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WDM Principle
�1 WDM Overview......................................................................................Page4
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1 WDM Overview......................................................................................Page4
�2 WDM Transmission Media....................................................................Page17
�3 WDM Key Technologies......................................................................Page 25
�4 Technology Specifications for WDM System........................................Page 49
WDM Principle
�About this course:
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About this course:
� This course mainly introduces the basic knowledge of WDM technologies,
expounds key technologies and optical transmission specification of WDM.
Through this course, you will have a relatively complete understanding of the
WDM knowledge and the development orientation of optical transmission
networks.
WDM Principle
�Reference:
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Reference:
�OTC000003 WDM principle
� ITU-T G.694.1 and G.694.2 (about the wavelength distribution)
� ITU-T G.671 (about the optical passive components)
� ITU-T G.652 , G.653 and G.655 (about the fiber)
WDM Principle
�Objectives for this chapter:
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Objectives for this chapter:
� To explain the basic concepts of WDM;
� To list the components of WDM and their functions;
� To outline the WDM background and technical features;
� To make comparison between CWDM and DWDM.
WDM Principle
� SDM increases the transmission capacity linearly by adding the number of optical
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SDM increases the transmission capacity linearly by adding the number of optical
fibers, and the transmission equipment will be increased linearly, too.
� TDM keeps the same transmission medium but increases the bit rate. The
equipment is getting more and more complicated and expensive. Additionally, the
maximum transported capability over a fiber pair is in the range of a few 10Gbps.
� The way to scale to higher transported capacity is WDM. This technology keeps the
same fiber, the same bit rate, but uses multiple colours to multiply transported
capacity.
�WDM is widely used in the national and metro backbone transmission systems.
WDM Principle
� Legend:
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Legend:
� Freeway: Fiber
� Patrol Car: Supervisory Signal
�Gas Station: Optical Relay
�Gray Car: Client Service
�Colored Car: Service in different channels (wavelength)
�Driveway: Optical Wavelength
�Wave Division Multiplexing is a technology that utilizes the properties of
refracted light to both combine and separate optical signals based on their
wavelengths within the optical spectrum.
WDM Principle
� The Greek letter lambda ( λ ) , is often used to designate individual wavelengths.
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The Greek letter lambda ( λ ) , is often used to designate individual wavelengths.
�Key word in the content is specific wavelength. How specific ? Please refer to ITU-T
series recommendations in chapter 4.
�WDM allows for a more efficient use of existing fiber by providing multiple optical
paths along a single (pair of) fiber (s).
�WDM allows for a greater range of protocol transmission better suited than legacy
network for data centric applications. (E.g.. GE, ESCON, Fiber Channel, D1 video)
WDM Principle
�OTU: Access the client service and convent the wavelength complied with ITU
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OTU: Access the client service and convent the wavelength complied with ITU
standards.
�OMU: Multiplex several services with different wavelength into one main path
signal.
�ODU: Demultiplex one main path signal into several individual signals.
�OA: Amplifies the optical signal.
�OLA: Optical Line Amplifier
�OSC: Optical Supervisory Channel
� ESC: Electrical Supervisory Channel
WDM Principle
�Unidirectional WDM system adopts two optical fibers. One only implements the
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Unidirectional WDM system adopts two optical fibers. One only implements the
transmission of signals in one direction while the other implements the transmission
of the signals in the opposite direction.
� This tansmission mode is widely used in the worldwide.
WDM Principle
�Bi-directional wave WDM system utilizes only one optical fiber. The single fiber
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Bi-directional wave WDM system utilizes only one optical fiber. The single fiber
transmits optical signals in both directions simultaneously, and the signals in the
different directions should be assigned on different wavelengths.
�Note:
� To MUX/DEMUX the signals in one fiber, circulator is recommended.
� This mode is usually used in the CWDM system to reduce the cost.
WDM Principle
� Open system has no special requirements for multiplex terminal optical interfaces,
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Open system has no special requirements for multiplex terminal optical interfaces,
the only requirement is that these interfaces meet the optical interface standards
defined in ITU-T.
WDM Principle
� Integrated system does not adopt the wavelength conversion technology, instead,
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Integrated system does not adopt the wavelength conversion technology, instead,
it requires that the wavelength of the optical signals at the multiplex terminal
conforms to the specifications for the WDM system.
� The optical interface in the client equipment that could provide standard
wavelength is called colored interface. Huawei series OSN products could support
this function.
� Thought:
�Can some channels use OTU and some channels use colored interface?
WDM Principle
�Up to know the capacity is 1920Gbps at most.
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Up to know the capacity is 1920Gbps at most.
