WDM (Wavelength Division Multiplexing)

 WDM Principle

 1. SDM increases the transmission capacity linearly by adding the number of optical fibers, and the transmission equipment will be increased linearly, too.

2. 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 capacity over a pair of fibers is 10Gbps.

3. 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.

4. WDM is widely used in the national and metro backbone transmission systems.

 

Wavelength 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.

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)

l  OTU: Access the client service and convert the wavelength complied with ITU standards.

l  OMU: Multiplex several services with different wavelength into one main path signal.

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

l  OA: Amplifies the optical signal.

l  OLA: Optical Line Amplifier

l  OSC: Optical Supervisory Channel

l  ESC: Electrical Supervisory Channel

 l  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.

l  This transmission mode is widely used in the worldwide.

l  Bi-directional 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:

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

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

l  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.

l  The optical interface in the client equipment that could provide standard wavelength is called colored interface. HUAWEI OSN series products could support this function.

 

 l  Up to know the capacity is 1920Gbps at most.

l  Data Transparency Transmission:  WDM doesn’t change the structure or any byte in the frame for the client signal.

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

l  Smooth expansion: modularization and no affect the existing services.

 

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

p  Fewer channels=cheaper hardware

p  No amplification=a lower-cost system and distance-limited system

l  Comparison between CWDM and DWDM

Types	CWDM	DWDM
Channel Spacing	20nm	100GHz/50GHz/25GHz
Band	1311~1611nm	C-band: 1529nm~1561nm
		L-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

 

 

 

l  OH：hydrogen

l	Band	Wavelength	Bandwidth (nm)
l	Original	1260~1360	100
l	Extended	1360~1460	100
l	Short	1460~1525	65
l	Conventional	1525~1565	40
l	Long	1565~1625	60
l	Ultra long	1625~1675	50

l  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.

Amplifier

l  The EDFA amplifier is widely used in WDM system.

l  Principle:

p  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.

p  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).

p  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.

 

l  TAP is used to split out a little part of energy and send it to the PD for detection.

l  ISO is used to make sure the signal transmit in one direction.

l  Pump laser has two types: with 980nm and with 1480nm.

l  If we want to get a high gain, we could cascade EDF and pumping laser.

l  Advantage:

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

l  Disadvantage:

p  Gain un-flatness

 

 

l  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 called 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.

l  Advantage:

p  The gain wavelength is determined by the pumping light wavelength.

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

p  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.

l  Disadvantage:

p  High power is harmful for body.

p  Be careful when put operation on Raman.

 

 l  According to its application:

p  BA: Booster amplifier, mainly used in the transmit end. In the hardware description, you will see OBU card.

p  LA: Line amplifier, mainly used in the amplifier station, could be recognized as BA+PA. In the hardware description, you will see OAU card.

p  PA: Pre-amplifier, mainly used in the receive end. In the hardware description, you will see OPU card.

 

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

l  Pumping wavelength of OA: 980nm or 1480nm.

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

l  OA fails, all signal lost, requires the supervisory signal continue to transmit alarms and other indications.

l  The receive sensitivity of the OSC unit is very good, up to -48dBm.

 

l  The optical transponder unit (OTU) multiplexes the supervisory information into the service channel for transmission.

l  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.