Raman solitons propagated in the anomalous-dispersion regime in a 5-m standard-dispersion single-mode optical fibre are studied by real-time acquisition of traces of two-photon-absorption frequency-resolved optical gating at wavelengths around 1500 nm. Rapid wavelength scanning based on a counter-rotating mirror is invented to realise real-time spectrogram measurements under background-free lock-in detection using a single-channel photodiode. The measurement period in this method is 1.8 s for a spectrogram of 128×128 pixels. Intuitive spectrograms of a fibre Raman soliton and a pedestal are acquired at various peak powers of an incident optical pulse to analyse walk-off of the fundamental-mode Raman soliton. The soliton walk-off is limited by 2nd-order chromatic dispersion in the fibre. A dispersion parameter of 9.3 ps/nm/km is obtained from the wavelength dependence of the soliton walk-off.