Compact all-fiber all-normal dispersion Yb-doped fiber ring laser is demonstrated based on nonlinear polarization rotation technique. Dissipative solitons (DSs) with bell-shaped spectra are observed in the fiber cavity with linear chirps for the first time to our knowledge. Stable pulse width is 9.7 ps with the pulse energy of 1.2 nJ, which can be compressed to near the transformed limited pulse duration of 225 fs. The experimental result agrees well with the numerical simulations. The theoretical modeling indicates that the formation of the bell-shaped spectra is due to the combined effect of normal dispersion, nonlinearity, gain, loss, and spectral filtering effect. Different from the pulse evolution in the mode-locked fiber laser based on the wave plate and the birefringent plate, conventional DSs with rectangular-shaped spectra can be obtained by varying the polarization orientation of the cavity.
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