UNIT 7: WDM Concepts and Components

The technology of combining a number of different independent information carrying wavelengths

onto the same fiber is known as wavelength division multiplexing or WDM.

Overview of WDM

With the advent of high quality light sources with extremely narrow spectral emission widths many

independent wavelength channels spaced less than a nanometer apart could be placed on the same

fiber. For eg. The modulated output of a DFB laser has spectral emission width of less than 310 nm

With such light sources the use of WDM allows a dramatic increase in the capacity of an optical fiber

compared to the original simple point to point link that carried only single wavelength.

Another advantage of WDM is that various optical channels can support different transmission

formats.

Thus, by using separate wavelengths differently formatted signals at any data rate can be sent

simultaneously and independently over the same fiber without the need for common signal structure.

Operational principles of WDM

The implementation of WDM networks requires that a variety of passive and active devices to

combine, distribute, isolate and amplify optical power at different wavelengths.

Passive devices require no external control for their operation. These components are mainly used to

split and combine or tap off optical signals.

Active devices are wavelength dependent and can be controlled electronically or optically, thereby

providing a large degree of flexibility. Active WDM components include tunable optical filters, tunable

sources and optical amplifiers.

Fig. Implementation of typical WDM Network containing various types of optical amplifiers

Fig. above shows the implementation of passive and active components in a typical WDM link.

At the transmitting end there are several independently modulated light sources, each emitting signals

at a unique wavelength.

Here a multiplexer is needed to combine these optical outputs into a continuous spectrum of signals

and couple them into a single fiber.

At the receiving end a demultiplexer is required to separate the optical signals into appropriate

detection channels for signal processing.

Fig. The attenuation vs. wavelength graph

Fig. shows there are many independent operating regions across the spectrum ranging from O-band

through the L-band in which narrow linewidth optical sources can be used simultaneously.

To find the optical bandwidth corresponding to a spectral width in these regions we use

2

cv

Where the frequency deviation v corresponds to the wavelength deviation around

The operational frequency band which is allocated to a particular light source normally ranges from 25

to 100 GHz.

Depending upon the frequency bands chosen for the optical transmission link, many operational

regions are available in the various spectral bands.

The engineering challenge for using such a large number of light sources is to ensure that each source

is spaced sufficiently far from its neighbours so as not to create interference.

WDM Standards

Since WDM is frequency division multiplexing at optical carrier frequencies, the WDM standards

developed by the International Telecommunication Union (ITU) specify channel spacing in terms of

frequency.

Recommendation G.692 was the first ITU-T specification for WDM. This document specifies selecting

the channels from a grid of frequencies referenced to 193.100THz and spacing them 100 GHz apart.

In 2002, the ITU-T released Recommendation G.694.1 for dense WDM (DWDM). This document

specifies WDM operation in the S, C and L bands for metro-area network (MAN) and wide area

network (WAN) services. It calls for narrow frequency spacing of 100 to 12.5 GHz which requires the

use of stable, high quality, frequency locked laser diode light sources.

To designate which C-band is under consideration ITU-T uses channel numbering convention. For this,

frequency 19N.M THz is designated as ITU channel number NM. Eg. The frequency 194.3 THz is ITU

channel 43.

The concept of coarse WDM (CWDM) emerged from the combination of the production of full

spectrum G.652C and G.652D fibers. In 2002, ITU-T released Recommendation G.694.2 which defines

the spectral grid for CWDM.

From Fig. above the CWDM grid is made up of 18 wavelengths defined within the range 1270nm to

1610nm spaced by 20nm with wavelength drift tolerances of 2nm.

The ITU-T Recommendation G.695 released in 2004 gives optical interface specifications for multi

channel CWDM over distances of 40 and 80 km. Both uni-directional and bidirectional are included in

the recommendation and covers all or part of 1270nm to 1610nm range.

Mach Zehnder Interferometer Multiplexers

Wavelength division multiplexers can also be made using Mach Zehnder Interferometry techniques

using active or passive devices.

Fig. Mach Zehnder Interferomater

Fig illustrates the constituents of MZI. This 2x2 MZI consists of three stages: an initial 3dB directional

coupler which splits the input signals, a central section where one of the waveguides is longer by L to

give a wavelength dependent phase shift between two arms, and another 3dB coupler which

recombines the signals at the output.

The propagation matrix couplerM for couplers of length d is

kdkdj

kdjkdM coupler

cossin

sincos

Where k is the coupling coefficient.

