Thulium-doped fiber laser with bidirectional output in a ring laser cavity

• A thulium-doped fiber laser with three output modes was proposed. • Two uniform fiber Bragg gratings were used as the wavelength-selected elements and the gain and loss was balanced by using a pair of translation stages. • The potential application of a strain sensor was demonstrated with a sensitivity of 1.33/1.22 pm/με under clockwise/counter-clockwise output. A thulium-doped fiber laser (TDFL) with bidirectional output was proposed and demonstrated herein. Clockwise (CW) and counter-clockwise (CCW) lasing output with different lasing wavelengths was realized by two uniform fiber Bragg gratings (UFBGs) and two optical circulators. The UFBGs located at 1941.238 and 2049.325 nm were incorporated in the laser cavity to act as the wavelength-selective element. Switchable operation was realized through tuning the gain and loss of the circulating light by applying curvature on a section of the single-mode fiber. When the TDFL was operated in the CW direction, single-wavelength output with a center wavelength, optical signal-to-noise ratio (OSNR), and maximal output power of 2049.335 nm, 33.175 dB, and 280.6 mW was obtained, and the fluctuation of the center wavelength and the output power were less than 0.041 nm and 1.400 dB within an observation time of 60 min. When the TDFL was operated in the CCW direction, single-wavelength output with a center wavelength, OSNR, and maximal output power of 1941.312 nm, 37.108 dB, and 486 mW could be obtained, and the maximal fluctuation of the center wavelength and the output power were 0.128 nm and 0.832 dB within an observation period of 60 min. In addition, the proposed TDFL can operate in both CC and CCW directions simultaneously with different output wavelengths. The potential application of a TDFL strain sensor with sensitivities of 1.33 and 1.22 pm/με in the CW and CCW directions was also demonstrated and the proposed TDFL may also well suited for use in medical and free-space optical communication systems.