Widely tunable multiwavelength thulium-doped fiber laser using a fiber interferometer and a tunable spatial mode-beating filter

We propose and demonstrate a widely tunable multiwavelength thulium-doped fiber laser using a hybrid filter in order to expand the tuning range of a multiwavelength laser. The hybrid filter consists of an all-fiber interferometer and a tunable spatial mode-beating filter (SMBF). The operation principles of the interferometer and the tunable filter are analyzed. With a pump power of 200 mW, the number of wavelength can be tuned from one to nine by adjusting the polarization controller in the interferometer, and the multiwavelength laser can be tuned from 1896 to 1936 nm by adjusting the polarization controller in the SMBF. The wavelength spacing is 0.35 nm, and the linewidth of a single-wavelength is 0.03 nm. When pump power increases to 1 W, the spacing of dual-wavelength laser can be tuned from 6 to 36 nm.