A novel twisted Er-doped fiber ring laser: Proposal, theory, and experiment

A novel Er-doped fiber ring laser having less mode instability due to polarization fluctuation is proposed and investigated. The Er-doped fiber is twisted to form a fiber ring laser. The right-hand and left-hand rotated circular waves induced by the twisting fiber are exchanged by a fiber type lambda/2 element placed in the fiber ring, which can fix the separation between two polarization resonances (i.e., two ESOP's resonances) and suppress the polarization mode instability. A simple theory of polarization characteristics of resonance for this novel fiber ring structure is presented, which shows that in the ideal case the separation between the two ESOP resonances can be fixed and is just half of the longitudinal mode space. Furthermore, an experiment is conducted which also confirms the results of analyses. In addition, the temperature effects of this novel fiber ring laser are experimentally investigated, showing that this fiber laser has a good performance of polarization stability. On the other hand, single mode operation of the fiber laser is obtained by using tunable optical filters. The frequency spectrum of the single mode fiber laser is measured using delayed self-heterodyne detection. The tunable range of the laser frequency is about 39 nm.