Abstract:We experimentally study the polarization properties of the outputs of different opencavity Raman fiber lasers based on spun fiber and a highly polarized pump, demonstrating controlled output polarization and improved threshold. Mirrorless fiber Raman lasers based on a distributed feedback via Rayleigh scattering [1] have attracted a great deal of attention in the last decade, as their lasing efficiency and beam quality are comparable to those of standard cavity lasers. Recently, in order to govern the state of polarization (SOP) of the laser output radiation and increase its efficiency, the use of polarized pumps and polarization-maintaining (PM) fiber have been proposed [2][3][4]. Here, we propose the complete control of the SOP of the output by means of the use of spun fiber. These fibers are created by rotating the fiber preform during the drawing processes, producing a fiber with a negligible average polarization mode dispersion (PMD).The schematic setup designed for our experiment consists of a 2 km piece of spun fiber pumped by a CW fiber laser at 1366nm, able to produce up to 38dBm. The pump is polarized by a polarizer prism, which inserts losses equal to approximately 3.6 dB, and injected into the fiber on one end, whereas a fiber Bragg grating (FBG), centered at 1454.5 nm, is initially situated at the opposite fiber end (scheme 1) or the same end (scheme 2). These two options are depicted in figure 1. The evolution of the forward-propagating (co-pumped) output spectrum for scheme 1, measured on the right side, is plotted in figure 2(a). We observe the generation of a supercontinuum at 1393 nm. When the open cavity is pumped with the depolarized pump (i.e., without the polarizer prism), this supercontinuum absorbs most of the energy, hence preventing the cavity from lasing for injected pump powers up to 35.5 dBm. The situation changes if we switch to the highly polarized pump. The evolution of the output spectrum can be divided in two stages. For pump powers below 33.5 dBm, the performance is dominated by XPM and parametric effects. The power of the supercontinuum, mentioned above, is increased with the pump power. For pump powers higher than 32 dBm, a new peak appears at 1453 nm growing until pump power reaches 33.5 dBm at which both peaks (1393 nm and 1453 nm) begin to decrease, as Raman effect becomes dominant and the device begins to lase at 1453 nm. Moreover, the power of the signal at 1393 nm is also reduced when the pump power is increased. Remarkably, lasing in this forward-propagating component occurs at the Raman gain peak, which does not fully overlap with the FBG wavelength. This result suggests that lasing is in this case independent on the presence of the FBG. This premise is confirmed by moving to scheme 2, in which the power threshold is similar, although the FBG is situated at the pumping end.