The authors experimentally demonstrate that the spectral flatness of chaotic light from an external-cavity feedback semiconductor laser is significantly improved using a fibre Bragg grating. The effects of detuning between the Bragg frequency and chaotic frequency on the bandwidth and spectral flatness are both investigated. The flatness of the power spectrum is enhanced to ±1.4 dB whilst maintaining the spectral bandwidth at around 8 GHz. The experimental results are in accordance with theoretical predictions which also have shown that the spectral flatness decreases initially then increases with increasing the 3 dB bandwidth of grating. A flattened spectrum with a low-pass energy distribution increases the utilisation efficiency of chaotic light in chaotic radar and reflectometry.