Contents I. Introduction
Submarine cable networks have acquired a tremendous transmission capacity by employing the wavelength-division multiplexing (WDM) technology and erbium-doped fiber amplifiers (EDFAs) as amplifier repeaters, since the first amplifier-based transoceanic transmission system, TPC-5CN (Trans-Pacific Cable No. 5 Cable Network), was put into operation in late 1996. A key technical challenge in employing EDFAs in the submarine cable systems is achieving long term reliability of high-power pump lasers, the only active optical devices in amplifier repeaters. A very effective and economical approach to this problem is to combine the outputs of two pump lasers in a 3-dB coupler, with the same coupler also splitting the pump power to feed two erbium-doped fibers (EDFs), one for uplink and the other for downlink [1]. In case of failure of one pump laser, the other pump laser can still deliver half of the initial pump power to EDFs, thus preventing a catastrophic drop in the system performance that the high loss of an unpumped EDF would cause. Furthermore, a theoretical investigation shows that a chain of EDF repeaters has a self-regulating mechanism, which, after a span of several repeaters, restores any local deviation in the repeater output power from its predetermined value [2]. Thus, the redundant pumping scheme using an optical coupler to combine a plurality of pump lasers is of practical importance.
