An All-Pole-Type Cavity Based on Smith Predictor to Achieve Single Longitudinal Mode Fiber Lasers

We propose and experimentally demonstrate an all-pole-type cavity structure to force single longitudinal mode (SLM) operation in fiber lasers. The proposed structure is based on the modified version of the Smith predictor, which compensates delay in a closed-loop feedback system. This structure is applicable to arbitrary wavelengths and gain media, does not limit the tunability range of the laser, and does not require few centimeters long loops or Fabry-Pérot tunable filters to achieve SLM operation. Thus, the major drawbacks of the conventional SLM forcing mechanisms are alleviated. Therefore, this structure can be considered as a universal approach to SLM operation in fiber lasers. To exemplify this structure, an erbium-doped fiber ring laser in continuous-wave mode is implemented. The SLM operation is confirmed from the RF spectrum over a 10 GHz bandwidth, and the laser is tunable over 62 nm from 1512 to 1574 nm.