WDM stands for Wavelength Division Multiplexing. WDM is the most important and most popular method to increase the capacity of a single strand of fiber.
As we all have learned from elementary school science, a white light beam can be separated into individual colored light beams by a prism, as shown in this picture. Vice versa, individual colored light beams can also be combined into a single white light beam by the prism, that is if we use the prism in the reverse direction.
WDM uses this same idea. Traditionally, only one colored light was used on a single strand of fiber to carry the information, such as 1550nm light. However, starting from the early 1990s, the Internet boom pushed service providers to find a method to increase the capacity on their network in the most economical way. That is when WDM devices were invented.
As shown in the right side picture, in a WDM system, many different colored lights are combined by a WDM multiplexing device and put into a single strand of fiber, each color is called a channel. On the receiving side, each color is separated into its own channel by a WDM de-multiplexing device.
It shows that a single fiber's capacity is increased by 40 times with a 40 channel WDM. The beauty of WDM is that you only need to upgrade the end equipment, no need to dig up trenches to bury more fibers, which is much more costly.
So how are WDM devices made?
This picture shows a 3 channel WDM device based on thin-film filters. Thin-film filters are a piece of flat surface glass with optical coatings on it. The optical coatings are designed to pass and reflect certain colored lights.
This is a de-multiplexing WDM device, which means that it separates combined colored lights from a single fiber into separate individual fibers. But if used in the reverse direction, it can also combine different colored lights from individual fibers into a single fiber.
As shown, three colored lights, which are 1510nm, 1530nm and 1570nm, come from a single strand of fiber 1. The GRIN lens collimates the light so it won't diverge into a large un-controlled beam.
Filter 1 is designed to pass 1530nm and 1510nm, but reflect 1570nm. So when the light hits filter 1, 1570nm is reflected and re-focused into fiber 2 by the GRIN lens. Now the light has 1530nm and 1510nm in it, and it keeps going until it hits filter 2.
Filter 2 is designed to pass 1510nm but reflect 1530nm, so 1510nm light passes and is focused into fiber 3 by the 2nd GRIN lens. On the other hand, 1530nm light is reflected by filter 2, passes filter 1 again, and is focused into fiber 4 by the 1st GRIN lens.
Why isn't 1530nm focused into fiber 2? That is because the filters have a thickness, so there is a offset on the vertical direction.
We just talked about a 3 channel WDM device. Thin-film filter based WDMs can be cascaded together to get higher channel counts, such as 4, 8, 16, 32 channels.
The right side picture shows a 8 channel WDM device. The left picture shows how it's made by cascading several WDMs together.
In additional to thin-film filter based WDMs, there are also fused fiber based WDMs, Arrayed Waveguide Grating based WDMs, Interleaver based WDMs, etc. They use different mechanisms to separate the colored lights.
So there you have it. Please don't forget to visit http://www.fiberoptics4sale.com for more free fiber optic tutorials.
I will see you in the next video!