�Data Transparency Transmission:
�WDM doesn’t change the structure or any byte in the frame for the client
signal.
� Long Haul transmission: 5000km without REG / 230km long hop.
� Smooth expansion: modularization and no affect the existing services.
WDM Principle
�CWDM greatly reduces the system cost while providing certain amount of
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CWDM greatly reduces the system cost while providing certain amount of
wavelengths and transmission distance within 100 km.
� Fewer channels=cheaper hardware
�No amplification=a lower-cost system and distance-limited system
�Comparison between CWDM and DWDM
Types CWDM DWDM
Channel Spacing 20nm 100GHz/50GHz/25GHz
Band 1311~1611nmC-band: 1529nm~1561nmL-band: 1570nm~1603nm
Capacity 16 x 2.5Gb/s = 40G 192 x 10Gb/s = 1920G
Laser Un-cooled Laser Cooled Laser
Cost 70% 100%
Application 100km 5000km
WDM Principle
� Fill in the blanks:
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Fill in the blanks:
1.WDM System includes:________, _________, _________ and __________;
2.CWDM system could use optical amplifiers (True or False) __________;
3.ESC means____________________________________. Need additional
wavelength to transmit in the fiber (True or False) _________.
4.Single fiber bidirectional transmission (can or can not )_________ use the
same wavelength for transmitting and receiving.
WDM Principle
�OTU, MUX/DeMUX, OA,OSC
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OTU, MUX/DeMUX, OA,OSC
� False
� Electrical Supervisory Channel,False
�Can not
WDM Principle
�Objectives for this chapter:
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Objectives for this chapter:
� List the characteristics of the fiber;
�Classify different types of the fiber;
�Outline the methods to against the factors.
WDM Principle
�An optical fiber consists of two different types of solid glass—the core and
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An optical fiber consists of two different types of solid glass—the core and
cladding—that are mixed with specific elements to adjust their refractive indices.
The difference between the refractive indices of the two materials causes most of
the transmitted light to bounce off the cladding and stay within the core. The
critical angle requirement is met by controlling the angle at which the light is
injected into the fiber. Two or more layers of protective coating around the
cladding ensure that the glass can be handled without damage.
�N1 and N2, which one is larger ?
WDM Principle
� Band Wavelength Bandwidth (nm)
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Band Wavelength Bandwidth (nm)
�Original 1260~1360 100
� Extended 1360~1460 100
� Short 1460~1525 65
�Conventional 1525~1565 40
� Long 1565~1625 60
�Ultra long 1625~1675 50
�Combining the above losses, the attenuation constant of single mode fiber at
1310nm and 1550nm wavelength areas is 0.3~0.4dB/km (1310nm) and
0.17~0.25dB/km (1550nm), respectively. As defined in ITU-T Recommendation
G.652, the attenuation constant at 1310nm and 1550nm should be less than
0.5dB/km and 0.4dB/km, respectively.
WDM Principle
�Dispersion in fiber refers to a physical phenomenon of signal distortion caused
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Dispersion in fiber refers to a physical phenomenon of signal distortion caused
when various modes carrying signal energy or different frequencies of the signal
have different group velocity and disperse from each other during propagation.
WDM Principle
�G.652 fiber is currently a single mode fiber for widely use, called 1310nm property
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G.652 fiber is currently a single mode fiber for widely use, called 1310nm property
optimal single mode fiber and also called dispersion unshifted fiber.
�G.653 fiber is called dispersion shifted fiber or 1550nm property optimal fiber. By
designing the refractive index cross section, the zero dispersion point of this kind of
fiber is shifted to the 1550nm window to match the minimum attenuation window.
This makes it possible to implement ultrahigh speed and ultra long distance optical
transmission.
�G.655 fiber, a nonzero dispersion shifted single mode optical fiber, is similar to
G.653 fiber and preserves certain dispersion near 1550nm to avoid four-wave
mixing phenomenon in DWDM transmission. It is suitable for DWDM system
applications.
WDM Principle
�DCF is one special kind of optical fiber, with the negative dispersion at 1550nm
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DCF is one special kind of optical fiber, with the negative dispersion at 1550nm
window.
� The dispersion coefficient is -90~-120ps/nm.km
�DCF can counter act positive dispersion while bring new insertion loss and
increasing of PMD.
