We report the first observation of birefringent in-phase supermode operation of a phase-locked multicore fiber laser. The in-phase mode operation of our 12-core rectangular-array microstructured fiber laser was confirmed by the near-field distribution, the far-field diffraction pattern, and the optical spectrum. The birefringence of the in-phase mode in propagation constant Δγ was measured as ~4×10-6 1/µm. The break of the polarization degeneracy indicates the possibility of single polarization operation of phase-locked multicore fiber lasers and amplifiers.
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