Introduction Intent This test procedure defines and updates the methodology for measuring the center (mean) wavelength, peak wavelength and the spectral width (rms1 ) of multilongitudinal mode (MLM), multitransverse mode (MTM), and single mode (SM) semiconductor lasers using a dispersive spectrophotometric method or other suitable methods. For SM lasers, only center wavelength should be used and it should be noted that other spectral characteristics, such as Side Mode Suppression Ratio (SMSR), Stop Band (SB) for distributed feedback (DFB) lasers for example, are not covered by this document. Other measures of spectral width for MM lasers are also retained from previous versions of this document. Recent studies on the measurement of the optical spectrum from various coherent sources into multimode fiber have highlighted issues with measurement repeatbility due to speckle pattern variations and launch conditioning of the optical source into the multimode optical fiber. In addition, using multitransverse mode sources, such as some VCSELs, with singelmode optical fiber can lead to undersampling of the complete optical spectrum. Hazards This procedure involves potentially hazardous operations as discussed in this section. During the measurement, a laser may emit non-visible light. Personnel are strongly cautioned never to look directly into the laser or any fiber optic cabling at any time. Although the optical output power is generally not very high, virtually all the power is concentrated into a narrow frequency band, which implies that the energy can be focused into a very intense spot on the retina by the lens within the viewer's eyes. For a complete review of laser safety guidelines, the reader is referred to the international standard IEC 60825-2.