High-sensitivity mode interferometer enabled precisely tunable and switchable thulium-doped fiber laser

This research proposes and experimentally demonstrates a highly stable, precisely controlled, tunable, and switchable multi-wavelength thulium-doped fiber laser (TDFL), utilizing a high-curvature-sensitivity interferometer. The curvature sensor was fabricated by splicing two identical sections of no-core fiber (NCF) between three sections of single-mode fiber (SMF), and the structural order was SMF-NCF-SMF-NCF-SMF (SNSNS). Using the beam propagation method to analyze the sensor's self-imaging distance, a curvature sensitivity of 13.43 nm/m^-1 can be obtained experimentally, demonstrating its functionality as a novel bending-actuated tuning element for the 2-μm-band. By bending the SNSNS with a step of 10 µm, precise and continuous tuning of seventeen distinct lasing channels with a uniform 0.33 nm interval can be obtained. The stability was monitored over 50 minutes without temperature control or vibration isolation, wavelength shifts and power fluctuations remained below 0.02 nm and 0.36 dB, respectively. Furthermore, as the pump power increased, adjusting the polarization controller enabled versatile switching among eighty-five single-wavelength, five dual-wavelength, and two three-wavelength operational states. Statistical analysis was conducted on the output wavelengths of TDFL and the peak wavelengths of the transmission spectrum of SNSNS, with the standard deviation below 0.12 nm, confirming the excellent tuning accuracy of the laser. These results demonstrate a robust, flexible, and high-precision multi-wavelength TDFL.