Transmissions photoniques Optical transmission in HFC networks Véronique Moeyaert [email protected]

Transmissions photoniques

Optical transmission in HFC networks

Véronique [email protected]

22Dr Ir Véronique MoeyaertDr Ir Véronique Moeyaert

References

[1] Photonic Aspects of CATV Networks, C. van der Plaats & T. Muys, ECOC ’98 short course 2 [2] Broadband Hybrid Fiber/Coax Access System Technologies, W. I. Way, Academic Press, ©1999, ISBN 0-

12-738755 [3] Article JCF [4] X.P. MAO, G.E.BODEEP, R.W.TKACH, A.R.CHRAPLYVY, T.E.DARCIE, R.M.DEROSIER, "Brillouin Scattering in

externally modulated lightwave AM-VSB CATV transmission systems", IEEE Photonics Technology Letters, vol.4, No 3, 1992, pp 287-289

[5] M.R.PHILLIPS, T.E.DARCIE, D.MARCUSE, G.E.BODEEP, N.J.FRIGO, "Nonlinear distortion generated by dispersive transmission of chirped intensity-modulated signals", IEEE Photonics Technology Letters, vol.3,No 5, 1991, pp 481-483

[6] F.COPPINGER, M.D.SELKER, D.PIEHLER, "The effect of SPM, EPM and sign of dispersion on the second order distortion in analog link", OFC 2001, 2001, WCC2/1-3

[7] M.C.WU, C.H.WANG, W.I.WAY, "CSO distortions due to the combined effects of self-and external-phase modulations in long distance 1550 nm CATV systems", IEEE Photonics Technology Letters, vol.11, No 6, 1999, pp 718-720

[8] Subscriber Multiplexing for Lightwave Networks and Video Distribution Systems, T. E. Darcie, IEEE Journal on Selected Areas on Communications, Vol. 8, n°7, september 1990, pp. 1240-1248


Outline

I - From coaxial CATV towards HFC II - General Technical Background III - Emitters and receivers for

downstream transmissions IV - Linear transmission effects V - Non-linear fiber transmission effects VI - Return path optical link VII - Future : from HFC to FTTx and

network segmentation with the use of WDM

I - From coaxial CATV towards HFC


HEADEND

Trunk linePrimary lines

Distribution lines

Classical coaxial topology


HEAD

END

Optical fibre

Fibre backbone topology


HFC topology

TAPTAPTAP

Upstream fibre

Downstream fibre

O/E

O/E

COHE

O/E

O/E

COHE

COHE

COHE

Optical fibre

Coaxial cable

Subscribers cluster

ONU PN

PN


HFC frequency allocation

Digital Video, Telephony, Data,...

Analogue Video

Data, Telephony,...

Downstream ServicesUpstreamServices

5 65 300 862

MHz


EO

DFB laser

EO

Photoreceiver

Photoreceiver

Fabry-Perot laser

OE

OE

Headend ONU

(Frequency

Multiplexing)

5-200 MHz

Optical transmission on HFC

II - General Technical Background


Outline

Analogue and digital video signal formats and standards

CATV channel allocation plan CNR, HD2, HD3, IMD2, IMD3, CSO &

CTB Definition Measurement


Analogue video format

Image carrier: AM-VSB modulation Sound carrier: FM modulation Color carrier:

phase modulation

PAL system NTSC system

videosignal

fcv

AMModulator

video IFcarrier

VSBfilter

fca

audio IFcarrier

audiosignal

+

+

IF AM-VSBTV signal

FMModulator


Inside an analogue channel, the spacing between carriers is normalised

Belgium case •625 interleaved lines (even & odd)• Negative Modulation• Noise bandwidth: 5 MHz • Espacement de 7 ou 8 MHz• PAL B/G & PAL+• FM & NICAM

PAL


Analogue VHF (Very High Frequency) channel: 7 MHz

7 MHz


Analogue UHF (Ultra High Frequency) channel: 8 MHz

8 MHz


Digital downstream transmission: DVB (Digital Video

Broadcast) DVB = market-led initiative since 1993 to standardize

digital broadcasting world wide. Concerns all media

220 membres (broadcast industry with head quarters in Europe) from 30 countries.

Work in an open standard concensus with ETSI/EBU/CENELEC/JTC and is published by ETSI/EBU.

DVB - SDVB - CDVB - CSDVB - TDVB - MS/MC

SatelliteCable

SMATVTerrestrial

MMDS


Worldwide DVB Standards Acceptation Process

(from http://www.dvb.org)


DVB - CGeneral Features

ITU-T J.83 Annex A, ETS 300 429 Broadcast application based on MPEG-2 TS (it may also

carry data). 1 program = 1.5, ..., 6 Mbit/s (depending on the quality).

