We describe the design, characterization, and calibration of a high-power and high-accuracy transfer standard for optical power measurements in fibers based on an integrating sphere radiometer working from -50 to +30 dBm. The integrating sphere radiometer has been calibrated in the spectral range 1250-1650 nm by use of an electrically calibrated pyroelectric radiometer and four tunable laser diodes. The total uncertainty obtained is less than +/-0.8% for these wavelength and power ranges.