We have experimentally studied the mode-locking dynamics of a polarization-maintaining figure-of-eight laser which has a semiconductor optical amplifier as gain medium. Self-starting mode-locking at the fundamental repetition rate of 18 MHz is obtained at the lasing threshold, and further increasing the bias current leads to the progressive emission of additional optical pulses in each round trip and eventually to mode-locked emission at increasingly high harmonics of the fundamental repetition rate, up to 2.45 GHz. The intensity autocorrelation of the amplified mode-locked pulses has a full width at half-maximum duration of 382 fs, which corresponds to a pulse duration of 247 fs. We have experimentally studied the mode-locking dynamics of a polarization-maintaining figure-of-eight laser which has semiconductor optical amplifier as gain medium. Self-starting mode-locking at the fundamental repetition rate of 18 MHz is obtained at the lasing threshold , and further increasing the bias current leads to the progressive emission of additional optical pulses in each round trip and eventually to mode-locked emission at increasingly high harmonics of the fundamental repetition rate, up to 2.45 GHz. The intensity autocorrelation of the amplified mode- locked pulses has a full width at half-maximum duration of 382 fs, which corresponds to a pulse duration of 247 fs.