Introduction to WDM and Investigation of Wavy characteristic
Optical Communication Team #1
9931207 Dae-Hun Yang, 0041143 Woo-Sung Kim, 0040277 Jung-Youn Han
Abstract-At the opening of 1970's, many people gave attention to the light communication.
Because if we use light to communicate, we would reduce the effect of channel because of low loss
through light fibers. And technologies of the light communication have been improved. Now, the
technology is greatly developed by introduction of Wavelength Division Multiplexing.
The light communication consists of receiver, channel, transmitter. There are many issues for each
parts. However, we are going to talk about WDM because not only our professor is teaching about
wave and field of electromagnetic, but also WDM is one of the most important and the latest
technology in the light communication.
1. Introduction to Optical Communication
Optical communication is the method that uses light. Light has wavy character proved by
Young's experiment 1. For that reason, optical communication could transmit signal to the receiver.
Basic concept of optical communication is explained below.
The innermost region of the fiber, or core, was used to transmit the light, while the glass
coating, or cladding, prevented the light from leaking out of the core by reflecting the light within
the boundaries of the core. This concept is explained by Snell’s Law which states that the angle
at which light is reflected is dependent on the refractive indices of the two materials 2 — in this
case, the core and the cladding. The lower refractive index of the cladding (with respect to the
core) causes the light to be angled back into the core as illustrated in Fig 1-1.
2. What's the WDM
The fiber optic industry first deployed single wavelength transmission links. As requirements
changed, the industry responded with wavelength-division multiplexing (WDM), which sends two
distinct signals per fiber, doubling transmission capacity. Two important considerations in a WDM
device are crosstalk 3 and channel separation. WDMs allow multiple independent data streams to
be sent over one fiber. The most common WDM system uses two wavelengths, although four or
more-wavelength systems are available.
Fig 2-1 illustrates two WDMs permitting two streams of data to be carried on a single fiber. The
type of data does not matter.
WDM enables the utilization of a significant portion of the available fiber bandwidth by allowing
many independent signals to be transmitted simultaneously on one fiber, with each signal located
at a different wavelength. Routing and detection of these signals can be accomplished
independently, with the wavelength determining the communication path by acting as the
signature address of the origin, destination or routing. Components are therefore required that
are wavelength selective, allowing for the transmission, recovery, or routing of specific
wavelengths.
2-1 Advanced Devices of WDM
- CWDM(Coarse Wavelength-division Multiplexing)
The development of CWDM, an intermediate technology, CWDM allows a modest(several) number
of channels, typically eight or less. Whereas DWDM systems use channel spacing as close to 0.4
nm, CWDM uses a spacing of 20 nm.
- DWDM(Dense Wavelength-division Multiplexing)
DWDM is a developed technology than CWDM. DWDM allows over 18 channels and uses very
narrow channel spacing(0.4nm). It means DWDM allows the transmission of e-mail, video,
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"Double Slit" experiment, demonstrating that light can be diffracted, and proving that light has wave properties.
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Crosstalk : 1. Undesired coupling from one circuit, part of a circuit, or channel to another. 2. Any phenomenon by which a si gnal
transmitted on one circuit or channel of a transmission system creates and undesired effect in another circuit or channel.
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multimedia, data, and voice—carried in Internet protocol (IP), asynchronous transfer mode (ATM),
and synchronous optical network/synchronous digital hierarchy (SONET/SDH), respectively, over
the optical layer.
- WWDM(Wide Wavelength Division Multiplexing)
WWDM was originally defined as a LAN WDM operating in the 1310nm window, while early
CWDM was also described as a LAN technology, but operational in the 850nm window.
