FOC_Unit_1

© Dr. Sangeetha R.G

Fiber Optic Communication and Networks (EEE536)

Dr. Sangeetha R.G

Associate Professor School of Electronics Engineering (SENSE)

1


Prerequisite Knowledge on Optical Fiber Communication,

Optical Components and

Devices.

Objectives Provides a deep insight on enabling

technology at a lever necessary to

understand the devices on which light wave

networks are built. It emphasizes on

methodology for network analysis, design,

control and management focusing on four

classes of optical networks.

Expected Outcome The students will have a strong understanding

of the enabling technology

and the devices that builds a light wave

network. They will be able to analyze and

design a light wave network of various classes.

2


Unit I Network Elements

Optical and Photonic Device Technology: Attenuation and dispersion,

Chirp, Dispersion Management, Couplers, Isolators, Circulators,

Multiplexers and Filters, EDFA, Raman Amplifier, SOA, SRA, Active

and Passive Optical Switches, Optical Cross Connects, Wavelength

Selective Cross Connects, Wavelength Converters, Optical Time

Domain Reflectometry (OTDR),Optical Spectrum Analysers

(OSA),WDM and Filters: dielectric, AWG and Fiber Bragg Grating (FBG)

devices, Nonlinear optical fibers.

3


Unit II Optical Modulators

Phenomenological theory of nonlinearities. Optics of anisotropic media.

Harmonic generation, mixing and parametric effects. Two-photon

absorption, saturated absorption and nonlinear refraction. Rayleigh, Brillouin

and Raman scattering. Self-focusing and self-phase-modulation. Self-

induced transparency. Solitons, Optical switching, Electro-Optic Effect and

Acousto-Optic effects. EO and AO modulators.

4


Unit III Detection and Receiver Design

Receiver Sensitivity – Bit-Error Rate, Eye Pattern,

Minimum received power, Quantum limit of photo

detection; Receiver Design – Front End, Linear channel,

Decision circuit, Integrated Receivers; Noise in

detection Circuit – Shot Noise, Thermal noise; Concept

of Carrier to Noise Analysis.

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Unit IV Network Architectures

and Topologies

The End To End Transmission Path, Loss And Dispersion Budgets

in Network Designing, Optical Signal Flow And Constraints,

Design of STAR, BUS, MESH and RING Topologies, Static

Multipoint Networks: The Broadcast Star, Multiplexing and

Multiple Access Schemes: TWDM/MA, Sub carriers, CDMA,

Capacity Allocation for Dedicated Connections, Demand

Assigned Connections.

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Unit V

Optical Networks

Optical Networks Architecture, SONET/SDH Optical Network,

WDM Optical Networks, Wavelength Routed Optical Network,

Routing Algorithms, Network Monitoring and Management, Fault

and Security Management, Routing Protocols, Intelligent Optical

Network (ION), FDDI, FTTH, Business Drivers for Next Generation-

Optical Networks, Coherent Optical Communication Systems and

Design Requirements, Dispersion Compensating Network

Designs, Optical Heterodyne Systems.

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BOOKS Text Books 1. Gerd Keiser, “Optical Fiber Communications” McGraw-Hill, 3rd Edition, 2000. 2. R. Ramaswami & K.N. Sivarajan, Morgan Kaufmann,” Optical Networks A practical perspective”, 2nd Edition, Pearson Education, 2000.

Reference Books

3.Govind P. Agrawal, “Fiber-Optic Communication Systems” , 3rd Ed., John

Wiley & Sons, 2002

4. John M. Senior “Optical Fiber Communications principles and practice“ 3rd

edition, PHI,2009.

5. Thomas E. Stern and Krishna Bala, “Multiwavelength Optical Networks A

Layered Approach”, Addison Wesley, 1996.

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Evaluation

•Mid-sem – 30%

•Seminar / Assignments – 25 %

•Term End Examination – 45 %

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Network Elements [2]

To introduce, transmission basics, common parameters, losses and devices used in optical Communication network

Transmission Basics: •Wavelengths •Frequencies •Channel spacing

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•WDM Signals – Wavelength/Frequency

•C=fλ

•To Characterize a WDM signal, we use either frequency (GHz/THz)or wavelength(nm(10-9)/μm(10-6)

•Optical window centered around 0.8, 1.3, 1.55μm (IR, invisible)

•Another parameter, channel spacing between 2 wavelengths/frequencies

•Accurate as long as wavelength spacing is small compared to actual channel wavelength, •λ0 = 1500nm, Δλ = 0.8nm, frequency spacing = 100GHz

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Common Parameters

•Digital information 1s and 0’s •Bit rate is inverse of period

•Equivalent representation in frequency domain – Energy spread across a set of frequencies – Power spectrum/Spectrum

•Signal BW is a measure of the width of the spectrum of the signal •BW can be measured either in wavelength or frequency, mostly frequency

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•BW (Hz)and bit rate are not same(bits/s), relationship depends on the modulation used

•Phone line offers 4KHz BW, sophisticated modulation technology allows to realize a bit rate of 56kb/s •The ratio of bit rate to available BW is called spectral efficiency

•Bit rate of 10Gb/s uses BW of 25GHz…Also, Signal BW needs to be sufficiently smaller than channel spacing otherwise we would have undesirable interference between the adjacent channels and distortion of signal itself

13


Wavelength standards [2]

14


•1.55μm - Inherent loss is optical fiber is the lowest - excellent optical amplifiers are available in this region

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Evolution of Light wave systems 1. Generation: The development of low-loss fibers and semiconductor lasers

(GaAs) in the 1970„s.

A Gallium Aresenide (GaAs) laser operates at a wavelength of 0.8μm. The optical

communication systems allowed a bit rate of 45Mbit/s and repeater spacing of

10km.

Example of a laser diode.

(Ref.: Infineon)


Evolution of Lightwave systems

2. Generation: The repeater spacing could be increased by operating the

lightwave system at 1.3μm. The attenuation of the optical fiber drops from 2-

3dB/km at 0.8μm down to 0.4dB/km at 1.3μm. Silica fibers have a local minima at

1.3μm.


3. Generation: Silica fibers have an absolute minima at 1.55μm. The attenuation

of a fiber is reduced to 0.2dB/km. Dispersion at a wavelength of 1.55μm

complicates the realization of lightwave systems. The dispersion could be

overcome by a dispersion-shifted fibers and by the use of lasers, which operate

only at single longitudinal modes. A bit rate of 4Gbit/s over a distance of 100km

was transmitted in the mid 1980„s. The major disadvantage of the 3. Generation

optical communication system is the fact that the signals are regenerated by

electrical means. The optical signal is transferred to an electrical signal, the signal

is regenerated and amplified before the signal is again transferred to an optical

fiber

Traditional long distance single channel fiber transmission system.

Ref.: H. J.R. Dutton, Understanding optical communications


4. Generation: The development of the optical amplifier lead to the 4. Generation of

optical communication systems.

Schematic sketch of an erbium-doped fiber amplifier (EDFA).

