A novel technique is proposed to characterize the power profile of short optical pulses using the transmission characteristic of a nonlinear optical loop mirror (NOLM). Under some assumptions, the power profile can be retrieved from pulse energy measurements at the NOLM output using a slow detection setup. As a fiber-based technique based on the Kerr effect, it does not require phase matching or tedious mechanical adjustments. The technique is able to characterize simultaneously pulse features from the picosecond to the nanosecond scale.