Publication Date: January 12, 2026
Authors: Young-Gyun Jeong, Trevor J. Stirling, Amr S. Helmy
Abstract:
We demonstrate second-harmonic generation frequency-resolved optical gating (SHG-FROG) characterization of Q-switched mode-locked (QML) pulses from a monolithic quantum-well Bragg reflection laser diode operating near 800 nm. Despite the low pulse energy (∼1 pJ) and complex temporal structure of QML emission, a complete FROG trace was successfully acquired using only standard optical components, without amplification or specialized detection schemes. Analysis of the seven distinct peaks in the autocorrelation trace and their corresponding FROG spectrograms reveals a temporal profile consisting of four mirror-symmetric mode-locked sub-pulses nested within a Q-switched base pulse. The sub-pulses exhibit systematic chirp: negative on the leading side and positive on the trailing side, with outer sub-pulses displaying larger chirp magnitudes and longer durations than inner sub-pulses. The estimated group-delay dispersion (GDD) of the mode-locked sub-pulses correlates with local intensity across the Q-switched base pulse, providing direct phase-sensitive insight into the temporal evolution of QML dynamics. Our laser platform generates 217-fs mode-locked pulses while supporting simultaneous Q-switching, offering a practical, compact source for integrated photonics, metrology, and quantum applications.
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