WDM Principle
� Fill in the blanks:
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Fill in the blanks:
1. The attenuation coefficient of G.652 fiber is __________; approximately
________ for engineering planning;
2. The dispersion coefficient of G.655 at 1550nm window
is_______________;
3. The dispersion coefficient of G.652 at 1310nm window is__________; at
1550nm window is___________;
WDM Principle
�Coating, Cladding, Core
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Coating, Cladding, Core
�G.652, G.653, G.655
�Attenuation, Dispersion, Nonlinear effect
WDM Principle
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WDM Principle
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WDM Principle
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WDM Principle
�Output laser is controlled by input current. The variation of the modulation current
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Output laser is controlled by input current. The variation of the modulation current
causes the variation of output wavelength.
� This variation, called modulation chirp, is actually a kind of wavelength (frequency)
jitter inevitable for direct modulation of the sources. The chirp broadens the
bandwidth of the emitting spectrum of the laser, deteriorates its spectrum
characteristics and limits the transmission rate and distance of the system.
� Transmission rate is limited to 2.5Gbit/s, and transmission distance is less than
100km.
� Similar Specification –This kind of modulator is Widely used in CWDM system.
WDM Principle
� EA modulator adopts different structure, use stable DC current to let LD output a
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EA modulator adopts different structure, use stable DC current to let LD output a
standard wavelength (complied with ITU-T). EA module act as a door that open
only happens to the current change. In this way, the information is modulated into
the wavelength.
� Less chirp = Support long haul transmission (2.5Gb/s > 600km)
�High Dispersion tolerance (2.5Gb/s: 7200~12800ps/nm)
�Most widely used in DWDM
WDM Principle
� This modulator separates the light input into two equal signals which enter the two
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optical branches of the modulator respectively. These two optical branches employ
an electro-optical material whose refractive index changes with the magnitude of
the external electrical signal applied to it. Changes of the refractive index of the
optical branches will result in the change variation of the signal phases. Hence,
when the signals from the two branches recombine at the output end, the
combined optical signal is an interference signal with varying intensity. With this
method, the frequency chirp of the separated external modulated laser can be
equal to zero.
� Long dispersion limited distance
�High cost with good performance
�Negligible chirp
�Not widely used.
WDM Principle
�As a maturing technology, direct modulator and indirector modulator are widely
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As a maturing technology, direct modulator and indirector modulator are widely
used in WDM system.
WDM Principle
� The EDFA amplifier is widely used in WDM system.
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The EDFA amplifier is widely used in WDM system.
WDM Principle
� Principle:
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Principle:
� The outer electrons of Er ions have 3 energy levels, where E1 is the basic
state energy level, E2 is the metastable state energy level and E3 is the high
energy level.
�When high-energy pump lasers are used to excite the EDF, lots of bound
electrons of the erbium ions are excited from E1 to E3 level, then soon
dropped to the E2 level via a non-radiation decay process (i.e. no photon but
heat is released).
�When a signal with the wavelength of 1550nm passes through this erbium-
doped fiber, particles in the metastable state are transited to the basic state
via stimulated radiation and generate photons identical to those in the
incident signal light.
WDM Principle
� TAP is used to spilt out a little part of energy and send it to the PD to detection.
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TAP is used to spilt out a little part of energy and send it to the PD to detection.
� ISO is used to make sure the signal transmit in one direction.
� Pump laser has two type: with 980nm and with 1480nm.
� If we want to get a high gain, we could cascade EDF and pumping laser
WDM Principle
�Advantage:
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Advantage:
� Fortunately, 1550nm is in the low attenuation window, the emergence of
EDFA greatly activate the development of WDM.
�Disadvantage:
�Gain un-flatness
WDM Principle
� If we cannot control the gain, optical surge generates.
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If we cannot control the gain, optical surge generates.
�With AGC function:
�When add wavelengths from 1 to 40, the gain will be not changed.
�When drop wavelengths from 40 to 1, the gain will be not changed also
�Key Component is the DSP that makes the nonlinear calculation.
WDM Principle
� Principle:
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Principle:
Fiber has wide SRS gain spectrum and a wide gain peak around a frequency 13THz
lower than that of the pumping light. If a weak signal and a strong pumping light
wave are transmitted through the fiber at the same time, and the wavelength of
the weak signal is set within the Raman gain bandwidth of the strong pumping
light, the weak signal can be amplified. Such SRS-based OA is call Raman optical
amplifier. Raman optical amplifier’s gain is the switch gain, that is, the difference
between the output power when the amplifier is on and that when the amplifier is
off.
�
WDM Principle
�Advantage:
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Advantage:
� The gain wavelength is determined by the pumping light wavelength.
� The gain medium is the transmission fiber itself, low noise.
� As the amplification is distributed along the fiber with the comparatively low
signal power, it reduces the interference from non-linear effect, especially
FWM effect.
�Disadvantage:
�High power is harmful for body.
