WDM

What is Wave Division Multiplexing(WDM)

WDM

We know that light comes in many different colors, there different colors can be combined in the same fiber. This goal is to have signals not interfere with each other.

WDM

There are several different types of WDM, the most common terms are Dense and Coarse. Essentially they both do the same thing in the same way, but the only difference is the channel spacing and the range of the optical spectrum they typically cover.

CWDM is loosely used to mean anything not DWDM, and the most "popular" meaning is 8 channels with 20nm spacing, which are centered on 1470/1490/1510/1530/1550/1570/1590/1610. Meanwhile, with low water peak fiber, the other 10 channels are possible, which are centered on 1270/1290/1310/1330/1350/1370/1390/1410/1430/1450.

The Dense WDM (DWDM) is a much more tightly packed WDM system, which is typically used for commercial long-haul system and usually based in the C-band. The specific channel sized are standardized in an "ITU Grid". Within C-band, these channel spacing are common.

  • 200GHz - 1.6nm spacing, 20 channels possible
  • 50GHz - 0.4nm spacing, 80 channels possible
  • 100GHz - 0.8nm spacing, 40 channels possible
  • 25GHz - 0.2nm spacing, 160 channels possible

For above 200GHz is an old technology, and 100GHz is the most common on plug-gables or cheaper systems. The 4Fiber DWDM SFP+/XFP transceivers widely applied in 100GHz application and support 50GHz for higher standard.

WDM Networking Components

WDM MUX/DEMUX

MUX is short for multiplexer, sometimes called a filter or prism, the term filter is how it actually works, by filtering specific colors, but most people understand a prism splitting light into spectrum. This is a simple device which combines or splits multiple colors of light into a single fiber, and this is a entirely passive device, requiring no power. A complete system requires a MUX+DEMUX for TX and RX, and most modern devices function the same in both directions. Therefore, many vendors combine the MUX+DEMUX into a single unit for simplicity but it is really 2 distinct components.

The OADM(Optical Add/Drop Multiplexer)

Selectively adds and drops certain WDM channels while passing other channels through without disruption. Where a MUX is used at the end-point of a WDM network to split all of the components wavelengths, an OADM used at a mid-point, often in a ring. With a well-constructed OADM ring, any node can reach any other node in the ring, potentially reusing the same wavelength multiple times across different portions of the ring. Some ROADMSs are multi-degree, instead of only bing able to "pass" or "drop", there are more 2 directions of "pass" to choose from. This allows you to build complex star topology at a purely optical level.

The Optical Amplifiers

Optical amplifiers increase the intensity of a signal, there are different types for different spectrum of light. And the most common is the Erbium Doped Fiber Amplifier. In an EDFA, a piece fo fiber si "doped": with Erbium ions. Additional laser power at 980nm add/or 1480nm is pumped in via a coupler. Then the interaction between the Erbium and the pump laser causes the emission of light in the C-band spectrum, amplifying the signal.

The Optical Amplifiers