Ref.: S.V. Kartalopoulos, Introduction to DWDM Technology


State of the Art optical communication system: Dense Wavelength Division

Multiplex (DWDM) in combination of optical amplifiers. The capacity of optical

communication systems doubles every 6 months. Bit rates of 10Tbit/s were

realized by 2001.

Ref.: S. Kartalopoulos, WDWM Networks, Devices and Technology

Evolution of Light wave systems


Propagation of Signals in Optical Fiber [1]

Advantages •Low loss ~0.2dB/km at 1550nm •Enormous bandwidth at least 25THz •Light weight •Flexible •Immunity to interferences •Low cost

Disadvantages and Impairments •Difficult to handle •Chromatic dispersion •Nonlinear Effects

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Refraction and reflection

Air(1.0)

Glass (1.5)

© Dr. Sangeetha R.G 23

ɸ2=90


Single fiber structure


Comparison of fiber structures


Acceptance Angle

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Low-order-mode fields


Signal Degradation in OF

•Absorbtion – fiber material

•Scattering- fiber with structural imperfections

•Radiative losses-perturbaions

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Signal Attenuation & Distortion in

Optical Fibers

•What are the loss or signal attenuation mechanism in a fiber?

•Why & to what degree do optical signals get distorted as they

propagate down a fiber?

•Signal attenuation (fiber loss) largely determines the maximum

repeater less separation between optical transmitter & receiver.

•Signal distortion cause that optical pulses to broaden as they

travel along a fiber, the overlap between neighboring pulses,

creating errors in the receiver output, resulting in the limitation of

information-carrying capacity of a fiber.

© Dr. Sangeetha R.G Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000

Optical fiber attenuation vs. wavelength


Power loss along a fiber:

The parameter is called fiber attenuation coefficient in a units of for example [1/km] or [nepers/km]. A more common unit is [dB/km] that is defined by:

Attenuation (fiber loss)

Z=0

P(0) mW

Z= l

lpePlP

)0()( mw

zpePzP

)0()(p

]km/1[343.4)(

)0(log

10]dB/km[ p

lP

P

l


Where [dBm] or dB milliwat is 10log(P [mW]).

Fiber loss in dB/km

z=0 Z=l

]km[]dB/km[]dBm)[0(]dBm)[( lPlP


Types of Attenuation

Absorption Loss:

Caused by the fibre itself or by impurities in

the fibre,such as water and metals.

Scattering Loss:

Intrinsic loss mechanism caused by the

interaction of photons with the glass itself.

Bending loss:

Loss induced by physical stress on the fibre.


Absorption

Absorption is caused by three different mechanisms

1- Impurities in fiber material: from transition metal ions (must

be in order of ppb) & particularly from OH ions with absorption

peaks at wavelengths 2700 nm, 400 nm, 950 nm & 725nm.

2- Intrinsic absorption (fundamental lower limit): electronic

absorption band (UV region) & atomic bond vibration band (IR

region) in basic SiO2.

3- Extrinsic absorption due to impurity atoms and Radiation

defects


Material Absorption Losses

• Material absorption is caused by absorption of photons

within the fibre.

– When a material is illuminated, photons can make the valence

electrons of an atom transition to higher energy levels

– Photon is destroyed, and the radiant energy is transformed into

electric potential energy. This energy can then

• re-emitted (scattering)

• Frees the electron (photoelectric effects) (not in fibers)

• Dissipated to the rest of the material (transformed into heat)

• In an optical fibre Material Absorption is the optical power that is

effectively converted to heat dissipation within the fibre.

• Two types of absorption exist:

– Intrinsic Absorption, caused by interaction with one or more of the

components of the glass

– Extrinsic Absorption, caused by impurities within the

glass


Intrinsic Absorption 1

Less significant than extrinsic absorption. For a pure

(no impurities) silica fibre a low loss window exists

between 800 nm and 1600 nm. Graph shows attenuation spectrum for

pure silica glass. Intrinsic absorption is

very low compared to other forms of loss.

It is for this reason that fibres are made up of silica and optical communications systems work between about 800 to 1600 nm.


Intrinsic Absorption 2

• Intrinsic absorption in the ultraviolet region is caused by

electronic absorption bands. Basically, absorption occurs when a

light particle (photon) interacts with an electron and excites it to

a higher energy level.

• The main cause of intrinsic absorption in the infrared region is

the characteristic vibration frequency of atomic bonds. In silica

glass, absorption is caused by the vibration of silicon-oxygen

(Si-O) bonds. The interaction between the vibrating bond and

the electromagnetic field of the optical signal causes intrinsic

absorption. Light energy is transferred from the electromagnetic

field to the bond.


Extrinsic Absorption (metallic ions)

Extrinsic absorption is much more significant than intrinsic

• Caused by impurities introduced into the fiber material during manufacture

– Iron, nickel, and chromium

• Caused by transition of metal ions to a higher energy level

• Modern fabrication techniques can reduce impurity levels below 1 part in 1010.

• For some of the more

common metallic impurities

in silica fibre the table shows

the peak attenuation

wavelength and the

attenuation caused by an

impurity concentration of 1 in

109


Extrinsic Absorption (OH ions) •Extrinsic absorption caused by dissolved water in the glass, as the hydroxyl

or OH ion.

•In this case absorption due to the same fundamental processes (between

2700 nm and 4200 nm) gives rise to so called absorption overtones at 1380,

950 and 720 nm.

•Typically a 1 part per million impurity level causes 1 dB/km of attenuation at

950 nm. Typical levels are a few parts per billion

Absorption Spectrum for OH in

Silica

Narrow windows circa 800, 1300

nm

and 1550 nm exist which are

unaffected by

this type of absorption.


Types of Scattering Loss in Fibre

Two basic types of scattering exist:

Linear scattering: Rayleigh and Mie

Non-linear scattering: Stimulated Brillouin and

Stimulated Raman.

Rayleigh is the dominant loss mechanism in the low

loss silica window between 800 nm and 1700 nm.

Raman scattering is an important issue in Dense

WDM systems


Rayleigh Scattering (I) Dominant scattering mechanism in silica fibres

Scattering causes by inhomogeneities in the glass, of a size smaller

than the wavelength.

Inhomogeneities manifested as refractive index variations, present in

the glass after manufacture.

Difficult to eliminate with present manufacturing methods

Rayleigh loss falls off as a function of the fourth power of wavelength:

in this empirical formula is expressed in microns (μm)

The Rayleigh scattering coefficient Ar is a constant for a given material.

For 1550 nm the loss is approximately 0.18 dB per km.

Scattering Loss

• Small (compared to wavelength) variation in material density, chemical

composition, and structural inhomogeneity scatter light in other directions

and absorb energy from guided optical wave.

