Systems and methods use single-fiber optical add/drop multiplexers (OADMs) to enable bi-directional data transmission in WDM systems over a single fiber. By reducing the required capacity for an optical network from dual fibers to a single fiber, significant cost and bandwidth efficiencies are achieved. The systems and methods also provide redundancy protection in single-fiber bidirectional line ring systems. In the event of a downstream fiber or device failure, the OADM module receives a signal on a first wavelength from a first direction, shifts the signal from the first wavelength to a second wavelength, and sends the signal back down the fiber it originated from on the second wavelength, thus maintaining the propagation of the signal in the ring system.