcps. This total transfer rate must be shared by the input channels. For all practically relevant
photon detection applications it is more than sufficient.
For maximum throughput, T3 mode data streams are normally written directly to disk. However, it is also
possible to analyze incoming data ”on the fly”. One such analysis method is the on–line correlation
implemented in the HydraHarp software. Other specialized analysis methods must be implemented by custom
software. The HydraHarp software installation CD contains demo programs showing how T3 mode files can be
read. The implementation of custom measurement programs requires the HydraHarp programming library,
which is available as a separate option. Another alternative for advanced TTTR data collection and analysis is
the SymPhoTime software offered by PicoQuant.
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