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We report on the detailed characterization of ultrashort pulses emitted from a 1.5- AlGaInAs/InP semiconductor passively mode-locked laser, operating at a repetition frequency of 35 GHz. Both the temporal and phase profiles of the pulses are retrieved using a sonogram technique that utilizes a highly-sensitive two-photon absorption waveguide detector. The system enables full characterization of pulses with energy as low as 10 fJ and peak power level of 5 mW, which is only inaccessible by a limited number of high-sensitivity measurement approaches. We show that the pulses exhibit a prevailing positive linear chirp across a wide range of biasing conditions. Its high sensitivity to the gain section current proves the dominant contribution of the gain conditions to the group delay characteristics of the emitted pulses.