Use of 16, 32 or 64-QAM modulation schemes in 8 MHz channels

Several programs per analogue channel. Randomization and protection against errors on 768 bits :

RS(204,188,8) and interleaver (I=12).

1.5 Mbit/s

2 to 3 Mbit/s

VHS quality video for film material

Sports

4 Mbit/s

6 Mbit/s

Most users detect no visible degradation

Broadcast quality


DVB - C Transmission & Reception Schemes

MPEGCoding

MPEGCoding

MPEGCoding

Multiplexing&

Scrambling

FEC, formating,filtering,

DAC

QAMModulation

Upconverter

DownconverterQAM

Demodulation

ADC, filtering

formating, FEC

Demultiplexing&

Descrambling

CABLE

MPEGDecoding

SELECTORSOURCE (DE)CODING

CHANNEL (DE)CODING


Digital UHF 64-QAM channels

BeTVCanal +

numérique

Zoom

Canal Z


Zoom on one of the BeTV channel

Symbol rate

Occupied bandwidth: 8 MHz 3dB bandwidth: symbol rate


Multichannel AM-VSB TV transmission

frequency

1

f1 - fIF

IF AM-VSBTV signal 1

2f2 - fIF

IF AM-VSBTV signal 2

N

fN - fIF

IF AM-VSBTV signal N

N

iiiii tftsmtx

1

)2cos()()(

x(t) : multichannel AM-CATV signalmi : modulation index of i-th channelsi(t) : normalized i-th modulation signalfi : carrier frequency of i-th channeli : carrier phase of i-th channelN : number of channels

It is a SCM modulation (= FDM)

[1]


‘Full Span’ IDEATEL spectrum: from 40 to 425 MHz

Band I analoguecarriers

FM radio carriers

Analogue VHF carriers

Analogue UHF carriers

Digital 64-QAM carriers

Pilot Tone

VHF : 30 à 300 MHz UHF : 300MHz à 3 GHz


DefinitionsSecond and Third Order Distortions

Non-linearSystem

f1, f2, f3 First order

3f1, 3f2, 3f3 3HDf1, f2,f3

f1 2f2, f1 2f3, f2 2f1 +-+- +-

f2 2f3, f3 2f1, f3 2f2 +-+- +-

f1 f2 f3+-+-

IMD3

3rd Order

Non-linearSystem

f1, f2 First order

2f1, 2f2 2HD

f1 f2 IMD2+-

f1 f2f1 f2 2f1 2f2f1 f2

+-

IMD22HD

f1, f2

2d Order


IMD2 & IMD3

2ω1-ω2ω2-ω1 2ω2-ω1 ω1+ω2ω1 ω2

IMD3 IMD2


Frequency plan allocation: CENELEC 42 test plan


Multichannel input signal: x(t) = mcos(1t + 1) + mcos(2t + 2) + ...

Non-linear transfer function: y(t) = 1 + x(t) + a x2(t) + b x3(t) + ...

y(t) will have the following intermodulation products:

Term Relativeampli-tude

Relativepower

Count

i (carrier) 1 1 -

2i (HD2) am/2 a2m2/4 1/4

i j (IM2) am a2m2 1

3i (HD3) bm2/4 b2m4/16 1/36

2i j (IM3) 3bm2/4 9b2m4/16 1/4

i j

k 6bm2/4 36b2m4/16 1

Distortions aggregation : CSO & CTB

Taylor(worst case)

or Volterratheory

coefficients


C

DCSO 2

C

D2 unmodulatedcarrier

Composite Second Order (CSO)

CSO: Composite Second Order intermodulation distortion = Spectrum measurement of the ratio of the carrier power C to the total power of the accumulation of second order distortion products for each second order distortion generated


C

D3

C

DCTB 3unmodulated

carrier

Composite Triple Beat (CTB)

CTB: Composite Triple Beat intermodulation distortion: Spectrum measurement of the ratio of the carrier power C to the total power of the accumulation of third order distortion products for each thrid order distortion generated

3030Dr Ir Véronique MoeyaertDr Ir Véronique MoeyaertJCvdP/TM/10/07/98 Photonic Aspects of CATV Networks

Typical system specifications

Actual deployment in Eastern Europe; HFC network with 1310 nm feeder with max. 3 coaxial amplifiers in cascade; System input signal specification:

CNR 50dB, CSO -65dBc, CTB -65dBc, Specification at subscriber wall-outlet:

CNR 46dB, CSO -57dBc, CTB -57dBc.


