Random Laser Operating at Near 1.67 µM Based on Bismuth-Doped Artificial Rayleigh Fiber

A narrow-linewidth bismuth-doped fiber laser (BDFL) with the random cavity and operational wavelength at 1.67 µm was demonstrated. The laser cavity was formed by an array of weakly reflecting fiber Bragg gratings inscribed in the active fiber core directly during the fiber drawing process. Taking into account the performed analysis of optical and laser properties, the viability of this approach as applied to Bi-doped fibers with high-GeO
$_{2}$
-SiO
$_{2}$
glass core is shown, even despite the high sensitivity of bismuth active centers (BACs) to laser irradiation, i.e. processing does not lead to the destruction of the BACs. The maximum output power of the developed BDFL in a simple linear configuration with the use of 200 m-long bismuth-doped active fiber was
$\sim$
20 mW at room temperature when pumped by Er-Yb fiber laser at a wavelength of 1568 nm and the total power of 450 mW. The achieved width of the laser emission line was narrower than 0.02 nm. We studied the laser behavior in various configurations, and it was revealed that the laser wavelength can vary within the spectral range of 1.669–1.674 µm in dependence on the length and the ambient temperature of the active fiber. It was most likely caused by inhomogeneous distribution of the written gratings structures along the active fiber length that was induced due to the peculiarities of the used active fiber, namely, a significant core ellipticity. In addition, we demonstrated the possibility of the output power scalability of this type of lasers by using a homemade Bi-doped fiber power amplifier. As a result, the optical power of the random BDFL was increased up to 240 mW.