2-2 Basic Operation of WDM and Transmission line Character of Optic Fiber
In a simple WDM system shown in Fig 2-2, each laser must emit light at a different wavelength,
with all the lasers’ light multiplexed together onto a single optical fiber. After being transmitted
through a high-bandwidth optical fiber, the combined optical signals must be demultiplexed at the
receiving end by distributing the total optical power to each output port and then requiring that
each receiver selectively recover only one wavelength by using a tunable optical filter. Each
laser is modulated at a given speed, and the total aggregate capacity being transmitted along the
high-bandwidth fiber is the sum total of the bit rates of the individual lasers. An example of the
system capacity enhancement is the situation in which ten 2.5 Gbps signals can be transmitted on
one fiber, producing a system capacity of 25 Gbps. This wavelength-parallelism circumvents the
problem of typical optoelectronic devices, which do not have bandwidths exceeding a few GHz
unless they are exotic and expensive. The speed requirements for the individual optoelectronic
components are, therefore, relaxed, even though a significant amount of total fiber bandwidth is
still being utilized.
The concept of wavelength demultiplexing using an optical filter is illustrated in Fig 2-3. In the
figure, four channels are input to an optical filter that has a nonideal transmission filtering
function. The filter transmission peak is centered over the desired channel, in this case, l 3,
thereby transmitting that channel and blocking all other channels. Because of the nonideal filter
transmission function, some optical energy of the neighboring channels leaks through the filter,
causing interchannel, interwavelength cross-talk. This cross-talk has the effect of reducing the
selected signal’s contrast ratio and can be minimized by increasing the spectral separation
between channels.
If the transmitted signals have various frequency, the optical fiber should be changed
transmission line characteristic impedance because optical fiber is a transmission line for the
light. Transmission characteristic impedance value is governed by electrical components such
that resistance, conductance, capacitance, inductance. WDM is to transmit several data types.
For example, voice signal is on 4Hz~20kHz and video signal is on the several tens of mega Hz.
So, if there is a lot of loss on the fiber, we can't send accurate data to the terminal. Because notfixed frequency causes transmission line loss. And the loss will make a thermal noise. Thermal
noise4 can be considered AWGN5 and loss may make distortion. For that reason, fiber which we
are going to use don't have to include R and G factor. But all kind of materials have their own
loss factor. On that account, we should think about noise effect a little.
WDM method is similar to FDM 6. If the velocity of wave is constant, one over wavelength is
equal to frequency. So, we can employ M-ary orthogonal signaling. Each type of signals that is
carried orthogonal basis don't interfere each other. After that, through the consideration of noise
effect and distortion, we are able to reach the goal that is to transmit correct data.
First, each signal passed by MUX has their own power. The unidirectional isolation is a
measure of the part of the optical power at each wavelength exiting from the port at wavelengths
different from the nominal wavelength relative to the power at the nominal wavelength. It is
given by the following formula:
IU = aiox – aioc
The terms aiox and aioc are elements of the logarithmic transfer matrix 7, where i is the input port
number, o is the output port number, c is the (channel) wavelength number associated with port o
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Thermal noise is the noise generated by thermal agitation of electrons in a conductor. The noise power, P , in watts, is given by P = 4kT Δf ,
where k is Boltzmann's constant in joules per kelvin, T is the conductor temperature in kelvins, and Δ f is the bandwidth in hertz.
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Additive White Gaussian Noise. White noise has equal power per hertz over the specified frequency band
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FDM(FREQUENCY-DIVISION MULTIPLEXING)
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A general logarithmic transfer matrix(for G.671:ITU-T Standard) is shown below
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and x is the isolation wavelength number, where x is any wavelength number not equal to c. In
each output port o there is one channel wavelength l c and k1 isolation wavelengths lx. This is
illustrated in Fig 2-4. lc is used in this concept to denote channel wavelength and not fibre cutoff wavelength.
Fig 2-5 illustrates an example using the transfer matrix, if powers P1, P2, P3, … Pk were
launched into a WDM DMUX device at wavelengths 1, 2, 3, … k respectively, then the signals
emerging from port x would be: t1x1P1, t1x2P2, t1x3P3, … t1xkPk. So the isolation of port 2 to
wavelength 3 would be a123 a121. In the fiber the variance value is fixed because noise variance
value is governed by channel's own material factor. So by the concept of power of signal passed
MUX, SNR could be controlled. By using SNR factor, we can decide received signal error
probability9. As you see below figure, the WDM is very row error performance method because
in the fiber noise 8variance is small.
