Diplomovka V upravenediplom.utc.sk/wan/719.pdf · ANOTAČNÝ ZÁZNAM - DIPLOMOVÁ PRÁCA Priezvisko, meno: Sabol, Dušan školský rok: 2005/2006 Názov práce: WDM systémy s DPSK

Abstract

Sabol, Dušan: WDM systémy s DPSK kľúčovaním [Diplomová práca]. Žilinská

univerzita v Žiline. Elektrotechnická fakulta; Katedra telekomunikácií. Školiteľ: Ing.

Jozef Dubovan. Stupeň odbornej kvalifikácie: inžinier (Ing.). Žilina: EF ŽU, 2006. 51 s.

Complex approach to optical communication is proposed in my work, where

limitation factors due to significant physical phenomena with cancellation techniques and

their suitability investigated. Economical fundamental factors are also involved following

trends in optical communication are proposed. Frequently used DPSK format is

theoretically described and impact of OSNR level and laser linewidth on its performance

is investigated by simulation technique provided in VPI simulation software. Behavior of

used modulation format within WDM system examined at four channel’s WDM and

affect of Kerr effect is qualitatively assessed.

Key words: DPSK, DQPSK, OOK, FWM, WDM, BER, limitation factors

ANOTAČNÝ ZÁZNAM - DIPLOMOVÁ PRÁCA

Priezvisko, meno: Sabol, Dušan školský rok: 2005/2006

Názov práce: WDM systémy s DPSK kĺúčovaním

Počet strán: 51 Počet obrázkov: 46 Počet tabuliek: 1

Počet grafov: 0 Počet príloh: 0 Použitá lit.: 17

Anotácia (slov. resp. český jazyk): Táto diplomová práca skúma limitujúce faktory

dnešných WDM sytémov. Popis DPSK modulácie a jej aplikácii, ako budúceho

modulačného formátu pre optické komunikácie ďalšej generácie. Práca obsahuje

simulácie popisujúce chybovosť jednotlivých typov modulácii a následne ich poškodenia

pôsobením Kerrovho efektu v štovkanálovom WDM systéme.

Anotácia v cudzom jazyku (anglický resp. nemecký): This diploma thesis investigates

limitation factors of current WDM systems. Description of DPSK modulation and its

applications as modulation format for optical communication of next generation. Thesis

contains simulation that describes BER of selected modulations and their distortions due

to Kerr effect at four channel WDM system.

Kľúčové slová: DPSK, DQPSK, OOK, FWM, WDM, BER, Limitujúce faktory

Vedúci práce: Ing. Jozef Dubovan

Recenzent práce: Prof. Ing. Milan Dado, PhD.

Dátum odovzdania práce: 19.5.2006

I

Content

List of Figures and Tables .........................................................................................III

List of Abbreviations .........................................................................................................VI

1 Introduction..................................................................................................................1

2 Complex approach to optical communication .............................................................2

2.1 Physical characteristic..........................................................................................2

2.2 Mitigation of impairments and distortions...........................................................3

2.2.1 Nonlinear effects..........................................................................................3

2.2.1.1 Kerr effect ................................................................................................3

2.2.1.2 Stimulated scatterings ..............................................................................7

2.2.2 Amplification ...............................................................................................9

2.2.3 Shaping ......................................................................................................12

2.2.4 Jitter............................................................................................................16

2.3 Modulation format .............................................................................................17

2.4 Economical aspects............................................................................................18

2.4.1 Catalysts of OT ..........................................................................................18

2.4.2 Inhibitors of OT .........................................................................................20

3 Differential phase-shift keyed format ........................................................................21

3.1 DPSK format......................................................................................................21

3.2 DPSK transmitter ...............................................................................................23

3.2.1 Transmitter evaluation ...............................................................................24

3.2.2 Pulse carver................................................................................................24

3.3 DPSK Receiver ..................................................................................................26

3.3.1 Balanced versus single-ended detection ....................................................27

3.3.2 Tolerance to Optical Filtering....................................................................29

3.4 DPSK transmission at 10 Gb/s...........................................................................30

3.5 DPSK transmission at 40 Gb/s...........................................................................31

3.6 DQPSK application............................................................................................32

4 Trends in the optical communication.........................................................................34

4.1 Transmission bands and spectral efficiency ......................................................34

4.2 Multiplexing techniques.....................................................................................34

4.2.1 Time division multiplex (OTDM) .............................................................34

II

4.2.2 Wavelength Division Multiplex.................................................................35

4.3 Simulation ..........................................................................................................35

4.3.1 DQPSK format...........................................................................................35

4.3.2 DPSK format..............................................................................................38

4.3.3 OOK format ...............................................................................................40

4.4 Impact of Kerr effect..........................................................................................40

4.4.1 Description of simulation scheme..............................................................40

4.4.2 OOK performance......................................................................................42

4.4.3 DPSK performance ....................................................................................44

4.4.4 DQPSK performance .................................................................................46

4.4.5 Potential of best setup ................................................................................48

5 Conclusion .................................................................................................................51

III

LIST OF FIGURES AND TABLES FIGURE 1: SPECTRAL VARIATION OF ATTENUATION; SOURCE [1]..........................................2

FIGURE 2: PRODUCTS OF KERR EFFECT; [6]. .........................................................................4

FIGURE 3: A) TEMPORAL VARIATION OF SPM-INDUCED PHASE SHIFT AND FREQUENCY CHIRP

FOR GAUSSIAN (DASHED CURVE) AND SUPER-GAUSSIAN (SOLID CURVE) PULSE. B)

EXPERIMENTALLY OBSERVED SPECTRA FOR A NEARLY GAUSSIAN PULSE AT THE

OUTPUT OF A 99 M LONG FIBRE. SPECTRA ARE LABELED BY MAXIMUM PHASE SHIFT

RELATED LINEARLY TO THE PEAK POWER; [4]. ..............................................................5

FIGURE 4: EYE DIAGRAM OF COMPLETE AND INCOMPLETE COLLISIONS; [5]..........................5

FIGURE 5: A) NONLINEAR CROSSTALK DUE TO FWM, B) EYE DIAGRAM OF AMPLITUDE NOISE

DUE TO FWM; [5]. ........................................................................................................6

FIGURE 6: EXAMPLES OF INTRACHANNEL EFFECTS; [7]. ........................................................7

FIGURE 7: RAMAN GAIN COEFFICIENT FOR PURE SILICA AND 1550 NM PUMP WAVELENGTH;

[8]. ................................................................................................................................8

FIGURE 8: THE OUTCOME OF SRS AT WDM SYSTEM; [5]. ....................................................8

FIGURE 9: BEHAVIOR OF INPUT AND OUTPUT POWER DUE TO SBS; [5]. ................................9

FIGURE 10: CHARACTERISTIC OF ERBIUM AMPLIFIER (LEFT) AND RAMAN AMPLIFIER

(RIGHT); [7].................................................................................................................11

FIGURE 11: VARIOUS HYBRID AMPLIFICATION SCHEMES; [7]. .............................................12

FIGURE 12: THE DISPERSION’S INFLUENCE; [7]. ..................................................................13

FIGURE 13: MATERIAL, WAVEGUIDE AND CHROMATIC DISPERSION FOR CURRENT FIBRES; .13

FIGURE 14: DISPERSION COMPENSATION BY FBG; [9]. .......................................................14

FIGURE 15: A), C) SYMMETRICAL DISPERSION MANAGEMENT; B), D) NON SYMMETRICAL

DISPERSION MANAGEMENT; [5]. ..................................................................................15

FIGURE 16: THE EFFECT OF DISPERSION SLOPE FOR BORDER AND CENTRAL FREQUENCIES;

[5]. ..............................................................................................................................15

FIGURE 17: PMD DUE TO BIREFRINGENCE EFFECT; [11]. ....................................................16

FIGURE 18: CANCELLATION OF TIMING JITTER WITH SYMMETRIC DISPERSION PROFILE; [5].

....................................................................................................................................17

FIGURE 19: COMPARISON OF IP TRAFFIC PREDICTIONS WITH INSTALLED CAPACITY; [1].....19

FIGURE 20: SIGNAL CONSTELLATION OF OOK AND DPSK MODULATION; [16] ..................22

FIGURE 21: TYPICAL DPSK TRANSMITTERS: A) IMPLEMENTATION WITH PM, B)

IMPLEMENTATION WITH MZM; [16] ...........................................................................24

IV

FIGURE 22: A) A TYPICAL RZ-DPSK TRANSMITTER. (B) OPTICAL INTENSITY AND PHASE

WAVEFORMS GENERATED BY AN IMPERFECT PULSE CARVER; [16]. .............................25

FIGURE 23: DPSK RECEIVER; [16]. .....................................................................................26

FIGURE 24: NUMERICAL CALCULATIONS FOR THE REQUIRED OSNR AT BER = 10-10 FOR

33% RZ-DPSK AND OOK AS A FUNCTION OF RECEIVER AMPLITUDE IMBALANCE Β;

[16].............................................................................................................................28

FIGURE 25: PENALTIES IN NON IDEAL RZ-DPSK RECEIVERS: (A) AMPLITUDE IMBALANCE IN

THE BALANCED DETECTOR (DASHED CURVE IS FOR A DELAY INTERFEROMETER WITH

AN EXTINCTION RATIO OF ONLY 10 DB), (B) PHASE IMBALANCE IN THE BALANCED

DETECTOR, (C) DELAY-TO-BIT RATE MISMATCH IN THE DELAY INTERFEROMETER, (D)

LASER FREQUENCY OFFSET FROM THE IDEAL AS SET BY THE INTERFEROMETER PHASE

DIFFERENCE (DASHED CURVE IS FOR 33% RZ-DPSK). CIRCLES ARE EXPERIMENTAL

RESULTS; [16]. ............................................................................................................29

