Beat frequency tuning in dual-polarization distributed feedback fiber laser using side polishing technique

We propose and demonstrate a method for tuning the beat frequency of a dual-polarization distributed feedback (DFB) fiber laser via fiber side polishing. This process can significantly alter the birefringence in DFB fiber lasers. Beat frequency evolutions in DFB fiber lasers were investigated, and the experimental results showed that the beat frequency tuning was dependent on polished thickness, roughness, and direction. The abrasive paper with a grain size of 1.8 µm was adopted to fine-tune the beat frequency. It was found that the beat frequency of DFB fiber lasers shifted toward higher frequencies with increasing polished thickness. However, the beat frequency shifted toward lower frequencies using a secondary side polishing process in the direction orthogonal to the first polished surface. As a result, the beat frequency of the DFB fiber laser was tuned in a wide frequency range from 475.5 MHz to 2080.4 MHz, which corresponds to a birefringence change of 1.2 × 10^-5. Side-polished DFB fiber lasers could provide a novel approach to frequency division multiplexing for a large number of fiber laser sensors.