4. Tunable lasers

Most long-distance or metropolitan area networks use Dense Wavelength Division Multiplexing (DWDM): lasers emit independently at standardized wavelengths, typically spaced at 100 GHz (i.e. 0.8 nm in wavelength) or 50 GHz (0.4 nm), thus multiplying the transmitted throughput by the number of wavelengths . For example, in the transmission window around 1.5 μm, we actually use lasers emitting in a band called C-band, ranging from 1,525 to 1,565 nm. This gives up to 100 transmission channels at 50 GHz spacing. Wavelength-tunable lasers have been developed for these applications. These are single-mode lasers, whose wavelength can be precisely...