Since we are considering the 3dB couplers which are dividing the powers equally, then 2kd=π/2, so

that

1

1

2

1

j

jM coupler

For a given phase difference the propagation matrix M for a phase shifter is

)2/exp(0

0)2/exp(

Ljk

LjkM

For optical output fields 1,outE and 2,outE from the two central arms can be related to the input fields

1,inE and 2,inE by

2,

1,

2,

1,

in

in

out

out

E

EM

E

E

Where M = couplerM x M x couplerM =

)2/sin(0)2/cos(

)2/cos()2/sin(

LjkLjk

LjkLjkj

Since we are building a multiplexer we want to have the inputs to the MZI at different wavelengths i.e

1,inE at 1 and 2,inE at

2 . Then the output fields are the sum of individual contributions from the

two input fields

1,outE = )2/cos()2/sin( 222,111, LkELkEj inin

2,outE = )2/sin()2/cos( 222,111, LkELkEj inin

The output powers are then found from the light intensity, which is the square of the field strengths.

Thus,

1,outP = 2,2

2

1,1

2 )2/(cos)2/(sin inin PLkPLk

1,outP = 2,2

2

1,1

2 )2/(sin)2/(cos inin PLkPLk

If we want that all the power from both the inputs to leave the same output port we need to have

2/1 Lk = π and 2/2 Lk = π/2 or

LnLkk eff

21

21

112

The length difference between the interferometer arms should be

vn

cnL

eff

eff

2

112

1

21 where v is the frequency separation of the two

wavelengths.

Using basic 2x2 MZIs, any N x N multiplexer can be constructed.

Fig. Four channel wavelength multiplexer

Fig illustrates a 4x4 multiplexer. Here the inputs to MZI1 are v+2 v and the inputs to MZI2 are v+ v

and v+3 v .

Since the signals in both the interferometers of the first stage are separated by 2 v , the path

difference satisfy the condition

)2(221

vn

cLL

eff

In the next stage, the inputs are separated by v . Consequently, we need to have

)(22 31

vn

cLL

eff

When these conditions are satisfied all four input powers will emerge from port C.

From this design example we can deduce that for an N to 1 MZI multiplexer where N=2n, the number

of multiplexer stages is n and the number of MZIs in stage j is 2n-j. The path difference in an

interferometer element of stage j is thus

)(2 vn

cL

eff

jnstagej

The N-to-1 multiplexer can also be used as 1-to-N demultiplexer by reversing the light propagation

direction.

Dielectric thin film filters

A dielectric thin film filter (TFF) is used as an optical bandpass filter. It allows a very narrow wavelength

band to pass through it and reflects all others.

The basis of these devices is Fabry-Perot filter structure, which is a cavity formed by two parallel highly

reflective mirror surface as shown in Fig. below. The other names of this structure are Fabry-Perot

Interferometer, etalon and thin film resonant cavity filter.

Consider a light signal that is incident on the left surface of the etalon. After light passes through the

cavity and hits the surface on the right, some of the light leaves the cavity and some is reflected.

The amount of light that is reflected depends on the reflectivity R of the surface.

If the roundtrip distance between the two mirrors is an integral multiple of a wavelength then all

light at those wavelength which pass through the right facet add in phase (constructive interference).

These wavelength are called the resonator wavelengths of the cavity. The etalon rejects all other

wavelengths.

The transmission T (power transfer fuction) of an ideal etalon in which there is no absorption by the

mirrors is an Airy function given by

1

2 2sin

1

41

R

RT

Where R is the reflectivity of the mirrors and

cos2

2nD is the roundtrip phase change of the light beam

N is the refractive index of the dielectric layer that forms the mirror,

D is the distance between the mirrors

is the angle to the normal of the incoming light beam.

Fig gives the generalized plot for an airy function over the range 33

Fig. The behavior of resonant wavelength in Fabry-Perot cavity for three values of mirror reflectivity based on Airy function

Fig shows that the power transfer function is periodic in f.

The peaks of the spacing called the passbands occur at those wavelengths that satisfy the condition N

=2nD, where N is an integer.

The distance between the adjacent peaks is called the free spectral range or FSR given by

nDFSR

2

2

The finesse F of the filter gives an approximation of the number of wavelength that a filter can

accommodate and given by the ratio of full spectrum range to the full width half maximum i.e.

R

RF

1

To create a wavelength multiplexing device for combining or separating N wavelength channels, one

needs to cascade N-1 thin film filters.

Fig. Multiplexing four wavelengths using thin film filters.

Fig. illustrates a multiplexing function for the four wavelengths 1, 2, 3, 4. Here the filters labeled

TFF2,TFF3and TFF4 pass wavelengths 2, 3 and 4 respectively and reflect all others.

