Investigation of the effect of injection locking between pump lasers on a redundant pumping scheme for erbium-doped fiber amplifiers

The stability of the redundant pumping scheme for an erbium-doped fiber amplifier is investigated. It is shown experimentally that an unstable operation occurs in the redundant pumping configuration and that the instability stems from the injection locking due to coupling between pump lasers. Two countermeasures for reducing the interaction between pump lasers are proposed for practical use-by using optical isolators and by using fiber gratings to obtain coherence collapse phenomenon. A theoretical model for the redundant pumping scheme is investigated, and the requirement for the isolation between the pump lasers to suppress the instability is verified.

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.