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A new coherence-free single-wavelength microwave photonic notch filter, which is widely tuneable, and which is compatible to be inserted in a conventional fiber-optic link, is presented. It is based on self-wavelength cross gain modulation in a semiconductor optical amplifier placed at an off-centre location in a Sagnac loop. The structure can realize a high free spectral-range negative tap notch filter response, and does not have any coherent interference and phase-noise limitations. Moreover, it enables widely tunable notch filtering to be simply realized. Experimental results are presented which demonstrate a large continuous filter tuning range of an octave, robust operation with a narrow linewidth source, and wide passband notch filtering operations.