On the Raman Threshold of Passive Large Mode Area Fibers

摘要

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: P_(th) =16A_(eff)/g_RL_(eff) . 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 (I_(th)-P_(th)/A_(eff) on the mode area. However, our calculations show otherwise. Additionally, we have obtained an improved Raman threshold formula valid for short LMA fibers.