Investigation of the Low Power Stage of an 1178 nm Raman System

An 1178 nm seeded and 1069 nm pumped Raman laser system where the second Stokes is amplified in a 1121 nm resonator defined by high reflector fiber Bragg gratings (FBGs) has the potential of producing high output power of narrow linewidth 1178 nm. However, 1121 nm power leakage out of the resonator cavity around the gratings was found to impact the performance of the laser and needs to be dealt with in order to obtain high 1178 nm output power levels. In order to address this problem, the causes of linewidth broadening must be understood. A fully nonlinear model has been built which involves propagation of the spectral wave shape via the nonlinear Schrdinger equation in addition to the Raman processes. It was found that increases in 1121 nm cavity power, fiber Bragg grating bandwidth, and the nonlinear index of refraction n2, as well as a decrease in group velocity dispersion 2 leads to an increase in linewidth broadening. It is concluded that the magnitude of linewidth broadening seen experimentally can only be explained if the spectral components outside the bandwidth of the FBGs are being amplified. Experimentally, 1121 nm power leakage can be handled by using a three wavelength WDM on either side of the rare earth doped amplifier. In addition, usage of a fiber having a high value for group velocity dispersion and/or a low value for nonlinear index of refraction n2 in addition to narrower bandwidth fiber Bragg gratings may help reduce the amount of linewidth broadening.