FIGURE 26: EXPERIMENTAL RESULTS OF PENALTIES FROM NARROW OPTICAL AND

ELECTRICAL FILTERING; [16] ......................................................................................29

FIGURE 27: DQPSK TRANSMITTER AND RECEIVER; [16]. ...................................................32

FIGURE 28: EXPERIMENTAL SETUP OF; [18]. .......................................................................36

FIGURE 29: SIMULATION SCHEME OF BER = ƒ (OSNR). .....................................................36

FIGURE 30: COMPARISON OF EXPERIMENTAL [18] AND SIMULATION OUTPUT.....................37

FIGURE 31: BER VS OSNR FOR 10 GB/S AND 40 GB/S DQPSK FORMAT. ..........................38

FIGURE 32: SIMULATION SCHEME OF BER = ƒ (OSNR). .....................................................39

FIGURE 33: BER VS OSNR FOR 10 GBS AND 40 GB/S DPSK FORMAT. ..............................40

FIGURE 34: SIMULATION SCHEME OF FWM GENERATION...................................................41

FIGURE 35: EYE DIAGRAM OF OOK TRANSMITTER, A) 2 MHZ LINEWIDTH, B) 500 KHZ

LINEWIDTH..................................................................................................................42

FIGURE 36: EYE DIAGRAM AFTER FIRST LOOP. ....................................................................43

FIGURE 37: SPECTRUM AFTER FIRST LOOP. .........................................................................43

FIGURE 38: DPSK BACK-TO-BACK EYE DIAGRAM OF A) 2 MHZ AND B) 500 KHZ LINEWIDTH

....................................................................................................................................44

FIGURE 39: DPSK EYE DIAGRAM AFTER THE FIRST LOOP....................................................45

FIGURE 40: DPSK SPECTRUM AFTER THE FIRST LOOP. ........................................................46

FIGURE 41: DQPSK EYE DIAGRAMS IN THE BACK-TO-BACK CONDITION A) 2 MHZ, B) 500

KHZ. ...........................................................................................................................47

FIGURE 42: DQPSK EYE DIAGRAM AFTER THE FIRST LOOP.................................................47

V

FIGURE 43: DQPSK SPECTRUM AFTER THE FIRST LOOP ......................................................48

FIGURE 44: DPSK EYE DIAGRAM AFTER FIRST LOOP...........................................................49

FIGURE 45: DPSK SPECTRUM AFTER FIRRST LOOP..............................................................49

FIGURE 46: SIGNAL POWER OF SELECTED CHANNELS RELATE ON THE NUMBER OF LOOP ....50

TABLE 1: PARAMETERS OF ULTRA HIGH SPEED ELECTRONICS; [16].....................................35

VI

List of Abbreviations

AMI Alternate-Mark Inversion ARMA Autoregresive Moving Average ASE Amplified Spontaneuos Emission ATM Asynchronous Transfer Mode AWGN Additive White Gaussian Noise B Channel bit rate BB Broad Band BER Bit Error Rate BoD Bandwidth on Demand CSRZ Carrier-Suppressed Return to Zero CT Communication Technologies CW Continuous Wave D Fibre Dispersion DBPSK Differential Binary Phase-Shift-Keyed DC Direct current DCF Dispersion Compensating Fibre DI Delay Interferometer DPSK Differential-Phase-Shift-Keyed DWDM Dense Wavelength Division Multiplex EDFA Erbium Doped Fibre Amplifier FBG Fibre Bragg Grating FEC Forwad Error Correction FTTx Fibre to the various structure FWM Four Wave Mixing HBT Heterojunction Bipolar Transistor HEMT High Electron Mobility Transistor iFWM Intra Four Wave Mixing IPoWDM IP over WDM IT Information Technologies iXPM Intra Cross Phase Modulation MA Moving Average MZM Mach-Zehnder Modulator NF Noise Figure NRZ Non Return to Zero NZ-DSF Non Zero - Dispersion Shifted Fibre OBS Optical Burst Switching OOK On-Off Keying OPS Optical Packet Switching OT Optical Communication OT Optical Technologies OTDM Optical Time Division Multiplex P0 Threshold Power PLC Planar-Lightwave-Circuit PM Phase Modulator

VII

PMD Polarization Mode Dispersion QoS Quality of Service RZ Return to Zero RZ-AMI Return to Zero Alternate-Mark Inversion RZ-DPSK Return to zero Differential-Phase-Shift-Keying RZ-Duobinary Return to Zero Duobinary SBS Stimulated Brillouin Scattering SDH Synchronous Digital Hierarchy SMF Single Mode Fibre SNR Signal to Noise Ratio SPM Self Phase Modulation SRS Stimulated Raman Scattering WDM Wavelength Division Multiplex xDSL various Digital Subscriber Line XPM Cross Phase Modulatio xPON various Passive Optical Network

Aeff Effective Core Area

|E|2 Optical Intensity Inside the Fibre

gB Peak Value of Brillouin-Gain Coefficient

gR Raman-Gain Coefficient

k0 Wave Number

k0i Wave Number of ith Carrier

L Fibre Length

Leff Effective Length

n (ω) Constant Part Given by Sellmeier Equation

n2 Nonlinear Refractive Index Coefficient Related to 3rd Order Susceptibility

β Propagation Constant

ΔnNL(i) Nonlinear Refractive Index of ith Wavelength in XPM

δω Frequency Chirp

ΦNL Nonlinear Phase Shift

ΦNL(i) Nonlinear Refractive Index of ith Wavelength in XPM

ω Anglular Frequency

1

1 Introduction The demand for higher transmission capacity and higher efficiency push optical

communication (OC) to create more complex designs and solutions. In this process, a lot

of various effects occur. Their impact on the transmission depends from particularly

conditions. Optimal setting is a trade-off between introduced distortions due to

phenomena and their relations in the optical fibre and our capabilities to suppress them.

The possibility of pure physical experiments is unacceptable from more points of views.

The response on the current situation is the application of simulation methods as one part

of developing and implementation process. Simulation roughly estimates optimal

parameters and then follows the tuning of physical device(s) or system(s). The simulation

accuracy and therefore success of whole project depends from simulation models and

conditions. At the beginning, credibility of simulation’s outputs should be verified with

basic experiments and then predicts results of desired design.

Second chapter regards the physical phenomena in the optical fibre and within

devices are briefly described. Their relative impact on the total transmission impairments

is evaluated and possibilities of mitigation are considered as well. The events and factors

with positive and also negative consequences on the OC are presumed Current massive

trend in the transmission systems reflects the demand of longer distance and higher bit

rates per channel. Substitution of modulation format has been following as one step to

fulfillment of higher requirements. Primary used on-off keying (OOK) is compared with

differential phase-shift keying (DPSK). Throughout diploma thesis, term DPSK refers to

differential binary PSK sometimes referred to as DBPSK. Properties and applications of

DPSK format consider chapter 3.

PSK detection sensitivity differs from the sensitivity of intensity detection. The

dominant limitation creates phase noise. Its origin results from the finite laser linewidth

and nonlinear phase induced by Kerr effect. The specification of major contributor gives

us system potential. This is the objective of chapter 4, where I want to specify OOK,

DPSK and DQPSK format performance dependents from varying values of amplitude and

phase noise represent by OSNR and laser linewidth for 10 Gb/s and 40 Gb/s. Then,

impact of Kerr effect within NZ-DSF and SMF is evaluated. Assessment will be realized

by simulation software for optical applications “VPI Photonics”, allowing wide range of

WDM and component design.

2

2 Complex approach to optical communication

2.1 Physical characteristic

Today’s optical communication uses transmission medium, that is ultra lossless in

the comparison to other technologies. It offers a bandwidth of 400 nm (equivalent of 54.5

THz), which is defined by loss <0.35 dB/km (see Figure 1) [1]. It is equivalent to 6.8125

billions B channels or 6.8125 millions of TV channels.

Limitations due to losses have got to do three mechanisms:

• Absorption: on the impurities and at IR area (Fig. 1)

• Scattering: predominantly formed by the Raleigh scattering

• Radiating losses: occur at the deep IR area and photonic crystal fibres

» Bending loss: energy emission from the fibre on the turnings, micro and macro

bends at very high wavelengths (Figure 1)

» Seepage loss: energy release through slim cladding, which creates photonic band

gap.

Figure 1: Spectral variation of attenuation; Source [1].

Today’s state of the art fibres are very close to their physical limit and next

enhancement will be able at hollow core fibre based on the band gap photonic crystal

fibre. It has got high losses due to simple manufacturing process. Water content in the

preform and the drawn fibres is too high [2]. Another problem is a manufacturing of the

finesse core surface. Conventional fibres improved over the past 20 years to their current

3

stage. First successful demonstration of hollow core fibre was in 1999 [3]. The question

that arises is how many times do the developers need to overcome processing problems?

2.2 Mitigation of impairments and distortions

Optical pulse is influenced by deterministic and stochastic processes as well during

the whole transmission and which impact the signal features (shape, amplitude and time).

The transfer functions describe deterministic part of the influence. Manufacturing and

also installing processes and their imperfections can be just excesses from calculated

value but they also introduce the stochastic behavior. The source of degradations has got

linear and nonlinear base. Linear processes have an effect on the time domain

characteristics. On other hand, nonlinearities processes influence the spectrum of the

signal and they occur in the case of high power in fibre, which might define more points

of view. Resulting effects have to be evaluated as a compact issue. The level of inevitable

regeneration depends on the working conditions of transmission system, especially on the

number of channels, bit rates and distance.

2.2.1 Nonlinear effects

The response of any dielectric to light becomes nonlinear and intense

electromagnetic fields and optical fibres are no exceptions. On a fundamental level, the

origin of nonlinear response is related to anharmonic motion of bound electrons under the

influence of applied field [4]. Nonlinearities are undesirable during transmission and

have the highest impact at the beginning of amplifier span. They are a limiting factor to

the long distance and high capacity transport systems. The source of nonlinearity is

transmission medium (fibre) and not amplifier [5]. Next, two groups of elastic and

inelastic nonlinear effects, which exist at fibre, will be described.

