Abstract: A bi-directional interface for transmitting signals between a circuit tester and a connection point proximate to a circuit to be tested includes a first and a second optical fiber link, a first directional gate for coupling both the input end of the first link and the output end of the second link to one endpoint of the interface and a second directional gate for coupling both the input end of the second link and the output end of the first link to the other endpoint of the interface. An optical fiber link for transmitting a signal between a circuit tester and a connection point proximate to a circuit to be tested includes an optical fiber for transmitting the signal, a light source electrically coupled to the link input and optically coupled to the input of the fiber, and a photodetector optically coupled to the fiber output and electrically coupled to the link output. The light source may include a light emitter and a driver stage electrically coupled to the light emitter.