We propose a parity-time (PT) symmetric fiber laser composed of two coupled ring cavities with gains and losses, which operates both in PT-symmetric and symmetry-broken regimes depending on the static phase shifts. We perform analytical and numerical analysis by the transfer matrix method taking into account gain saturation and predict laser bistability in the PT-symmetric regime in contrast to a symmetry-broken single-mode operation. In the PT-broken regime, the generation power counterintuitively increases with an increase of the cavity losses.

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In this work, we propose a concept of a coupled fiber laser exhibiting PT-symmetry properties. We consider a system operated via Raman gain. The scheme comprises two identical fiber loops (ring cavities) connected by means of two fiber couplers with variable phase shift between them. We show that by changing the phase shift one can switch between generation regimes, realizing either PT-symmetric or PT-broken solution. Furthermore, the paper investigates some peculiarities of the system such as power oscillations and the role of nonlinear phase shift in fiber rings.

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