& cplSiteName &

Wavelength Division Multiplexing (WDM)

Light Reading
Beginners' Guides
Light Reading
8/2/2001
50%
50%

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
(149)  | 
Comment  | 
Print  | 
Newest First  |  Oldest First  |  Threaded View        ADD A COMMENT
Page 1 / 15   >   >>
rubyliu
50%
50%
rubyliu,
User Rank: Light Beer
5/29/2014 | 4:05:26 AM
re: Wavelength Division Multiplexing (WDM)
In fiber-optic communications, wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carrier signals onto a single optical fiber by using different wavelengths (colours) of laser light. This technique enables bidirectional communications over one strand of fiber, as well as multiplication of capacity.@fiberstore.com
zataki
50%
50%
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
50%
50%
tony33,
User Rank: Light Beer
12/5/2012 | 1:19:32 AM
re: Wavelength Division Multiplexing (WDM)
test
alvind
50%
50%
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
50%
50%
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
50%
50%
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
50%
50%
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
50%
50%
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
50%
50%
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
50%
50%
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.
Page 1 / 15   >   >>
Light Reading’s Upskill U is a FREE, interactive, online educational resource that delivers must-have education on themes that relate to the overall business transformation taking place in the communications industry.
NEXT COURSE
Friday, September 30, 1:00PM EDT
Gigabit & the Great Migration
Robert Howald, Vice President, Network Architecture, Comcast
UPCOMING COURSE SCHEDULE
Wednesday, October 5, 1:00PM EDT
Gigabit & Smart Cities
Joe Kochan, COO & Co-Founder, US Ignite
Friday, October 7, 1:00PM EDT
Gigabit & DOCSIS 3.1
Ty Pearman, Director, Access Architecture, Comcast
Wednesday, October 19, 1:00PM EDT
Securing a Virtual World
Rita Marty, Executive Director, Mobility and Cloud Security, Chief Security Office, AT&T
in association with:
From The Founder
Light Reading today starts a new voyage as part of a larger Enterprise.
Flash Poll
Live Streaming Video
Charting the CSP's Future
Six different communications service providers join to debate their visions of the future CSP, following a landmark presentation from AT&T on its massive virtualization efforts and a look back on where the telecom industry has been and where it's going from two industry veterans.
LRTV Interviews
CenturyLink: SDN/NFV Pose New Interconnection Possibilities

9|28|16   |   04:37   |   (0) comments


Network operators should develop new APIs and business processes for reselling virtual assets to each other, says CenturyLink's Bill Walker. That will enable them to build digital business portfolios that help them avoid becoming commodity transport providers.
LRTV Interviews
Level 3: Overcoming Terror of Being Supplier, Integrator & Developer

9|28|16   |     |   (0) comments


At Light Reading's NFV & Carrier SDN event in Denver, Travis Ewert of Level 3 Communications said there is terror in becoming supplier, integrator and developer, but it can be overcome and be cost effective.
LRTV Custom TV
Introducing IoT World News

9|27|16   |   01:43   |   (0) comments


Self-driving cars, medical sensors, smart cities... and refrigerators. In order to address the huge scope of IoT, KNect365 has created a unique online community that will help businesses to understand and monetize the opportunities that live within the IoT market. We look forward to welcoming you to IoT World News -- your gateway to a better connected future.
LRTV Interviews
AT&T: Re-usable Functions Next NFV Key

9|27|16   |   06:03   |   (0) comments


The next generation of NFV has to break functions down into re-usable software chunks, making everything much more cloud-like.
LRTV Interviews
Masergy on Security: Attackers Gaining Upper Hand

9|27|16   |   5:10   |   (2) comments


At Light Reading's NFV & Carrier SDN event in Denver, Ray Watson, vice president of Global Technology at Masergy, says that because of the growth in virtualization, the threat landscape is shifting in favor of the attackers. As a result, service providers need to think beyond just defending the perimeter and take a more holistic approach to security.
LRTV Interviews
Verizon Takes Next Step on Biz Virtualization Journey

