Nonreciprocal optical devices, allowing transmission of light with different efficiencies in opposite directions, are key elements for modern optical communication and even quantum information technologies, but elusive to be integrated on a chip to date. Such devices exploring nonlinearity can tackle this integration issue without using a magnetic field. However, they are constrained by narrow bandwidth or large signal power because of the weak nonlinear strength of materials. Here, by combining the strong gain-saturation nonlinearity of an Er-doped optical fiber and the asymmetric inputs, we experimentally demonstrate a broadband resonator-free nonreciprocal optical device. This nonmagnetic device operating at telecom wavelength shows nonreciprocal transmission with contrast of $19.97\mathrm{dB}$ over a bandwidth of $33.3\mathrm{nm}$ and reaches a maximum contrast of $23.91\mathrm{dB}$. With the potential of extending to on-chip Er-doped optical waveguides with currently existing experimental techniques, this simple design paves the way for integrated broadband nonreciprocity devices and can boost the functions of photonic chips.

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• Non-reciprocal propagation
• Non-reciprocal transmission
• Nonlinear optics