Decoupled tip-tilt detection and steering of beamlets in a fiber laser array using sequential tagging lasers.

The tiled fiber laser array is a promising architecture for power scaling and mitigation of atmospheric turbulence. Precise tip-tilt detection and steering of the beamlets in the array to guarantee effective overlap in the far-field is essential for high beam qualities. However, the beamlets are tightly coupled in the far-field, making it a major challenge to identify each other. Consequently, most researchers turn to indirect methods without tip-tilt detection. We propose what we believe to be a novel approach of decoupled tip-tilt detection and steering of the beamlets for a tiled fiber laser array. A pulsed tagging laser is divided and combined into each beamlet with difference pulse sequences. By synchronizing the tagging laser pulses and exposures of the far-field camera, only one spot of the tagging laser is detected in each image, whose location is coincident with its corresponding beamlet. Thereby, the beamlets are decoupled by different pulses. The pointing of each beamlet is then readily detected and steered individually. The proposed method is validated by experiments with a seven-beamlet adaptive fiber collimator array and proved efficient. This is the first, to the best of the authors' knowledge, demonstration of direct and decoupled beamlet tip-tilt detection and steering in the far-field for a tiled fiber laser array.