Reliability of 1.55 μm DFB lasers for use in dense-WDM systems

The invention of the Er-doped optical amplifier has revolutionized the world of long-wavelength transmission systems. With the use of 1.55 μm optical line amplifiers, transmission systems utilizing 1.55 μm DFB lasers can reach span distances of 200 km (direct modulation) to 1000 km (external modulation) before the need for electrical regeneration of the transmitted signal. Further, the extreme bandwidth (>30 nm) of these Er-doped line amplifiers has enabled the use of several densely populated WDM carriers on the same optical line, sharing the same amplifier chain. Current systems employ carrier wavelength spacings on the order of 0.8 nm (100 GHz), while proposed systems may employ spacings as small as 0.2 nm (25 GHz). In order for these systems to operate with minimal impairment, the transmission sources employed (cooled DFB lasers) must be stable enough to prevent excessive loss in multiplexers/demultiplexers or cross-channel interference. For a typical 100 GHz system, this implies a required wavelength stability of 0.2 nm (25 GHz)