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Professor Xueming Liu's group published a paper on LPR to reveal the entire buildup of harmonic mode-locking
Source : Administrator   Time : 2019-08-02

       Recently, our group successfully tracked the formation and evolution of passive harmonic mode-locked (HML) in an ultrafast laser by using the time-stretch dispersive Fourier transform technique (TS-DFT). It is unveiled that the whole process of HML buildup successively undergoes seven different ultrafast phases. This work was published in the latest issue of Laser & Photonics Review [Laser & Photonics Reviews 13, 1800333 (2019)], entitled "Revealing the Buildup Dynamics of Harmonic Mode-Locking States in Ultrafast Lasers."

       Ultrafast lasers with high repetition frequency have many potential applications in ultra-precision spectroscopy, microwave photonics, high-speed optical sampling and data storage. To increase the pulse repetition rate of fiber lasers, a less technically challenging and more convenient way is the harmonic mode-locking (HML) scheme, where multiple pulses are evenly spaced in the cavity. Harmonically mode-locked fiber laser can be realized through active mode-locking, passive mode-locking and hybrid mode-locking. The passive HML scheme has, from a practical point of view, the intrinsic advantage of repetition-rate self-stabilization.

       However, the problem of how harmonics are generated and the mechanism by which self-stability is achieved has plagued researchers for many years. The solution to these problems can provide great guidance value for the design and implementation of harmonic lasers.

Fig. 1 Conceptual diagram of seven stages in the process of harmonic mode-locking

       The spacing between multiple pulses operated in HML undergoes a random change due to the presence of temperature variations and mechanical vibrations. As a result, the HML operation in the ordinary non-polarization-maintaining (non-PM) fiber lasers can only remain stable in a short time-scale, and even such operation is discontinued by these perturbations. The controllable HML operation of passively mode-locked fiber lasers, on the other hand, has been proved quite hard to be achieved. The all-PM fiber lasers can be immune to these variations and vibrations. Coupling between cavity modes and the acoustic resonance in a fiber provides strong and optomechanical interactions, resulting in “control elements” that enable controllable and stable HML operation of lasers.

       Based on the TS-DFT technique, combined with the specially designed full polarization-maintaining optical cavity, our team conducted a detailed experimental study on the formation dynamics of passive harmonic mode-locking. We experimentally demonstrate that the buildup process of HML includes such stages as the raised relaxation oscillation, beating dynamics, birth of a giant pulse, self-phase modulation (SPM)-induced instability, pulse splitting, repulsion and separation of multiple pulses, and the stable HML state. The seven stages of the harmonic mode-locking state are conceptually shown in Figure 1.

Fig. 2 (a) DFT measurement results of the complete harmonic mode-locking process (b) close-up of local region in (a)

       Figure 2 demonstrates the entire evolution process from noisy lasing to single‐pulse mode-locking, eventually to stable fifth HML. Obviously, a big corner is observed at ~1.06×10^5 RTs, before and after which the evolution trajectories of the intra‐cavity pulses are quite different, indicating a quite obvious decrease of the pulse peak power at this corner.

Fig. 3 Repulsion and separation in the beginning stage of fifth HML buildup.

       By analyzing the autocorrelation trajectory at the corner, it is shown that the harmonic mode-locking of the laser originates from the splitting of a single large pulse. At the same time, in the early stage of the harmonic mode-locking process, multiple pulses are separated one by one, and a breathing pattern appears, eventually forming a stable harmonic mode-locking, as shown in Figure 3. The numerical results confirm that the effects of dispersive wave, gain depletion and recovery, and acoustic wave play key roles in the earlier, middle, and later stages of this HML buildup process, respectively; as well, the acoustic resonance in the single-mode fiber stabilizes the final HML state of lasers. By means of the optoacoustic effect that induces a trapping potential, the acoustic resonance can stabilize the mode-locking of laser at different harmonics (from the first to sixth order at the appropriate pumping strength) with perfect long-term stability.

       The work was supported by the National Natural Science Foundation of China.

       Original link: https://onlinelibrary.wiley.com/doi/10.1002/lpor.201800333