Measurements of fibre optic power using photodiodes with and without an integrating sphere

We have studied two kinds of reference detectors to be used for calibration of fibre optic power meters. The first type consists of an InGaAs photodiode mounted directly on an adapter using a standard FC connector. In the second type, a similar photodiode is fitted into a 5 cm diameter integrating sphere. The plain photodiodes are used for low power levels and the sphere detector for high power levels because of the low throughput (∼0.7%) of the sphere. The detectors, with their FC connectors removed, are calibrated using collimated laser beams, using as the reference a spectrally flat pyroelectric radiometer traceable to a cryogenic absolute radiometer. The spatial non-uniformities of the plain photodiodes are of the order of 0.3% throughout the active area of 5 mm diameter. The angular responsivity varies less than ±0.1% within the angle of ±8°, which covers 99% of the radiation emitted from the fibre end. The angular non-uniformity of the sphere detector is less than 0.1%. The results indicate that a plain photodiode can be used as a reference detector of fibre optic power. The calibration uncertainty at HUT is presently of the order of 1% ( k = 2).