First filter TFF2 reflects 1 and allows 2 to pass through. These two signals then are reflected from

TFF3 where they are joined by 3. After similar process at TFF4 the four wavelengths can be coupled

into a fiber by means of lens mechanism.

To separate the four wavelengths from one fiber into four independent channels the direction of the

arrows in Fig. are reversed.

Optical isolators

Optical isolators are devices that allow light to pass through them in only one direction.

This prevents backward travelling light from entering a laser diode which causes instabilities in the

optical output.

An optical isolator should be independent of the state of polarization (SOP) since light in an optical link

normally is not polarized.

Fig. Polarization independent isolator

Fig. shows a design for a polarization independent isolatorthat is made of three optical components:

an input birefringent wedge (with its ordinary polarization direction vertical and its extraordinary

polarization direction horizontal), a Faraday rotator, and an output birefringent wedge (with its

ordinary polarization direction at 45°, and its extraordinary polarization direction at −45°).

Light traveling in the forward direction is split by the input birefringent wedge into its vertical (0°) and

horizontal (90°) components, called the ordinary ray (o-ray) and the extraordinary ray (e-ray)

respectively. The Faraday rotator rotates both the o-ray and e-ray by 45°. This means the o-ray is now

at 45°, and the e-ray is at −45°. The output birefringent wedge then recombines the two components.

Light traveling in the backward direction is separated into the o-ray at 45, and the e-ray at −45° by the

birefringent wedge. The Faraday Rotator again rotates both the rays by 45°. Now the o-ray is at 90°,

and the e-ray is at 0°. Instead of being focused by the second birefringent wedge, the rays diverge.

Optical Circulators

Fig. Optical Circulator

An optical isolator is a non-reciprocal multi-port passive device that directs light sequentially from port

to port in only one direction.

This device is used in optical amplifiers, add/drop multiplexers etc.

Fig. shows a three port circulator. Here an input on port 1 is sent out on port 2, an input on port 2 is

sent out on port 3, and an input on port 3 is sent out on port 1.

These devices have low insertion loss, high isolation over a wide wavelength range, minimal

polarization dependent loss and low polarization mode dispersion.

Tunable Light Sources:

Modern WDM systems require single mode, narrow linewidth lasers whose wavelength remains fixed

over time.

DFB lasers satisfy this requirement but their wavelength stability comes at the expense of tenability.

The large number of DFB lasers used inside a WDM transmitter makes the design and maintenance of

such lightwave system expensive and impractical.

The availability of lasers whose wavelength can be tuned over a wide range would solve this problem

The basic tuning operations include the following

o Wavelength tuning of a laser by means of temperature or current variations

o Use of frequency tunable laser such as multi section laser or external cavity laser

o Frequency locking to a particular lasing mode in a Fabry-Perot laser

o Spectral slicing by means of a fixed or tunable narrow band optical filter.

Frequency tunable lasers are based on DFB or DBR structures which have a waveguide type grating

filter in the lasing cavity. Frequency tuning is achieved either by changing the temperature of the

device or by altering the injection current into the active section or passive section which results a

change in the effective refractive index which causes a shift in the peak output wavelength.

Fig. Tuning range of an injection tunable three section DBR laser

Fig. illustrates the tuning range of an injection tunable three section DBR laser.

The tuning range tune can be estimated by

eff

efftune

n

n

Where effn is the change in the effective refractive index.

Practically the maximum index change is around 1 percent, resulting in a tuning range of 10-15nm.

Fig. Relationship between tuning range, channel spacing, and source spectral width

Fig. depicts the relationship between tuning range, channel spacing and source spectral width. To

avoid crosstalk between adjacent channels, a channel spacing of 10times the source spectral width is

often specified i.e. signalchannel 10

Thus, the maximum number of channels N that can be placed in the tuning range tune is

channel

tuneN

External cavity laser designs include the use of Littman and Littrow cavities.

o The Littman cavity schemes uses a grating and a MEMS based tuning mirror to deliver a

high level of side mode suppression with narrow linewidth.

o The Littrow cavity method uses a grating to offer an increase in optical output power but

with a slight reduction in side mode suppression.

o In both devices coarse tuning is achieved by manual adjustment of a high precision

adjuster and further fine tuning is achieved by means of a piezoelectric actuator.

Variable multiple section tunable laser include a Bragg reflector, a gain portion, a passive phase

correction section and a coarse tuning section. Modulating a Bragg reflector provides a series, or comb

of wavelength peaks.By using an external control current, the coarse tuner then selects one of these

peaks over a 32nm range C-band.

