The output power of fiber optic laser systems has been exponentially increasing in the last years. However, non-linear
effects, and in particular stimulated Raman scattering (SRS), are threatening to seriously limit the development pace in
the near future.
SRS can take place anywhere along the laser system, however it is actually the passive delivery fiber at the end of the
system, the section where SRS is most likely to occur. The common way to combat this problem is to use the so-called
Large Mode Area (LMA) fibers. However, these fibers are expensive and have a multimode nature that will either reduce
the beam quality of the laser output or require a careful excitation of the fundamental mode. Furthermore, the larger the
core area, the more complicated it will be to sustain single-mode operation. Therefore, it is becoming increasingly
important to be able to determine which is the minimum core area required in the delivery fiber to avoid SRS.
This calculation is usually carried out using the conventional formula for the Raman Threshold published by R.G. Smith
in 1972: Pth =16Aeff gRLeff . In this work we demonstrate that this formula and the conclusions derived from it are
inaccurate for short (several meters long) LMA fibers. For example, one widely spread belief (obtained from this
expression) is that there is no dependence of the Raman intensity threshold (Ith=Pth/Aeff) on the mode area. However, our
calculations show otherwise. Additionally, we have obtained an improved Raman threshold formula valid for short LMA