• The essential mechanism is the Rayleigh scattering. Since the black body

radiation classically is proportional to (this is true for wavelength

typically greater than 5 micrometer), the attenuation coefficient due to

Rayleigh scattering is approximately proportional to . This seems to be

not precise, where the attenuation of fibers at 1.3 & 1.55 micrometer can be

exactly predicted with Planck’s formula & can not be described with

Rayleigh-Jeans law. Therefore the more accurate formula for scattering loss

is

4

4

1

5 )exp(

Tk

hc

B

scat

eTemperatur : ,JK 103806.1 Js, 10626.6 -12334 Tkh B


Absorption & scattering losses in fibers

Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000


Bending Loss (Macrobending & Microbending)

Macrobending Loss: The curvature

of the bend is much larger than fiber

diameter. Lightwave suffers sever

loss due to radiation of the

evanescent field in the cladding

region. As the radius of the curvature

decreases, the loss increases

exponentially until it reaches at a

certain critical radius. For any radius

a bit smaller than this point, the

losses suddenly becomes extremely

large. Higher order modes radiate

away faster than lower order modes.



Microbending Loss

Microbending Loss: microscopic

bends of the fiber axis that can

arise when the fibers are

incorporated into cables. The

power is dissipated through the

microbended fiber, because of the

repetitive coupling of energy

between guided modes & the

leaky or radiation modes in the

fiber.


Group Velocity

• Wave Velocities:

1- Plane wave velocity: For a plane wave propagating along z-axis in an unbounded homogeneous region of refractive index , which is represented by , the velocity of constant phase plane is:

2- Modal wave phase velocity: For a modal wave propagating along z-axis represented by , the velocity of constant phase plane is:

3- For transmission system operation the most important & useful type of velocity is the group velocity, . This is the actual velocity which the signal information & energy is traveling down the fiber. It is always less than the speed of light in the medium. The observable delay experiences by the optical signal waveform & energy, when traveling a length of l along the fiber is commonly referred to as group delay.

1n

)ωexp( 1zjktj

11 n

c

kv

)ωexp( zjtj

ωpv

[3-4]

[3-5]

gV


Dispersion in Optical Fibers

•Dispersion: Any phenomenon in which the velocity of propagation of any electromagnetic wave is wavelength dependent. •In communication, dispersion is used to describe any process by which any electromagnetic signal propagating in a physical medium is degraded because the various wave characteristics (i.e., frequencies) of the signal have different propagation velocities within the physical medium. •There are 3 dispersion types in the optical fibers, in general: 1- Material Dispersion 2- Waveguide Dispersion 3- Polarization-Mode Dispersion Material & waveguide dispersions are main causes of Intramodal Dispersion.


Intramodal Dispersion •As we have seen from Input/output signal relationship in optical fiber, the

output is proportional to the delayed version of the input signal, and the delay

is inversely proportional to the group velocity of the wave. Since the

propagation constant, , is frequency dependent over band width

sitting at the center frequency , at each frequency, we have one propagation

constant resulting in a specific delay time. As the output signal is collectively

represented by group velocity & group delay this phenomenon is called

intramodal dispersion or Group Velocity Dispersion (GVD). This

phenomenon arises due to a finite bandwidth of the optical source,

dependency of refractive index on the wavelength and the modal

dependency of the group velocity.

•In the case of optical pulse propagation down the fiber, GVD causes pulse

broadening, leading to Inter Symbol Interference (ISI).

)ω( ω

cω


Polarization mode dispersion •In practice, fibers are not perfectly circularly symmetric, and the two orthogonally polarized modes have slightly different propagation constants; that is, practical fibers are slightly birefringent. •Since the light energy of a pulse propagating in a fiber will usually be split between these two modes, this birefringence gives rise to pulse spreading. This phenomenon is called polarization-mode dispersion (PMD). •This is similar, in principle, to pulse spreading in the case of multimode fibers, but the effect is much weaker.

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Chromatic Dispersion •Chromatic dispersion is the term given to the phenomenon by which different spectral components of a pulse travel at different velocities. • Chromatic dispersion arises for two reasons. • The first is that the refractive index of silica, the material used to make optical fiber, is frequency dependent. Thus different frequency components travel at different speeds in silica. This component of chromatic dispersion is termed material dispersion. •The second component, called waveguide dispersion. To understand the physical origin of waveguide dispersion, recall that the light energy of a mode propagates partly in the core and partly in the cladding.

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•Also recall that the effective index of a mode lies between the refractive indices of the cladding and the core. The actual value of the effective index between these two limits depends on the proportion of power that is contained in the cladding and the core. •The power distribution of a mode between the core and cladding of the fiber is itself a function of the wavelength. •More accurately, the longer the wavelength, the more power in the cladding. Thus, even in the absence of material dispersion—so that the refractive indices of the core and cladding are independent of wavelength—if the wavelength changes, this power distribution changes, causing the effective index or propagation constant of the mode to change. This is the physical explanation for waveguide dispersion.

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Chirp [2]

• specific family of pulses changes shape as they propagate along a length of single-mode optical fiber. The pulses we consider are called chirped Gaussian pulses.

• Gaussian - envelope of the launched pulse

• Chirped - the frequency of the launched pulse changes with time

• We consider chirped pulses for three reasons.

- The pulses emitted by semiconductor lasers, when

they are directly modulated are considerably chirped

- The second reason, is that some nonlinear effects may cause unchirped pulses to acquire a chirp. It then becomes important to study the effect of chromatic dispersion on such pulses.

- The third reason is that the best transmission performance is achieved today by the use of Gaussian pulses that are deliberately chirped

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Dispersion Management

• Graded index fiber can combat modal dispersion • Group velocity dispersion is commonly expressed in terms of the chromatic • dispersion parameter D that is related to β2 as D = −(2πc/λ2)β2. • The chromatic dispersion parameter is measured in units of ps/nm-km since it

expresses the temporal spread (ps) per unit propagation distance (km), per unit pulse spectral width (nm).

• D = DM +DW, DM : material dispersion and DW : waveguide dispersion • DM, DW, and D for standard single-mode fiber. DM increases monotonically with λ

and equals 0 for λ = 1.276 μm. • On the other hand, DW decreases monotonically with λ and is always negative.

The total chromatic dispersion D is zero around • λ = 1.31 μm; thus the waveguide dispersion shifts the zero-dispersion wavelength by a few tens of nanometers. Around the zero-dispersion wavelength, D may be approximated by a straight line whose slope is called the chromatic dispersion slope of the fiber.

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• For standard single-mode fiber, the chromatic dispersion effects are small in the 1.3 μm band, and systems operating in this wavelength range are loss limited

• On the other hand, most optical communication systems operate in

the 1.55 μm band today because of the low loss in this region and the well-developed erbium-doped fiber amplifier technology.

• But as we have already seen, optical communication systems in this

band are chromatic dispersion limited • This limitation can be reduced if somehow the zero-dispersion

wavelength were shifted to the 1.55 μm band.

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DSF

• The waveguide dispersion can be varied by varying the refractive index profile of the fiber, that is, the variation of refractive index in the fiber core and cladding.

• Such a variation in refractive index profile leads to a single-mode fiber

with a dispersion zero in the 1.55 μm band.