In spectral slicing an integrated combination of an optical source, a waveguide grating multiplexer and

optical amplifier are used. In this method, broad spectral output is sliced by waveguide grating to

produce a comb of precisely spaced optical frequencies. These spectral slices are then fed into a

sequence of individually addressable wavelength channels that can be externally modulated.

Active Optical Components:

Active optical components require some type of external energy either to perform their functions or to

be used over a wide operating range, therby offering greater application flexibility.

These device include variable optical attenuators, tunable optical filters, dynamic gain equalizers,

optical add/drop multiplexers, polarization controllers and dispersion compensators.

Many of the active optical components are based on using micro-electical-mechanical systems of

MEMS technology.

MEMS Technology:

MEMS is the acronym for micro electro mechanical systems. These are miniature devices that can

combine mechanical, electrical and optical components to provide sensing and actuation functions by

means of micro gears or movable levers, shutters or mirrors.

These devices are widely used in automobile air-bag deployment systems, in ink-jet printer heads, for

monitoring mechanical shock etc.

Fig. A simple example of MEMS actuation method

Fig. shows a simple example of MEMS actuation method. At the top of the device there is thin

suspended polysilicon beam that has typical length, width and thickness of 80µm, 10µm and 0.5µm.

At the bottom there is silicon ground plane which is covered by an insulator material. There is a gap of

nominally 0.6µm between the beam and the insulator.

When a voltage is applied between the silicon ground plane and the polysilicon beam, the electric

force pulls the beam down so that it makes contact with the lower structure.

Initially MEMS devices were based on silicon technology which is very stiff material that require higher

voltages to achieve a given mechanical deflection.

To reduce these required forces, current MEMS devices are being made with soft rubber like material

called elastomer. This is referred to as compliant MEMS or CMEMS.

Variable optical attenuators:

A variable optical attenuator (VOA) offers dynamic signal level control. This device attenuates optical

power by various means to control signal levels precisely without disturbing other properties of a light

signal.

They are polarization independent and have a dynamic range of 15 to 30dB.

The control methods include mechanical, thermo-optic, MEMS or electro optic techniques.

The mechanical control methods are reliable but have a low dynamic range and slow response time.

Thermo-optic methods have high dynamic range, but are slow and require the use of thermoelectric

cooler which may not be desirable.

The two most popular control methods are MEMS based and electro-optic based techniques.

For MEMS techniques an electrostatic actuation method is used where a voltage change across a pair

of electrodes provides an electrostatic actuation force. This requires lower power levels than other

methods and is the fastest.

Tunable optical filters:

The two key technologies to make a tunable filter are MEMS based and Bragg grating based devices.

MEMS based tunable filters have the advantage of wide tuning range and design flexibility.

o The MEMS based device consists of two sets of epitaxially grown semiconductor layers

that form a single Fabry-Perot cavity.

o The device operation is based on allowing one of the two mirrors to be moved precisely by

an actuator. This enables a change in the distance between the two cavity mirrors, thereby

resulting in the selection of different wavelengths to be filtered.

Fiber Bragg gratings are wavelength selective reflective filters with steep spectral profiles as shown

in Fig.

o These filters involve a stretching and relaxation process of the spacing in the fiber grating.

o Since glass is a slightly stretchable medium, as an optical fiber is stretched with the grating

inside of it, the spacing of the index perturbation and refractive index will change.

o This process induces a change in the Bragg wavelength thereby changing the center

wavelength of the filter.

o Such optical fibers can be made for the S,C and L bands and for operation in the 1310nm

region.

The stretching can be done by thermo-mechanical, piezoelectric and stepper motor means as

shown in Fig.

Fig. three methods for adjusting the wavelength of a tunable Bragg grating

The thermo-mechanical method use a bimetal differential expansion element which changes its shape

as its temperature varies.This method is expensive but it is slow, takes time to stabilize and has limited

tuning range.

The piezoelectric techniques use a material that changes its length when a voltage is applied. Although

this method provides precise wavelength resolution, it is more expensive, complex to implement and

has limited tuning range.

The stepper motor method changes the length of the fiber grating by pulling or relaxing one end of the

structure. It has a moderate cost, is reliable, and has reasonable tuning speed.

Dynamic Gain Equalizers:

Dynamic gain equalizer (DGE) is used to reduce the attenuation of the individual wavelengths

within a spectral band.

These devices are also called dynamic channel equalizers or dynamic spectral equalizers.

They are used for flattening the nonlinear gain profile of an optical fiber, compensation for

variation in transmission losses on individual channels across a given spectral band within a

link, and attenuating, adding or dropping selective wavelengths.

