  We report an actively tunable topological edge mode laser in a
one-dimensional Su-Schrieffer-Heeger (SSH) laser chain, where the SSH chain is
realized in an electrically-injected Fabry-Perot (FP) laser chain. A
non-Hermitian SSH model is developed to investigate the SSH laser chain with
tunable active topological defect. We theoretically demonstrate topological
edge mode phase transition in the SSH laser chain. The phase transition
manifested in the tight binding SSH laser chain is observed experimentally and
agreed well with the theoretical predictions. Finally, by electronically tuning
the gain and loss, a lossy topological mode is obtained, and lasing is
experimentally observed in the lossy time-reversal configuration. This work
presents a versatile platform to investigate novel concepts, such as
topological mode, for main stream photonic applications.