Frequency count of non-linearities: CENELEC


C

n = noise power density [dBm/Hz]

eBn

CCNR

unmodulatedcarrier

Be = 5 MHz (PAL), 4 MHz (NTSC)

Carrier-to-Noise Ratio (CNR)

CNR: Carrier-to-Noise Ratio: Sprectrum measurement of the ratio of the carrier power C to the noise power in 5 MHz bandwidth (NTSC: 4 MHz).


Experimental Setup for CNR, CSO & CTB measurements

AttenuatorDFBLaser

Optical receiver

Multicarrier generator

Spectrum analyserPC

AND/OR

Optical fibre

GPIB

DUT


CNR, CSO, CTB measurements & OMD

CNR CSO & CTB

RBW 100 kHz 30 kHz

VBW 1 kHz 1 kHz

SPAN 3.5 MHz 500 kHz

Spectrum parameters CSO & CTB measurements

Optical Modulation Depth

In a 5 MHz BW

*

* Not EN50083-7 compliant

OMDP

Pjj

0


Real CTB measurement (zoom in)

-30

-20

-10

0

10

20

30

40

48.244 48.246 48.248 48.25 48.252 48.254 48.256

Frequency [MHz]

Po

we

r D

en

sit

y

[1]

III - Emitters and receivers for downstream

transmissions


Outline

IntroductionCNR calculation

Receiver noises Transmitters types Transmitter RIN

Laser Chirp Clipping


Outline





LinearLaser

LinearPhotodiode

))(1( txP0

))(1)(( txIII thbth RR

R

R

Ph

qPI

txI

r

))(1(L

))(1( txPR 0R PLP

Multi-channel

AM-CATV signal

PL

Ith Ib

P

I

AM-SCM transmission system

IR : average photodiode current [A] : quantum efficiency of photodiodeq : electron charge 1.610-19 CPR : average received optical power [W]h : Planck’s constant 6.626 10-34 Js : optical frequency [Hz]rphotodiode responsivity [A/W]

[1]

4040Dr Ir Véronique MoeyaertDr Ir Véronique MoeyaertJCvdP/TM/10/07/98 35Photonic Aspects of CATV Networks

High power low RIN 1.55 (or 1.3) m Continuous Wave (CW) laser with linearized (using predistorter) external LiNbO3 Mach-Zehnder amplitude modulator - no chirp - expensivePredistorter

Directly modulated (predistorter optional) analogue (medium power, linear and low RIN) 1.3 (or 1.55) m DFB laser - chirp (1.55) - low cost

Predistorter

Transmitter types[1]


Outline





Receiver noise sources

Schottky noise: noise due to the fact that the PIN or APD

photodiode acts as a junction.

Thermal noise: Due to resistance and leaking currents Characterized by Iéqu [pA/(Hz1/2)], an

equivalent noise current at the photodiode output

ReSchottky IqAverage receivedcurrent


Emitter noise source: RIN

Hypothesis: monomode transmission! RIN = relative intensity noise [dB/Hz]

Inherent noise due to the intrinsic instabilities of oscillation conditions inside the laser cavity

Ratio between the square of the optical noise power density and the average optical power emitted by the laser

2o

2

P

tPfRIN

The RIN depends on frequency!


Optical budget [dB]

Definition: maximum loss of the link (fibre loss + connectors loss + …).

Depends on the laser output power and the receiver sensitivity (minimum input power to obtain a given quality)

B

laser

receiver


Optical modulation index: OMI

mi , the optical modulation index of the ith carrier, is defined as the ratio maximal variation of optical power due to the ith carrier and the average emitted power P0

All carriers have generally the same OMI --> mi = m

0

ii P

Pm


CNR calculation (I)

Received power due to a single carrier:

Received noise due to laser RIN:

Received noise due to the photodiode Schottky noise:

B

PrrPI 0

RR B

mPrI 0

carrier1R 2

0carrier1R

2B

mPrP

2

20

2

PP B2

RIN.P

B0

R

B

PqrI.q 0

RI,Schottky R


CNR calculation (II)

Received noise due to the photodiode thermal noise:

Noises are not correlated and are integrated in f = 5 MHz (PAL) noise equivalent bandwidth:

2

I2equ

I,thermal R

2

I

B

r.P.q

B2

r.P.RIN.f.2i

2equ0

2

202

2

I

Br.P.q

B2r.P.RIN

.f.2

2B

rP.m

log.10CNR2equ0

2

20

2

0

10


CNR vs B in a reference situation

m = 4% P0 = 6 mW r = 0.8 A/W RIN = -155 dB/Hz Iequ = 12 pA/(Hz1/2)

13


CNR vs B with RIN as parameter

RIN = -160 dB/HzRIN = -155 dB/HzRIN = -150 dB/Hz

If the RIN decreases, CNR improves only for low optical budgets. From a given OB budget, there is no improvement anymore.