The simplest way how to cope with nonlinear effects is to prevent them. This can

be achieved via design of new networks, where proper fibre with large effective area can

be applied. Otherwise other methods for specific issue at hand have to be used.

2.2.1.1 Kerr effect

It is elastic in the sense that no energy is exchanged between electromagnetic

field and the dielectric medium. It describes the intensity dependence of the refractive

index. In the simplest form, the refractive index can be written as

2 22( , ) ( ) ( )n E n n Eω ω= + (2.1)

4

The intensity dependence of the refractive index leads to a large number of nonlinear

effects and the two most widely studied are self-phase modulation (SPM), cross-phase

modulation (XPM) and four-wave mixing (FWM) [4].

Figure 2: Products of Kerr effect; [6].

Self-phase modulation

SPM refers to the self-induced phase shift experienced by an optical field during

its propagation. It is responsible for spectral broadening of pulses as a consequence of the

time dependence on nonlinear phase shift ΦNL.

22 0NL n k L Eφ = (2.2)

The time dependence of its time derivation (marked δω) refers to frequency chirping,

Figure 3 a). The Chirp induced by SPM increases in magnitude with the propagated

distance Figure 3 b). The extent of spectral broadening depends on the pulse shape.

5

a) b)

Figure 3: a) Temporal variation of SPM-induced phase shift and frequency chirp for Gaussian

(dashed curve) and super-Gaussian (solid curve) pulse. b) Experimentally observed spectra for a

nearly Gaussian pulse at the output of a 99 m long fibre. Spectra are labeled by maximum phase shift

related linearly to the peak power; [4].

Cross-phase modulation

XPM is always accompanied by SPM when at least two optical frequencies are

propagated simultaneously in the fibre. XPM stands for equally intense optical fields of

different wavelengths the contribution of XPM to the nonlinear phase shift is twice that

SPM. Nonlinear phase shift for the field at ωi is given by

( ) ( )2 0

i iNL i NLn k nφ = Δ (2.3)

22( )

2 2N

iNL i j

j i

n n E E≠

⎡ ⎤Δ = +⎢ ⎥

⎣ ⎦∑ (2.4)

Figure 4: Eye diagram of complete and incomplete collisions; [5].

6

XPM induces timing jitter shown in Figure 4 [5] and it can lead to modulation instability,

asymmetric spectral and temporal changes of copropagating optical pulses [4].

Four wave mixing

FWM transfers energy from strong pump waves to new waves describes

following equations:

4 1 2 3ω ω ω ω= + − (2.5)

4 1 2 3β β β β= + − (2.6)

FWM induces amplitude noise, which is a source of serious degradation shown by Figure

5. However its efficiency is very sensitive on the phase matching [4].

a) b)

Figure 5: a) nonlinear crosstalk due to FWM, b) eye diagram of amplitude noise due to FWM; [5].

Limitations due to Kerr effect have got different nature:

• Interchannel effects: involve phenomena mentioned above and dominate at 10 the

Gb/s channel. These effects have got counterpart at anomalous dispersion regime and

trade-off between both features is strong instrument of how to get optimal conditions

of complex transmission system. Soliton transmission is possible and therefore very

robust pulse over long distance can be achieved. But it has got a lot of issues that have

to be overcome. More modulation techniques are available with the influence on the

final result.

• Intrachannel effects: pulse spreading is high and nonlinear intersymbol interference

becomes the major single-channel penalty, Figure 6. They dominate at 40 Gb/s and

higher bit rate channels [7]. Dispersion induces intrachannel effects, so it is the

limiting factor no more. Different design of dispersion map, which employs fibre

7

spans consisting of three or higher number of concatenated sections is useful. Another

possibility supposes different modulation scheme based on the DPSK format [vsetky

o DPSK]

Figure 6: examples of intrachannel effects; [7].

2.2.1.2 Stimulated scatterings

Optical field transfers part of its energy to the nonlinear medium. Stimulated

Raman scattering (SRS) and stimulated Brillouin scattering (SBS) belong to this group.

They are related to vibrational excitation modes of silica. The main difference is that

optical photons participate in SRS while acoustic photons participate in SBS. In a simple

quantum-mechanical model applicable to both SRS and SBS, a photon of incident field is

annihilated to create a photon at a lower frequency and a photon with the right energy and

momentum to converse the energy and the momentum. Even SRS and SBS are very

similar in their origin, different dispersion relations for acoustic and optical photons lead

to some basic differences.

Stimulated Raman scattering

In any molecular medium, spontaneous Raman scattering can transfer a small

fraction of power (~ 10-6) from one optical field to another, whose frequency is

downshifted by an amount determined by the vibrational modes of the medium. Raman

gain spectrum is specific for the material and for fused silica as shown in Figure 4. This

behavior is caused by amorphous nature of the silica glass and it can be tuned by

dopands.

8

Figure 7: Raman gain coefficient for pure silica and 1550 nm pump wavelength; [8].

SRS occurs when the pump power exceeds a threshold value and then builds up

almost exponentially. The threshold power is explained as follows [4]:

0 16 eff

eff R

AP

L g≅ (2.7)

Typical value of Raman threshold is approximately 500 mW of whole optical power. The

influence of SRS on the WDM system is shown by Figure 5. Impact of SRS is not able to

be reduced by dispersion as not Kerr effect by itself but the dispersion changes result due

to intense presence of Kerr effect [4].

The way to suppress SRS is through power management in optical fibre. First

decision is set the fibre type, mainly its Aeff. Then variation of number of channels and

therefore power level is available. SRS is a challenge and offers next dimension at WDM

design.

Figure 8: The outcome of SRS at WDM system; [5].

9

Stimulated Brillouin scattering

The pump field generates an acoustic wave through the process of

electrostriction. The acoustic wave in turn modulates the refractive index of the medium.

This pump-induced index grating scatters the pump light through Bragg diffraction.

Scattered light is downshifted in frequency due to Doppler shift and propagates in the

backward direction. SBS saturates maximum power per channel in the optical fibre

(Figure 6).

Figure 9: Behavior of input and output power due to SBS; [5].

SBS is a very narrow-band process and its spectral width of gain spectrum is ~ 10 MHz

because it is related to the damping time of acoustic waves. Thus SBS occurs efficiently

for CW pump or pump pulses whose spectral width is smaller than the gain bandwidth.

The threshold power is explained similarly like that of the SRS threshold

0 21 eff

eff B

AP

L g≅ (2.8)

The threshold level predicted by (2.8) is only approximate. The effective Brillouin gain

can be reduced by many factors and therefore SBS can be as low as ~1 mW for a CW

pump and nearly ceases to occur for short pump pulses (width <10 ns) [4].

SBS can be suppressed by dithering laser frequency [5], but is not a limiting

factor of today’s transmission systems.

2.2.2 Amplification

A signal after the passing of specific distance or device has to be amplified on the

required level. Amplifiers set up the real part of device’s transfer function and they are

employed due to compensating losses or setting required power level. Qualitative

characteristics of the amplifier determine the features of transmission system:

10

• Noise figure: describes an inherent process of every amplifier. The noise generated

during amplification has got broadband characteristic and its presence in the signal

band decrease OSNR and transmission capabilities. The noise figure is defined:

IN

OUT

SNRNFSNR

= (2.9)

The maximum number of amplifier spans determines beginning value OSNR, noise

figure of used amplifiers and required OSNR at receiver. Total transmission distance

can be tuned by varying mentioned parameters.

• Bandwidth: is defined by frequencies that belong to 3 dB fall from the peak gain. It

limits the number of channels at WDM system.

• Gain: specifies the level of amplification for the current frequency and therefore

amplifier span or repeater spacing. Longer repeater spans usually mean shorter total

distance due to higher level of ASE. So long haul systems can have a half or less

amplifier span against short haul.

• Gain flatness: it is spectral characteristic of the gain and describes difference of the

lowest and the highest gain from the mean gain. High value induces strong self-

filtering effect, which limits the using of concatenated amplifiers. It will lead to

extremely high dynamic of received signals and drowning of low amplified signals in

the noise. This parameter can be suppressed also after manufacturing by optical gain

flatness filters which can be integrated at amplifier or connected on the amplifier’s

output.

The two amplification’s schemes are suitable to use:

1. Lumped amplification: the amplifiers’ footprint is small comparing to the

transmission size. It is up to 1 km length of fibre. Amplifying is based on the 3 or

4 level model.

2. Distributed amplification: the transmission fibre is also an amplifier

simultaneously. Raman Effect and 3 or 4 levels model is considered as well.

Doped fibre amplifiers

The stimulated emission of the proper quantum jump, which has to be similar to

the transmitted pulses, conducts to the amplification. Rare earth elements are necessary

11

part of structure. It is tuned by appropriate dopants. Today’s knowledge of dopants and

designs reduces their applications on the C, L, O and S band (see Figure 1, Figure 7).

These amplifiers have got high gain and power conversion and the number of

operating windows is sufficient for traffic demand. On the other hand, the minimum level

of noise figure is 3 dB, what can be limit for transoceanic and transcontinental

applications. It influences the length of amplifier’s span. Thus, more amplifiers have to be

deployed with the impact on the cost and reliability. The issue of distributed doped

amplifiers still has been only at theoretical models.

Raman amplifiers

Raman amplifier employs SRS and consists from any transmission fibre and group

of pump lasers lasing on the proper frequencies, which are shifted to upper values to

match maximum Raman gain, 14 THz for silica glass (Figure 8). It can be created by one

laser as well, what depends on the desired parameters. Its operating band is set up by

pump’s wavelength and spectral characteristics are influenced by fibre properties

(dopants and refractive index profile). Raman amplifier is able to amplify whatever band

limited only available laser wavelengths and it is not any barrier.