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Positive and negative dispersion fiber

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Components [2]

• The components used in modern optical networks include

o couplers

o lasers

o photodetectors

o optical amplifiers

o optical switches

o filters

o multiplexers.

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• Couplers are the building blocks for several other optical devices

• the use of directional couplers in modulators and switches

• Couplers are also the principal components used to construct Mach-Zehnder interferometers, which can be used as optical filters, multiplexers/demultiplexers, or as building blocks for optical modulators, switches, and wavelength converters.

60

Couplers • A directional coupler is used to combine and split signals in an

optical network. • The most commonly used couplers are made by fusing two

fibers together • in the middle—these are called fused fiber couplers. Couplers

can also be fabricated using waveguides in integrated optics. • A 2 × 2 coupler, takes a fraction α of the power from input 1

and places it on output 1 and the remaining fraction 1 − α on output 2. Similarly, a fraction 1 − α of the power from input 2 is distributed to output 1 and the remaining power to output 2. We call α the coupling ratio.

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Applications

• The same coupler can also be used to separate/couple the two signals coming in on a common fiber.

• Wavelength-dependent couplers are also used to combine 980 nm or 1480

nm pump signals along with a 1550 nm signal into an erbium-doped fiber amplifier;

• the excess loss is the loss of the device above the fundamental loss

introduced by the coupling ratio α.

• For example, a 3 dB coupler has a nominal loss of 3 dB but may introduce additional losses of, say, 0.2 dB.

• In addition, we also need to maintain low polarization-dependent loss (PDL) for most applications.

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Principle of operation

• The net result of this analysis is that the electric fields, Eo1 and Eo2, at the outputs of a directional coupler may be expressed in terms of the electric fields at the inputs Ei1 and Ei2, as follows

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• l denotes the coupling length the propagation constant in each of the two waveguides of the directional coupler.

• The quantity κ is called the coupling coefficient and is a function of the width of the waveguides, the refractive indices of the waveguiding region (core) and the substrate, and the proximity of the two waveguides

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• If the directional coupler is lossless, the power in the output fields must equal the power in the input fields so that

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Isolators and Circulators

• Couplers and most other passive optical devices are reciprocal devices in that the devices work exactly the same way if their inputs and outputs are reversed

• However, in many systems there is a need for a passive nonreciprocal device

• An isolator is an example of such a device. Its main function is to allow transmission in one direction through it but block all transmission in the other direction

• Isolators are used in systems at the output of optical amplifiers and lasers primarily to prevent reflections from entering these devices, which would otherwise degrade their performance

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The two key parameters of an isolator are

-insertion loss: loss in the forward direction which should be as small as possible (1dB)

-isolation :loss in the reverse direction which should be as large as possible (40–50 dB)

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• At any time, the electric field vector can be expressed as a linear combination of the two orthogonal linear polarizations supported by the fiber horizontal and vertical modes

68

Multiplexers and Filters

• Optical filters are essential components in transmission systems

- to multiplex and demultiplex wavelengths in a WDM system—these devices are called multiplexers/demultiplexers

- to provide equalization of the gain and filtering of noise in optical amplifiers.

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• A simple filter is a two-port device that selects one wavelength and rejects all others.

• It may have an additional third port on which the rejected wavelengths can be obtained.

• A multiplexer combines signals at different wavelengths on its input ports onto a common output port, and a demultiplexer performs the opposite function.

• Multiplexers and demultiplexers are used in WDM terminals as well as in larger wavelength crossconnects and wavelength add/drop multiplexers

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• Demultiplexers and multiplexers can be cascaded to realize static wavelength cross-connects (WXCs)

• In a static WXC, the cross-connect pattern is fixed at the device is made and cannot be changed dynamically

• The device routes signals from an input port to an output port based on the wavelength

• Dynamic WXCs can be constructed by combining optical switches with multiplexers and demultiplexers

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Key Characteristics 1.Good optical filters should have low insertion losses. The insertion loss is the input-to-output loss of the filter.

2. The loss should be independent of the state of polarization of the input signals. The state of polarization varies randomly with time in most systems, and if the filter has a polarization-dependent loss, the output power will vary with time as well—an undesirable feature.

3. The passband of a filter should be insensitive to variations in ambient temperature.

4. As more and more filters are cascaded in a WDM system, the passband becomes progressively narrower. To ensure reasonably broad passbands at the end of the cascade, the individual filters should have very flat passbands, so as to accommodate small changes in operating wavelengths of the lasers over time

5. At the same time, the passband skirts should be sharp to reduce the amount of energy passed through from adjacent channels. This energy is seen as crosstalk and degrades the system performance. The crosstalk suppression, or isolation of the filter, is an important parameter as well

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Gratings

• The term grating is used to describe almost any device whose operation involves interference among multiple optical signals originating from the same source but with different relative phase shifts.

• An electromagnetic wave (light) of angular frequency ω propagating, say, in the z direction has a dependence on z and t of the form cos(ωt − βz). Here, β is the propagation constant and depends on the medium. The phase of the wave is ωt −βz.

• Thus a relative phase shift between two waves from the same source can be achieved if they traverse two paths of different lengths.

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• In WDM communication systems, gratings are used as demultiplexers to separate the individual wavelengths or as multiplexers to combine them

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• Note that the energy at a single wavelength is distributed over all the discrete angles that satisfy the grating equation at this wavelength.

• When the grating is used as a demultiplexer in a WDM system, light is collected from only one of these angles, and the remaining energy in the other orders is lost.

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Blazing

• The light energy at zeroth-order interference maximum is wasted since the wavelengths are not separated

• Thus gratings must be designed so that the light energy is maximum at one of the other interference maxima. This is done using a technique called blazing

• If the reflecting slits are inclined at an angle α to the grating plane. This has the effect of maximizing the light energy in the interference maximum whose order corresponds to the blazing angle.

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Fiber Gratings

Applications • Fiber gratings are used for a variety of applications, • Filtering • add/drop functions • compensating for accumulated dispersion Advantages • low loss • ease of coupling (with other fibers) • polarization insensitivity • low temperature-coefficient • simple packaging • extremely low-cost devices

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• Gratings are written in fibers by making use of the photosensitivity of certain types of optical fibers

• A conventional silica fiber doped with germanium becomes extremely photosensitive

• Exposing this fiber to ultraviolet (UV) light causes changes in the refractive index within the fiber core

• A grating can be written in such a fiber by exposing its core to two interfering UV beams. This causes the radiation intensity to vary periodically along the length of the fiber. Where the intensity is high, the refractive index is increased; where it is low, the refractive index is unchanged.

• The change in refractive index needed to obtain gratings is quite small—around 10−4.

• Other techniques, such as phase masks, can also be used to produce gratings.

• A phase mask is a diffractive optical element. When it is illuminated by a light beam, it splits the beams into different diffractive orders, which then interfere with one another to write the grating into the fiber.

80

• Fiber gratings are classified as either short-period or long-period gratings, based on the period of the grating.

• Short-period gratings are also called Bragg gratings and have periods that are comparable to the wavelength, typically around 0.5 μm.

