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The temperature dependence of differential gain dG/dn for 1.3-μm InGaAsP-InP FP and DFB lasers with two profiles of p-doping was obtained from RIN measurements within the temperature range of 25/spl deg/C-65/spl deg/C. Experiments showed that the change of the active region doping level from 3/spl middot/10/sup 17/ cm/sup -3/ to 3/spl middot/10/sup 18/ cm/sup -3/ leads to a 50% increase of the differential gain for FP lasers at 25/spl deg/C. Heavily doped devices also exhibit more rapid reduction of the differential gain with increasing temperature. The effect of active region doping on the energy separation between the electron Fermi level and electronic states coupled into the laser mode explains the observations. The temperature dependence of differential gain for DFB devices strongly depends on the detuning of the lasing wavelength from the gain peak.