Low-noise active fiber optical amplifier with pumping power reflector

An optical amplifier of the active fiber type for amplifying optical communication signals having a predetermined wavelength, said amplifier comprising:

• an optical coupler having a first input for receiving said communication signals at the wavelength of said communication signals, a second input for receiving pumping light energy and an output for transmitting both said communication signals and said pumping light energy;
• an active optical fiber which is long relative to its cross section and having a core and a cladding therearound, whereby said communication signals and said pumping light energy are guided along said core, said core being connected at one end thereof to said output for receiving both said communication signals and said pumping light energy and containing Erbium ions as laser light emitting dopant, which, when subjected to said pumping light energy and to said communication signals, emits light at said predetermined wavelength, said active optical fiber also having another end remote from said one end thereof from which light at said predetermined wavelength is emitted;
• a pumping light source having a wavelength lower than said predetermined communication signal wavelength, coupled to said second input of said optical coupler for supplying pumping light energy into said active fiber at said one end thereof, whereby said pumping light energy travels in said core in the direction of the longitudinal axis of said core and said Erbium ions are excited by said pumping light energy to a laser emitting state, from which state they decay to the ground state when subjected to said communication signals to cause amplified light emission from said other end of said active fiber at said predetermined wavelength of said communication signals, the light from said pumping light energy source being absorbed in said active fiber in an amount depending on the distance travelled in said active fiber from said one end of the latter toward said other end of the latter and said active fiber having a length between said one end and said other end such that said light from said pumping source is only partially absorbed in said active optical fiber;
• and a selective mirror element coupled to said other end of said active optical fiber, said mirror element reflecting light at said pumping wavelength into said other end of said active fiber and permitting light at said wavelength of said communication signals to pass therethrough;
• and
• said length between said one end of said active fiber and said other end of said active fiber being selected so that the pumping light energy at all points along the length of said fiber is greater than the pumping light energy in the absence of said selective mirror element;
• and is greater than the saturation power at all points along the length of said fiber.