All-fiber butterfly-shaped Mach–Zehnder interferometer: an efficient spectral filter for selecting the output performance of erbium-doped fiber lasers

With the rapidly increasing demand for optical communication systems, it is highly desirable to develop filter devices integrated into a single fiber laser cavity for generating tunable and switchable multi-wavelengths. We demonstrate a distinct butterfly-shaped optical fiber configuration, and its filtering potential for generating tunable single-, tunable/switchable dual-, and switchable multi-wavelength erbium-doped fiber laser (EDFL) is achieved. The butterfly-shaped configuration-based Mach–Zehnder interferometer (MZI) was constructed by wrapping a single-mode fiber to create two loops on either side of the midpoint of a capillary tube: one loop represents the upper wing (UW), whereas the other represents the lower wing (LW) of the butterfly configuration. Each wing functions as a distinct MZI. By finely tuning the bending diameters of the wings within the EDFL cavity, the MZIs’ transmission spectrum is altered, allowing the laser to operate in tunable single- and dual-wavelength modes. Initially, as the UW bending diameter decreased from 35 to 15 mm, a tunable single-wavelength EDFL was achieved with a maximum tuning range of 8.7 nm. Subsequently, the dual-wavelength operation was obtained by setting the UW and LW bending diameters to ∼35 mm. In addition, a tunable dual-wavelength lasing operation was successfully performed by gradually reducing the diameters of both wings from 35 to 20 mm. Finally, multi-wavelength switching was achieved by modifying the polarization controller (PC) settings inside the cavity. The findings demonstrate that the proposed filter can serve various fiber laser applications.