Correlated photons produced by spontaneous parametric downconversion can be used to absolutely measure detector external quantum efficiency (q.e).

Figure 1 shows a schematic of how spontaneous parametric downconversion can be used to measure quantum efficiency.

Figure 1: NPL schematic of quantum efficiency measurements
PBS = Polarising Beam Splitter, HWP = Half Wave Plate, BBO = beta barium borate
BP = Band Pass, DUT = Device Under Test, TRIG = Trigger

In this set up, the detectors capture the correlated photons of a particular wavelength selected by spectral filtering elements placed in the trigger path. The count rate is recorded on each counter (equations (4) and (5)), the number of coincidences (number of correlated photon pairs) is given by equation 5, where η is the external quantum efficiency, N is the total number of counts, T is the transmittance from point of downconversion to the detector, N_c is the number of coincidence counts:

Rearranging the equations above gives the quantum efficiency of the device under test (DUT):

The quantum efficiency of the trigger detector does not need to be known, and this makes the measurement inherently absolute.

Ongoing research at NMIs aims to significantly reduce the current uncertainty limits.

Taking experimental factors into account, equation (4) becomes:

T_DUT = optical losses in path from point of downconversion to DUT
N_c = apparent number of coincidence events
N_Acc = number of accidental coincidence events
N_trigger = apparent number of trigger events
N_false = number of false trigger events

With appropriate downconversion media and pump laser the technique can be used to calibrate infra-red detectors with respect to visible photon counting detectors where performance is greater.

Photon counting detector characterisation capabilities:

• Afterpulsing
• Spatial uniformity
• Jitter
• Timing resolution
• Spectral responsivity
• Transmittance losses

For further information, please contact: Jessica Cheung