Useful zone forCATV transmissions--> laser RIN not important!

Useful zone forstudio transmissions


CNR vs B with OMI as parameter

m = 5%m = 4%m = 3%

If OMI increases, CNR improves


CNR vs B with P0 as parameter

P0 = 8 mW

If the laser output power increases, CNR improves only for high optical budgets

P0 = 6 mWP0 = 4 mW


CNR vs B with r as parameter

r = 0.9 A/W

If the photodiode responsivity increases, CNR improves only for high optical budgets but the improvement is not important

r = 0.8 A/Wr = 0.7 A/W


CNR vs B with Iequ as parameter

Iequ = 5 pA/(Hz1/2)

If the photodiode equivalent noise current increases, CNR degrades for high optical budgets

Iequ = 10 pA/(Hz1/2)

Iequ = 7 pA/(Hz1/2)


CNR Measurements vs. Frequency & Optical Budget

48.25

133.25

154.25

175.25

196.25

217.25

238.25

259.25

8.89.8

10.811.8

12.813.8

14.815.8

16.817.8

18.819.8

20.821.8

25272931333537394143454749

51

53

55

53-55

51-53

49-51

47-49

45-47

43-45

41-43

39-41

37-39

35-37

33-35

31-33

29-31

27-29

25-27

Car

rier

to

No

ise

Rat

io [

dB

]

Optical budget [dB]

Frequency [MHz]25 carriers

(IDEATEL)


RIN & Iequ Computation from CNR theory

Least squares fit results

51 carriers@ 407.25 MHzP0 = 5 mWm = 3.2 %r = 0.86 A/W

Experimental parameters:

RIN = -159.6 dB/HzIeq = 5.6 pA/(Hz1/2)

2

I

Br.P.q

B2r.P.RIN

.f.2

2B

rP.m

log.10CNR2equ0

2

20

2

0

10


RIN & Ieq Computation vs. Frequency

-165

-164

-163

-162

-161

-160

-159

-158

-157

-156

-155

-154

-153

-152

-151

-150

-149

-148

-147

-146

-145

48.25 62.25 140.25 154.25 168.25 182.25 196.25 210.25 224.25 238.25 252.25 266.25

RIN [dB/Hz]

3

4

5

6

7

8

9

10

11

12

13

14

15

48.25 62.25 140.25 154.25 168.25 182.25 196.25 210.25 224.25 238.25 252.25 266.25

Ieq [pA/(Hz)^0.5]

RIN computation

Iequ computation


Outline





Laser chirp in direct modulation

The laser chirp comes from a AM-FM conversion of the modulation signal

It is due to a variation of refractive index of the laser cavity due to the injected carrier density (modulation).

The variation of refractive index modifies the propagation constant.

tsintjmo

moetcosm1EE Field intensity


Spectrum due to chirping

But,

(Bessel 1st order)

=> Chirping results in a discrete spectrum

n

jnn

sin.x.j e).x(Je

fm = 150 MHz

0 mA(mod. Current)

0.76 mA

1.4 mA

1.84 mA

2.24 mA

2.88 mA

3.44 mA4 mA


Outline





Laser clipping

P

Vb V

PM

P

IbIth I

IL(t)

PL

PL(t) PM(t)

VM(t)

Directly Modulated Laser External Modulator


SCM modelling statistic

Perfect linear transmitters still have a clipping distortion limit.

The amplitude distribution of the driving multi-channel current or voltage can be approximated by a Gaussian distribution if the number of channels N >7.

(a) (b) (c)

(d) (e)

Pdf of:•(a) 1•(b) 2•(c) 3•(d) 5•(e) 10

carriers


Clipping limit (worst case) Clipping starts to occur when N·m > 1, where N is the

number of channels and m the OMI. The clipping distortion is largely a function of the

standard deviation of the amplitude distribution of the driving signal:

Darcie limit [8]:2

Nm

2213

2

e61

2NLD

C

C/NLD = 55 dB if µ = 0.246

N

348,0OMD


Clipping as a distortion generator or as an impulse noise generator

Impulse noise generator

IntermodulationOutside the band

Distortions generator

"Theoretical and Experimental Analysis of Clipping-Induced Impulsive Noise in AM–VSB Subcarrier Multiplexed Lightwave Systems"; Stephen Lai and Jan Conradi, Senior Member, IEEE; JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 15, NO. 1, JANUARY 1997

IntermodulationOutside the band

IntermodulationsIntside the band


Laser clipping as distortion generator - CSO

-60 dB

7.7 %


Laser clipping as distortion generator - CTB

-60 dB

7.3 %

IV - Linear transmission effects


Outline

Multi-Path InterferenceChromatic DispersionPolarisation Mode DispersionEDFA amplification


Outline

Multi-Path InterferenceChromatic Dispersion Polarisation Mode DispersionEDFA amplification


Multipath Interference (MPI)

54Photonic Aspects of CATV Networks

Two or more reflections in the optical transmission path cause interference at the receiver of the direct signal (POUT0 ) the doubly reflected version of itself (POUT2). If R1R2 << 1, the higher order (POUT4, POUT6, …) reflected versions of the signal can in general be neglected.