�Be careful when put operation on Raman.
WDM Principle
�According to its application:
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According to its application:
�BA: Booster amplifier, mainly used in the transmit end. For the hardware
description, you will see OBU card.
� LA: Line amplifier, mainly used in the amplifier station, could be recognized
as BA+PA. For the hardware description, you will see OAU card.
� PA: Pre-amplifier, mainly used in the receive end. For the hardware
description, you will see OPU card.
WDM Principle
� For all the optical lights are bidirectional, the mechanisms of multiplexer and
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For all the optical lights are bidirectional, the mechanisms of multiplexer and
demultiplexer are the same. Here in after we just discuss about the multiplexer, if
you reverse the direction, it could also be considered as a demultiplexer.
WDM Principle
� Film Filter offers good stability and isolation between channels at moderate cost,
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Film Filter offers good stability and isolation between channels at moderate cost,
but with a high insertion loss.
� So the number of dropping wavelength is limited.
WDM Principle
� The waveguides are connected to cavities at the input and output. When the light
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The waveguides are connected to cavities at the input and output. When the light
enters the input cavity, it is diffracted and enters the waveguide array. There the
optical length difference of each waveguide introduces phase delays in the output
cavity, where an array of fibers is coupled. The process results in different
wavelengths having maximal interference at different locations, which correspond
to the output ports.
WDM Principle
�OSC is often used in the backbone wavelength system,and ESC is normally used in
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OSC is often used in the backbone wavelength system,and ESC is normally used in
metropolitan system.
WDM Principle
� Pumping wavelength of OA: 980nm or 1480nm.
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Pumping wavelength of OA: 980nm or 1480nm.
�1310nm already defined by ITU-T for future use.
�OA fails, all signal lost, requires the supervisory signal continue to transmit alarms
and other indications.
� The receive sensitivity of the OSC unit is very good, up to -48dBm.
WDM Principle
� FA: Frame alignment.
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FA: Frame alignment.
� E1 E2 : Orderwire.
�ALC: Automatic Level Control.
� F1 F2 F3 : transparent serials data.
�D1-D12: DCC bytes, data communication channel.
WDM Principle
� The optical transponder unit (OTU) multiplexes the supervisory information into the
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The optical transponder unit (OTU) multiplexes the supervisory information into the
service channel for transmission.
� The ESC reduces the investment of the OSC. It also deletes the insertion loss of the
FIU. This lowers the cost and the power budget of optical channels.
WDM Principle
� Fill in the blanks:
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Fill in the blanks:
1.EDFA means:______________________; its pumping wavelength
is___________; We can calculate noise figure by _________。
2.AWG means:______________________; TFF
means:________________________;
3.OSC signal’s frame structure is_____________, (can, can not) by amplified by
OA.
4.ESC support OLA station ?_______(True, False)
WDM Principle
� LD, EA, M-Z
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LD, EA, M-Z
� EDFA, Raman
� TFF, AWG
�OSC, ESC
WDM Principle
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WDM Principle
� ITU-G.692 – Optical Interfaces for Multi-Channel Systems with Optical Amplifiers
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ITU-G.692 – Optical Interfaces for Multi-Channel Systems with Optical Amplifiers
� This recommendation specifies multi-channel optical line system interfaces for
the purpose of providing future transverse compatibility among such systems.
The current recommendation defines interface parameters for systems of four,
eight, and sixteen channels operating at bit rates of up to STM-16 on fibers,
as described in Recommendations G.652, G.653, and G.655 with nominal
span lengths of 80 km, 120 km, and 160 km and target distances between
regenerators of up to 640 km. A frequency grid anchored at 193.1 THz with
inter-channel spacing at integer multiples of 50 GHz and 100 GHz is specified
as the basis for selecting channel central frequencies.
WDM Principle
� The WDM system in the above figure has the following reference points:
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The WDM system in the above figure has the following reference points:
� S1…Sn: The reference points on the fiber at transmitter optical output
connector in channels 1…n;
�RM1 RMn: The reference points on the fiber at OM/OA optical input
connector in channels 1…n;
�MPI-S: A reference point on the optical fiber just behind the OM/OA optical
output connector;
� S': A reference point on the optical fiber behind the optical output connector
of the optical line amplifier;
�R': A reference point on the optical fiber in front of the optical input
connector of the optical line amplifier;
�MPI-R: A reference point on the optical fiber in front of the OA/OD input
optical connector;
� SD1…SDn: The reference points at the OA/OD optical output connector;
�R1…Rn: The reference points at receiver optical transmitter input connector.
WDM Principle
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WDM Principle
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WDM Principle
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