WDM/ DWDM Systems &

Components

J.K.Chhabra, Ex Scientist

CSIO Chandigarh

Chhabra_jk@yahoo.com

POSSIBLE SOLUTIONS

Install more optical fiber

Increase TDM speed (shorter pulses)

Wavelength Division Multiplexing (WDM)

Optical Code Division Multiplexing(OCDMA)

4

OVERVIEW

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Single Pair of Fibers

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

Digital Transceiver

WDM MUX WDM MUX

Single Pair of Fibers

Traditional Digital Fiber Optic Transport

Digital Fiber Optic Transport using WDM

5

OVERVIEW

WDM COMPONENTS

Optical Multiplexer

Optical De-multiplexer

Optical Add/Drop Multiplexer

(OADM)

l1

l2

l3

l1

l2

l3

Transponder

850/1310 15xx

l1

l2

l3

l1...n

l1...n

MULTIPLEXING

Time-division multiplexing

• Interleaves signals at slower rates

Frequency-division multiplexing

• Combines signals at different frequencies electronically to make composite signal

Wavelength-division multiplexing

• Sends signals at different wavelengths

• Like frequency multiplexing of radio spectrum

6

Problem:

Demand for massive increases in capacity

Immediate Solution:

Dense Wavelength Division Multiplexing

Longer term Solution:

Optical Fibre Networks

PROBLEMS AND SOLUTIONS

Multiple channels of information carried over the same fibre, each using an

individual wavelength

Attractive multiplexing technique

High aggregate bit rate without high speed electronics or modulation

Low dispersion penalty for aggregate bit rate

Very useful for upgrades to installed fibres

Realisable using commercial components, unlike OTDM

Loss, crosstalk and non-linear effects are potential problems

Wavelength

Division

Multiplexer

Wavelength

Division

Demultiplexerl

1A

l

2l

3

B

C

l

1X

l

2l

3

Y

Zl1 + l2 + l3

Fibre

WDM Overview

DWDM FUNCTIONAL SCHEMATIC

Transmitters

DWDM

Multiplexer

Power

AmpLine

Amp

Receive

Preamp

200 km

DWDM

DeMultiplexe

r

Each wavelength still behaves as if it has it own "virtual fibre"

Wavelengths can be added and dropped as required at

some intermediate location

ReceiversAdd/Drop

Mux/Demux

Optical

fibre

DWDM SYSTEM WITH ADD-DROP

Trend is toward smaller channel spacings, to incease the channel count

ITU channel spacings are 0.4 nm, 0.8 nm and 1.6 nm (50, 100 and 200 GHz)

Proposed spacings of 0.2 nm (25 GHz) and even 0.1 nm (12.5 GHz)

Requires laser sources with excellent long term wavelength stability, better than 10

pm

One target is to allow more channels in the C-band without other upgrades

Wavelength in nm

1550 15541551 1552 15531553

0.8 nm

Channel Spacing

Speed of Light assumed to be 2.99792458 x 108 m/s

1552.52

1553.33

1554.13

1554.94

1555.75

1556.55

1557.36

1558.17

1546.52

1547.32

1548.11

1548.91

1549.72

1550.52

1551.32

1552.12

1540.56

1541.35

1542.14

1542.94

1543.73

1544.53

1545.32

1546.12

1534.64

1535.43

1536.22

1537.00

1537.79

1538.58

1539.37

1540.16

1528.77

1529.55

1530.33

1531.12

1531.90

1532.68

1533.47

1534.25

1558.98

1559.79

1560.61

All Wavelengths in nm

ITU DWDM Channel Plan

0.8 nm Spacing (100 GHz)

Speed of Light assumed to be 2.99792458 x 108

m/s

All Wavelengths in nm

1552.52

1552.93

1553.33

1553.73

1554.13

1554.54

1554.94

1555.34

1555.75

1556.15

1556.55

1556.96

1557.36

1557.77

1558.17

1546.52

1546.92

1547.32

1547.72

1548.11

1548.51

1548.91

1549.32

1549.72

1550.12

1550.52

1550.92

1551.32

1551.72

1552.12

1540.56

1540.95

1541.35

1541.75

1542.14

1542.54

1542.94

1543.33

1543.73

1544.13

1544.53

1544.92

1545.32

1545.72

1546.12

1534.64

1535.04

1535.43

1535.82

1536.22

1536.61

1537.00

1537.40

1537.79

1538.19

1538.58

1538.98

1539.37

1539.77

1540.16

1528.77

1529.16

1529.55

1529.94

1530.33

1530.72

1531.12

1531.51

1531.90

1532.29

1532.68

1533.07

1533.47

1533.86

1534.25

1558.58

1558.98

1559.39

1559.79

1560.20

1560.61

So called

ITU C-Band

81 channels defined

Another band called

the L-band exists

above 1565 nm

ITU DWDM Channel Plan

0.4 nm Spacing (50 GHz)

