All-fiber theta cavity multi-wavelength Tm-doped fiber laser based on a dual biconical taper filter

We report experimentally a multi-wavelength Thulium-doped fiber (TDF) laser based on all-fiber theta cavity and a dual biconical taper filter without isolator. The dual biconical taper with waist of 50 inm and length of 188.05 mm is implemented as a narrow band filter. Three stable laser output states are successfully obtained with one, two or three wavelengths by increasing the pump power and properly adjusting the polarization controller. For single-wavelength output at 1978.80 nm, the full width at half maximum (FWHM) is 0.23 nm with optical signal-to-noise ratios (OSNRs) larger than 19 dB. For dual-wavelength output at 1978.80 nm and 1991.80 nm, the FWHMs are 0.24 nm and 0.19 nm with OSNRs of 30 dB and 19 dB, respectively. For triple-wavelength operation at 1978.8 nm, 1990.80 nm and 1991.60 nm, the FWHMs are 0.2 similar to 0.26 nm and OSNRs of 27 dB, 20 dB and 22 dB, respectively. The stable multiple wavelengths laser emission output with maximal (minimal) interval of 13 nm (1 nm) is achieved at 2 inm region. There is negligible wavelength shift for over one hour. It can be verified that this scheme using a dual biconical fiber taper filter is feasible and can be an alternative solution for all-fiberized stable narrow-bandwidth multi-wavelength TDF laser.