Figure 10: Characteristic of Erbium amplifier (left) and Raman amplifier (right); [7].

If Raman amplifiers want to reach sufficient gain, they will have to use very high

pump power. Therefore pump efficiency is lower against doped fibre amplifiers moreover

strong Kerr effect occurs with high powers and WDM crosstalk is another issue. On the

other hand they allow to provide negative effective NF in the case of amplification within

the transmission fibre [8].

12

The latest trends in optical amplification have tended to focus on Raman

amplifiers and doped fibre amplifiers are on the edge of interest. Both types have got

unique features, which don’t exclude co-operative realization. Hybrid schemes don’t have

adequate attention. They are able to offer outstanding results (Figure 4) and their potential

should be explored deeper. It will probably happen later, when the requirements on the

transmission will reach higher levels.

Figure 11: Various hybrid amplification schemes; [7].

2.2.3 Shaping

The shape describes modulated envelope applied on the optical carrier. Pulses

used at OC have got finite linewidth and therefore compose from the band of frequencies.

Optical part of spectrum is affected by the dispersion at any material. The dispersion hits

only time domain characteristic of the pulse that assigns individual velocity to every

frequency. It broadens the pulse width during its transmission while the spectrum has

been unaffected It would result to intersymbol interference. It is metaphorically

demonstrated by Figure 5.

13

Figure 12: The dispersion’s influence; [7].

The sequence of frequencies depends from the dispersion regime:

• Anomalous dispersion regime: blue part of pulse goes faster than red part.

• Normal dispersion regime: red part of pulse goes faster than blue part.

The dispersive influence of the optical fibre is called Chromatic dispersion that consists

from two parts:

• Material dispersion: characterizes dispersion properties of the current material.

• Waveguide dispersion: is caused by different refractive index in the core and

cladding due to pulse propagation in the core and also in the cladding, where it has

different velocities. It is variable and permits the tuning by the refractive index

profile. It describes Figure 13.

Figure 13: Material, waveguide and chromatic dispersion for current fibres;

The issue of chromatic dispersion is more important as the bit rates per channel

increase. For a system penalty of 1 dB, the bit rate, dispersion and distance are related as

follows [10]: The dependence of system

2 5 210 / ( / )B DL ps nm Gb s≈ (2.10)

14

It means that four times higher bit rate allows sixteen times smaller tolerance than earlier

value of tolerance. The need of dispersion compensation can accomplish more

technologies:

• Dispersion compensating fibres: use fibre with negative chromatic dispersion joined

after typical fibre and creates part of route. Dispersion is tuned by waveguide

dispersion and modern DCF offer similar value of attenuation and high negative

chromatic dispersion but effective core area is smaller due to refractive index profile.

Recently DCF has overcome other way of dispersion compensation and it is the most

frequent technique.

• Fibre Bragg grating: short piece of fibre up to one meter with concatenated

refractive index pattern, which is designed to refract the incoming frequencies in the

aim to clear their group velocity dispersion. FBG imposes degradation, which can

express NF and it has got high thermal sensitivity and small effective core area [red

book]. It has to use the circulator for proper work.

Figure 14: Dispersion compensation by FBG; [9].

• Optical filters: contain several filter approaches. One of them is already mentioned

FBG, which represents ARMA response and other possibilities is planar waveguide

that meet MA and optical all-pass filter with constant magnitude response. Those

alternatives are used mainly at dispersion equalization [10].

• Soliton transmission: use optical pulses with special shape and power level, where

SPM and dispersion are at equilibrium. Technical issues haven’t allowed commercial

deployment.

Spread pulse

circulator

Chirped FBG

15

Dispersion compensation can perform symmetrical or non symmetrical management. The

first possibility pre-compensates on the inverse value of dispersion maximum. On the

other hand, non symmetrical technique works only at positive interval. On the Figure 10,

there are the results of simulation (left one) and experiment (right one) of symmetrical

and non symmetrical dispersion compensation in the centre (upper row) and in the end

(lower row) of the span [5]. It is clearly seen that symmetrical compensation gives better

conditions for transmission.

......................................... a) b) c) d)

Figure 15: a), c) symmetrical dispersion management; b), d) non symmetrical dispersion

management; [5].

• Dispersion slope: is third member in the Tayler expansion of the propagation

constant. It assigns different value of the dispersion for the various frequencies Figure

11. It introduces next parameter, which characterizes property optical fibre and it has

to be compensated as well. The dispersion compensation doesn’t mean dispersion

slope compensation and it is equalized at the end of the transmission.

Figure 16: The effect of dispersion slope for border and central frequencies; [5].

16

• Polarization mode dispersion: PMD is has got different principle against

phenomena mentioned above. Its stochastic character gives atypical features compare

with previous effects. Whole impact is described by:

» 1st order PMD: wavelength independent, time variant and increases with root of the

length.

» 2nd order PMD: wavelength dependent and occurs after suppression of 1st order

PMD.

The source of PMD is random birefringence along the link and in the components. It

causes various velocities of both parts of linearly polarized mode. Consequently the

pulse spreading occurs (Figure 12). It becomes serious issue from 10 Gb/s and it can

induce temporally high increase of BER. Thus, PMD is important limitation factor at

high speed networks.

Figure 17: PMD due to birefringence effect; [11].

The best way of PMD suppression is to prevent induction of birefringence by the

using of precise technological and installation techniques. Then PMD equalizers can be

employed at the end of link for single channel [4] or low speed polarization-scramblers at

the beginning of the fibre, which should decrease the PMD on the tolerant level [12],

[13]. The last possibility is more efficient than PMD equalizers due to its application on

the whole WDM band.

2.2.4 Jitter

Jitter is stochastic process of every component within transmission system. This

section considers just optical contribution to the jitter. PMD is the strong contributor of

jitter and its origin and suppression was described earlier. Frequency jitter of laser

17

linewidth is transferred to the timing jitter by dispersion of the fibre. Thus, precise and

stable lasers and filters will produce less timing jitter [5]. Another component is

fluctuations of refractive index due to high powers and interaction with dispersion.

Symmetrical dispersion compensation is suitable to remove this effect showed by Figure

13. But this dispersion scheme has to design with amplification scheme and consider

amplifier position due to maintenance of low Kerr effect. Bit-pattern-dependent effects

can remove modulation format with constant power level at every state.

Figure 18: Cancellation of timing jitter with symmetric dispersion profile; [5].

2.3 Modulation format

The information can transmit signal’s amplitude, phase, frequency or combination

of those features. Huge bandwidth offered by optical fibre and high value of laser’s

linewidth have established OOK like gold standard. It offers three variations, NRZ, RZ

and CSRZ, which are more resilient to some effects and also more sensitive to some of

them.

• NRZ OOK: the most tolerant on the chromatic dispersion and optical filtering.

• RZ OOK: the best performance against intra-channel nonlinear effects after

transmission due to the best performance in back-to-back.

• CSRZ OOK: the superior resilience to nonlinearities and represents satisfactory

opinion to meet all key requirements for future all-optical networks [14].

It seems that basic features of the OOK don’t match demands for future all-optical

networks. More precise lasers with narrower linewidth have opened possibilities of phase

shift keying. Today’s situation is suitable for the developing of DPSK transmitters with

18

balanced receiver due to its economical efficiency. DPSK overcomes OOK at following

points:

• 3 dB lower required OSNR for the same BER (theoretically)

• 3 dB lower peak power

• Sensitive on the phase noise

• More resistant against nonlinear effects

• More robust at higher bit rates

2.4 Economical aspects

If we don’t want to do research for research, optimal implementation of every

technology will have to meet following rule, where optical communication (OC) and its

basic, pillar optical technologies (OT), is not an exception:

Solution = ƒ (service)

OC is a part of global market and tries to meet its demand. The predictions of

development are very volatile and whole process of the creating of estimation is difficult.

I specify main factors affects on the OT’s boom and recession.

2.4.1 Catalysts of OT

Traffic growth

Since the year 2000, global traffic has been dominated by internet data. Studies of

traffic of the year 2005 indicated a global traffic growth of 115% per a year. Estimation

of Atlantic traffic through 2025 is illustrated by the Figure 19. Other driving factors

would be globalization of FTTx-xPON access for 10-100-1000 Mb/s BB solutions and

installation and services cost reduction to the current level of xDSL as we are witnesses

nowadays [1].

19

Figure 19: Comparison of IP traffic predictions with installed capacity; [1].

The open issue has been the conditions of IP traffic growth, which stimulates demand for

higher quality multimedia. There will probably be the ultimate limit that average user will

be able to comprehend, but this has yet to be reached [15].

Service convergence and their adaptability to NGN

Sectors of IT and CT have converged to the common platform due to removing of

interworking restrictions and creating open platform environment by the way. This

process has developed with success and disappointment as well. The cheapest alternative

based on IP has been chosen as the basic transport protocol that is not optimal solution for

switching circuit services requiring guaranteed parameters of transmission. Therefore

upgrades to the connection oriented services have to be implemented and they need

approximately up to twice bandwidth compared to legacy systems and adequate margin in

the congestion protection.

Those factors have brought better managing and flexibility of services, unclear

savings and on other hand higher requirement on the capacity of core network.

Introduction of new services

This area includes the roll-out of new services or improving their existing quality.

The main objects of interest are multimedia and real time applications. The latest

demonstration is Triple Play. Its strategy is to offer voice, Internet and video through one

data access. The potential is huge and there will be a lot of modifications.

Dimension of that service is not able to realize without OT. There is also

opportunity for radio and metallic connections but only as final section between customer

and multiplexer.

20

2.4.2 Inhibitors of OT

Providers’ concentration on the short-time objectives

Low-cost solution with fast market returns have become the key driving concerns

of OC innovation. This emphasis prevents industrial and academic research from

exploring technologies that are deemed to immature for short-term deployment and that

have no immediately business value. The phenomenal growth of capacity in recent

wireless BB services has provided a perception that bandwidth is infinite.

