Nature of fiber-coupled detector responsivity measurements at 0.1% using a primary standard

We demonstrate the capability to measure the absolute power responsivity of optical fiber-coupled detectors at an expanded uncertainty of 0.1%, by direct comparison with a cryogenic primary standard. To facilitate synchronous power measurements, commercial all-fiber beam-splitters direct laser diode light simultaneously to the device under test and the primary standard. We investigate the use of single-mode, polarisation maintaining, and photonic crystal fibers to access the cryogenic standard, and report a reduction in the temperature dependent effective refractive index of these fibers of 0.1%, 0.15% and 0.3% respectively in going from room temperature to 5 K. We also evaluate the polarisation dependent loss of the beam-splitters, the stability of the beam-splitter ratio between the cryogenic detector and the device under test and the temporal and modal stability of the Fabry-Pérot laser diode sources. It is shown that the stability of the optical fiber beam-splitters limits the overall performance of the measurement system to an expanded uncertainty of 0.1%.