9|26|16   |   4:38   |   (2) comments


At September's NFV & Carrier SDN event in Denver, Light Reading sat down with Victoria Lonker, director of Product and New Business Innovation at Verizon, to chat about where the carrier is with delivering virtualized services to business customers.
LRTV Interviews
Global Services: The $40B Face-Off

9|26|16   |   05:53   |   (1) comment


More service providers than ever before are battling it out to win a slice of what is now a $40 billion global communications services pie, explains Ovum Principal Analyst David Molony.
LRTV Documentaries
MEC Congress: The Key Takeaways

9|22|16   |   03:25   |   (3) comments


Three key takeaways from the Mobile Edge Computing (MEC) Congress in Munich, Germany.
Wagner’s Ring
Time to Shut Up About 'Dumb Pipes'

9|22|16   |     |   (12) comments


Service providers can't compete with OTT players. It just isn't in their DNA. Instead, service providers need to embrace what they're good at -- providing reliable, secure connectivity.
Wagner’s Ring
Keeping Your Tech Career Going After 50

9|21|16   |     |   (13) comments


How do you keep your career moving forward when you're past the half-century mark?
LRTV Interviews
Peering Into the Digital Future

9|20|16   |   04:25   |   (0) comments


Nick Thomas, practice leader of digital media at Ovum, talks about how digital transformation in the technology, media and telecom sectors will enable the development of a new range of applications and services for enterprises and consumers and how the upcoming Digital Futures event in London will examine ...
LRTV Custom TV
Napatech Tackles NFV's Major Challenge

9|7|16   |   08:42   |   (0) comments


One of the main challenges for network operators introducing NFV is to combine performance and flexibility in a cost-effective way, but there is a solution, explains Napatech's Dan Joe Barry.
Upcoming Live Events
November 3, 2016, The Montcalm Marble Arch, London
November 30, 2016, The Westin Times Square, New York City
December 1, 2016, The Westin Times Square, New York, NY
December 6-8, 2016, The Westin Excelsior, Rome
May 16-17, 2017, Austin Convention Center, Austin, TX
All Upcoming Live Events
Infographics
Hot Topics
Verizon CFO: Eat Our (Fixed) 5G Dust!
Dan Jones, Mobile Editor, 9/22/2016
WiCipedia: The Women Helping Women Edition
Eryn Leavens, Special Features & Copy Editor, 9/23/2016
Eurobites: Telefónica Taps Juniper for Network Security
Paul Rainford, Assistant Editor, Europe, 9/26/2016
Open Source Getting on My Nerves
Carol Wilson, Editor-at-large, 9/26/2016
Google, Facebook Gaining Network Equipment Clout
Patrick Donegan, Chief Analyst, Heavy Reading, 9/26/2016
Like Us on Facebook
Twitter Feed
BETWEEN THE CEOs - Executive Interviews
Light Reading CEO Steve Saunders and UXP Systems CEO Gemini Waghmare discuss the strategic importance of digital identity for operators in the midst of transformation.
Join us for an in-depth interview between Steve Saunders of Light Reading and Alexis Black Bjorlin of Intel as they discuss the release of the company's Silicon Photonics platform, its performance, long-term prospects, customer expectations and much more.
Animals with Phones
There's Nothing Like Missing a Full Minute of Pokémon Go Click Here
Live Digital Audio

A vital part of increasing the number of women in comms is transforming the ways companies can support and empower women. While progressive company policies that support both men and women in achieving work-life balance are a step in the right direction, creating a company culture that supports those policies can at times be more challenging.

During this show, we'll talk to Lynn Comp, Senior Director of Industry and Sales Enabling (ISE) in the Network Platforms Group at Intel, about why those challenges exist and how companies can overcome them. She'll provide insight into how Intel has worked to create a culture that supports work-life balance, and provide steps and guidance for other companies wishing to do the same. We will also leave plenty of time to get your questions answered live on the air.