Single mode optical fiber

Optical fiber adapted for guiding electromagnetic radiation, guided radiation comprising a LP.sub.01 mode and, possibly, a LP.sub.11 mode, the fiber being a single mode fiber for electromagnetic radiation of an operating wavelength, the optical fiber to be referred to as the first fiber, the first fiber comprising

• (a) a core region having an equivalent radius a and an equivalent maximum refractive index n.sub.c ;
• (b) a first cladding region surrounding the core and having an equivalent outer radius R.sub.1o, an equivalent inner radius R.sub.1i, and an equivalent refractive index n.sub.1, with n.sub.1 <n.sub.c ;
• and
• (c) a second cladding region surrounding the first cladding region and having an equivalent inner radius R.sub.2i and an equivalent refractive index n.sub.2, with n.sub.2 .gtoreq.n.sub.1 ;
• characterized in that
• the first fiber further comprises a third cladding region having an equivalent inner radius R.sub.3i, an equivalent outer radius R.sub.3o, and an equivalent refractive index n.sub.3, with R.sub.3o .ltoreq.R.sub.2i, R.sub.1o .ltoreq.R.sub.3i, and n.sub.3 >n.sub.1 ;
• wherein n.sub.3, R.sub.3i and R.sub.3o are chosen such that
• (d) at R.sub.3i the square of the amplitude of the LP.sub.01 mode is at most 10.sup.-5 of the square of the amplitude of the LP.sub.01 mode at the center of the core, for radiation of the operating wavelength of the optical fiber;
• (e) at R.sub.2i in the first fiber, the square of the amplitude of the LP.sub.11 mode is at least 50% larger than the square of the amplitude of the LP.sub.11 mode in a second fiber at a radius equal to R.sub.2i, the second fiber being identical to the first fiber except that in the second fiber R.sub.3i =R.sub.3o, the third cladding region thus being absent in the second fiber, the second fiber having a cut-off wavelength, the squares of the amplitudes being compared at the cut-off wavelength of the second fiber;
• and
• (f) at R.sub.2i and at the operating wavelength, the square of the amplitude of the LP.sub.01 mode in the first fiber is at most 50% larger than the square of the amplitude of the LP.sub.01 mode in the second fiber, at the same wavelength and the same radius.