ERBIUM-DOPED FIBER AMPLIFIERS - THEORY OF UP-CONVERSION-PUMPED OPERATION WITH A SIGNAL WAVELENGTH OF 850 NM

A model is presented for erbium-doped fiber amplifiers that operate by an upconversion process in which two pump photons at approximately 800 nm are absorbed and can provide an inverted population and hence the ability to amplify signal photons at approximately 850 nm. The most important spectroscopic features of the amplifier are identified and used to formulate a theory that predicts how the amplifier gain varies with fiber length and doping density, as well as with launched pump and signal powers. The limitation that amplified spontaneous emission imposes on the gain is investigated, and useful approximate formulas are derived in the limit where amplified spontaneous emission is negligible.