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We demonstrate the generation of dark square pulses in a ring cavity with a 207-m long tellurite single-mode fiber. The duration of the pulse is tuned by adjusting the linear phase delay bias in the cavity. We obtain the dark-square pulse with a maximum full width at half maximum of 112 ns. The dark pulse is formed by the combined actions of the bright-square-pulse generation, birefringence, and polarizing effect in the cavity. The tellurite fiber can simultaneously provide the high nonlinearity, high birefringence, and large normal dispersion, which dominate the formation of a dark pulse in the cavity.