WDM normally requires a separate light source for each wavelength. Tunable lasers do not eliminate that requirement; they just simplify the logistics of stocking and sparing separate parts for each wavelength. Some developers are already looking a step beyond tunable lasers to light sources that could simultaneously generate OCs at many separate wavelengths on the WDM grid. Some have already been demonstrated, but the technology is still in the early stages and applications remain quite limited [6].

The general goal is to generate a comb of regularly spaced optical wavelengths or frequencies on standard optical channels (see Fig. 9.8) [6]. A few approaches include ways to modulate the carriers directly with a signal, but so far most merely generate the wavelength comb.

Most multiwavelength sources fall into three basic categories. One simple concept is to integrate diode lasers oscillating at different wavelengths on a single chip, but this merely integrates multiple lasers on a single substrate, and will not be discussed further. A second approach is to generate a continuous spectrum covering a broad range of wavelengths, then slice the broadband emission into a number of discrete optical channels that can then be modulated with signals. A third alternative is to create a type of optical cavity that allows a laser source to oscillate simultaneously on multiple wavelengths [6].

9.6.1. Ultrafast Sources and Bandwidth

One way for a laser to generate ...

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