But new driving factors to appear in the next 3-7 years, should steer OC industry,

namely: exploding bandwidth demand, lightwave capacity exhaustion and facing ultimate

technology limits [1].

Establishing of the new layer model

One way how to satisfy the demand for higher capacity is revolutionist change of

interconnection’s model and direct mapping from 3rd layer onto physical medium with

new non redundant management either IP or optical layer. This approach represents

IPoWDM and it would replace mapping through ATM and SDH to the WDM layer.

This technique crashes on the deep penetration of the current technology and it

would be alternative solution in the extreme cases.

Technological development at cooperative sectors

OC is a one ring in chain, which brings the service from the server to the

customer’s application. The reliability of service providing determinates the crucial

process as the chain’s strength depends from the weakest ring.

This idea includes negative and also positive development of the similar

technologies. Huge progress on the field of DSP at 1990s meant stop for commercial

deployment of ultra high speed optical systems due to enormous video compression.

Insufficient development of server performance would restrict data traffic and it will

decrease progress of whole ICT sector.

Other reasons

This section should describe phenomena that haven’t got rational base or I haven’t

recognized. One example of my ideas is technological deflation at the end of 20th

Century. It still has been finding the lost goodwill of the investors.

21

3 Differential phase-shift keyed format Phase-shift-keyed (PSK) formats carry the information in the optical phase itself. The

receiver has to compare detected signal with reference signal and extract information.

Due to the lack of an absolute phase reference in direct-detection receivers, the phase of

the preceding bit is used as a relative phase reference for demodulation. This results in

DPSK formats, which carry the information in optical phase changes between bits. DPSK

has got several advantages against ASK modulation thus it is not surprising, that many of

the recent long-haul WDM transmission records at per-channel rates of 10 and 40 Gb/s

are now held by systems based on DPSK.

Optical systems based on DPSK are not new. DPSK was extensively studied in the

late 1980s and early 1990s for use mainly in single-span fiber-optic systems employing

coherent receivers as well as in the context of free-space optical communications, where

the 3-dB sensitivity advantage over OOK could be exploited. When erbium-doped fiber

amplifiers (EDFAs) were introduced, interest in coherent systems declined. For about a

decade, OOK-based WDM systems using optical-amplifier repeaters dominated the

research in long-haul optical communications. Interest in DPSK reemerged several years

ago, as WDM systems were pushed to ever-higher levels of performance [16].

3.1 DPSK format

In the DPSK format, optical power appears in each bit slot, with the binary data

encoded as either a 0 or π optical phase shift between adjacent bits. The optical power in

each bit can occupy the entire bit slot (NRZ-DPSK) or can appear as an optical pulse

(RZ-DPSK). The most obvious benefit of DPSK when compared to OOK is the ~ 3-dB

lower OSNR required to reach a given BER. This can be understood by comparing the

signal constellations for DPSK and OOK, as shown in Figure 20. For the same average

optical power, the symbol distance in DPSK (expressed in terms of the optical field) is

increased by √2. Therefore, only half the average optical power should be needed for

DPSK as compared to OOK to achieve the same symbol distance. This ~ 3 dB benefit of

22

Figure 20: Signal constellation of OOK and DPSK modulation; [16]

DPSK modulation can be only extracted by using balanced detection. In practice, and

neglecting the loss of the optical preamplifier’s input optical isolator, a receiver

sensitivity of 60 photons/bit has been reported for a 10-Gb/s RZ-OOK signal. Using an

RZ-DPSK signal and a balanced-photodiode detection scheme, the sensitivity was

improved to 30 photons/bit. At 42.7 Gb/s, a sensitivity of about 38 photons/bit has been

reported using RZ-DPSK. Again, this is approximately 3 dB better than the best OOK

results of 78 photons/bit. The lower OSNR requirement of DPSK can be used to extend

transmission distance, reduce optical power requirements or relax component

specifications.

DPSK with balanced detection has been demonstrated to offer large tolerance to

signal power fluctuations in the receiver decision circuit because the decision threshold is

independent of the input power. DPSK is more robust to narrow-band optical filtering

than OOK, especially when balanced detection is employed. Numerical simulations and

experiments have shown DPSK to be more resilient than OOK to some nonlinear effects.

This results from the fact that: i) the optical power is more evenly distributed than in

OOK (power is present in every bit slot for DPSK, which reduces bit-pattern-dependent

nonlinear effects) and ii) the optical peak power is 3 dB lower for DPSK than for OOK

for the same average optical power. Finally, an extension to differential quadrature phase-

shift keying (DQPSK) and other multilevel formats should enable higher spectral

efficiency and greater tolerance to chromatic- and polarization-mode dispersion [16].

23

3.2 DPSK transmitter

Two commonly used RZ-DPSK transmitter setups are shown in Figure 21. The

transmitters consist of a continuously oscillating laser followed by one or two external

modulators, typically based on technology. Phase modulation can either be performed by

a straight-line phase modulator [PM, Figure 21 (a)] or by a Mach–Zehnder modulator

[MZM, Figure 21 (b)]. A PM only modulates the phase of the optical field, resulting in

aconstant-envelope optical signal [see measured sampling scope traces in Figure 21 (a)].

Since phase modulation does not occur instantaneously, a PM inevitably introduces chirp

across bit transitions [see symbol diagram in Figure 21 (a)]. A sinusoidally driven second

modulator (“pulse carver”) may be used to carve pulses out of the phase-modulated

signal, thus generating RZ-DPSK. The inset to Figure 21 (a) shows the resulting optical

power waveform. (The seemingly limited extinction ratio of the RZ-DPSK pulses is a

measurement artifact, caused by detecting a 40-Gb/s signal using a 32-GHz-bandwidth

photodiode.)

When using a MZM for phase modulation, the modulator is biased at its

transmission null, and is driven at twice the switching voltage required for OOK

modulation. If a z-cut MZM is used, it is driven in push-pull configuration to minimize

chirp, whereas an x-cut modulator requires only a single electrical drive. Since the phase

of the optical field changes its sign upon transitioning through a minimum in the MZMs

power transmission curve, two neighboring intensity transmission maxima have opposite

optical phase, and a near-perfect 180 phase shift is obtained, independent of the drive

voltage swing. As can be seen from the symbol diagram in Figure 21 (b), the benefit of

highly accurate phase modulation comes at the expense of some residual amplitude

modulation at the transition of two bits, with the width of the resulting intensity dips

depending on the drive signal’s bandwidth and voltage. However, since DPSK encodes

information in the optical phase rather than in the intensity, these dips are of reduced

importance, especially for RZ-DPSK, where the pulse carver cuts out the amplitude-

modulation-free center portions of the bits only, and thus largely eliminates any residual

dips [16].

24

Figure 21: Typical DPSK transmitters: a) implementation with PM, b) implementation with MZM;

[16]

3.2.1 Transmitter evaluation

Transient chirp for PM-based DPSK transmitters and intensity dips for MZM-

based transmitters, the effects of drive waveform imperfections are worth mentioning. The

nonlinear (cosine) transmission curve of the MZM ameliorates the impact of drive-

waveform overshoots or of limited drive-signal rise times. Any remaining imperfections

are only translated into optical intensity variations, but the information-bearing optical

phase is left intact. On the other hand, using a PM for phase modulation, any drive-

waveform imperfections get directly mapped onto the optical phase, thus potentially

degrading performance. Difference of two modulators will arrive, if the driver output

power and the combined driver-plus-modulator bandwidth cannot be chosen arbitrarily

high, which is the case in practice, especially for high-data-rate systems. [16].

3.2.2 Pulse carver

Since DPSK carries information in the phase of the optical signal, optical phase

distortions (such as chirp) will have a severe impact on DPSK receiver performance. At

the transmitter, phase distortions may be caused by imperfect pulse carvers. In order to

operate chirp-free, a dual-drive MZM pulse carver has to have infinite DC extinction, and

25

has to work in perfect push-pull operation, i.e., the sinusoidal drive amplitudes have to be

of the same amplitude and of opposite phase. Any deviation from this ideal condition

inevitably produces chirp. Figure 22 (a) shows three commonly used ways of pulse

carving by applying a sinusoidal drive signal to a MZM-based pulse carver.

Figure 22: a) A typical RZ-DPSK transmitter. (b) Optical intensity and phase waveforms generated

by an imperfect pulse carver; [16].

Three important facts are evident from the optical intensity and phase waveforms

shown in Figure 22 (b): First, when sinusoidally carving at the data rate (50% RZ), the

residual optical phase variations are identical for each bit, while they are different for

adjacent bits when carving at half the data rate (33% and 67% RZ). Since it is the

difference between the optical phase of two adjacent bits that is used to decode DPSK

signals at the receiver, higher degradations due to pulse carver chirp are found for 33%

and 67% duty cycle RZ-DPSK than for 50% RZ-DPSK. Second, we see from the

opposite phase curvatures (50% and 67%) or slope (33%) that chirp due to finite DC

extinction ratios of the MZM can partially be compensated by imbalancing the drive

amplitudes. Third, we notice that for 33% RZ a drive-signal amplitude imbalance leads to

linear phase transitions (i.e., to optical frequency shifts) at pulse center, while a drive-

signal phase error produces a phase offset at pulse center. Since pure bit-alternating

frequency offsets do not disturb the phase difference between adjacent bits at pulse center

(where the intensity is highest, and thus the contribution to the demodulated signal is

largest), a higher tolerance is found for drive amplitude imbalance than for drive phase

errors in the case of 33% RZ. For 67% RZ, the situation is opposite, and we find a higher

26

tolerance to drive phase errors than to drive amplitude imbalance. Experimental as well as

numerical quantifications of pulse carver tolerances for RZ-DPSK can be found in [16].

