It is widely known that one of the most serious problems in the measurement of multi-mode fiber attenuations is the influence of the excitation conditions at the input of the fiber. We have developed a precision instrument that can measure the optical power of a stable source with an error of less than 0.05%, implying that power changes in individual modes can be detected. This capability allows us to accurately study the effect of modal distributions upon attenuation, and for the first time makes it possible to accurately measure the loss of even a single low-loss (e.g. fusion) splice that is excited by the steady-state modal distribution of one fiber and "pumps" another long fiber in which steady-state is once more reached before measurement. In this paper, we briefly outline the essential features of this instrument, and go on to describe a measurement technique that can be used to minimize the influence of cladding as well as leaky modes. Moreover, the use of a simple correction factor allows the determination of the attenuation of the fiber both for selective and for complete excitation.
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