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Wavelength Division Multiplexing (WDM)

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Beginners' Guides
Light Reading
8/2/2001
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Before reading this you may find the following tutorials useful:
Optical Networks, Optical Fiber

As an optical network consists of optical fibers carrying flashes of light from a laser, you can improve the speed of information transfer by increasing the number of laser light flashes per second (increasing the bit-rate). However, a point comes at which the technology of lasers cannot meet the demands of an optical network. Just as with the lone, dirty old man that flashes at passersby in the park — he can only flash so fast. But what if this man were to invite a group of his dirty old friends? Now they could all flash at the same time and vastly increase the amount of information they are transmitting to the innocent people walking by.

In an optical network, you can increase the number of lasers and have them all sending their light down the optical fiber at the same time. However, there is a catch. If the dirty old man and his friends were all wearing the same color of trench coat, then people would not be able to distinguish the different sources of information. So each flasher would need to have his own color of trench coat, to make sure that his information is not confused with that of the others. Similarly, all the different lasers must give out different colors (different wavelengths) of light so that their information can be separated at the other end of the network. The sending of many different wavelengths down the same optical fiber is known as Wavelength Division Multiplexing (WDM).

Modern networks in which individual lasers can transmit at 10 Gigabits per second can now have several different lasers each giving out 10 Gbit/s through the same fiber at the same time. The number of wavelengths is usually a power of 2 for some reason. So WDM systems will use two different wavelengths, or 4, 16, 32, 64, 128, etc. Systems being deployed at present will usually have no more than maybe 32 wavelengths, but technology advancements will continue to make a higher number of wavelengths possible.

Wavelength Division Multiplexing

The act of combining several different wavelengths on the same fiber is known as multiplexing. At the receiving end, these wavelengths need to be separated again, which is known, logically enough, as demultiplexing. Each wavelength will then need its own light detector to convert it back into useful information.

A WDM System

The exact wavelengths of light being used are usually around the 1550 nanometer region, the wavelength region in which optical fiber performs the best (it has very “low loss” or “low attenuation” at this wavelength). Each different wavelength will be separated by a multiple of 0.8nm (sometimes referred to as “100GHz spacing,” which is the frequency separation; or as the “ITU-Grid,” named after the standards body that set the figure). So if you have four wavelengths you may have them at 1549.2nm, 1550nm, 1550.8nm, and 1551.6nm. However, you could also separate each by 1.6nm, or even 2.4nm, as long as it is some multiple of 0.8nm. Newer designs that aim to cram even more wavelengths into an even tighter space, may even have half the regular spacing (0.4nm) or a quarter (0.2nm). There can be problems with wavelengths spreading out (known as dispersion) and affecting neighboring wavelengths; so this and other more complicated issues need to be considered carefully when designing a WDM system.

Key Points

  • Increases capacity of optical fibers
  • Different wavelength lasers each transmitting at same time down same fiber
  • 'Multiplexing' is combining wavelengths; 'demultiplexing' is splitting wavelengths
  • Usually in powers of 2 — 2, 4, 8, 16, 32, 64, 128, etc. wavelengths
  • Wavelengths separated by multiples of 0.8nm (100GHz, ITU-Grid)


Further Reading

Laser Basics, Tunable Lasers, Nonlinear Effects, Optical Amplification
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zataki
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zataki,
User Rank: Light Beer
12/5/2012 | 3:27:28 AM
re: Wavelength Division Multiplexing (WDM)
Another option for this is to have a WDM coupler such as a 1310/1550 coupler. with this device it does not matter what direction the traffic is travelling in. The TX and RX can easily co-exist on the same fiber.
tony33
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tony33,
User Rank: Light Beer
12/5/2012 | 1:19:32 AM
re: Wavelength Division Multiplexing (WDM)
test
alvind
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alvind,
User Rank: Light Beer
12/5/2012 | 1:12:31 AM
re: Wavelength Division Multiplexing (WDM)
If I send different wavelength lasers from either ends on the same fibre simultaneousely, will it be possible to recover them on the other sides using refraction or something ?
PO
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PO,
User Rank: Light Beer
12/5/2012 | 1:12:18 AM
re: Wavelength Division Multiplexing (WDM)
You'll want to look into a device called an optical circulator.

From one website's description, an "optical circulator is a multi-port passive device, which routes one incoming optical signal from port 1 to port 2 and another signal from port 2 to port 3."

