Eleven-wavelength-switchable thulium-doped fiber laser with a sandwich four-mode fiber Mach–Zehnder interferometer

An eleven-wavelength-switchable thulium-doped fiber laser was proposed and demonstrated, incorporating an improved geta-shaped cavity providing bidirectional amplification, and a sandwich four-mode fiber Mach–Zehnder interferometer filter with curvature sensitivity. The filter was fabricated by splicing a segment of four-mode fiber (FMF) between two segments of a no-core fiber (NCF), with two segments of a single mode fiber (SMF) fused outside the two NCFs, and the structural order was SMF-NCF-FMF-NCF-SMF (SNFNS). The mode interference of the SNFNS filter was investigated theoretically in detail. The curvature characteristic was measured experimentally, and the maximum curvature sensitivity of the filter was 5.83 dB/m−1 in the ranges of 1.51 ∼ 2.26 m−1. By adjusting the curvature of the filter and polarization controllers, switchable eleven single-wavelength output can be obtained. The stability of the single-wavelength output was monitored with the fluctuations of the center wavelength and output power being less than 0.05 nm and 0.90 dB, respectively. Additionally, a slope efficiency of 9.797 % was achieved using a bidirectional amplifier configuration. The proposed laser may be suitable for mode-division multiplexing systems and optical sensor applications.