Second, if there is a loss on the transmission line, transmitted signal could be changed their own
shape. It is a distortion. If we know about the information of filter and power loss when signal is
transmitted. The filter is made for the wanted purpose. So the only factor we should consider is
power loss in the transmission line. It is the reduction in optical power between an input and
output port of a WDM device in decibels (dB). It is defined as:
IL = -10log(Pout / Pin)
where, Pin is the optical power launched into the input port and P out the optical power received
from the output port.
For a WDM device, it is an element a iow of the n*n*k element logarithmic transfer matrix. Here i
is the input port number, o is the output port number and w is the wavelength numb er associated
with port i or o, n is the total number of input + output ports and kis the total number of
wavelengths of the logarithmic transfer matrix. For WWDM devices, it shall be specified as a
maximum value and a minimum value at each operating wavelength range. For DWDM and CWDM
devices, it shall be specified as a maximum value and a minimum value within the wavelength
range as illustrated in Fig 2-6. Obtained power loss and known information about filter will help
to equalize. Because each type of WDM method has different power loss, as shown in Fig 2-6, it
should be applied to accurate value and equalizer.
Recent research interests in WDM networks include network control and management, fault
management, multicasting, physical-layer issues, IP over WDM, traffic grooming, and optical
packet switching, just to name a few.
3. Conclusion
By investigation we’ve done, WDM is a good solution to improve potical communication.
Especially, WDM is the most efficient method in the ultra-frequency and multi-communication field.
If we try to improve these forms, without distribution optical infrastructure we can achieve the goal
to transmit variety data types and high quality data. After that, this technology will be the best
solution in communication field.
4. References
1)Book references
Physics for Scientist and Engineers-Fishbane, Gasiorowicz, Thornton. Chapter 36 [Wavy Character of Light]
Signal & System Openheim Chapter 8.[FDM, Frequency of Data types]
Cheng Chapter 9-[Transmission line]
Digital Communication-Proakis Chapter 4. Chapter 5[M-ary orthogonal signaling, Error performance]
Introduction to Digital Communication-Bernard Sklar Chapter 3[Equalizing]
fiber-optics communication third Gorvind P. AGRAWAL Ch.8, Ch.1
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where aio is the optical power reduction in decibels out of port o with unit power into port i, i.e.:
aio = 10 log (tio)
where tio is the transfer matrix coefficient. Similarly, for the off state, aoio = 10 log (toio).
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Noise generated in an optical fiber system combination of mode-dependent optical losses and fluctuation in the distribution of optical
energy among the guided modes or in the relative phases of the guided modes
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2)Internet references
http://www.itu.int/sancho/0_ENG_OLES/G671_3233&19134def.doc
http://www.itu.int/sancho/0_ENG_OLES/G671_335&19140def.doc
http://www.itu.int/sancho/0_ENG_OLES/G671_329&19111def.doc
http://computing-dictionary.thefreedictionary.com/
http://www.ntt.co.jp/RD/OFIS/active/2002pdfe/nw_e.pdf
http://www2.rad.com/networks/1999/wdm/wdm.htm
http://www.fiber-optics.info/
http://www.iec.org/online/tutorials/dwdm/index.html
5. Figures
Fig2-1 WDM Application
Fig 1-1 Basic
Communication
concept
of
Optical
Fig 2-2 Basic Operation
Fig 2-3 The concept of wavelength
demultiplexing
Fig 2-4 Illustration of unidirectional (farend) isolation of a WDM device
Fig 2-5 Example of WDM demultiplexer
device
Fig 2-6 Illustration of maximum and
minimum insertion loss of a WDM device
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