Experimental Demonstration of Central-Lobe Energy Enhancement Based on Amplitude Modulation of Beamlets in 19 Elements Fiber Laser Phased Array

Abstract

Coherent beam combination of fiber laser phased array is a promising technique to achieve high-brightness laser output. In this paper, we focus on central-lobe energy enhancement of the combined beam in the far-field by theoretically and experimentally investigating the relationship between the power-in-the-bucket (PIB) of the combined beam in the far-field and the beamlets amplitude modulation in the near-field. We employ a 19-elements fiber laser phased array integrating adaptive fiber-optic collimators with a beamlet filling factor of 0.903 and achieve phase-locking and beamlet pointing correction among fiber lasers using the SPGD algorithm. The amplitude modulation applied to the tiled sub-apertures is used to control the shape of the emissive array's amplitude envelope to further change the PIB of the combined beam under the same total output power. According to theoretical analysis, the fiber laser phased array achieves PIB = 0.51 for a zero truncation ratio (Tr), which is increased to PIB = 0.57 for Tr = 1.3. Experimentally, we observe PIB = 0.49 for Tr = 0 and PIB= 0.56 for Tr = 1.3. The experimental results are in good agreement with the theoretical analysis, suggesting that the amplitude modulation is helpful for further improving the energy density of central-lobe.

Discussion on Piston-Type Phase Ambiguity in a Coherent Beam Combining System

“…(a) (c) (e) On the emissive plane, the sub-apertures are usually arranged in a centrosymmetric shape, which is beneficial to reduce the sidelobe energy and obtain high-quality combined beams [33][34][35][36][37][38]. However, the centrosymmetric arrangement will cause phase ambiguity.…”

“…It is obvious that x−n = −xn and y−n = −yn. The overall near-field complex amplitude modulated by the piston phase is equal to the sum of each sub-aperture, which can be expressed as On the emissive plane, the sub-apertures are usually arranged in a centrosymmetric shape, which is beneficial to reduce the sidelobe energy and obtain high-quality combined beams [33][34][35][36][37][38]. However, the centrosymmetric arrangement will cause phase ambiguity.…”

Optical Field Transmission Analysis of an All-Fiber Signal Combiner With a Dumbbell Shape

“…However, there are many challenges to further improve the output power of a single fiber laser [7][8] . To solve this problem, laser beam combining is supposed to be an effective method to further improve the output power [9][10][11] . Fiber combiner is an all-fiber device that can combine multiple fiber laser s into one fiber for output.…”