Wavelength Division Multiplexing www swedishcr weebly com Multiplexing

Wavelength Division Multiplexing www. swedishcr. weebly. com

Multiplexing • Multiplexing – a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium • Types – Time division multiplexing – Frequency division multiplexing • Optically – Time division multiplexing – Wavelength division multiplexing

WDM wavelength-division multiplexing (WDM) is a technology which multiplexes a number of optical carriers signals onto a single optical fiber by using different Wavelengths (i. e. colors) of laser light. This technique enables bidirectional communications over one strand of fiber.

Problems and Solutions Problem: Demand for massive increases in capacity Immediate Solution: Dense Wavelength Division Multiplexing Longer term Solution: Optical Fibre Networks

WDM Overview A B l 1 l 2 Wavelength Division Multiplexer Fibre Wavelength Division Demultiplexer l 1 + l 2 l 1 l 2 X Y Ÿ Multiple channels of information carried over the same fibre, each using an individual wavelength Ÿ A communicates with X and B with Y as if a dedicated fibre is used for each signal Ÿ Typically one channel utilises 1320 nm and the other 1550 nm Ÿ Broad channel spacing, several hundred nm Ÿ Recently WDM has become known as Coarse WDM or CWDM to distinguish it from DWDM

WDM Overview l 1 A Wavelength Division Multiplexer Fibre Wavelength Division Demultiplexer l 2 B l 3 C l 1 l 2 l 1 + l 2 + l 3 X Y Z Ÿ Multiple channels of information carried over the same fibre, each using an individual wavelength Ÿ Attractive multiplexing technique Ø High aggregate bit rate without high speed electronics or modulation Ø Low dispersion penalty for aggregate bit rate Ø Very useful for upgrades to installed fibres Ø Realisable using commercial components, unlike OTDM Ÿ Loss, crosstalk and non-linear effects are potential problems

Simple DWDM System T 1 T 2 TN l 1 Wavelength Division Multiplexer Fibre Wavelength Division Demultiplexer l 2 l. N l 1 l 2 l 1 + l 2. . . l. N R 1 R 2 RN Multiple channels of information carried over the same fibre, each using an individual wavelength Unlike CWDM channels are much closer together Transmitter T 1 communicates with Receiver R 1 as if connected by a dedicated fibre as does T 2 and R 2 and so on Source: Master 7_4

DWDM ŸDense WDM is WDM utilising closely spaced channels ŸChannel spacing reduced to 1. 6 nm and less ŸCost effective way of increasing capacity without replacing fibre ŸCommercial systems available with capacities of 32 channels and upwards; > 80 Gb/s per fibre ŸAllows new optical network topologies, for example high speed metropolitian rings ŸOptical amplifiers are also a key component www. swedishcr. weebly. com

DWDM Advantages and Disadvantages

DWDM Advantages ŸGreater fiber capacity ŸEasier network expansion Ø No new fibre needed Ø Just add a new wavelength Ø Incremental cost for a new channel is low Ø No need to replace many components such as optical amplifiers

DWDM Components

DWDM System Receivers DWDM Multiplexer Optical fibre Power Amp Transmitters Line Amp Receive Preamp DWDM De. Multiplexer 200 km Ÿ Each wavelength behaves as if it has it own "virtual fibre" Ÿ Optical amplifiers needed to overcome losses in mux/demux and long fibre spans

Receivers DWDM Multiplexer Optical fibre Power Amp Transmitters Line Amp Receive Preamp DWDM De. Multiplexer

DWDM: Typical Components ŸPassive Components: Ø Gain equalisation filter for fibre amplifiers Ø Bragg gratings based demultiplexer Ø Array Waveguide multiplexers/demultiplexers Ø Add/Drop Coupler ŸActive Components/Subsystems: Ø Transceivers and Transponders Ø DFB lasers at ITU specified wavelengths Ø DWDM flat Erbium Fibre amplifiers

DWDM System Receivers DWDM Multiplexer Optical fibre Power Amp Transmitters Line Amp Receive Preamp DWDM De. Multiplexer 200 km ŸEach wavelength behaves as if it has it own "virtual fibre" ŸOptical amplifiers needed to overcome losses in mux/demux and long fibre spans

DWDM System with Add-Drop Add/Drop Mux/Demux DWDM Multiplexer Power Amp Transmitters Optical fibre Receivers Line Amp Receive Preamp DWDM De. Multiplexer 200 km ŸEach wavelength still behaves as if it has it own "virtual fibre" ŸWavelengths can be added and dropped as required at some intermediate location

Typical DWDM Systems Manufacturer & System Number of Channels Channel Spacing Channel Speeds Maximum Bit Rate Tb/s Nortel OPtera 1600 OLS 160 0. 4 nm 2. 5 or 10 Gb/s 1. 6 Tbs/s Lucent 40 2. 5 Alcatel Marconi PLT 40/80/160 0. 4, 0. 8 nm 2. 5 or 10 Gb/s 1. 6 Tb/s

Optical Amplifiers DWDM System Spans R P 160 -200 km P L L P Power/Booster Amp R Receive Preamp L Line Amp R up to 600 -700 km P L 3 R Regen L R 700 + km Animation

Nortel DWDM Nortel S/DMS Transport System ŸAggregate span capacities up to 320 Gbits/sec (160 Gbits/sec per direction) possible ŸRed band = 1547. 5 to 1561 nm, blue band = 1527. 5 to 1542. 5 nm