• the other hand, have periods that are much greater than the wavelength, ranging from a few hundred micrometers to a few millimeters.

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Fiber brag gartings

82

Mach-zehnder Interferometer

83

• Consider the operation of the MZI as a demultiplexer; so only one input, say, input 1, has a signal

• After the first directional coupler, the input signal power is divided equally between the two arms of the MZI, but the signal in one arm has a phase shift of π/2 with respect to the other. Specifically, the signal in the lower arm lags the one in the upper arm in phase by π/2

• Since there is a length difference of L between the two arms, there is a further phase lag of βL introduced in the signal in the lower arm

• In the second directional coupler, the signal from the lower arm undergoes another phase delay of π/2 in going to the first output relative to the signal from the upper arm

• Thus the total relative phase difference at the first or upper output between the • two signals is π/2 + βL + π/2 • At the output directional coupler, in going to the second output, the signal from

the upper arm lags the signal from the lower arm in phase by π/2 • Thus the total relative phase difference at the second or lower output between the

two signals is π/2 + βL − π/2 = βL

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Transfer functions of MZI

85

Arrayed waveguide Grating (AWG)

• An arrayed waveguide grating (AWG) is a generalization of the Mach-Zehnder interferometer

• It consists of two multiport couplers interconnected by an array of waveguides

• The MZI can be viewed as a device where two copies of the same signal, but shifted in phase by different amounts,are added together

• The AWG is a device where several copies of the same signal, but shifted in phase by different amounts, are added together

86

87


Here, n1 is the refractive index in the input and output directional couplers, and n2 is the refractive index in the arrayed waveguides. From input i, those wavelengths λ, for which φij k , k = 1, . . . ,m, differ by a multiple of 2π will add in phase at output j

88

Optical Switching

• Switching speed of electronics cannot keep up with the transmission capacity of optics

• Optical network systems supports multiplexing 10 to 100 wavelengths with 10Gb/s each.

• Drawbacks of Electronics switching

Needs O/E conversion

Electronic components dependent on data rate and protocol

Addition/Replacement of component is required

89 09.10.2012

90

Optical switch

• PARAMETERS OF SWITCH – Insertion loss

– Cross talk

– Extinction ratio

– Polarization dependent loss

– Reliability

– Scalability

– Energy Usage

– Temperature Resistance

Attractions of Optical Switching

• Elimination of O/E conversion

• Independent of data rate and protocol

• Reduction in the network equipment

• Increase in switching speed and thus throughput

• Decrease in operating power

• Decrease in cost

91 09.10.2012

Limitations of Optical Switching

• Lack of bit level processing

• Lack of efficient buffering

Several solutions are currently under research but the common goal is the transitions to the switching systems in which optical technology plays a major role

92 09.10.2012

93

Switching Technologies

• Optomechanical Switches

• Micro-electromechanical

System Devices

– 2-D MEMS Device

– 3-D MEMS Device

• Electro-optic Switches

• Thermo-optic Switches

– Interferometric Switches

– Digital Optic Switches

• Liquid Crystal Switches

• Acoustooptic Switches

• Semiconductor Optical

Amplifier Switches

Micro-Optic

(MEMS)

Bubble

Waveguide

Free Space

Indium

Phosphide

SiO 2 / Si

Fibre

( acousto -optic)

Mechanical

Liquid

High Loss

Crystal

Can be configured in two or three

dimensional architectures

Poor Reliability

Not Scalable

Polarization Dependent

WDM Optical Networking Cannes 2000 Jacqueline Edwards, Nortel

Optical

Switching

Element

Technologies

Optical Switching Element Technologies

LiNbO 3

Thermo-

optic

Gel/oil based

SOA

Basic Structure of the Switch

A

B (b)

Switched

State

A A

B

, ,

B

(a)

Through

State

/2 /2 FLC cell (+E)

BD BD

A

B

,

FLC cell (-E)

/2 /2

BD BD

96

Optomechanical Switches:

• Prisms, Mirrors, Directional Couplers

• Advantages:

– Low insertion loss

– Low cross talk

– Relatively inexpensive

• Disadvantages:

– Lack of scalability

– Switching speed is less

97

:

• 2-D MEMS

– Advantages:

• Any number of channels can be added or dropped

– Disadvantages:

• Optical loss

• 3-D MEMS

– Advantage:

• Large port count optical switches

– Disadvantages:

• Complexity

• Feedback system is required to maintain the position of the mirror

Micro-ElectroMechanical System Devices

MEMS

98

MEMS stands for "Micro-ElectroMechanical System"

Systems are mechanical but very small

Fabricated in silicon using established semiconductor processes

MEMS first used in automotive, sensing and other applications

Optical MEMS switch uses a movable micro mirror

Fundamentally a space division switching element

Two axis motion

Micro mirror

MEMS based Optical Switch

Input fibre

Output fibre

Mirrors have only two possible positions

Light is routed in a 2D plane

For N inputs and N outputs we need N2

mirrors

Loss increases rapidly with N SEM photo of 2D MEMS mirrors

2D MEMS based Optical Switch Matrix

101

• Lithium niobate (LiNbO3)

– Coupling ratio of Directional coupler is

changed to vary refractive index of the

material

• Advantage:

– switching speed is comparatively high

• Disadvantages:

– High insertion loss

– Polarization dependance

Electro-Optic Switches

102

Variation of refractive index by varying the

temperature of the material itself

Advantages:

small in size

allows integration of variable attenuators and the

wavelength selective elements on the same chip

Disadvantages:

High driving power characteristics

Requires forced air cooling facility

Thermo-optic switches

103

• Liquid Crystal State is phase exhibited by large number

of organic materials over certain temperature ranges

• Advantages:

– Reliable

– optical performance is satisfactory

• Disadvantages:

– Affected by extreme temperature if not properly designed

Liquid Crystal Switches

Total Internal Reflection LC Switch

105

• Operation is based on Acousto-Optic Effect

– Interaction between sound & light

• Advantages:

– Simple

• Disadvantages:

– Switching speed is limited by the speed of the sound

Acousto-optic switches

106

Semiconductor Optical Switches:

• An SOA can be used as an On/OFF Switch by varying

the bias voltage

• Advantages:

– Simple

– Inexpensive

• Disadvantages:

– Polarization dependent

107

• Switch Sizes larger than 2 X 2 can be realised by appropriately cascading small switches

• Main Considerations: – Number of switches required

– Loss of uniformity

– Number of crossovers

– Blocking characteristics • Blocking

• Non-blocking

– Wide sense non blocking (without any additional rerouting)

– Strict sense non blocking (regardless of how previous connections)

– Rearrangeably non-blocking (re-routing)

Large Switches

Applications of Multistage networks

Information exchange between the processors and memory elements

• High performance large scale information systems like supercomputers

• Telecommunication core routers

• High data storage systems

108 09.10.2012

Optical Packet Switching (OPS)