Fig. DGE Equalization

Fig. shows how a DGE equalizes the gain profile of an erbium doped fiber amplifier.

The operation of these devices can be controlled electronically and configured by software residing in

a microprocessor.

The control is based on feedback information from a performance monitoring card that provides the

parameter values needed to adjust and adapt to required link specifications.

This allows a high degree of agility in responding to optical power fluctuations that may result from

changing network conditions.

Optical Add/Drop Multiplexers

The function of optical add/drop multiplexers (OADM) is to insert (add) or extract (drop) one or more

selected wavelengths at a designated point in an optical network.

Fig. Add/Drop Multiplexer

Fig. shows a simple OADM configuration that has four input and four output ports.

The add/drop functions are controlled by MEMS based mirrors that are activated selectively to

connect the desired paths.

When no mirrors are activated, each incoming channel passes through the switch to the output port.

Incoming signal can be dropped from the traffic flow by activating appropriate mirror pair. Eg. To drop

the signal carried on wavelength 3 entering port 3 to port 2D the mirrors are activated as shown in

Fig.

When an optical signal is dropped another path is established simultaneously allowing a new signal to

be added from port 2A to the traffic flow.

Polarization Controllers:

These devices dynamically adjust any incoming state of polarization to an arbitrary output state of

polarization. This is done through electronically control voltages that are applied independently to

adjustable polarization retardation plates.

Application of polarization controllers include polarization mode dispersion (PMD) compensation,

polarization scrambling, and polarization multiplexing.

Chromatic Dispersion Compensators:

A critical factor in optical links operating above 2.5Gbps is compensating for chromatic dispersion that

cause pulse broadening which leads to increased bit error rates.

To reduce this, first dispersion management method using dispersion compensating fiber is carried out

over a wide spectral range.

Then fine tuning is done by means of tunable dispersion compensator that works over a narrow

spectral band to correct for any residual or variable dispersion.

The device for achieving this fine tuning is called dispersion compensating module (DCM).

This module can be tuned manually, remotely or dynamically.

o Manual tuning is done by network technician prior to or after installation of the module in

telecommunication racks.

o By using network management software it can be tuned remotely from a central

management console by a network operator.

o Dynamic tuning is done by the module itself without any human intervention.

Fig. Dynamic Chromatic dispersion may be accomplished with chirped Bragg grating

Fig. shows one method of achieving dynamic chromatic dispersion through the use of chirped fiber

Bragg grating (FBG).

Here the grating spacing varies linearly over the length of the grating which creates chirped grating.

His creates a range of wavelength that satisfies the Bragg condition for reflection.

As the spacing decreases along the fiber Bragg wavelength decreases with distance along the grating

length.

The shorter wavelength component of a pulse travel farther into the fiber before being reflected.

Thereby they experience more delay in going through the grating than the longer wavelength

components.

This results in dispersion compensation since it compresses the pulse.

2 x 2 Fiber Coupler:

A device with two inputs and two outputs is called as 2 x 2 coupler. Fig. shows 2 x2 fiber coupler.

Fused biconically tapered technique is used to fabricate multiport couplers.

The input and output port has long tapered section of length ‘L’.

The tapered section gradually reduced and fused together to form coupling region of length ‘W’.

o Input optical power : P0.

o Throughtputpower : P1.

o Coupled power : P2.

o Cross talk : P3.

o Power due to refelction : P4.

The gradual tapered section determines the reflection of optical power to the input port, hence the

device is called as directional coupler.

The optical power coupled from on fiber to other is dependent on-

Axial length of coupling region where the fields from fiber interact.

Radius of fiber in coupling region.

The difference in radii of two fibers in coupling region.

Performance Parameters of Optical Coupler

1. Splitting ratio / coupling ratio

Splitting ratio is defined as –

2. Excess loss:

Excess loss is defined as ratio of input power to the total output power. Excess is expressed in

decibels.

3. Insertion loss:

Insertion loss refers to the loss for a particular port to port path. For path from input port I to

output port j.

4. Cross talk :

Cross talk is a measure of degree of isolation between input port and power scattered or

reflected back to other input port.

8 x 8 Star Coupler:

An 8 x 8 star coupler can be formed by interconnecting 2 x 2 couplers. It requires twelve 2 x 2

couplers.

Excess loss in dB is given as

where FT is fraction of power traversing each coupler element.

Splitting loss = 10 log N

Total loss = Splitting loss + Excess loss

= 10 (1 – 3.32 log FT)log N