ABSTRACT
While InGaAsP/1nP-based Bragg grating and distributed-feedback (DFB) lasers have been the sources for long-haul, 1.55-um optical-fiber backbone networks for the past three decades and clearly meet the distance criteria in Figure 14.1, their cost is still far too high to meet the demands of hundreds of million lasers that might be utilized in MANs and LANs in a modern data communication network architecture. In addition to the lowcost VCSELs, there is an additional network challenge in order to realize low-cost, high-bandwidth networks, and that is that a much greater portion of the low-loss fiber bandwidth must be made accessible to enable coarsewavelength division multiplexing (CWDM) to be utilized. This requirement comes from the necessity to operate the lasers uncooled (i.e., no thermoelectric cooler) and directly modulated. These two requirements translate into much larger bandwidths required per channel (..=',25 nm) compared with current dense-wavelength division multiplexing (DWDM) systems (.0.1 nm) employed in the fiber backbone. The primary driving force for DWDM systems was development of the erbium-doped fiber amplifier (EDFA), which enabled all channels within its gain region to be amplified in parallel. Unfortunately, the EDFA gain region is only about 10% of the available low-loss fiber region, which is illustrated in Figure 14.2 [2]. While there is work ongoing to extend the EDFA gain into the L-band, there are still large
mi 0.4 • _Fiber Loss ..
