Correlated Photon Radiometry at NIST

Correlated Photon Radiometry Home> Determining the absolute responsivity of photon counting detectors The responsivity (1) of a photon counting detector can be determined using pairs of correlated photons by positioning two detectors to intercept each of photons in the pair. The counting rate of each detector (N1 and N2) is recorded along with the coincidence rate (NC) between the two detectors. The ratio of the coincidence rate to the single rate of one detector is the absolute quantum efficiency of the other detector and vice versa. The rate of photon production (N) cancels in this procedure.  Put another way, the output pulses of one of the detectors can be thought of as a trigger which indicates the existence of a second photon headed for the other detector. The quantum efficiency of the detector is then just the fraction of photons detected at the second detector in conjunction with a trigger from the first. Figure 1: Schematic of the absolute responsivity scheme To test this method, a parametric down-conversion source has been set up to allow the absolute spatial responsivity of a photomultiplier to be measured at a range of wavelengths in the visible. The method was verified using independent calibration methods available within the division. The goal is to determine the ultimate accuracy that may be achieved with this method. The first results showed agreement at the 0.5% level, which was the estimated 1-sigma uncertainty of that comparison. Improvements are underway to test this comparison at the 0.1% level.

For technical information or questions, contact:
Alan Migdall Phone: (301) 975-2331 Fax: (301) 869-5700 Email: amigdall@nist.gov
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