3.3 DPSK Receiver

A typical balanced DPSK receiver is shown in Figure 23. The optical signal is first

passed through a Mach-Zehnder delay-interferometer (DI), whose differential delay is

equal to the bit period. This optical preprocessing is necessary in direct-detection

receivers to accomplish demodulation, since photodetection is inherently insensitive to

the optical phase; a detector only converts the optical signal power into an electrical

signal. In a direct-detection DPSK receiver, the DI lets two adjacent bits interfere with

each other its output ports. This interference leads to the presence (absence) of power at a

DI output port if two adjacent bits interfere constructively (destructively) with each other.

Thus, the preceding bit in a DPSK-encoded bit stream acts as the phase reference for

demodulating the current bit. Ideally, one of the DI output ports is adjusted for destructive

interference in the absence of phase modulation (“destructive port”), while the other

output port then automatically exhibits constructive interference due to energy

conservation (“constructive port”). For the same reason, the two DI output ports will

carry identical, but logically inverted data streams under DPSK modulation.

Figure 23: DPSK receiver; [16].

27

Careful analysis of the optically demodulated signals at the DI output reveals that the

constructive port carries duobinary modulation, whereas the destructive port carries

alternate-mark inversion (AMI). Today, technical difficulties in implementing stable

delay interferometers have been overcome, and DIs have been demonstrated both in fiber-

based and in planar-lightwave-circuit (PLC) technologies. Fine-tuning of the differential

delay to match the laser center frequency and achieve good interference quality is

typically achieved using a heating element on one of the interferometer arms. Also,

polarization-dependent phase shifts within the DI have to be avoided.

Since both DI outputs ports carry the full (logically conjugated) information, they

can be either detected by themselves (“single-ended detection”), or connected to two

photodiodes using a balanced receiver (see Figure 23). Identical path lengths between the

output coupler of the DI and the point of subtraction within the balanced receiver can be

achieved using variable optical delay units or photonic integration of the detectors with

the DI. Alternatively, separate detection of both output ports in combination with joint

digital signal processing can be applied. The 42.7-Gb/s eye pattern at the output of the

balanced-detector circuit obtained in our experiments is shown in Figure 23 [16].

3.3.1 Balanced versus single-ended detection

Introduction of a detector amplitude imbalance, β, is defined as

A B

A B

S SS S

β −=

+ (3.1)

where SA and SB are the overall opto-electronic conversion factors for the destructive (A)

and constructive (B) DI output ports, respectively. Balanced detection is achieved for

SA=SB, while detection of the constructive (destructive) port alone is found for β=1(β=-1).

Figure 24 (solid curve) shows numerical calculations for the required OSNR at BER = 10-

10 for 33% RZ-DPSK as a function of receiver amplitude imbalance β. It can be seen that

a balanced DPSK receiver performs about 2.7 dB better than its single-ended counterpart.

Also shown (dashed curve) is the required OSNR at BER = 10-10 for OOK, which is (by

definition) independent of β, and comparable to the OSNR needed for single-ended

detection of DPSK. This shows that the frequently cited “3-dB benefit” of DPSK over

OOK, neither is exactly 3 dB, nor is a property of DPSK alone; it is a property of the

modulation format in combination with the detection scheme.

28

Figure 24: numerical calculations for the required OSNR at BER = 10-10 for 33% RZ-DPSK and

OOK as a function of receiver amplitude imbalance β; [16]

Balanced detection in a beat-noise-limited scenario has to be numerically modeled using

the exact probability density functions (PDFs) of detection noise rather than Gaussian

approximations to these PDFs, as is common practice and works well for single-ended

OOK receivers. If Gaussian PDFs are used to represent the noise statistics at the decision

gate, we obtain the dotted curve in Figure 24. While being reasonably accurate for single-

ended DPSK detection, the Gaussian approximation (as well as all simulation techniques

based on the Gaussian noise assumption, such as a standard -factor analysis) is bound to

fail in predicting balanced DPSK receiver performance. The reason for this important

simulation aspect rests in the fact that the tails of the exact (chi-square-like) PDFs differ

significantly from the tails of the Gaussian distributions. For single-ended detection of

DPSK as well as for OOK, this difference in the PDFs, by pure numerical coincidence,

cancels to a high degree of accuracy when calculating BER. In contrast, this beneficial

cancellation is not found for balanced receivers, owing to the different nature of

detection: a single-ended receiver compares a single, noisy signal against a deterministic

(non noisy) threshold to retrieve the digital data, while a balanced receiver essentially

compares two noisy signals against each other [16].

Qualitative physical aspects affected on the detector imbalance include amplitude

imbalance, temporal receiver imbalance, interferometer extinction, delay, phase error,

frequency offsets. Their quantitative impact is shown by Figure 25.

29

Figure 25: Penalties in non ideal RZ-DPSK receivers: (a) Amplitude imbalance in the balanced

detector (dashed curve is for a delay interferometer with an extinction ratio of only 10 dB), (b) Phase

imbalance in the balanced detector, (c) Delay-to-bit rate mismatch in the delay interferometer, (d)

Laser frequency offset from the ideal as set by the interferometer phase difference (dashed curve is

for 33% RZ-DPSK). Circles are experimental results; [16].

3.3.2 Tolerance to Optical Filtering

As mentioned previously, DPSK is more tolerant of tight optical filtering than

OOK. The reason for the good performance can be attributed to the use of ISI-tolerant RZ

coding and higher robustness of balanced DPSK to reduced optical filter bandwidths.

Figure 26: Experimental results of penalties from narrow optical and electrical filtering; [16]

30

Figure 26 shows 40-Gbit/s measurement results for 33% RZ-DPSK. The BER target was

10-9. The gain of balanced DPSK reception over single-ended detection is seen to be some

4 dB, and increases to over 5 dB at low optical bandwidths, where both OOK and the

destructive DI output port show severe penalties [16].

3.4 DPSK transmission at 10 Gb/s

In a linear system employing optical amplifiers, the 3-dB DPSK advantage over

OOK would double the achievable distance by allowing the accumulation of twice as

much amplified-spontaneous-emission (ASE) noise. Limitations from chromatic

dispersion (CD) and polarization-mode dispersion (PMD) are similar for DPSK and OOK

signaling, some chromatic dispersion advantage has been reported for NRZ-DPSK.

However, transmission performance in fiber is also affected by the Kerr nonlinearity. This

is exhibited as four-wave mixing (FWM), self-phase modulation (SPM) and cross-phase

modulation (XPM).The extent of these effects depends on several system design factors,

including average optical power, peak optical power, modulation format, transmission

regime (pulse-preserved or pulse-overlapped), and the nonlinear interaction of signal with

ASE noise.

As in OOK systems, dispersion management can be used in DPSK systems to

reduce the FWM efficiency among WDM channels to low levels. Therefore, interchannel

FWM is generally not a concern. SPM and XPM affect DPSK signals somewhat

differently than OOK signals. In SPM, the intensity variations of an optical signal

modulate the signal’s optical phase via the nonlinear refractive index, causing a red shift

on the rising edges of pulses, and a blue shift on the falling edges. The effect is to broaden

the signal spectrum. The broadened signal spectrum, combined with dispersion, then

broadens the received pulses, introducing a transmission penalty (although we note that

solitons balance SPM and dispersion to maintain pulse shape). For DPSK signals, an

additional effect is important, because the information is carried by the optical phase.

Noise-induced power fluctuations are converted into phase fluctuations by SPM, and

become a source of transmission penalty. This nonlinear interaction of signal and noise is

referred to as the Gordon–Mollenauer effect. In the nonlinear regime, performance

should be substantially different in the two cases for a DPSK signal, while remaining

essentially unchanged for an OOK signal. After nonlinear transmission, the “Q”-factor of

the DPSK signal depended strongly on the transmitter OSNR, whereas the “Q”-factor for

the OOK signal did not.

31

In XPM, the intensity of one signal modulates the phase of another. In OOK systems, the

“collisions” of WDM signals passing through each other impart phase variations that,

when combined with dispersion, result in pattern-dependent timing jitter of the received

pulses. Complete collisions at near-constant power cause less harm, as the phase

variations caused during the first half of the collision are largely undone in the second

half. However, as WDM channels are placed closer together, the difference in

propagation speed between adjacent channels becomes lower, the pulses move through

each other more slowly, and partial collisions increase the XPM penalty. DPSK signals,

however, exhibit power in every bit slot. Therefore, all pulses in a given WDM channel

experience similar collisions, mitigating the XPM effect. Of course, a second-order effect

is expected, as noise-induced amplitude fluctuations on pulses in one channel cause phase

fluctuations in another. Generally speaking, it appears that long-haul 10-Gb/s single-

channel OOK systems can outperform DPSK systems, which are limited by the Gordon-

Mollenauer effect, although we stress that single-channel performance is highly system-

dependent, and that there can be cases in which DPSK will outperform OOK. In 10-Gb/s

WDM systems, both experimental measurements and computer simulations indicate that

DPSK and OOK perform similarly at a spectral efficiency of 0.2 b/s/Hz. At a spectral

efficiency of 0.4 b/s/Hz and higher, DPSK, due to its increased robustness to XPM, can

outperform OOK. However, we again stress that system performance is dependent on

many factors, including channel power and dispersion map [16].

