Comparison between spectra from the facet of a semiconductor laser before andafter AR coated under the same bias condition has led to the establishment of the variationcurve of the facet reflectivity vs. the wavelength.,This method is applicable to a certainwavelength range, and overcomes the difficulty encountered when Kaminow's methodis implemented to determine the very low reflectivity of the diode with both facetS ARcoated.As a result, a reflectivity of less than 8×10-5 at the second coated facet has been measured.