Multi-wavelength sub-100 Hz linewidth Brillouin fiber laser with cascaded SBS polarization parity-time symmetry

A multi-wavelength sub-100 Hz linewidth Brillouin fiber laser (BFL) with cascaded stimulated Brillouin scattering (SBS) polarization parity-time (PT) symmetry is proposed and experimentally investigated. Multi -order Stokes wave generation using a structure consisting of a main fiber ring (MFR) and a sub fiber loop (SFL). The MFR is used to excite the Stokes laser and to guarantee single longitudinal mode (SLM) operation of the generated stokes laser of each order and the SFL cascades each Stokes lasing emission to act as a Brillouin pump for the next order of Stokes generation. A Sagnac loop in the MFR consisting of optical coupler (OC), polarization beam splitter (PBS) and two polarization controllers (PCs), is used to achieve cascaded SBS polarization PT symmetry. By tuning the PC to control the polarization state of each order linearly polarized light injected into PBS at each order, thus controlling the gain and loss of the loop corresponding to the two backpropagating light waves. When the gain and loss of the coupling loops are the same in magnitude and greater than the coupling coefficient, the cascaded SBS polarization PT symmetry is broken, resulting in SLM operation for each order of Stokes waves. Experimental results reveal that thirteen Stokes waves were obtained from two different output ports at an erbium-doped fiber amplifier (EDFA) output power of 28 dBm. The measured linewidths at - 20 dB power were 0.34, 0.39, 0.33, 0.4, 0.39 and 0.43 kHz for the 1st-6th order Stokes emission, corresponding to linewidths of 17.1, 19.6, 16.6, 20.1 19.6 and 21.6 Hz, respectively, while the maximum side mode suppression ratios (SMSRs) were measured to be 47, 45, 49, 44, 41 and 41 dB for the 1st-6th order Stokes emission. Within 60 min of the stability experiment, the wavelength and power stability were +/- 2 pm and +/- 0.6 dB, respectively. Compared with previous studies, the present design guarantees the operation of SLM for each order of Stokes waves with sub-100 Hz laser linewidth.