Switchable and Reciprocal Soliton Bound States Enabled by Continuously Tunable Local Modal-Birefringence in a Mode-Locked Fiber Laser

How to identically manage pulse dynamics in passively mode-locked fiber lasers has long been an important yet troublesome issue. Manipulations of bound state of solitons or even more broadly defined soliton molecules/crystals are much more challenging. This results from that the soliton interactions are typically complex and highly sensitive to various intrinsic or external disturbances of the used fiber devices or the overall fiber system. In this paper, we demonstrate an opto-mechanical scheme in a passively mode-locked fiber laser that can enable the switching of soliton bound state from one to another reciprocally and repeatably, i.e., in a highly identical way. It is achieved through intra-cavity continuously tuning the local modal-birefringence (LMB) of a house-made polarization-maintaining fiber taper (PMFT). The LMB tunability is enabled by axially tensioning the PMFT, which results in elongation of the PMFT. Such elongations are typically limited to hundreds of micrometers. We further analyze that such a minor variation only induces significant change on the PMFT's LMB without any evident variation to the overall cavity birefringence, in both theory and experiment. With only the adjustment of LMB, most of the influences from distribution factors can be eliminated. Thus, the undesired distortions during adjustment can be largely excluded, ensuring the repeatability during manipulations of pulse dynamics.