Generation of High Power Levels of Narrow Linewidth 1178 Nm via Raman Amplification of the Second Stokes

There is interest in frequency doubling a 1178 nm narrow linewidth fiber laser source for sodium guidestar laser applications. 1178 nm can be generated via the following methods documented in the scientific literature and patents: 1. Direct lasing of Yb doped fiber [1]; 2. Direct lasing of photonic bandgap fiber [2]; 3. Generation of 1178 nm via the Raman process [3]; and 4. Lasing of a semiconductor heterostructure [4]. A major constraint on the approach is the requirement for a very narrow linewidth, i.e., on the order of 3 GHz or less depending on the approach, as well as the need for a linearly polarized beam. The highest power level of narrow linewidth 1178 nm obtained to date, 39 W, was generated by the European Southern Observatory via conversion of 1121 to 1178 nm in a very long Raman fiber [3]. Power levels of 1178 nm were limited in this system by power constraints associated with the wavelength division multiplexer (WDM), low intensities of pump in the fiber, low 1178 nm seed level, as well as Stimulated Brillouin Scattering (SBS). In order to obtain power levels of 1178 nm sufficient to generate about 50 W of 589 nm via second harmonic generation, coherent beam combination was required [5]. Generation of 1178 nm via photonic bandgap fiber is also promising with 24.6 W with a linewidth of 320 kHz having recently been reported [2].