Frequency response of a 1.55 μm distributed feedback (DFB) laser is measured by injecting light which has the orthogonal polarization to the laser’s lasing mode. The injected light, here we call it as orthogonal polarization light, is carefully selected not to couple to the lasing mode by adjusting its polarization to be orthogonal to that of the lasing mode and by setting its wavelength slightly different from that of the lasing mode. The wavelength of the orthogonal polarization light, however, is set within the range where the laser’s active layer has a gain. The gain saturation coefficients of a distributed feedback (DFB) laser are estimated from the measurement and numerical analysis. The gain saturation coefficients for the lasing light is estimated to be 4.7 × 10-23 cm2 and that by the injected orthogonal polarization light (almost same wavelength as lasing mode) is 3.7 × 10-23 cm2, respectively. The resonant peak intensity and the resonant frequency show almost same behaviors as those in injection current modulation.