• Allows WDM techniques

• OPS supports high speed, data rate transparency and configurability

• Packet streams can be multiplexed

• Each packet has header and payload

• Switches/Nodes analyse headers to forward the packets

• Packets travelling in OPS network has variant delays

109 09.10.2012

• Requirements

High speed

Low latency

High throughput

Fault tolerance

• Limitations

Optical buffering

Lack of bit level processing

110

Multistage interconnection Networks

09.10.2012

Wide sense non-blocking

• Cross-bar

• Shortest path is 1 and longest path is (2n-1)

• Can be fabricated without crossovers

111

Strict sense blocking

• Clos

• We use three parameters, m, k, and p. Let n = mk. The first and third stage consist of k (m× p)switches. The middle stage consists of p (k × k) switches

112

Rearrangeably non-blocking

• Benes

• Not wide-sense non-blocking

• Number of crossovers

113

Passive and Active optical switches

• Stated simply, active access networks contain an active element, a switch aggregator, between the central office or head end switch and customer-premises

[http://www.broadbandproperties.com/2004issues/may04issues/fibercity.pdf]

• Flexible so it can handle multiple services from many service providers,

• Scalable so that it cost-effectively provides the bandwidth needed today and can economically provision additional bandwidth with the addition of new applications tomorrow,

• Secure so content can not be diverted, corrupted or split, and

• Serviceable so that faults can be isolated and failure groups minimized.

Active networks have additional intelligence located closer to the subscriber that can reduce latency, flexibly add bandwidth, isolate faults, switch, schedule and queue traffic— and maximize bandwidth utilization between the switch aggregator and central office.

114

Applications of Optical Switching

• Optical cross-connects

• Reliable protection switching

• Optical Add/Drop multiplexing

• Optical Signal Monitoring

• Network provisioning

115 09.10.2012

Optical Cross Connect Cross connects groom and optimize transmission data paths.

Optical switch requirements for OXCs include

Scalability

High-port-count switches

The ability to switch with high reliability, low loss, good uniformity

of optical signals independent on path length

The ability to switch to a specific optical path without disrupting

the other optical paths

The difficulty in displacing the electrical with the optical lies in the

necessity of performance monitoring and high port counts afforded by

electric matrices.

Optical Add/Drop Multiplexing An OADM extracts optical wavelengths from the optical transmission

stream as well as inserts optical wavelengths into the optical

transmission stream at the processing node before the processed

transmission stream exits the same node.

Within a long-haul WDM-based network, OADM may require the added

optical signal to resemble the dropped optical signal in optical power

level to prevent the amplifier profiles from being altered. This power

stability requirement between the add and drop channels drives the need

for good optical switch uniformity across a wavelength range.

Low insertion loss and small physical size of the OADM optical switch

are important.

Wavelength selective switches!

Optical Spectral Monitoring Optical spectral monitoring receives a small optically tapped portion

of the aggregated WDM signal, separates the tapped signal into its

individual wavelengths, and monitors each channel’s optical spectra for

wavelength accuracy, optical power levels, and optical crosstalk.

OSM usually wraps software processing around optical switches,

optical filters and optical-to-electrical converters.

The optical switch size depends on the system wavelength density

and desired monitoring thoroughness. Usually ranges from a series of

small port count optical switches to a medium size optical switch.

It is important in the OSM application, because the tapped optical

signal is very low in optical signal power, that the optical switch has a

high extinction ratio (low interference between paths), low insertion loss,

and good uniformity.

Ultra-fast and ultra-short optical pulse generation

High speed modulation and detection

High capacity multiplexing

Wavelength division multiplexing

Optical time division multiplexing

Wideband optical amplification

Optical switching and routing

Optical clock extraction and regeneration

Ultra-low dispersion and low non-linearity fibre

Optical Functions Required

Parameters of an Optical Switch Switching time

Insertion loss: the fraction of signal power that is lost because of the switch. Usually measured in decibels and must be as small as possible. The insertion loss of a switch should be about the same for all input-output connections (loss uniformity).

Crosstalk: the ratio of the power at a specific output from the desired input to the power from all other inputs.

Extinction ratio: the ratio of the output power in the on-state to the output power in the off-state. This ratio should be as large as possible.

Polarization-dependent loss (PDL): if the loss of the switch is not equal for both states of polarization of the optical signal, the switch is said to have polarization-dependent loss. It is desirable that optical switches have low PDL.

Other parameters: reliability, energy usage, scalability (ability to build switches with large port counts that perform adequately), and temperature resistance.

Wavelength converters [2]

• A wavelength converter is a device that converts data from one incoming wavelength to another outgoing wavelength.

• Wavelength converters are useful components in WDM networks for three major reasons.

• First, data may enter the network at a wavelength that is not suitable for use within the network.

For example, the first-generation networks commonly transmit data in the 1310 nm wavelength window, using LEDs or Fabry-Perot lasers. Neither the wavelength nor the type of laser is compatible with WDM networks. So at the inputs and outputs of the network, data must be converted from these wavelengths to narrow-band WDM signals in the 1550 nm wavelength range. A wavelength converter used to perform this function is sometimes called a transponder.

121

• Second, wavelength converters may be needed within the network to improve the utilization of the available wavelengths on the network links.

• Finally, wavelength converters may be needed at boundaries between different networks if the different networks are managed by different entities and these entities do not coordinate the allocation of wavelengths in their networks.

122

123

Wavelength Converters

• Improve utilization of available wavelengths on links

• All-optical WCs being developed • Greatly reduce blocking probabilities

No converters

1

2 3

New request

1 3

1

2 3

New request

1 3

With converters

WC

Ways to achieve wavelength conversion

(1) optoelectronic

(2) optical gating

(3) interferometric

(4) wave mixing.

The latter three approaches are all-optical but not yet mature enough for commercial use.

Optoelectronic converters today offer substantially better performance at lower cost than comparable all-optical wavelength converters.

124

Optical Fibre Test and

Measurement

Overview

Optical Test and Measurement

Optical measurements takes place at a variety

of levels

Design & research laboratories

Production and manufacture

Component characterization

Network test and measurement

Network performance monitoring

Transmission characterization

Optical Test and Measurement

Equipment

Optical source and power meter

Optical test set (source and power meter combined)

Optical Time Domain Reflectometer (OTDR)

Optical spectrum analyser (OSA)

Optical waveform analyser/optical oscilloscope

Dispersion analyser

Polarization mode dispersion analyser

Optical return loss test sets

Fibre talk sets

Connector inspection microscopes

Wide variety of equipment is in use......

128

Link Characterization Using the OTDR OTDR General Theory

What is an optical time-domain reflectometer (OTDR)?

