Fabrication and Characterisation of Few Mode DBR Brillouin-Raman Lasers

Stimulated Raman scattering (SRS) fiber lasers remain an area of interest as they provide a simple and compact method of creating lasers at wavelengths beyond the rare-earth ion bands. The authors and others have demonstrated that short $\pi$ -phase shifted distributed-feedback (DFB) cavity structures can be used to create single-frequency lasers [1]–[3]. To reach longer wavelengths, cascading SRS cavities are typically used, which usually involve many-meter-long distributed Bragg reflector (DBR) Fabry-Pérot cavities with 100s or 1000s of longitudinal modes. However, to keep the laser system compact, these cavities must be as short as possible, which results in few longitudinal modes and a different behaviour. Siekiera et al. have reported such short DBRs which consist of two 50mm gratings separated by 120mm in a high germania fiber with an effective area of 7.3 $\mu \mathrm{m}^{2}$ [4]. This produced a laser with a threshold of 4W with a maximum output of 700mW for 8W of pump with the expected 607MHz beat frequency between the few longitudinal modesm and 20 dB side mode suppression.