Multi-wavelength Brillouin random fiber lasers (MBRFLs) are a new type of laser based on a random distributed feedback resonant cavity and the gain of the stimulated Brillouin scattering (SBS) effect. Because the SBS effect in MBRFLs have excellent properties, such as higher gain, lower threshold, narrower gain spectrum width, and higher sensitivity to environmental factors, it has been widely utilized in the study of fiber lasers. However, most studies are limited to non-polarization parameters, including wavelength, laser linewidth, intensity noise, and phase noise, and are rarely related to the polarization characteristics. In this study, we propose two novel orthogonal polarization interleaving multi-wavelength Brillouin random fiber lasers (OPI-MWBRFLs) that emit orthogonally polarized multi-wavelength light with single and double Brillouin frequency shift (BFS) intervals, based on the axial polarization pulling property of the SBS effect in polarization-maintaining fibers (PMFs). This system yields highly stable orthogonally polarized light, with an adjacent polarization extinction ratio as high as 33 dB. Compared with conventional MBRFLs, OPI-MWBRFLs can provide multi-wavelength lasing light with orthogonal polarizations between adjacent wavelengths, thus effectively eliminating inter-channel interference in dense wavelength division multiplexing (DWDM) systems with potential application in the fields of fiber sensing, optical fiber communication, and optical spectrum analysis.

In this study, we design two novel OPI-MWBRFLs that emit orthogonally polarized multi-wavelength light with single and double BFS intervals. First, based on the polarization vector propagation equation and the simplified intensity equation of the pump and signal lights of the SBS effect in PMFs, which theoretically indicate that the SBS effect in the PMF has an explicit axial polarization-pulling behavior. Second, we deduce the relationship between the traction direction and the state of polarization(SOP)of incident pump light, SBS gain, pump light polarization state, and pump light power. Finally, we realize a single BFS OPI-MWBRFL using a 3 km long PMF as the SBS gain medium, and demonstrate a double BFS OPI-MWBRFL by cascading a 21 km long single mode fiber(SMF)random cavity and a 3 km long PMF random cavity in the feedback loop of the single BFS OPI-MWBRFLs. In the double-BFS OPI-MWBRFL, we use a tunable laser (TLS) to output the pump light with a center wavelength of 1553.73 nm, then the pump light is adjusted by a polarization controller (PC1) and launched into 21 km long SMF through an ordinary SMF circulator (Cir1). In the random cavity, the SMF acts as a Brillouin gain medium and excitation of even-order Stokes light occurs in the opposite direction, of which 10% is output through the optical coupler (C2), and the rest is amplified by an erbium-doped fiber amplifier (EDFA1) and launched into a 3 km long PMF to stimulate higher-order Stokes light. Thus, by controlling the polarization state of the triggered Stokes light in the feedback loop, orthogonally polarized multi-wavelength lasers with single and double BFS intervals are output.

For single BFS OPI-MWBRFLs, the number of output Stokes light wavelengths is positively correlated with the pump optical power. The total output spectrum when the EDFA is in the range of 80‒170 mW is measured (Fig.3). When the output power of the EDFA is set to 80 mW and 100 mW, six and seven wavelengths, respectively, are observed with a space of 0.088 nm (Fig.2). Under these power settings, four odd-orders of Stokes lights resonating at sign (p^in⋅β^l)β^l are measured, and the even-order Stokes light resonating at -sign(p^in⋅β^l)β^l are measured. Further, the extinction ratio of the polarization component exceeds 33 dB [Fig.2(c)]. In contrast, a double BFS OPI-MWBRFL is demonstrated by cascading a 21 km long SMF random cavity and a 3 km long PMF random cavity in the feedback loop of a single BFS OPI-MWBRFL. An orthogonally polarized 4-wavelength lasing light with a double BFS interval is observed, and the adjacent polarization extinction ratio is as high as 34 dB (Fig.6). Finally, after monitoring and measuring the power fluctuation and wavelength shift of the output laser of both schemes for 1 h without any mechanical and temperature stabilization measures, we observe that the maximum power fluctuation and maximum wavelength shift are within 0.21 dB and 0.02 nm (Figs.4 and 7), respectively, indicating the good stability of the system.

In this study, two novel OPI-MWBRFLs are proposed and implemented based on the axial polarization pulling effect of the SBS in PMFs. First, we analyze and discuss the polarization-mode operating region of the PMF-BRFL system and the corresponding operating conditions. Second, two experimental systems that can output polarization multi-wavelength light are realized using different random laser cavities, and the polarization extinction ratio is higher than 33 dB. Finally, the polarization orthogonality of these systems is guaranteed by the natural nonlinear axial polarization pulling effect of the SBS in the PMFs, rather than by the artificial precise polarization control of the systems; thus, the two OPI-MWBRFLs exhibit excellent working stability in experiments in the absence of mechanical or temperature control. The results of these experiments are highly consistent with expectations and have broad application prospects in the fields of optical fiber sensing, DWDM optical fiber communication, and spectral detection.