Optimization of fiber coupling in ultra-high power pump modules at λ=980 nm

This work presents some aspects of development of ultra-high power single-mode pump modules at lambda = 980 nm for erbium-doped fiber amplifiers. We report here on the results of numerical simulations and experimental data of modifications to the laser waveguide structure with a focus on improving the fiber coupling efficiency. The so-called integrated fiber wedge lens was used as a coupling element in the present investigation. Our simulations showed that between the two most widely used laser waveguide types: large optical cavity (LOC) and separate confinement (SCH or GRICC) heterostructures the difference in coupling efficiency can be as high as ten absolute percent We achieved an experimental coupling efficiency of 93 percent for LOC-like lasers structure. The SCH-based lasers showed maximum coupling efficiency of 83 percent. However, in spite of superior coupling efficiency, use of LOC-based lasers in pump modules does not bring any benefits because of subpar electro-optical performance. To improve the situation we had to find a reasonable compromise between LOC and SCH structures. Lasers resulting from this approach gave a coupling efficiency around 90 percent. The laser diodes based on the optimized structure achieve more than 3 W of output power and more than 2 W of kink-free power in CW regime at room temperature. They also demonstrate differential quantum efficiency above 85% and laser power conversion efficiency above 60 percent at use conditions. Thanks to the combination of all these factors pump modules built on these lasers produce 1W of wavelength-stabilized power at an operating current below 1.3 A. Maximum kink-free, wavelength-stabilized output from the pump module reached 1.8 W at room temperature.