The lasing mode behavior of a multiple quantum well (MQW) distributed feedback (DFB) laser was measured when intensity-modulated orthogonally polarized (transverse magnetic mode: TM mode) light was injected. The 3 dB bandwidth of the frequency response showed a trend different from that observed with conventional bias current modulation: at high bias currents, it decreased with increasing bias current. The maximum 3 dB bandwidth was observed when the normalized bias current was 4, and it reached 16 GHz at this bias current. The gain saturation coefficients for the transverse electric (TE) and TM modes estimated from these results were (formula omitted) 2.5 x 10-17 cm3 and (formula omitted)5. 7 x 10-18 cm3 for the TE mode, and (formula omitted) 6. 0 x 10“17 cm3 and (formula omitted) 2. 0 x 10-14 cm3 for the TM mode.