Tunable and Switchable Dual-Wavelength Single-Longitudinal-Mode Erbium-Doped Fiber Lasers

Two types of erbium-doped fiber lasers based on Sagnac interferometer incorporated with fiber grating are proposed and experimentally demonstrated, which effectively support tunable and switchable dual-wavelength single-longitudinal-mode laser operation. The mode selection is implemented by use of the Sagnac interferometer with fiber grating, an umpumped erbium-doped fiber, and a tunable fiber Bragg grating. The Sagnac interferometer with fiber grating plays the role as a narrowband comb filter, while the unpumped erbium-doped fiber together with the tunable fiber Bragg grating essentially forms a passive self-tracking filter to ensure a stable single-longitudinal-mode fiber laser operation. By use of the Sagnac interferometer with chirped fiber Bragg grating, tunable microwave signals can be generated by beating the dual wavelengths, within the frequency range between ∼9.8 and ∼14 GHz.