Photon Counting System

The Photon Counting System consists of the Avalanche Photodiode Detectors (APD's), Detector Overload Protectors, a National Instruments data acquisition board, an FCS acquisition board, an External Bin Timer, and a PC. For image acquisition, pulses (representing individual detected photons) from the APD's are counted using hardware counters on the National Instruments data acquisition board. Data from the counters is read at regular intervals (the bin time) by applying pulses to the counters' GATE inputs. The GATE pulses are hardware-generated and synchronized to the stage control software. All of the binned counts of a line are buffered for maximum counting efficiency, and the buffer is read at the end of each line (while the stage moves up to the next line).

For acquiring data from points of interest in the image, the National Instruments data acquisition board can be used in a manner similar to image acquisition, or the FCS acquisition board can be used to acquire Fluorescence Correlation Spectroscopy data (FCS). For both boards it is also possible to acquire the time between photon arrivals, allowing post-acquisition bin interval selection.

Data Acquisition Boards:

National Instruments PCI-6025E board

Photon Counting Channels: 1 or 2 Buffered (for two the External Bin Timer is used)

Bin Time Range: 100ns to 800ms (longer bin times are possible if needed)

Bin Time Resolution: 50ns

Binning Dead Time (Inter-Bin Time): 100ns

Maximum Number of Counts per Scanned Line: 16.7million per detector channel

Note: For counting operations the NI 6602 board is a better choice but it has no analog I/O.

Correlator.com FLEX01-12D FCS board

Photon Counting Channels: 1 or 2 Buffered

Time Resolution: 12.5ns

Real time hardware autocorrelation and cross-correlation modes

Photon history (inter-photon arrival time) acquisition mode

USB interface

External Bin Timer: The External Bin Timer is a custom clock-generation circuit that is used to provide a gating signal to the National Instruments board counters when both counters are in use for photon counting. The clock period is selected and triggered by digital I/O lines from the National Instruments board.

Bin Time Range: 100ns to 500ms

Bin Time Resolution: 50ns

Binning Dead Time (Inter-Bin Time): 50ns

Re-configurable CPLD design

Display indicator of bin time code

Detector Overload Protection Modules: The Detector Overload Protection modules monitor the count rate from the detectors and send a signal to the shutter controller if the count rate is above an adjustable threshold. The shutter is then closed until the user resets the unit. The module also outputs a TTL-level pulse for each input pulse from the detector. A power jack on the rear panel provides power for the detector, insuring that the protection circuitry is ON if the detectors are on.

Overload threshold adjustable from 1MHz to 3MHz.

Adjustable input pulse threshold, with hysteresis.

Pulse output can source or sink over 100mA into a 50 ohm load at full count rate.

LEDs to indicate power, high count rate, and trip conditions.

Last updated August 10, 2003