The development of optical amplifiers that operate at 1550 nm, along with the lower loss that that window, drove the development of DWDM systems. With DWDM, you have 30 or 40 different optical transmitters operating at different frequencies. DWDM stacks them up together like radio stations in your car. In addition to exponentially increasing the available capacity on embedded fiber, DWDM adds even more doubloon to the carriers' golden stashes, because they don't have to get rid of their existing terminating equipment to implement it. In addition, DWDM systems can pick up traffic from many different types of transmission equipment, including Sonet and asynchronous gear.
Open systems are interoperable with multiple vendors' Sonet and other transmission equipment.
Laying It Down The DWDM transponder is the device that consists of circuit boards and lasers that 'densely' pack different colored wavelengths onto a single fiber. Unfortunately, like all telecommunications signals, they degrade as the distance they travel increases. Strategically spaced across a fiber span, in-line EDFAs amplify the DWDM signals in toto and boost them on their way to the next amplifier, or to a termination point. Very near the end of the link, the signals are boosted one last time by a receive pre-amplifier that sends them a short distance into the terminating Sonet system or other transmission gear. EDFAs amplify the whole kit and caboodle.