The finest level of WDM involves using multiple sources near 1550 nm on a single fiber, each with accurately controlled wavelengths. Laser sources on the ITU wavelength grid are readily available. Couplers to combine and separate these wavelengths are also readily available. However, to accurately control the wavelength of the source and response of the filters in the couplers is complex. These devices are fairly expensive today, although costs are decreasing rapidly.
Current technology can achieve acceptable analog performance with eight wavelengths per fiber.
This is the assumption used in this analysis. It is clear that this will be increased to 16 and 32 wavelengths in the future, making this option more attractive. The most basic digital return is to digitize the analog waveform. Achieving acceptable dynamic range requires an eight or 10 bit analog to digital converter and a sampling rate near 100 MHz. Therefore, a 1.8 G bit per second digital link can transmit two independent returns.