• Single-ended measurement tool

• Provides a detailed picture of section-by-section loss

• Operates by sending a high-power pulse of light down the fiber and

measuring the light reflected back

• Uses the time it takes for individual reflections to return to determine

the distance of each event

• Measures/characterizes:

Fiber attenuation

Attenuation example (new G.652.C fibers)

0.33 dB/km at 1310 nm (0.35 dB/km for worst case)



129


Measures/characterizes:

– Reflection and optical loss caused by every event in the link

• Connectors

• Splices

– Fiber ends

– Detectable faults

• Misalignments and mismatches

• Dirt on connectors

• Fiber breaks

• Macrobends

130

OTDR

131

OTDR Basic Principles

An OTDR sends short pulses of light into a fiber. Light scattering occurs in the

fiber due to discontinuities such as connectors, splices, bends, and faults. An

OTDR then detects and analyzes the backscattered signals. The signal strength

is measured for specific intervals of time and is used to characterize events.

The OTDR to calculate distances as follows:

Distance = c/n * t/2

c = speed of light in a vacuum (2.998 x 108 m/s)

t = time delay from the launch of the pulse to the reception of the pulse

n = index of refraction of the fiber under test (as specified by the manufacturer)


132


133


An OTDR uses the effects of Rayleigh scattering and Fresnel reflection

to measure the fiber's condition, but the Fresnel reflection is tens of

thousands of times greater in power level than the backscatter.

Rayleigh scattering occurs when a pulse travels down the fiber and small

variations in the material, such as variations and discontinuities in the index of

refraction, cause light to be scattered in all directions. However, the

phenomenon of small amounts of light being reflected directly back toward the

transmitter is called backscattering.

Fresnel reflections occur when the light traveling down the fiber encounters

abrupt changes in material density that may occur at connections or breaks

where an air gap exists. A very large quantity of light is reflected, as compared

with the Rayleigh scattering. The strength of the reflection depends on the

degree of change in the index of refraction.

134

Link Characterization Using the OTDR

OTDR General Theory

Reflectometry theory

• The OTDR launches short light pulses

(from 5 ns to 20 µs)

• Measuring the difference between the

launching time and the time of arrival of

the returned signal, it determines the

distance between the launching point

and the event.

135

Link Characterization Using the OTDR

OTDR General Theory

• Rayleigh backscattering

• Comes from the fiber‟s “natural” reflectiveness

• The OTDR uses the Rayleigh back reflections to measure fiber

attenuation (dB/km)

• Back reflection level around -75 dB

• Higher wavelengths are less attenuated by the Rayleigh backscattering

136


•Distance: corresponds to the distance range of the fiber span

to be tested according to the selected measurement units

•Pulse: corresponds to the pulse width for the test. A longer pulse

allows you to probe further along the fiber, but results in less

resolution. A shorter pulse width provides higher resolution, but less

distance range.

•Time: corresponds to the acquisition duration (period during which

results will be averaged). Generally, longer acquisition times generate

cleaner traces (long-distance traces) because as the acquisition time

increases, more of the noise is averaged out. This averaging increases

the signal-to-noise ratio (SNR) and the OTDR's ability to detect small

events.

137


Simplified OTDR trace

138

http://www.porta-optica.org


• Loss in fiber is wavelength-dependent

139


Link Characterization Using the OTDR Testing Techniques

Launch cables

• A launch cable is recommended if the user wants to characterize the first

or last connector of an optical link.

• It allows the OTDR to have a power reference before and after the

connector in order to characterize it.

• Standard available lengths vary from 200 meters to 1500 meters

140



Without a pulse suppressor box

141



Four-point events: loss measurement

142



Acquisition parameter settings

143



To take acqusition just press START

C:/Program Files/EXFO/Toolbox/ToolBox6.exe

Optical Spectrum Analyser (OSA)

144

Power levels of the individual carriers • to ensure even power distribution over the entire bandwidth

• to notice immediately whether any channels have dropped out

Channel wavelength / Channel spacing • to indicate possible wavelength shifts for individual laser sources

Signal-to-noise ratio • to ensure that error-free transmission is possible in each data channel

Overall power • to check the optical fiber amplifiers of the system

• to check against safety limit of currently +17 dBm

Crosstalk • to provide a quality indication for the system components (couplers,

...)

DWDM test parameters

DWDM spectrum and corresponding test parameters

W

D

M

W

D

M

TX

TX

TX

TX

1

n

OSA-155

1...

n

1...

n

OFA

1

n

RX

RX

RX

RX

OFA

W a n d e l & G o l t e r m a n n

A D V A N C E D

N E T W O R K T E S T E R

A N T - 2 0


F i b e r F O X W G O F T - 1 0

M a in

S e t

?

S t a r tS t o p

DWDM measurements with an optical spectrum analyzer


A D V A N C E D N E T W O R K T E S T E R

A N T - 2 0



M a in

S e t

?

S t a r tS t o p

OFA monitor output: 8 data signals and ASE spurious spectrum

DWDM system with the recommended reference test points

from ITU-T Rec. G.692

100 GHz channel grid in the range 1530 to 1565 nm from ITU-T Rec. G.692

RX RX TX TX

1 n...

1

n

W

D

M

W

D

M

OFA

1...

n

OFA

OSA-155

1...

n


A D V A N C E D

N E T W O R K T E S T E R

A N T - 2 0



M a in

S e t

?

S t a r tS t o p

Optical add/drop multiplexing principle and monitoring of

system parameters

Optical add/drop multiplexing principle and monitoring of

system parameters

4-channel DWDM signal with characteristic noise floors

and ASE spurious spectrum

W

D

M

W

D

M

TX

TX

TX

TX

1

n

OSA-155

ANT-20

1...

n

1...

n

i

OFA



A N T - 2 0

1

n

RX

RX

RX

RX

OFA



A N T - 2 0



M a in

S e t

?

S t a r tS t o p

Quality monitoring in DWDM systems

Channel-No.

Actual

Power / -stability

S/N / -stability



A N T - 2 0



M a in

S e t

?

S t a r tS t o p

Cyclical recording and stability measurement of

Due to the irregular gain/

wavelength response of

todays EDFAs cascading

of optical amplifiers

result

in power shifts !

To minimize crosstalk

effects ( Rx side) of each

DWDM channel power

equalization becomes

necessary.



A N T - 2 0



M a in

S e t

?

S t a r tS t o p

Spectrum analysis and power leveladjustment in DWDM systems

Quality monitoring through wavelength-selective bit error analysis

(automatic evaluation of bit error histograms)

Future Challenges

Multiple channels / wavelengths

A variety of new parameters to measure

Must be completed quickly

May need to be carried out remotely

Will require a high degree of automation

High optical power levels > +20 dBm

Extensive data reporting/recording abilities

Optical network measurement challenges

Optical Amplifiers

• In an optical communication system, the optical signals from the transmitter are attenuated by the optical fiber as they propagate through it.

• Other optical components, such as multiplexers and couplers, also add loss.

• After some distance, the cumulative loss of signal strength causes the signal to become too weak to be detected.

• Before this happens, the signal strength has to be restored. Prior to the advent of optical amplifiers over the last decade, the only option was to regenerate the signal, that is, receive the signal and retransmit it. This process is accomplished by regenerators.

• A regenerator converts the optical signal to an electrical signal, cleans it up, and converts it back into an optical signal for onward transmission.

