Widely Tunable Narrow Linewidth Dual-Wavelength Fiber Laser Using In-Line Six-Mode Fiber Filter With a Double-Ring Compound Cavity (DRCC)

This work experimentally demonstrates a single-longitudinal-mode (SLM) based narrow linewidth tunable dual-wavelength erbium-doped fiber (EDF) laser. The dual-wavelength at 1550.01 ( $\lambda _{1}$ ) and 1550.17 ( $\lambda _{2}$ ) nm with a free spectral range (FSR) of 0.16 nm was obtained by incorporating an in-line comb filter based on the six-mode fiber. The SLM operation was observed by integrating a double-ring compound cavity (DRCC) filter, which helps to achieve a narrow linewidth dual-wavelength with a high optical signal-to-noise ratio (OSNR) of around 71 dB. A stable dual-wavelength fiber laser with power fluctuation and wavelength drift of less than 0.5 dB and 0.02 nm was obtained. The proposed laser offers a broader tunability range of 44 nm. Furthermore, the performance of SLM was examined with and without the DRCC filter and unpumped EDF, where the unpumped EDF was utilized as the saturable absorber (SA) to control the mode fluctuation and enhance the stability. The linewidth of $\lambda _{1}$ and $\lambda _{2}$ was measured using the delayed self-heterodyne technique (DSH), where the narrow linewidths around 496 and 479 Hz were obtained for $\lambda _{1}$ and $\lambda _{2}$ , respectively.