Direct extraction of the chirp parameter and higher order frequency chirp of ultrafast pulses via modulation-based mode-locked pulse characterization

We present experimental results of direct extraction of chirp parameter made possible by temporal phase retrieval measurements of ultrafast pulses. Results for a multi-spatial-mode mode-locked semiconductor laser at various bias points are presented. The comparison with the normally used root-mean-square (r.m.s.) chirp parameter reveals that the correlation of the r.m.s. chirp parameter with the actual pulse chirp is limited, especially for complex pulse shapes that may involve non-Gaussian spectral profiles and higher order chirps. The measurement methods and results are important in studying both the semiconductor laser mode-locking dynamics and the application of such ultrafast pulses.