Besides discrete reflections from connectors, components, etc., also Rayleigh backscatter in the fiber causes MPI.

PINPOUT0

POUT2

POUT4

POUT1

POUT3

R1POUT1’ R2Lc ,

c

Core

Envelope

7171Dr Ir Véronique MoeyaertDr Ir Véronique MoeyaertJCvdP/TM/10/07/98 55Photonic Aspects of CATV Networks

)2t(j)t(jc0eqc

2eq

)2t(jc021

2c

)t(j0

c0212c0

*RR

2

RR

t

0

thbFM0

n0

)2t(jc021

2c

)t(j0R

c

c

c

ee))2t(x1))(t(x1(ReLPrR2)2t(IR)t(I

e))2t(x1(LPRRLe))t(x1(LPrRe2

))2t(x1(rLPRRL))t(x1(rLP)t(E)t(rEtEr)t(I

modulationdirectford)(x)II(2t2

modulationexternalfor)t(t2)t(with

e))2t(x1(LPRRLe))t(x1(LP)t(E

MPI - Phase to intensity noise conversion

Original signal

Delayed and attenuated

version of original signal

Beating of original signal with a delayed and attenuated version of itself,

baseband intensity noise if 2c >> coh [9][24][25]

and distortion if 2c < coh [10][39]

[1]Link loss Lc = Cavity loss (1 trip)


RIN in the case of N connectors and Rayleigh equivalent RIN

N connectors (same return loss, same distance between connectors)

Rayleigh backscattering Calculation (complex formula) of an equivalent

RIN depending on the spectral linewidth, the electrical modulation frequency, the link length and the backscattering coefficient

N222

2

sNconnector11N

1f

4RfRIN

Equivalent to Lc


Laser linewidth influence on CNR (RIN) due to Rayleigh backscat.

If => reflection influence => CNR

=> Alwaysmodulate to lower RIN!

()


How to evaluate the equivalent Rayleigh backscattering RIN?

Core

Envelope

Car

rier

to

No

ise

Rat

io [

dB

]

Optical budget [dB]

20

25

30

35

40

45

50

55

10

.7

11

.7

12

.7

13

.7

14

.7

15

.7

16

.7

17

.7

18

.7

19

.7

20

.7

21

.7

22

.7

23

.7

24

.7

25

.7

26

.7

27

.7

28

.7

29

.7

With fiber (25 km)

Without fiber

OMD = 3 %, 26 carriers, = 1312 nm

Ps = 11.8 dBm, r = 0.73 A/W

Phenomenon

Results

Computation of RIN value due to the fibre

@48.25 MHz

RINsource = -160.5 dB/Hz (Iequ = 5.6pA/(Hz1/2))RINsource+fibre = -151.5 dB/HzEquivalent RINfibre = -152 dB/Hz


Outline



Chirp + chromatic dispersion lead to distortion generation

In the case of a 1.55 m (1.3 m) directly modulated transmitter, the laser chirp in combination with the dispersion of standard SMF (dispersion shifted SMF) generates second-order (CSO) and some negligible third-order (CTB << CSO) distortion.

with fd the distortion frequency, D the fiber dispersion [ps/nmkm], L the fiber length [km].

In general, the distortions generated by chromatic dispersion for externally modulated systems can be neglected.

22

)(2

thbFMdCSODISP IIDL

cmfNCSO

[1]


-75.0

-70.0

-65.0

-60.0

-55.0

-50.0

-45.0

-40.0

-35.0

-30.0

0 100 200 300 400 500 600 700 800 900

50 km

25 km

10 km

IMD

2 [d

Bc]

Frequency [MHz]

CSO a OMD N

CTB a Kwith a D L

cI I

d CSOFM a th

20 10

20 200

2

00

log . . log

log log. .

6 dB

carriers = 1312 nm

DS fibreD = -17.81 ps/(nm*km)

6 dB

Chromatic Dispersion & Laser Chirp - CSO measurement


Given a 1550 nm DFB laser with FM = 100 MHz/mA and (Ib - Ith) = 30 mA used in a directly modulated system with BK600 frequency plan, OMI = 5%. If we require CSO < -60 dBc, what is then the maximum length L of standard non-dispersion shifted SMF (D = 17 ps/nmkm) over which we can transmit the AM-CATV signal?

