Nondestructive measurement of length dependence of gain characteristics in fiber amplifiers

The authors describe a nondestructive method for determining the length dependence of the gain of a fiber amplifier. The method measures rapidly the length dependence of fiber-amplifier gain characteristics without resorting to cutback techniques. By extracting small amounts of signal power with a macrobend placed along the length of a fiber amplifier, the optical amplifier´s gain, output power, pump efficiency, and optimal length can be determined without cutting the fiber. The macrobend was formed by bending the fiber over a rod. Transmitting only one modulated signal through the amplifier without the fiber being pumped makes it possible to measure the absorption of the fiber at any particular wavelength. By fitting to these curves, the absorption length of the pump, signal, or any other wavelength can easily be determined.<>