INSTITUTE OF PHYSICS PUBLISHING METROLOGIA Metrologia 43 (2006) S145–S150 doi:10.1088/0026-1394/43/2/S29 An integrating sphere radiometer as a solution for high power calibrations in fibre optics Ana Carrasco-Sanz 1 , F´ elix Rodr´ ıguez-Barrios 1 , Pedro Corredera 1 , Sonia Mart´ ın-L´ opez 1 , Miguel Gonz ´ alez-Herr´ aez 1,2 and Mar´ ıa Luisa Hernanz 1 1 Department de Metrolog´ ıa, Instituto de F´ ısica Aplicada (IFA) CSIC, Serrano 144, 28006 Madrid, Spain 2 Department de Electr ´ onica, Escuela Polit´ ecnica Superior, U. de Alcal´ a, Campus Universitario, 28871, Alcal´ a de Henares, Madrid, Spain Received 19 October 2005 Published 29 March 2006 Online at stacks.iop.org/Met/43/S145 Abstract This paper describes the design, characterization and calibration of a high power transfer standard for optical power measurements in optical fibres based on an integrating sphere radiometer. This radiometer, based on two detectors (Si and InGaAs), can measure powers between 100 nW and 10 W within the wavelength range of (400–1700) nm. The radiometer has been calibrated over the total spectral range of use against an electrically calibrated pyroelectric radiometer and different fibre laser diodes and ion lasers. The total uncertainty obtained is lower than ±1.5% for these wavelengths and power ranges (excluding the water absorption region). 1. Introduction The proliferation of high power fibre lasers used for pumping Raman amplifiers whose output powers are several watts requires an improvement in the calibration capabilities for absolute power measurements in optical fibres at levels from 100 mW to 10 W. Integrating sphere radiometers (ISRs) have been demon- strated as a realizable system for developing fibre optic power scales [1–3] and NIR spectral responsivity scales us- ing monochromator-based cryogenic radiometers [4] or laser- based cryogenic radiometer facilities [5]. Three national stan- dards laboratories have realized high fibre optic power scales, all of them based on ISR radiometers with an uncertainty of 1.3% for a (1–200) mW power level [6]. In order to fully benefit from the high accuracy of the cryogenic radiometer, it is important that the transfer radiometers have the best possible radiometric properties. These include good long-term stability, a high degree of spatial uniformity, a small variation of responsivity with angle of incidence, a small temperature variation of the responsivity and a high degree of linearity. In a previous paper [7], we have demonstrated the possibility of creating a sphere radiometer of high accuracy for optical fibres useful for powers up to 0.8 W in the range of wavelengths from 1250 nm to 1650 nm. In this paper, a further development of this type of radiometer is presented. The new radiometer has been designed for working in the (400–1700) nm wavelength range. In order to cover this entire spectral range we put together Si and InGaAs detectors in an integrating sphere. The characterization of the new radiometer was performed in two steps: firstly, we made a complete characterization of all the single elements (sphere material, Si and InGaAs detectors) before they were coupled into the sphere radiometer. Secondly, we calibrated the ensemble sphere radiometer at 1 mW (0 dB m), and using the linearity figure of the detectors and the properties measured of the sphere, we extended the results and estimated the uncertainties in the optical power range from 100 nW to 10 W (−40 dB m to +40 dB m). 2. Design of the integrating sphere radiometer The structure of the sphere radiometer designed is shown in figure 1. The radiometer consists of a Spectralon ® integrating sphere manufactured by Labsphere, Inc., and two detectors. 0026-1394/06/020145+06$30.00 © 2006 BIPM and IOP Publishing Ltd Printed in the UK S145