Maximum CSO distortion occurs in channel K25 with carrier frequency 503.25 MHz. The NCSO = 6.25 for this channel at frequency fd = 504.25 MHz.

CSOthbFMd N

CSO

IIDmf

cL max

2max)(2

Lmax = 6.18 km

Example[1]


Chromatic dispersion & chirp countermeasures?

Electronic compensation in transmitterInsertion of Dispersion Compensating

Fiber (DCF)Use of the fiber with the right 0 for

chromatic dispersionUse of externally instead of directly

modulated transmitter


Outline



PMD generates CSO

Two different PMD related mechanisms generate second-order distortion: Interaction of PMD and laser chirp generates CSO, in fibers

with coupling between the polarization modes, which scales with the square of the distortion frequency fd,

Interaction of PMD, laser chirp, and Polarization Dependent Loss (PDL) generates CSO independent of the distortion frequency.

The fiber PMD fluctuates with time because it is dependent on the polarization mode coupling in the fiber which is sensitive to ambient temperature and mechanical perturbations.


-80

-75

-70

-65

-60

-55

-50

-45

-40

42 72 92 119 147 169 202.5 224 265.5 343.5 399.5 455.5 511.5C

SO

[d

Bc

]

Frequency [MHz]

Pigtails

Standard fibre alone

Standard fibre + disp. comp. fibre

PhenomenonAnalogue to chromatic dispersion but fluctuates with time due to the random polarisation mode coupling -> CSO generation

PMD (2.5 ps) generated by a 4.2 km DCF to compensate for 14.77 km CF, o = 1544.5 nm

FM = 260 MHz/mAOMD = 3%(Iao - Ith) = 38 mA

Theoretical result fd = 56 MHz, NCSO = 26

CSO = -61 dB

Polarization Mode Dispersion & Laser Chirp


Outline



Measurement of the distortions due to the insertion of the home-made EDFA along an optical link

-75.00

-70.00

-65.00

-60.00

-55.00

-50.00

0.00 100.00 200.00 300.00 400.00 500.00 600.00

Frequency [MHz]

CS

O [

dB

c]

LEVEL of link distortions without EDFA [dBc]

LEVEL of link distortions with EDFA [dBc]

Without EDFA

With EDFA

Insertion of an EDFA - Influence on Second Order Distortions


Insertion of an EDFA - Influence on Second Order Distortions

Measurement of the distortions due to the insertion of the home-made EDFA along an optical link

-95.00

-90.00

-85.00

-80.00

-75.00

-70.00

-65.00

-60.00

-55.00

-50.00

0.00 100.00 200.00 300.00 400.00 500.00 600.00

Frequency [MHz]

CS

O [

dB

c]

LEVEL of EDFA (home-made) distortions [dBc]

CSO due to the EDFA gain tilt

around o combined with

the laser chirp.

V - Non-linear fiber transmission effects


Outline

Stimulated Brillouin Scattering (SBS)Self-Phase Modulation (SPM)

… in external modulation


Direct modulation vs External modulation in CATV transmission

Direct Modulation: Laser intensity is directly modulated by the injected current

• induces laser chirp (intensity modulation induces phase modulation) Main limitation : combined effect of CD and laser chirp on standard

monomode fibre (SMF)• generates second and third order distortions

– Limit distance = +/- 35 km @ 1550 nm Long distance transmission @ 1550 nm is not possible

External Modulation (complexity >>>): Intensity modulation is realized by an external modulator Transmitter using external modulator presents low residual chirp

• combined effect of CD and laser chirp is not significant allows to realize long distance transmission @ 1550 nm (up to 100 km

on SMF) takes advantage of the smallest fibre attenuation (0.2 dB/km @ 1550

nm) and the use of optical amplification (EDFA)


Limiting effects on external modulation – Brillouin effect

Optical fibre presents Stimulated Brillouin Scattering is a non-linear effect in optical fibre (increases with optical power) consists of a distributed backscattering of the input optical power appears only when input optical power is greater than a power threshold

(Brillouin threshold = PSBS)

• In direct modulation with 67.25 MHz as input signal, PSBS = 30 dBm [4] (due to beneficial effect of laser chirp)

• In external modulation with 67.25 MHz as input signal, PSBS = 6 dBm [4] (due to very low residual chirp)

Impossible to use optical amplification Brillouin threshold must be increased in external modulation

transmission

Pth = threshold depending on fibre properties [W]l = bandwidth of the modulated source [Hz]

b = Brillouin bandwidth of the fibre depending on

fibre properties (between 20 MHz and 100 MHz)


