WDM Principle

WDM Principle

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1 WDM Overview......................................................................................Page4

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2 WDM Transmission Media....................................................................Page17

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3 WDM Key Technologies......................................................................Page 25

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4 Technology Specifications for WDM System........................................Page 49

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WDM Principle

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Reference: 

OTC000003 WDM principle

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ITU-T G.694.1 and G.694.2 (about the wavelength distribution)

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ITU-T G.671 (about the optical passive components)

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ITU-T G.652 , G.653 and G.655 (about the fiber)


WDM Principle

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Reference: 

OTC000003 WDM principle

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ITU-T G.694.1 and G.694.2 (about the wavelength distribution)

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ITU-T G.671 (about the optical passive components)

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ITU-T G.652 , G.653 and G.655 (about the fiber)


WDM Principle

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Objectives for this chapter: 

To explain the basic concepts of WDM;

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To list the components components of WDM and their functions;

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To outline the WDM background and technical features;

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To make comparison between CWDM and DWDM.


WDM Principle

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SDM increases the transmission capacity 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 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 us ed in the national and metro backbone transmission transmission systems.


WDM Principle

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Legend: 

Freeway: Fiber

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Patrol Car: Supervisory Signal

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Gas Station: Optical Relay

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Gray Car: Client Service

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Colored Car: Service in different channels (wavelength)

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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

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The Greek letter lambda (  ) , 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

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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.

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ODU: Demultiplex one main path signal into several individual signals.

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OA: Amplifies the optical signal.

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OLA: Optical Line Amplifier

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OSC: Optical Supervisory Channel

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ESC: Electrical Supervisory Channel


WDM Principle

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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

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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:

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To MUX/DEMUX the signals in one fiber, circulator is recommended.

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This mode is usually used in the CWDM system to reduce the cost.


WDM Principle

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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

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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

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Up to know the capacity is 1920Gbps at most.

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Data Transparency Transmission: 

WDM doesn’t change the structure or any byte in the frame for the client signal.

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Long Haul transmission: 5000km without REG / 230km long hop.

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Smooth expansion: modularization and no affect the existing services.


WDM Principle

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CWDM greatly reduces the system cost while providing certain amount of wavelengths and transmission distance within 100 km. 

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Fewer channels=cheaper hardware No amplification=a lower-cost system and distance-limited system

Comparison between CWDM and DWDM

Types Channel Spacing

CWDM

DWDM 100GHz/50GHz/25GHz C-band: 1529nm~1561nm L-band: 1570nm~1603nm

20nm

Band

1311~1611nm

Capacity Laser

16 x 2.5Gb/s = 40G Un-cooled Laser

192 x 10Gb/s = 1920G Cooled Laser

Cost

70%

100%

Application

100km

5000km


WDM Principle

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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

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OTU, MUX/DeMUX, OA,OSC

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False

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Electrical Supervisory Channel,False

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Can not


WDM Principle

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Objectives for this chapter: 

List the characteristics of the fiber;

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Classify different types of the fiber;

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Outline the methods to against the factors.


WDM Principle

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Band

Wavelength

Bandwidth (nm)

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Original

1260~1360

100

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Extended

1360~1460

100

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Short

1460~1525

65

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Conventional

1525~1565

40

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Long

1565~1625

60

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Ultra long

1625~1675

50

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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

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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

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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

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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

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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

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Coating, Cladding, Core

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G.652, G.653, G.655

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Attenuation, Dispersion, Nonlinear effect


WDM Principle


WDM Principle


WDM Principle


WDM Principle

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The EDFA amplifier is widely used in WDM system.


WDM Principle

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Advantage: 

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Fortunately, 1550nm is in the low attenuation window, the emergence of EDFA greatly activate the development of WDM.

Disadvantage: 

Gain un-flatness


WDM Principle

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If we cannot control the gain, optical surge generates.

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With AGC function:

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When add wavelengths from 1 to 40, the gain will be not changed.

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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

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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.

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WDM Principle

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Advantage: 

The gain wavelength is determined by the pumping light wavelength.

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The gain medium is the transmission fiber itself, low noise.

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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.

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Be careful when put operation on Raman.


WDM Principle

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According to its application: 

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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

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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

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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

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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

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OSC is often used in the backbone wavelength system,and ESC is normally used in metropolitan system.


WDM Principle

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Pumping wavelength of OA: 980nm or 1480nm.

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1310nm already defined by ITU-T for future use.

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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

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FA: Frame alignment.

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E1 E2 : Orderwire.

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ALC: Automatic Level Control.

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F1 F2 F3 : transparent serials data.

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D1-D12: DCC bytes, data communication channel.


WDM Principle

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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

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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

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LD, EA, M-Z

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EDFA, Raman

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TFF, AWG

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OSC, ESC


WDM Principle


WDM Principle

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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

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The WDM system in the above figure has the following reference points: 

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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


WDM Principle


WDM Principle

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