Nortel DWDM Coupler Ÿ 8 wavelengths used (4 in each direction). 200 Ghz frequency spacing ŸIncorporates a Dispersion Compensation Module (DCM) ŸExpansion ports available to allow denser multiplexing Red band = 1547. 5 to 1561 nm, blue band = 1527. 5 to 1542. 5 nm

Sixteen Channel Multiplexing Ÿ 16 wavelengths used (8 in each direction). Remains 200 Ghz frequency spacing ŸFurther expansion ports available to allow even denser multiplexing Red band = 1547. 5 to 1561 nm, blue band = 1527. 5 to 1542. 5 nm

32 Channel Multiplexing Ÿ 32 wavelengths used (16 in each direction). 100 Ghz ITU frequency spacing ŸPer band dispersion compensation Red band = 1547. 5 to 1561 nm, blue band = 1527. 5 to 1542. 5 nm

DWDM Transceivers and Transponders

DWDM Transceivers Line Amp DWDM Multiplexer DWDM De. Multiplexer Receive Preamp Line Amp Power Amp Transmitters Transceiver Power Amp Receivers DWDM De. Multiplexer DWDM Multiplexer ŸTransmission in both directions needed. ŸIn practice each end has transmitters and receivers ŸCombination of transmitter and receiver for a particular wavelength is a "transceiver"

Transceivers. V. Transponders 1550 nm SDH C band signal l 1550 nm SDH Ÿ In a "classic" system inputs/outputs to/from transceivers are electrical Ÿ In practice inputs/outputs are SDH, so they are optical, wavelength around 1550 nm Ÿ In effect we need wavelength convertors not transceivers Ÿ Such convertors are called transponders.

DWDM Transponders (II) Ÿ Full 3 R transponders: (power, shape and time) Ÿ Regenerate data clock Ÿ Bit rate specific Ÿ More sensitive - longer range Ÿ 2 R transponders also available: (power, shape) Ÿ Bit rate flexible Ÿ Less electronics Ÿ Less sensitive - shorter range Luminet DWDM Transponder

Bidirectional Transmission using WDM Source: Master 7_4

Conventional (Simplex) Transmission ŸMost common approach is "one fibre / one direction" ŸThis is called "simplex" transmission Ÿ Linking two locations will involve two fibres and two transceivers Transmitter Receiver Local Transceiver Transmitter Fibres x 2 Distant Transceiver Source: Master 7_4

Bi-directional using WDM ŸSignificant savings possible with so called bi-directional transmission using WDM ŸThis is called "full-duplex" transmission ŸIndividual wavelengths used for each direction ŸLinking two locations will involve only one fibres, two WDM mux/demuxs and two transceivers Transmitter Receiver l. A WDM Mux/Demux A l. B Local Transceiver WDM Mux/Demux B l. A l. B Fibre l. B l. A Transmitter Receiver Distant Transceiver

Bi-directional DWDM ŸDifferent wavelength bands are used for transmission in each direction ŸTypcially the bands are called: The "Red Band", upper half of the C-band to 1560 nm ØThe "Blue Band", lower half of the C-band from 1528 nm Ø Transmitter l 1 R l 1 B Transmitter l 2 R l 2 B Transmitter ln. B Transmitter Red Band Transmitter Receiver ln. R l 1 B l 2 B ln. B DWDM Mux/Demux Blue Band Fibre l 1 R l 2 R ln. R Receiver

The need for a Guard Band ŸTo avoid interference red and blue bands must be separated ŸThis separation is called a "guard band" ŸGuard band is typically about 5 nm ŸGuard band wastes spectral space, disadvantage of bi-directional DWDM Blue channel band 1528 nm G u a r d B a n d Red channel band 1560 nm

DWDM Issues Crosstalk between Channels

Non-linear Effects and Crosstalk ŸWith DWDM the aggregate optical power on a single fibre is high because: Ø Simultaneous transmission of multiple optical channels Ø Optical amplification is used ŸWhen the optical power level reaches a point where the fibre is non-linear spurious extra components are generated, causing interference, called "crosstalk" ŸCommon non-linear effects: Ø Four wave mixing (FWM) Ø Stimulated Raman Scattering (SRS) ŸNon-linear effects are all dependent on optical power levels, channels spacing etc.

Coarse Wavelength Division Multiplexing

Coarse Wavelength Division Multiplexing Ÿ WDM with wider channel spacing (typical 20 nm) Ÿ More cost effective than DWDM Ÿ Driven by: Ø Cost-conscious telecommunications environment Ø Need to better utilize existing infrastructure Ÿ Main deployment is foreseen on: Ø Single mode fibres meeting ITU Rec. G. 652. Ø Metro networks

Why CWDM? ŸCWDM is a cheaper and simpler alternative to DWDM, estimates point to savings up to 30% ŸWhy is CWDM more cost effective? Ø Ø Less expensive uncooled lasers may be used - wide channel spacing. Lasers used require less precise wavelength control, Passive components, such as multiplexers, are lower-cost CWDM components use less space on PCBs - lower cost DFB laser, typical temperature drift 0. 08 nm per deg. C For a 70 degree temperature range drift is 5. 6 nm

CWDM Migration to DWDM Ÿ A clear migration route from CWDM to DWDM is essential Ÿ Migration will occur with serious upturn in demand for bandwidth along with a reduction in DWDM costs Ÿ Approach involves replacing CWDM single channel space with DWDM "band" Ÿ May render DWDM band specs such as S, C and L redundant?