Optical amplifier with bent, single-mode active fiber

An optical amplifier for the amplification of optical transmission signals at a first predetermined wavelength propagating in an optical fiber line, said amplifier comprising:

• a pumping energy source for providing pumping energy at a second predetermined wavelength shorter than said first predetermined wavelength;
• a dichroic coupler having two inputs and an output, one of said inputs being coupled to said pumping energy source and the other of said inputs being adapted to be connected to said optical fiber line in which said optical transmission signals are propagated;
• an active fiber which is long relative to its width and which contains a fluorescent dopant which can be pumped at said second predetermined wavelength and which emits radiation at said first predetermined wavelength, said fiber, in substantially rectilinear configuration, providing single-mode propagation of signals at said first predetermined wavelengths and multi-mode propagation of energy at said second predetermined wavelength and said fiber being coupled at one end to said output of said dichroic coupler and being bent transversely to its length into a curved configuration for at least 70% of its length with a curvature radius such that said fiber in curved configuration provides substantially only single mode propagation therein at said second predetermined wavelength;
• and
• said amplifier being without means for feeding radiation energy at said first predetermined wavelength from the other end of said active fiber to said one end of said active fiber.