In a slanted Bragg grating, coupling between the fundamental guided mode and the counterpropagative cladding modes result in discrete resonances in the transmission spectrum. These resonances are a drawback when Slanted Bragg Gratings are used for gain flattening of fibres amplifiers. A new method based on a chemical etching of the cladding is proposed leading to an overlap of the resonances and a reduction of the amplitude of the modulation. This method can be applied for any value of photo induced modulation amplitude in the SBG.