Solutions to increase Brillouin threshold in external modulation

Main Idea : increase the bandwidth of the modulated source (spread the total optical power on a greater bandwidth)=> l increases => PSBS increases

Techniques Dithering

• consists in directly modulating the laser with an unmodulated electrical tone (to induce chirp)

External Phase Modulation (EPM)• optical carrier is modulated in phase by a frequency fEPM

– Total optical power » is not only content in a single carrier» is spread on several optical carriers spaced by fEPM

(first order Bessel function)• allows to increase SBS threshold by 10 dB (PSBS 16 – 17 dBm)

=> compatible with optical booster amplification

SC laserTone

External mod.

SC laserTone

External mod.


f0-fm f0+fm0 f0

0.25Ap

0.35Ap

0.5Ap

0.35Ap

0.5Ap

0.35Ap 0.35Ap

0.15Ap0.15Ap

0.05Ap0.05Ap

f0-4fm

f0-5fmf0-3fm

f0-2fm f0+2fm

f0+3fm

f0+4fm

f0+5fmfrequency

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15-0.5

-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Modulation index

J i(

)

J1

J0

J2J3 J4 J5

= 3

Classical spectrum of a phase modulated signal

f0 = carrier

fm = modulating signal

modulation index = 3)


External modulator presents 2 inputs CATV RF signal for intensity modulation Frequency fEPM for phase modulation

To avoid spectral overlapping : fEPM > 2 * fmax => fEPM > 2 GHz

laserExternal Modulator

CATV-RF fEPM

f0

f0-2fEPM f0 f0+fEPM f0+2fEPM

0 0 fEPMfmax = 862 MHz

Electrical spectrumWith EPM Without EPM

Optical spectrum

External modulation with EPM: optical spectrum output

Optical spectrum


Limiting effects on external modulation – Self Phase Modulation

SPM is a non-linear phenomenon of the optical fibre (increases

with optical power) is due to the dependence of the fibre refractive index with

the optical intensity• induces a phase modulation of the optical carrier (is equivalent

to a distributed chirp into the fibre) => leads to a spectral broadening of the transmitted signal

In external modulation using external phase modulation (to avoid SBS threshold), injected power can reach 15 dBm=> SPM effect can not be neglected

Intensity variation of optical signal

Variation of fibre refractive index

Phase variation of the optical signal

SPM


Intuitive analysis of CD effect in external modulation transmission

As for direct modulation, if the optical signal presents a spectral broadening (due to SPM or/and EPM), the presence of CD induces distortions

Influence of CD and SPM• SPM induces spectral spreading => SPM + CD induces distortions

Influence of CD and EPM• EPM induces spectral spreading => EPM + CD induces distortions

=> In practice : combined effect of CD+EPM and CD+SPM


Combined effect of CD and SPM : analytical results and simulations : HD2 and HD3 [5]

Evolution of distortion/carrier (HD2 and HD3 in the case of 1 tone as RF input signal) versus fibre length in external modulation transmission system with SPM (and without EPM) Comments:

Analytical results give separated contributions forCombined effect of SPM and CD

(‘NL’ curve)Combined effect of CD and the

modulating RF signal (‘DSP’ curve)Distortions increase with fibre lengthHD3 is more than 60 dB below HD2

There is no experimental results because, in practice, EPM is present to avoid Brillouin Scattering

RF input = 500 MHz ; dispersion = 17 ps/nm/km

fibre length


Combined effect of CD, EPM and SPM – experimental results : HD2 [3]

Phase Modulation @ 3 GHz with = 2.4 CATV-RF signal composed by only one tone @ 375 MHz Optical link composed by 3 EDFAs and 3 fibre spans of 50 km => HD2 (@ 750 MHz) is measured for different fibre lengths


Combined effect of CD, EPM and SPM - experimental results : HD2 [3]

The combined effect of CD, EPM and SPM allows to obtain a minimum (a cancellation) of HD2 for a specific fibre length

This experimental result is confirmed by simulation

Minimum CSO position and curve shape depend on several parameters : fibre parameters (chromatic dispersion, non-linear coefficient)phase modulation parameters (frequency and modulation index )intensity modulation parameters (RF frequency, OMI,…)laser parameters (output power, wavelength,…)


Combined effect of CD, EPM and SPM : experimental results : CSO [5]

Phase Modulation frequency : 1.9, 4 and 6 GHz (simulations) phase modulation index : between 2.5 and 6 (simulations and experiments)

CATV-RF signal composed by NTSC plan (78 tones) Optical link composed by 5 stages (EDFA + 60 km fibre link) => CSO @ 548.5 MHz (channel 78) is measured in function of fibre link

for different phase modulation frequency for different phase modulation index


Combined effect of CD, EPM and SPM – experimental results : CSO [7]

Simulation of CSO evolution versus fibre length for different phase modulation frequency.

