Mode Selection in Large-Mode-Area Step-Index Multicore Fiber Laser and Amplifier

We present mode-selection in a laser and an amplifier from a multicore Ytterbium (Yb) doped step-index fiber with six large mode area and low numerical aperture (NA) cores, fabricated using a combination of chelate delivery system (CDS) and stack and draw technique. The six cores are placed in a ring shape with each core having a diameter of $\sim 19~\mu \text{m}$ and NA of ~0.068, or V-number of ~3.8 at $1.06~\mu \text{m}$ . The cores are placed with a 1.1 pitch-to-core ratio, which efficiently promotes supermode generation and creates a good modal separation to facilitate mode selection. Mode selection of the multicore fiber is investigated in a laser as well as an amplifier configuration. With an intracavity pinhole, we are able to select the fundamental in-phase supermode for lasing up to 33.9W output power with a 52% slope efficiency. In the amplifier configuration, a Gaussian beam from a seed source is successfully coupled to the fundamental supermode in the multicore fiber. The seed source is amplified to ~43.5W from 0.14 W input power.