Abstract: The present invention concerns a monomode optical fiber comprising an optical core for guiding the majority of light waves, surrounded by an optical sheath, the difference between the maximum refraction coefficient of the core and that of the optical sheath being indicated as .DELTA.n and the radius of the core being indicated as a, characterized in that:a .epsilon.?0.6a.sub.c :1.1a.sub.c !, a.sub.c being provided, in .mu.m, by the formula: ##EQU1## where k=2.pi./.lambda., .lambda. being (in .mu.m) the wavelength of the transmission, and where n.sub.g is the coefficient of said optical sheath, equal within 10.sup.-3 to that of pure silica,.DELTA.n is greater than or equal to 0.01,and in that, .phi. being the diameter of said optical sheath, in .mu.m: .phi..epsilon.?.phi..sub.min :100!, .phi..sub.min being provided, in .mu.

Abstract: A method of detecting variation in the diameter of an optical fiber includes the following steps:transversely illuminating a portion of the fiber by means of an incident light beam,measuring a radiation pattern produced by the fiber portion in a predetermined acute angular sector around the fiber, anddetecting variation in the diameter of the optical fiber causing modification of the angular radiation pattern produced in this way.The angular sector is defined by angles of 90.degree. and 105.degree. or by angles of 255.degree. and 270.degree..

Abstract: A telecommunications system having erbium-doped fiber optical amplifiers for transmitting signals including at least one signal at a wavelength of about 1550 nm, wherein the amplifiers are fibers whose cores are doped with erbium and germanium and are free from aluminum; and wherein for each of the amplifiers, the length of the fiber and the pumping power which is injected therein are such that the noise generated by the fiber is generally emitted around 1550 nm. As can be seen in FIG. 4 , the signal-to-noise ratio remains satisfactory after 10,000 km of propagation without using an intermediate filter.

Abstract: An erbium-doped fiber optical amplifier, characterized in that said fiber has a mode radius which is not greater than 2.7 .mu.m at a wavelength of 1.55 .mu.m, and in that it has at least one pump wavelength situated in the following regions of the spectrum: 0.98 .mu.m.+-.0.4 .mu.m and 1.48 .mu.m.+-.0.4 .mu.m; the region of the spectrum of at least one signal to be amplified being 1.5 .mu.m to 1.6 .mu.m.

Abstract: A DFB laser (2) is coupled to a monomode line fiber (14) via an isolator constituted by a Faraday rotator crystal (4) and a polarizing fiber (10) which is welded to a fiber to be coupled (14). A focusing lens (8) is integrated in an intermediate block including the rotator crystal (4) and a permanent magnet (6). The invention is particularly suitable for making light-emitting heads for use in optical fiber telecommunications systems.

Abstract: An optical fiber for frequency conversion, has an annular index profile structure for obtaining zero order phase matching and optimum overlap between the fibers's resonant propagation modes, one propagating the wave w and the other the wave kw where 2.ltoreq.k.ltoreq.4 for at least one wavelength lying in the range 0.4 .mu.m to 2.6 .mu.m, with the index step .delta.n in the ring lying in the range 2.times.10.sup.-2 and 5.times.10.sup.-2, the inside radius r.sub.i of the ring lying in the range 0.5 .mu.m to 1.5 .mu.m, and the outside radius r.sub.e of the ring lying in the range 2 .mu.m to 3.5 .mu.m.

Abstract: The core (2) of a monomode fiber is surrounded by a slightly monomode annular waveguide (6) guiding masking light having the same wavelength as the light conveyed by the core. The invention is particularly suitable for telecommunications.

Abstract: An erbium-doped fiber laser amplifier (1), characterized by the fact that said fiber (3) is doped to a concentration lying in the range 5 ppm to 50 ppm, that the length of said fiber lies in the range 250 m to 30 m depending on the selected concentration of doping, and that the radial position of the doped zone substantially coincides with the energy maximum of the set of propagation modes in said fiber at the pumping wavelength.