Analytical and simulation results of CSO evolution versus fibre length (without EPM : = 0) => measurement is not possible

Comments:Minimum CSO position appears for

shorter fibre length when phase modulation frequency increases

For fibre length lower than the CSO minimum, all curves follow the curve without EPM (analytical curve)


Combined effect of CD, EPM and SPM - experimental results : CSO (ref. [4] – Wu et al.)

Simulation and experimental measurements of CSO evolution versus fibre length for different phase modulation index

Comments:Minimum CSO position appears for

shorter fibre length when phase modulation index increases

For fibre length lower than the CSO minimum, all curves follow the curve without EPM (analytical curve)


Effect of modulation frequency and on the minimum CSO position

These 2 last results are coherent with the evolution of the spectral bandwidth of the phase modulation signal for different phase modulation frequency phase modulation index

Indeed, effective spectral bandwidth of the phase modulation signal increases when modulation frequency increases (for same modulation index) when modulation index increases (for same modulation frequency)

because there are more significant tones in the modulation signal spectrum

modulation frequency increases or modulation index increases

Effective spectral bandwidth increases

Same effect for CSO evolution

Minimum CSO position appears for shorter fibre length


Influence of modulation frequency on a phase modulated signal spectrum

(f0 = 100 kHz, modulation index = 5)

ffmm = = 20 k20 kHzHz

ffmm = = 40 k40 kHzHz

BP = 400 kHz

BP = 800 kHz

- Shape is the same (same number of significant tones)

- Space between tones is different


Influence of modulation index on a phase modulated signal spectrum

(f0 = 100 kHz, modulation frequency = 20 kHz)

= 2.5= 2.5

= 1.5= 1.5

-Space between tones is the same -Shape is different (not same number

of significant tones)

BP = 200 kHz

BP = 120 kHz


Our experimental setup and simulation tool

External modulator

Multi-tones generator

External modulator with phase Modulation frequency variable (2 GHz or 6 GHz) phase modulation index variable

CATV-RF signal composed by only one tone @ 375.25 MHz Cenelec 42 carriers plan

Optical link composed by 3 EDFAs and 3 fibre spans of 50 km

EDFA EDFA EDFA

VI - Return path optical link


Laser types

Return signals need less performance than downstream signals: Return :

• QPSK or 16-QAM signal• SNR = 16 or 23 dB (10-9)• SNR = 13 or 19 dB (10-5)

Downstream: CNR = 55 dBReturn lasers costs are shared among less

subscribers than downstream=> return lasers have lower quality (DFB or

FP, sometimes no isolator or no cooler)


Laser types

Return lasers are in cabinets => they are subject to temperature change if not cooled

Optical Dispaching

Batteries

ONU

Dow

nstream

receiver

Up

stream em

itter

4 coaxial amp

lifiers F

or 4 distrib

ution

lines


OMI variation with temperature

OMI variation if no cooling (very often) or active polarisation current research

25°C

85°C

P0

P0

2P

2P

I

VII - Future : from HFC to FTTx and network

segmentation with the use of WDM


Aim of network segmentation

Increase throughput per user for the same bandwidth

Decrease noise level at the CMTS

Upstream fibre

Downstream fibre

TAPTAPTAP

O/EO/E

ONU

PN

TAPTAPTAPTAPTAPTAP

TAPTAPTAP

One Subscriber cluster


Network segmentation using frequency stacking

Use of return path laser full bandwidth by subscriber cluster bandwidth frequency up-convertion

Upstream fibre

Downstream fibre

TAPTAPTAP

O/EO/E

ONU

PN

TAPTAPTAPTAPTAPTAP

TAPTAPTAP

4 Subscriber clusters

ElectricalFrequency [MHz]0 200

Cluster1

Cluster2

Cluster3

Cluster4


Network segmentation using WDM Benefit of fiber bandwidth. Each subscriber cluster

modulated its own return path lasers centered on different wavelength.

Upstream fibre

Downstream fibre

TAPTAPTAP

O/EO/E

ONU

PN

TAPTAPTAPTAPTAPTAP

TAPTAPTAP

4 Subscriber clusters

OpticalWavelength [nm].

Cluster1

Cluster2

Cluster3

Cluster4

Coarse WDMor

Dense WDM

Documents

Transmissions photoniques Optical transmission in HFC networks Véronique Moeyaert [email protected]