3.5 DPSK transmission at 40 Gb/s

At 40-Gb/s, single-channel effects in the pulse-overlapped (pseudolinear) regime

mainly limit signal transmission. In particular, intrachannel FWM (iFWM) transfers

power between bit slots as pulses disperse into each other and mix due to fiber

nonlinearity. The effect in OOK is amplitude fluctuations on the “1s”, and “ghost pulses”

(residual power) on “0s”. In DPSK systems, the phase fluctuations from this mixing are

more detrimental than amplitude fluctuations. In intra-channel XPM (iXPM), intensity

fluctuations of the dispersed, overlapped pulses modulate the optical phase. The effect in

OOK is timing jitter when combined with dispersion, while in DPSK, both the timing

jitter and the phase fluctuations are detrimental. As mentioned earlier, undistorted DPSK

has 3-dB lower peak power than OOK for a given average power, due to having power in

each bit slot. In the pulse-overlapped regime, nonlinear DPSK penalties can be reduced

because of this more smoothly distributed power. Also, correlation between the nonlinear

32

phase shifts experienced by adjacent bits (due to experiencing a similar environment in

transmission), combined with differential detection and should reduce nonlinear DPSK

penalties. Experimental results have consistently shown better performance for DPSK

than OOK in 40-Gb/s single-channel and WDM systems [16].

3.6 DQPSK application

There have been a number of applications that have been proposed and

demonstrated for PSK. One of these is to increase spectral efficiency through the use of

multilevel signaling. In particular, DQPSK has recently received intense study. The most

widely used implementation of a DQPSK transmitter and receiver is shown in Figure 27.

The transmitter consists of two parallel DPSK modulators that are integrated together in

order to achieve phase stability (a serial arrangement is also possible, and has been used

Figure 27: DQPSK transmitter and receiver; [16].

in experimental demonstrations). The receiver essentially consists of two DPSK receivers,

although the phase difference in the arms of the delay interferometers is now set to +π/4

and -π/4. The benefit of DQPSK is that, for the same data rate, the symbol rate is reduced

by a factor of two. Consequently, the spectral occupancy is reduced, the transmitter and

receiver bandwidth requirements are reduced, and the chromatic dispersion and PMD

limitations are extended. As compared to DPSK, the required OSNR to reach a given

BER is increased by about 1–2 dB, depending on the BER. Also, the frequency offset

tolerance between the laser and the delay interferometer is about six times less than for

DPSK, making the DI design and stabilization somewhat challenging. Even higher

spectral efficiency can be achieved using various combinations of phase- and amplitude-

shift keying. Such multilevel modulation can also improve system tolerance to chromatic

33

dispersion and PMD. However, these schemes quickly become quite complicated to

implement, require higher OSNR, and are sensitive to nonlinear phase noise [16].

34

4 Trends in the optical communication Development of OT has got good drive although it doesn’t reach its maximum from

the last decade of 20th Century. History, the newest requires and economical background

have formed design of OC. One clearly defined direction is price cut and rising bit rates

per channel up to 40 Gb/s. Those hints can realize more ways and their short description

follows.

4.1 Transmission bands and spectral efficiency

The choice of transmission band is not very difficult. Physical properties of existing

infrastructure and compensation technique lead to the C and L band exploitation. They

are feasible in the case of required capacity, availability and reliability. Higher

requirements can handle two alternatives that represent the using of another band or

improving spectral efficiency. Second variant has applied till now. It includes narrower

channel spacing and multilevel modulation. Its potential is finite and occupying of next

band will come sooner or later.

4.2 Multiplexing techniques

The growing demand for higher bit rates can accommodate two different

multiplexing techniques, which have got different approaches of the exploitation of

offered bandwidth. Another fact is enhanced characteristics of available sources including

narrower linewidth.

4.2.1 Time division multiplex (OTDM)

OTDM is serial transmission of number time delayed channels with lower bit rates

to the common channel at one wavelength. Key parameters are at time domain. Its huge

bandwidth is very sensitive on the value of total dispersion and timing jitter. This one

channel hasn’t got enough power to induce nonlinear effects. OTDM allows dynamic

allocation of offering transmission capacity, BoD service. Then, it supports QoS and

optical burst switching (OBS) and optical packet switching (OPS) are supposed but every

device has to work at one data format and their number may be high.

35

Optimal deployment of OTDM is metropolitan and access networks, where bit

rate flexibility is required.

Four times higher speed WDM requires four times higher number of link

terminated devices against the same number of link terminated devices at OTDM. On the

other hand, OTDM requires precise dispersion management with low PMD and higher

OSNR. The transmission capacity of OTDM reaches only fragment of DWDM

performance due to maximum frequency values and lack of capabilities for processing in

optical domain. The parameters of the state of the art electronics is shown by Table 1

[17]. HEMT HBT

GaAs InP SiGe InP

Mux/Demux Gb/s 45 1441 1322 1203

Driver @ 40G VPP 8 - 5 124

1 Fujitsu, 2 IBM, 3 NTT, 4 Lucent

Table 1: Parameters of ultra high speed electronics; [17]

OTDM hasn’t reached required feasibility and the metro and access networks use

others technological solutions.

4.2.2 Wavelength Division Multiplex

WDM is parallel transmission of number of channels within one fibre. The key

parameters are at spectral domain. Thus, nonlinear effects and linewidth of sources are

cardinal issues. The chromatic dispersion and PMD become crucial at long haul systems.

Basically, it is circuit switching technique, which forms its characteristics such as higher

employed bandwidth of the fibre, the channel independence, data format transparency.

The optimal application for WDM deployment is at core and backbone networks.

4.3 Simulation

4.3.1 DQPSK format

BER curve of PSK modulation describes Figure 30 a). There are experimental

results of DQPSK format [18], and the object of investigation was impact of various

values of OSNR and laser linewidth on the narrowband filtered DQPSK modulation in

36

the back-to-back configuration. It represents BER of the transmitter. I have used it as

verification of my simulation model. Its experimental setup is shown by Figure 28.

Figure 28: Experimental setup of; [18].

I created equivalent setup to its scheme and it is shown by Figure 29. I used

generator of additive white Gaussian noise (AWGN) instead of optical amplifier.

Figure 29: Simulation scheme of BER = ƒ (OSNR).

NRZ-DQPSK source consists from CW laser followed by two dual stage Mach-

Zehnder modulators. The used bit rate was 10 Gb/s. The source output power and power

density of AWGN was set on the values, that corresponded to OSNR = 5 dB. Attenuator

modified level of OSNR in the range from 5 to 20 dB. Signal passed through 1. order

Gaussian bandpass filter with 6 GHz linewidth (spectral efficiency equals to 1.67 b/s/Hz),

what was identical with experiment. Signal was demodulated in the optical delay line

demodulator and detected at balanced receiver. At the end, BER was assigned to every

value of OSNR. It was done for 0, 10, 20, 50 and 100 MHz laser linewidth. The

simulation results are shown by Figure 30 b).

37

a) b)

Figure 30: Comparison of experimental [18] and simulation output.

Maximum deviation was in the range of one order, what gave sufficient

likelihood. Next step involved increasing of bit rate on the 40 Gb/s and filter bandwidth

expansion on the equal value of spectral efficiency. The verification was based on the two

assumptions:

1. theoretical prediction that the same BER value for 40 Gb/s transmission requires 6

dB higher OSNR level compared to 10 Gb/s transmission due to noise power

2. application of previous prediction and obtained results from first simulation.

The results for 10 and 40 Gb/s are shown by Figure 31.

38

Figure 31: BER vs OSNR for 10 Gb/s and 40 Gb/s DQPSK format.

The previously mentioned assumptions were not accomplished completely. Only

ideal source met prediction completely. Other curves haven’t got typical inflexed point

and don’t converge to typical value for current phase noise (corresponds to linewidth). It

is partially satisfied approximately until 15 dB or BER = 10-4. The variant with 40 Gb/s

bit rate was not estimated with desired likelihood. The reason of that act may be various.

One possibility could be small time window and only small part of frequencies had been

generated and processed in the BER estimator. Another variant comprises problem(s) at

simulation model. Incorrect conditions, impertinent algorithm or mistake in the algorithm,

which can skip noise at calculation, because proper dependence occurs in the convex part

of curve (until 15 dB) and the absence of inflexed point concave part of curve. The first

one couldn’t be tested due to lack of available memory. Second one is the good question

to designers of software.

Finally, the 40 Gb/s case hasn’t been considered.

4.3.2 DPSK format

Another simulation investigated DPSK modulation. The setup was modified by

DPSK components. DPSK source consisted from CW laser and one dual stage Mach-

39

Zehnder modulator. Laser power was set on one half of the previous value. Filter has got

double bandwidth due to the same spectral efficiency. One delay line interferometer with

single balanced receiver was employed and BER estimation provided the same module.

The scheme shows Figure 32.

Figure 32: Simulation scheme of BER = ƒ (OSNR).

I haven’t got experimental data from DPSK measurements. Thus I used theoretical

predictions and previously verified DQPSK simulation. DPSK should have identical

behavior like DQPSK (shape of BER curve). Two signal’s states of DPSK against four

signal’s states of DQPSK are able to reach the same BER level at worse conditions,

because the linewidth has to be much lower than modulation bandwidth.

The transmission bit rate was 10 Gb/s. The output data corresponds to theoretical

predictions that DPSK is more resistant against phase noise. The divergence of

particularly curves is not rapid, but typical flat waveform occurs for 1 GHz linewidth. The

OSNR difference between DQPSK and DPSK shows better performance of DPSK. Its

benefit increases with lower BER. Identical trends but different setup is reported too [18].

40

Figure 33: BER vs OSNR for 10 Gbs and 40 Gb/s DPSK format.

The DPSK source generated 40 Gb/s stream didn’t repeat the specific behavior of

coherent systems with phase noise. The Flat waveform is missing but convex part is

assessed very well, but only one curve with inflexed point and it fused with others. This

case excludes possibility of low value of time window, because source with 1 GHz

linewidth was described correctly at 10 Gb/s bit rate. It indicates problem at simulation

model.

The simulation variant 40 Gb/s DPSK will not be joined at next considerations.

4.3.3 OOK format

The simulation of OOK format didn’t give reasonable BER values. It has

displayed the same number for different values of OSNR. So, OOK is not evaluated.