Port 2 is the rx/tx multiplex port; port 1 is tx, port 3 is rx.

Is this what you intended?
lightmaniac
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lightmaniac,
User Rank: Light Beer
12/4/2012 | 11:04:20 PM
re: Wavelength Division Multiplexing (WDM)
L=C/F is a hyperbola function, so by any means there is no such linear relationship between delta L and delta F. If we apply differential of F on both sides, we can get dL/dF = -c/F^2 = -L^2/c.

This means the channel spacing is around 0.78nm at wavelength of 1530nm, and 0.81nm at wavelength of 1560nm.
dwdm2
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dwdm2,
User Rank: Light Beer
12/4/2012 | 10:54:00 PM
re: Wavelength Division Multiplexing (WDM)
Apparently this thread died before I spotted. However, it raised my curiosity enough to jump in even this late.

I tried to do the calculation in the following way. The basic formula that relates the frequency, f, wavelength, l, speed of light, c, and refractive index of propagating medium, n, is

nfl = c ....... (1)

Where, c = 299792.458 THz.nm.

Also, ITU grid frequeny is given by,

f_N = 190.000 + 0.1*N (THz), N = 0, 1, 2, ...
(2)
where, N is a given ITU channel number.

For N=0, the wavelength from Eq. (1) can be found to be,

l = c/f = 1577.855 nm (ITU # 0)... (3)

Eq. (3) produces correct value of wavelengths corresponding to each ITU frequency. For 100 MHz (0.8nm) channel spacing, one then expects to be able to write Eq. (2) in terms of ITU wavelength as

l_N = 1577.855 + 0.8N, N = 0, 1, 2, ....
(4)

However, that is not the case! If you compute the wavelengths using Eq. (4), they do not correspond to the ITU grid wavelenths! In fact if you compute delta-l from the values obtained from Eq. (3), youGÇÖll find that at ITU #1, delta-l = 0.83 and it is only ~ 0.77 at the other edge of C-band. This matches with lightmanicGÇÖs analysis.

Regards,

AR
dwdm2
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dwdm2,
User Rank: Light Beer
12/4/2012 | 10:52:54 PM
re: Wavelength Division Multiplexing (WDM)
In fact if one plots the delta-l vs. ITU channel#, it can be expressed as

delta-l = 0.83 - 0.0008*N (nm), N is an ITU channel#.

This shows that, delta-l = 0.8 nm only at ITU# 36.

However, most 100GHz systems have a passband of only +/- 0.1 nm (i.e., 25 GHz). So even 0.75 nm channel spacing is not bad, there is plenty of room.

Can anyone comment on 50 GHz or 25 GHz systems. Is there an ITU definition for these spacing?

Thanks.
debasish71
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debasish71,
User Rank: Light Beer
12/4/2012 | 10:49:39 PM
re: Wavelength Division Multiplexing (WDM)
What is the maximum electrical speed that can be transported over the fibre without resorting to WDM?Suppose the signal to be transported over the fibre exceeds that value; how can it be a candidate for WDM? because, it still is a single electical signal, it is not a bunch of signals having varying speeds.. so that each signal can be converted to the optical context by parellel lasers and can be input to a DWM box which can mux them?If DWM is normally the mux of 32 signals, what is this value for DWDM?
Thanks,
dwdm2
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dwdm2,
User Rank: Light Beer
12/4/2012 | 10:48:50 PM
re: Wavelength Division Multiplexing (WDM)
debasish71:

What is the maximum electrical speed that can be transported over the fibre without resorting to WDM?
-------

Well, debasish71, fiber does not transport electrical signal. Perhaps someone can/would try to explain if you try to rephrase/ask a question.
debasish71
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debasish71,
User Rank: Light Beer
12/4/2012 | 10:48:01 PM
re: Wavelength Division Multiplexing (WDM)
The concept explained in this article says that when the signal to be transported exceeds a certain upper limit, it can no longer simply be converted to an optical signal by a laser and transported over the fibre, as can be done with lower speed signal. Hence WDM is used .

In this context, my doubt is, dear friend, how can a single higher bit-rate signal be wavelength division multiplexed? By multiplexing, the concept is to produce a signal which is the resultant of multiple input signals, but in this case we have only one signal. In other words, how do you use WDM to bypass this limitation of the laser that it can operate only upto a certain limit?
Thanks,
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