For calibration, a reference fiber optic cable with a known length and attenuation is required. They are directly related to more than 15 IEC International Standards accurately optical power from fibre optic sources. As the components like fiber, connectors, splices, LED or laser sources, detectors and receivers are being developed, testing confirms their performance specifications and helps. In this article, we explore why fiber optic cable testing is essential, delve into three key testing methods, and explain how to determine the best approach for your needs. To augment the absolute power measurements NIST provides nonlinearity, spectral responsivity, and uniformity measurements.