WDM, DWDM, and CWDM

Wavelength-division Multiplexing (WDM) is a technology we can put several laser light beams into the same optical fiber. The light beams are identified by their unique physical properties: wavelengths. Because of this, all such technologies are collectively called Wavelength-division Multiplexing.

By TotoBaggins on Wikipedia.

Notation

How can we draw a diagram describing different light beams going through the same fiber? Some smart people came up with a brilliant idea. They analogize wavelengths to visible lights. Most people have ever observed the dispersion of visible light when it is going through a prism. Our human eyes will interpret different wavelengths as colors, just like the colors of a rainbow. That’s why we always use “colors” to describe different wavelengths of WDM light beams.

Dispersion of light running through prism.
Created by Joanjoc on Wikipedia

However, this is only an analogy. The laser lights used in fiber communication system are totally invisible. These laser lights are with wavelengths around 1550 nanometer. Human visible lights are within range of 390~700 nanometer. Therefore, you will never see those laser lights with bare eyes, and please do not even try it because long exposure of laser lights might damage your irises.

Wavelength? Why not Frequency-division?

Another question people might ask is why we use the name “wavelength-division” instead of “frequency division”? We all know this physical equation: (Wavelength) x (Frequency) = (Light speed in fiber). In this equation we know frequency is just the reciprocal of wavelength timed to constant light speed, so wavelength-division is exactly the same as frequency-division in physics.

I think this is the result of the convenience of writing. Number and unit are smaller when we use wavelengths to describe laser beams used in fiber communication. Consider the same example of wavelength of 1550 nanometer, its equivalent frequency is calculated as (200,000 kilometers per second) / (1550 nanometers) ~= 1.29032258 × 10^14 Hertz, or, 129 Terahertz. Would you write 129 Terahertz instead of 1550 nm? I think 1550 nm is much easier for me to remember and to write it down. When we discuss light waves, we usually use wavelengths.

On the other hand in radio communication, we usually use “frequency” instead of wavelengths. For example, in LTE we have FDD-LTE (Frequency-division Duplexing LTE).


Erbium Doped Fiber Amplifier (EDFA)

Before we discuss how dense and how coarse they are, we must know the technology of amplification of laser lights. A special element is call Erbium. Scientists discovered in lab that when fiber is doped with Erbium, and powered by electricity, any laser beams running through this doped fiber will be amplified.

Schematic diagram of a simple Doped Fibre Amplifier.
By Dcoetzee on Wikipedia.

Boosted with this amplifier in the middle, we can now transmit laser beams in fibers of hundreds of kilometers. This is how we build long distance fiber communication systems today.

However, there is one limitation of this amplification device. It only amplifies laser beams with wavelengths from 1525 to 1565 nanometer. Unless we have breakthroughs in newer materials, if we want to leverage EDFA as amplifier today, we can only choose wavelengths of laser beams in this range. There is a formal name for this range of wavelength: C-Band.

Dense WDM (DWDM)

With typical spacing of around 0.8 nanometer, we can put 40 different wavelengths in the range of 1525 to 1565 nanometer. If a fiber communication is running within this range, it is called Dense Wavelength-division Multiplexing. How dense could it be? Let's move on to the next technology before we visualize it.

Coarse WDM (CWDM)

Typical spacing for Coarse WDM is 20 nanometer. Compared to 0.8 nanometer of DWDM, it is much wider and coarser.

With technology of DWDM, why should we create another similar technology of CWDM? One reason behind it is because of the cost. DWDM devices are much more expensive than CWDM devices. If we only need small degree of multiplexing, CWDM is much more cost-effective.

Below is an illustration I copied from Cisco’s web site. We can easily see how coarse and how dense they are. In this illustration, DWDM is squeezed inside the range of 1530~1560 nanometer, but CWDM is spread from 1470 to 1610 nanometer spaced with 20 nanometer.

Cisco Enhanced Wavelength Division Multiplexing (EWDM) concept.
Copied on Cisco.com.

There are eight smaller wavelengths in this illustration. This is to explain the working principle of Cisco’s “Enhanced Wavelength Division Multiplexing Product Line.” By adding 8 more non-overlapping DWDM wavelengths, Cisco can double the capacity for existing CWDM-class systems. But this is not relevant to my purpose to visually compare how dense and coarse they are.


Conclusion

With WDM, we can put more lanes of data transmission in single fiber. DWDM is more expensive, and is designed for longer distances with EDFA as amplifier. On the other hand, CWDM is much cheaper but only good for shorter distances for lack of effective amplifiers. They are both WDM technologies.