164

Repeaters Vs Regenerators

• On one hand, regenerators are specific to the bit rate and modulation format used by the communication system.

• On the other hand, optical amplifiers are insensitive to the bit rate or signal formats.

• Thus a system using optical amplifiers can be more easily upgraded, for example, to a higher bit rate, without replacing the amplifiers. In contrast, in a system using regenerators, such an upgrade would require all the regenerators to be replaced.

• Furthermore, optical amplifiers have fairly large gain bandwidths, and as a consequence, a single amplifier can simultaneously amplify several WDM signals.

• In contrast, we would need a regenerator for each wavelength. Thus optical amplifiers have become essential components in high-performance optical communication systems.

165

• Amplifiers, are not perfect devices. • They introduce additional noise, and this noise accumulates

as the signal passes through multiple amplifiers along its path due to the analog nature of the amplifier.

• The spectral shape of the gain, the output power, and the transient behaviour of the amplifier are also important considerations for system applications.

• Ideally, we would like to have a sufficiently high output power to meet the needs of the network application.

• We would also like the gain to be flat over the operating wavelength range and to be insensitive to variations in input power of the signal

166

Traditional Optical Communication System

Loss compensation: Repeaters at every 20-50 km

Classification

• Erbium-doped fiber amplifiers (EDFA)

• Raman amplifiers (RA)

• semiconductor optical amplifiers (SOA)

168

Key Phenomenon stimulated emission

• stimulated emission of radiation by atoms in the presence of an electromagnetic field.

• consider an atom and two of its energy levels, E1 and E2, with E2 > E1.

• An electromagnetic field whose frequency fc satisfies hfc = E2 − E1 induces transitions of atoms between the energy levels E1 and E2.

• E1 →E2 and E2 → E1 is possible. E1 →E2 transitions are accompanied by absorption of photons from the incident electromagnetic field. E2 → E1 transitions are accompanied by the emission of photons of energy hfc, the same energy as that of the incident photons.

• Increase in the number of photons of energy hfc and an amplification of the signal.

169

• It follows from the theory of quantum mechanics that the rate of the E1 → E2

transitions per atom equals the rate of the E2 → E1 transitions per atom. Let this

common rate be denoted by r.

• If the populations (number of atoms) in the energy levels E1 and E2 are N1 and N2,

respectively, we have a net increase in power (energy per unit time) of (N2 −N1)

rhfc.

• Clearly, for amplification to occur, this must be positive, that is, N2 > N1. This

condition is known as population inversion.

• The reason for this term is that, at thermal equilibrium, lower energy levels are

more highly populated, that is, N2 < N1.

• Therefore, at thermal equilibrium, we have only absorption of the input signal. In

order for amplification to occur, we must invert the relationship between the

populations of levels E1 and E2 that prevails under thermal equilibrium.

• Population inversion can be achieved by supplying additional energy in a suitable

form to pump the electrons to the higher energy level. This additional energy can

be in optical or electrical form

170

Spontaneous emission

• Independent of any external radiation that may be present, atoms in energy level E2 transit to the lower energy level E1, emitting a photon of energy hfc.

• The spontaneous emission rate per atom from level E2 to level E1 is a characteristic of the system, and its reciprocal, denoted by τ21,is called the spontaneous emission lifetime.

• Thus, if there are N2 atoms in level E2, the rate of spontaneous emission is N2/τ21, and the spontaneous emission power is hfcN2/τ21. The amplifier treats

• spontaneous emission radiation as another electromagnetic field at the frequency hfc, and the spontaneous emission also gets amplified, in addition to the incident optical signal.

• This amplified spontaneous emission (ASE) appears as noise at the output of the amplifier. 171

EDFA

• It consists of a length of silica fiber whose core is doped with ionized atoms (ions), Er3+

• This fiber is pumped using a pump signal from a laser, typically at a wavelength of 980 nm or 1480 nm.

• In order to combine the output of the pump laser with the input signal, the doped fiber is preceded by a wavelength-selective coupler.

• EDFA the amplifier of choice in today’s optical communication systems:

(1) the availability of compact and reliable high-power semiconductor pump lasers

(2) the fact that it is an all-fiber device making it polarization independent and easy to couple light in and out of it

(3) the simplicity of the device

(4) the fact that it introduces no crosstalk when amplifying WDM signals

172

• Er ions introduced in si –starksplitting

• Macroscopically, that is, when viewed

as a collection of ions, this has the effect

of spreading each discrete energy level

of an erbium ion into a continuous energy band.

• Recall 2 energy level, only an optical signal at the frequency fc satisfying hfc = E2 −E1 could be amplified

• Pumping at 980nm

173

τ21,

~1550 nm

980 nm

Radiationless Decay

~1550 nm

Pump

Signal

Output

Optical Pumping to Higher Energy levels Rapid Relaxation to "metastable" State

Stimulated Emission and Amplification

N 1

N 2

N 3

N 1

N 2

N 3

N 1

N 2

N 3

Amplification Process of EDFA

Fig. 11-4: Erbium energy-level diagram

τ32,

EDFA configurations

Co-Directional Pumping

Counter Directional

Dual Pumping

177

Amplified Spontaneous Emission (ASE) Noise

Semiconductor Optical Amplifier (SOA)

• Semiconductor optical amplifiers are similar in construction to

semiconductor lasers. They consist of a gain (active) section and a

passive section constructed of a semiconductor material such as

indium phosphide. The main difference is that SOAs are made

with layers of antireflection coatings to prevent light from

reflecting back into the circuit.

• Optical gain occurs as excited electrons in the semiconductor

material are stimulated by incoming light signals; when current is

applied across the p-n junction the process causes the photons to

replicate, producing signal gain. The gain medium can be either a

bulk or a multiple-quantum-well active layer.

Semiconductor optical amplifiers, like their semiconductor-laser,

consist of gain and passive regions. Layers of antireflective coatings

prevent light from reflecting back into the circuit while the incoming

signal stimulates electrons in the gain region

SOA

• Types:

– Resonant , Febry Perot amplifier (partially reflective end mirrors, requires very carefull temperature and injection current)

– No-resonant, Traveling wave amplifier (TWA) (The end facets are anti-reflection coating or cleaved at an angle so no internal reflection)

– External pumping

181

182

Optical confinement factor, a= gain constant

Raman and Brillouin fiber amplifier

• Non-linear effects within the fiber may be provided to obtain optical amplification

• Stimulated scattering, stimulated brillouin scattering or four photon mixing, giving gain by injecting a high power laser beam into undoped amplifier

• Raman amplifier exhibits self phase matching between pump and signal together-WDM systems

• Simultaneous amplification using 60mW pump power, gain-BW is 20-30nm 183

Raman gain GR

184

Observations about Raman amplifiers

• Gain is a function of fiber length for standard 10µm core single mode fiber with input power 1.6W

185

• Raman gain becomes larger as the fiber length increases around 50km

• Raman gain can be obtained using low loss fibers

• Raman gain increased as fiber core diameter decreased

• Pump power required for amplification is high

186

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