Influence of Clad-to-Core Ratio on the Performance of Nd-Doped LMA Fiber Amplifiers at 915 nm

Over the past decade, there has been a growing interest in high-power continuous wave and pulsed Nd-doped fibers (NDF) operating on the 3-level transition
${{}^{4}\mathsf{F}}_{3/2}\rightarrow{{}^{4}\mathsf{I}}_{9/2}$
in the 880–930 nm wavelength range. Due to their efficiency and compactness compared to a Ti:Sa laser, they are very suitable for two-photon excitation microscopy, but also for the generation of high-power deep blue and UV laser light after non-linear frequency conversion. These wavelengths are indeed of great interest in various fields of application ranging from underwater LIDAR [1] to laser tattoo removal [2]. However, because of the 4-level
${{}^{4}\mathsf{F}}_{3/2}\rightarrow{{}^{4}\mathsf{I}}_{11/2}$
transition near 1050 nm, efficient laser emission based on the 3-level transition near 900 nm is difficult to achieve. Hence, the fiber design must be carefully optimized to lower the gain at 1050 nm and prevent spurious lasing at this wavelength. In high power laser regime, the best results have been obtained with large mode area (LMA) fibers. As for example, a NDF LMA fiber with a 30/125 geometry corresponding to a clad-to-core ratio of 4 and a very low NA of 0.05 allowed to obtain a record output power of 83 W at 910 nm [3]. On the other hand, the core exhibits slightly multimode behavior with a V parameter of 4.63 and it is then difficult to maintain a good beam quality with M2 < 1.5 in a fully fiberized configuration.