Abstract: In a data communications network comprising a series of nodes connected by optical fiber links providing direct bidirectional communication between adjacent nodes, and communication between non-adjacent nodes in both directions through the same set of intermediate nodes, failure of a single link or of a single node is overcome by protection switching to a standby transmisison loop, including a standby link connecting the head end and tail end nodes of the series. The network provides survivability while also taking advantage of the ability of add/drop multiplexers to allow reuse of bandwidth. Rapid switchover to the protection loop is achieved by providing each node with the capability of detecting and responding to adjacent faults and, in normal operation, maintaining a high-speed by-pass path for the protection loop through all of the nodes except the head end node.

Abstract: The apparent differential output power or quantum efficiency of a semiconductor laser, such as a double heterostructure laser diode, is reduced by means of a circuit which shunts the laser and includes a threshold device. The I-V characteristic of the shunt circuit intersects that of the laser at V.sub.x such that at voltages below V.sub.x the current through the laser exceeds that through the shunt, and conversely above V.sub.x. Specific embodiments of the shunt circuit include a resistor in series with either a zener diode or one or more forward-biased conventional diodes.