WDM Components

Components

Dispersion Shifted fiber

Special Stable Sources

Wave Guide or Fiber Couples Splitters/ Combiners

Optical Amplifiers

Add Drop Switches

Demultiplexers

WDM OPTICS

Multiplexing: combine optical channels

Demultiplexing: separating optical channels

• Detectors can't tell wavelengths apart

Separate optical channels

• When closely spaced

• Maintain low crosstalk

Separated channels may be switched

Wavelengths may be converted17

MULTIPLEXING

Coupler Configuration

P1P2

P3

P1P2

P3

l1

l2

l3

l1

l2

ln

l1 ……ln

Integrated Optics

20

WDM DEMUX TECHNOLOGIES

Interference filters

• Interference filters for DWDM

• Diffraction gratings

Fiber Bragg gratings

Mach-Zehnder interferometers

Planar waveguide arrays

21

INTERFERENCE FILTERS FOR WDM

Multiple filters needed for WDM

Pick-off one wavelength at a time

Pick bands, then individual wavelengths

High resolution possible

Many components

Scalable in increments

22

ONE WAVELENGTH AT A TIME

23

l1, l2, l3, l4, l5, l6 l1

l2, l3, l4, l5, l6

l2

l3, l4, l5, l6 l3

l4, l5, l6

l4

l5, l6 l 5

l6

Input

Interference Filter Based WDM

FIBER BRAGG GRATINGS

Function analogous to interference filters

Zones of high refractive index scatter light

Selectively reflect one wavelength

• Transmit other wavelengths

Like filters, can be grouped to drop one wavelength at a time

Require an optical circulator

Fiber components

25

Calculate Grating Period:

Bragg Wavelength: 1550 nm

Index of Refraction: 1.4 [

Period of Bragg Grating: 553.57142 857nm

When a Bragg grating exists in an optical fiber, it will reflect a

specific wavelength dependent on the period of the Bragg grating

and the index of refraction of the optical fiber. This calculator

finds the period of Bragg grating needed for a predetermined

wavelength and index of refraction.

wavelength

For a given grating period a particular wavelength

(frequency) of light is reflected. In this case yellow light will

be reflected

If the grating spacing is changed (e.g. reduced due to

compression of the fibre or a drop in temperature} the

wavelength of the reflected light changes. In this case it

becomes higher and reflects blue light

In practice the colour shifts will be much finer than those illustrated

Optical fibre

Grating pattern etched

into body of fibre

Detector

Fiber Bragg Gratings (FBG)

ADD/DROP WITH FIBER GRATING

31

Fiber Bragg grating

Reflects l8

Multichannel input

Reflected channel only

Optical

circulator

l1-l7

l1-l8

l1-l8

l8

l8l8

l1-l8

Transmits

Circulator directs reflected

light to third port

With DWDM the aggregate optical power on a single fibre is

high

With the use of amplifiers the optical power level can rise to

point where non-linear effects occur:

Four wave mixing (FWM): spurious components are created

interfering with wanted signals

Stimulated Raman Scattering (SRS)

Non-linear effects are dependent on optical power levels,

channels spacing etc:

DWDM PROBLEMS

INITIAL WDM SIGNAL

33

All channels roughly equal power

FINAL WDM SIGNAL

34

After a series of amplifiers

Signal to noise reduced

Some channels stronger than others

GAIN TILT AND GAIN

SLOPE

OPTICAL AMPLIFIERS

1/8/2015 36

J.K.Chhabra, Scientist, CSIO

Chandigarh

Optical Amplifier

Pout = GPinPin

• EDFA amplifiers

• Separate amplifiers for C-band and L-band

• Source of optical noise

• Simple

G

EDFA

Erbium Doped Fiber Amplifier

“Simple” device consisting of four parts:

• Erbium-doped fiber

• An optical pump (to invert the population).

• A coupler

• An isolator to cut off backpropagating noise

Isolator Coupler IsolatorCoupler

Erbium-Doped

Fiber (10–50m)

Pump

Laser

Pump

Laser

Bidirectional Transmission using

WDM

Source: Master 7_4

TRANSMITTE

R

A

Fibre, connectors and splices

RECEIVER

B

TRANSMITTE

R

B

RECEIVER

A

WDM

Mux/Demux

WDM

Mux/Demux

1330 nm

1550 nm

Transmitter A communicates with Receiver A using a signal on 1330 nm

Transmitter B communicates with Receiver B using a signal on 1550 nm

WDM Mux/Demux filters out the wanted wavelength so that for example

Receiver A only receives a 1330 nm signal

Bi-directional Transmission using

WDM

To avoid interference red and blue bands must be separated

This separation is called a "guard band"

Guard band is typically about 5 nm

Guard band wastes spectral space, disadvantage of bi-directional

DWDM

Blue

channel

band

Red

channel

band

1528 nm 1560 nm

G

u

a

r

d

B

a

n

d

The need for a Guard Band

WDM SUMMARY

Several viable technologies

Can be used in conjunction

• Interleavers for fine resolution

• Filters for coarser resolution

Each has attractions for some applications

Costs hard to pin down

A real technology horse race

45

CONCLUSIONS

• From modest seeds great trees grow.

• Complementary innovations lead to unexpected consequences.

• Understanding of physical processes continues to be key. The industrial world is not all software.

• The impact of photonics far exceed expectations of 40 years ago.

• We have not seen the end!

Thank you