Dual-wavelength fiber Bragg grating laser with independently controllable output polarization

Abstract

By incorporating an inline filter based on a polarization-diversified loop structure composed of two fiber Bragg gratings (FBGs) with different resonant wavelengths and three quarter-wave plates (QWPs), we propose a dual-wavelength FBG laser that can independently control the output polarization of dual-lasing lines oscillating at two Bragg resonances. At a special combination of the orientation angles of the QWPs, the inline filter can be arranged so that its transmittance depends on input polarization, and it can also select the output polarization states of reflection spectra of two FBGs independently according to its input polarization. For example, their reflection spectra chosen by the inline filter can be linear horizontally polarized at one Bragg wavelength and linear vertically polarized at the other Bragg wavelength, and vice versa. Moreover, linear horizontally or vertically polarized reflection spectra at both Bragg resonances can also be realized. By controlling the QWPs and a cavity polarizer used as an input polarizer of the filter, switching operation among four possible output polarization sets (i.e., two kinds of orthogonal output polarization sets and two kinds of parallel ones at two Bragg resonances) could be implemented in the proposed fiber laser with the measured polarization extinction ratio of each lasing line more than 20.0 dB.