  Physics phenomena of multi-soliton complexes have enriched the life of
dissipative solitons in fiber lasers. By developing a birefringence-enhanced
fiber laser, we report the first experimental observation of
group-velocity-locked vector soliton (GVLVS) molecules. The
birefringence-enhanced fiber laser facilitates the generation of GVLVSs, where
the two orthogonally polarized components are coupled together to form a
multi-soliton complex. Moreover, the interaction of repulsive and attractive
forces between multiple pulses binds the particle-like GVLVSs together in time
domain to further form compound multi-soliton complexes, namely GVLVS
molecules. By adopting the polarization-resolved measurement, we show that the
two orthogonally polarized components of the GVLVS molecules are both soliton
molecules supported by the strongly modulated spectral fringes and the
double-humped intensity profiles. Additionally, GVLVS molecules with various
soliton separations are also observed by adjusting the pump power and the
polarization controller.