4.4 Impact of Kerr effect

4.4.1 Description of simulation scheme

Kerr effect occurs in the presence of high power. Its influence is significant until

effective fibre length. Therefore my scheme is adapted on the investigation of Kerr effect

(Figure 34). Four channels WDM system was set to catch the FWM influence. This

41

simulation is the extension of previously investigated DPSK, DQPSK and OOK formats.

It will comprise the BER of transmitter affected by Kerr effect within part of fibre span.

Then, the resilient of current format can be derived.

The main part of parameters is identical with previous simulations. Channel

spacing is set on the 50 GHz. The optical fibre is arranged in the 4 km long fibre loop.

DCF is insert between transmission fibre and measurement devices. Demultiplexing

provides the identical filter like in the previous case. Optical pulse passes five times

through a loop and after every circle is created set of measurements. It includes optical

spectrum, eye diagrams of signal’s channels and power levels of selected signal channel

and two lower, two upper induced bands. Simulation set comprises two common types of

optical fibres (SMF, NZ-DSF), three modulation formats examined previously and two

various sources. First one demonstrates common telecommunication (DFB) laser type

with 2 MHz linewidth and second one represents today’s high-end (DFB) laser but

ordinary one after certain time (it is hard deal to specify that time, I guess until two years)

with 500 kHz linewidth. Better DFB lasers are available as well.

Figure 34: Simulation scheme of FWM generation.

Computing resources were not adequate to software requirements. It led to the

crashes of whole simulation. Lower global parameters had to be used and consequently

BER estimation is not available. Therefore quality assessment has to use only eye

diagrams, which don’t reflect real affects on the BER. On the pictures, there will be cases

that deploy NZ-DSF, what means worse alternative.

42

4.4.2 OOK performance

OOK should be the less resilient format and simulation confirmed it. Bit-pattern-

dependence introduced the highest amount of amplitude noise to eye diagram. Narrower

linewidth has no effect on the performance; see Figure 35, Figure 36.

a) b)

Figure 35: Eye diagram of OOK transmitter, a) 2 MHz linewidth, b) 500 kHz linewidth

43

Figure 36: Eye diagram after first loop.

Figure 37: Spectrum after first loop.

44

4.4.3 DPSK performance

This format is more sensitive on the laser linewidth, Figure 38. On the other

Figures, there are dependences with 500 kHz linewidth. The eye diagrams (Figure 39) are

smoother than OOK ones. Signal’s bands in the spectrum have got approximately the

same amplitude for OOK and DPSK as well. The nearest noise bands are higher for

DPSK (Figure 40) compared to OOK spectrum (Figure 37) but on the other hand, other

FWM products have to be higher in the OOK spectrum, due to energy conservation. The

eye diagrams opening are approximately identical, but in the back-to-back condition

OOK has got advantage of ~2.5 mW higher contrast (difference between low and high

level). It means that DPSK is more resilient than OOK.

a) b)

Figure 38: DPSK back-to-back eye diagram of a) 2 MHz and b) 500 kHz linewidth

45

Figure 39: DPSK eye diagram after the first loop.

46

Figure 40: DPSK spectrum after the first loop.

4.4.4 DQPSK performance

Investigation of modulated DQPSK format suffers bit interference at symbol. The

distortion assessment on the DQPSK may bring the focusing on the bottom curve in the

eye diagram (Figure 42). It shows similar distortion as DPSK (Figure 39). Certainly the

final impairment is more serious than DPSK case and it has to consider also fidelity of

demodulation. The FWM products generation (Figure 43) has got similar behavior

compared to OOK format (Figure 37).

47

Figure 41: DQPSK eye diagrams in the back-to-back condition a) 2 MHz, b) 500 kHz.

Figure 42: DQPSK eye diagram after the first loop.

48

Figure 43: DQPSK spectrum after the first loop

4.4.5 Potential of best setup

DPSK format deployed 500 kHz laser linewidth reached the best performance, which

can be enhanced by using new type of optical fibre with large effective area and sources

with even narrower linewidth. Following Figures (44-46) show the best results of my

simulation.

49

Figure 44: DPSK eye diagram after first loop

Figure 45: DPSK spectrum after firrst loop

50

Figure 46: Signal power of selected channels relate on the number of loop

51

5 Conclusion My diploma thesis reflected limiting factors of today’s optical communication

systems emerged from relevant physical phenomena in the optical fibre and optical

devices with possibilities of their suppression as well. The domination of positive mood

followed from economical aspects. Consequently, proposed trends in the optical

communication involved wide spectrum of issues and their answers will be given by

particularly situation. DPSK format, basic component of outstanding solutions, was

described with its properties and applications.

Performance of DPSK and DQPSK transmitters was examined for various values of

OSNR and laser linewidth for today’s 10 Gb/s and future 40 Gb/s gold standard bit rates.

Model deployed higher bit rate is not absolutely relevant, because it doesn’t contain

inflexed point and concave part of characteristic BER curve. It was probably caused by

settings or nature of simulation algorithms. Model of lower bit rate estimation has worked

reliably and it was used for the evaluation of Kerr effect on the DPSK and OOK

modulation formats for two types of optical fibre and two laser linewitdhs. This influence

hasn’t been simulated due to high software requirements on the memory. Then, only

qualitative analyze was realized from eye diagram opening.

Modulation formats were evaluated within four channel’s WDM system, where

DPSK format was more resistant against Kerr effect than OOK. The nature of spectrum

FWM products is narrower with high levels at next adjacent noise channels compared

with OOK, which noise products affects wider spectrum. Binary DPSK modulation

outperforms DQPSK and not only in the case of wider laser linewidths but also required

difference of OSNR for the same BER rises with increasing BER value.

The models used in this diploma thesis evaluated impact of transmission channel on

the three modulation types of optical signal. It is good preparation to complex assessment

of particularly component limitation contribution to whole BER value.

Higher computing force will be needed to get better results and collaboration with

VPIsupport seems to be essential on the road to the successful results of simulations.

References:

[1] E. Desurvire, Optical Communiations in 2025, Glasgow: ECOC 2005

[2] Crystal Fibre: the fibre of the future?, Opto & Laser Europe, IOP Publishing

Limited, Bristol, UK, December, 2001

[3] Photonic fibre finds its first applications, Opto & Laser Europe, IOP Publishing

Limited, Bristol, UK, April, 2004

[4] G. P. Agrawal: Nonlinear Fiber Optics Third Edition, London, Academic Press,

2001.

[5] A. Mecozzi, Modeling of high-data rate optical communication systems:

propagation, detection and noise, Glasgow: Short Course6 - ECOC 2005

[6] A. K. Dutta, N. K. Dutta, M. Fujiwara: WDM Technologies: Optical networks,

London, Elsevier Acadmic Press, 2004.

[7] S. Bigo, Optimizing terrestrial systems for 40 Gbit/s channel bit rates, Glasgow:

ECOC 2005

[8] VPIcomponentMakerTM, Optical Amplifier User’s Manual, Berlin, Germany,

VPIphotonics, 2005,

[9] https://vzdelavanie.utc.sk/moodle/mod/resource/view.php?id=4621

[10] Ch. K. Madsen, J. H. Zhao: Optical filter design and analysis A signal processing

approach, Toronto, Canada, John Wiley & Sons, Inc., 1999

[11] https://vzdelavanie.utc.sk/moodle/mod/resource/view.php?id=1162

[12] N. Yoshikane, I. Morita, 1.14 b/s/Hz Spectrally Efficient 50 85.4-Gb/s Transmission

Over 300 km Using Copolarized RZ-DQPSK Signals, Journal of Lightwave

Technology, Volume 23, Issue 1, 108-114, January, 2005

[13] G. Charlet, E. Corbel, J. Lazaro, A. Klekamp, R. Dischler, P. Tran, W. Idler, H.

Mardoyan, A. Konczykowska, F. Jorge, S. Bigo, WDM Transmission at 6-Tbit/s

Capacity Over Transatlantic Distance, Using 42.7- Gb/s Differential Phase-Shift

Keying Without Pulse Carver, Journal of Lightwave Technology, Volume 23, Issue

1, January, 2005

[14] E. Pincemin, Y. Guilloux, A. Bezard, T. Vargas, Robustness of the OOK

Modulation Formats at 40 Gbit/s in the Practical System Infrastructure, Glasgow:

ECOC 2005

[15] VPItransmissionMakerTMWDM, User’s manual, Berlin, Germany, VPIphotonics,

2005,

[16] A. H. Gnauck, P. J. Winzer, Optical PSK transmission, Journal of Lightwave

Technology, Volume 23, Issue 1, January, 2005

[17] R. Ludwig, Ultrafast Transmission Technology, Glasgow: ECOC 2005

[18] N. S. Avlonitis, E. M. Yeatman, Performance Evaluation of Optical DQPSK Using

Saddle Point Approximation, Journal of Lightwave Technology, Volume 24, Issue

3, March, 2006

Statutory declaration

ČESTNÉ VYHLÁSENIE

Vyhlasujem, že som zadanú diplomovú prácu vypracoval samostatne, pod

odborným vedením vedúceho diplomovej práce Ing. Jozefa Dubovana a používal som len

literatúru uvedenú v práci.

Súhlasím so zapožičiavaním diplomovej práce.

V Žiline dňa .............................. podpis diplomanta..............................

Acknowledgment

POĎAKOVANIE

Touto cestou sa chcem poďakovať všetkým, ktorí sa akýmkoľvek spôsobom

podieľali na realizácii tejto diplomovej práce. Špeciálne by som sa chcel poďakovať

najmä, Ing. Jozefovi Dubovanovi, Prof. Ing. Milanovi Dadovi, PhD. a Ing. Miroslavovi

Bystrianskemu ktorí mi venovali svoj čas a energiu. Navyše rodine a blízkym ďakujem za

vytvorenie vhodných podmienok nielen počas tvorbe tejto práce, ale celého štúdia a ich

podporu v ťažkých obdobiach.

Autor