A single-mode optical fiber supports only its fundamental spatial guided normal mode at the operating wavelength, with possible distinct states of polarization of an electromagnetic wave. The designation must include wavelength: MIT's fibre laboratory explicitly notes that a fibre sold for single-mode operation can support several normal modes at its helium–neon laser wavelength.[ref]
Optical Waveguides and Fibers (MIT 6.161 laboratory exercise). Optical Waveguides and Fibers, MIT 6.161 Modern Optics Project Laboratory, Fall 2005, Laboratory Exercise No. 7, p. 8, §7.4(a): operating-wavelength qualification
