WDM and its light sources

Wavelength Division Multiplexing (WDM) is the technique of sending several optical signals at the same time over one fibre, each on a different wavelength (colour) of light. It is FDM at optical frequencies and multiplies the capacity of an installed fibre.

 l1 -->|\                    /|--> l1
 l2 -->| MUX ==== fibre ==== DEMUX|--> l2
 l3 -->|/   (l1+l2+...+lN)   \|--> l3
 lN -->                         --> lN
 l = wavelength (lambda)

Each channel drives its own source at $\lambda_1, \lambda_2, \ldots$; a passive optical multiplexer (grating, prism or thin-film filter) combines them and a demultiplexer separates them. CWDM uses 20 nm spacing; DWDM uses 0.8 nm (100 GHz) or closer, with EDFAs amplifying all channels together.

Light sources and their characteristics

Two semiconductor sources are used: the LED (light-emitting diode) and the LD (injection laser diode). A good optical source must:

• emit at a wavelength where fibre loss and dispersion are low (850, 1310, 1550 nm);
• have a narrow spectral width, so dispersion is low and channels can be closely packed in WDM;
• couple enough power into the small fibre core;
• be easy to modulate at high speed with a linear light–current characteristic;
• be small, reliable, long-lived and stable with temperature.

LED: works by spontaneous emission when electrons and holes recombine in a forward-biased p-n junction. Output is incoherent with a wide spectrum (30–60 nm) and wide beam, so coupling to fibre is poor. It is cheap, simple and reliable, and is used with multimode fibre over short distances.

Laser diode: works by stimulated emission in a p-n junction with an optical cavity. Above the threshold current the output rises steeply and is coherent and narrow (1–3 nm; less than 0.1 nm for DFB lasers). It couples much more power into single-mode fibre and can be modulated at many Gbps, so it is the source used for DWDM and long-haul links.

 Optical
 power       LD /
   |           /
   |          /      LED
   |         /   .-------
   |        / .-'
   |    _.-/'
   +--------+-----------> current
          I_th (threshold of LD)

Difference between LED and laser diode

For WDM, laser diodes (especially DFB lasers with temperature control) are needed because only they give the narrow, stable wavelengths that allow many closely spaced channels.