Simulation and Experimental Study of SCM/WDM Optical

Systems

Renxiang Huang University of Kansas

May 23, 2001

OutlineOutline

■■ Introduction to SCM technologyIntroduction to SCM technology ■■ Demand for more bandwidthDemand for more bandwidth

■■ TDM,WDM strategiesTDM,WDM strategies

■■ SCM and WDM combinationSCM and WDM combination

■■ Numerical simulation modelNumerical simulation model ■■ Goals and general requirementsGoals and general requirements

■■ Description of the simulation modelDescription of the simulation model ■ Transmitter ■ Optical fiber nonlinear model ■ Receiver

OutlineOutline

■ Simulation results ■ BPSK, 4-subcarrier each with 2.5Gb/s datarate, self-coherent

detection ■ QPSK, 2-subcarrier each with 2x2.5Gb/s datarate, self-

coherent detection ■ ASK, 4-subcarriers each with 2.5Gb/s datarate, direct

detection using narrowband optical filter ■ Traditional NRZ modulated OC192 system

■ Experiment

■ Conclusions and future work

Introduction: Introduction: Demand for more BandwidthDemand for more Bandwidth

■ Faster Internet access ■ High speed data transmission ■ Video Conferencing ■ High resolution image transmission ■ Other new services

■■ More fiber everywhereMore fiber everywhere

■■ Higher data-rate each wavelength----------TDMHigher data-rate each wavelength----------TDM

■■ More wavelengths in the fiber--------------- WDMMore wavelengths in the fiber--------------- WDM

Introduction: TDM,WDM strategiesIntroduction: TDM,WDM strategies

■■ TDM strategyTDM strategy ■■ Simple modulation format: IMDD binary systemSimple modulation format: IMDD binary system

■■ Short bit length, more bits in unit timeShort bit length, more bits in unit time

■■ Electrical TDM and optical TDM, Electrical TDM and optical TDM, solitonsoliton

■■ Grooming may be a big problemGrooming may be a big problem

■■ Difficult in transmission through fiber: chromatic dispersion,Difficult in transmission through fiber: chromatic dispersion, PMD, fiber nonlinearityPMD, fiber nonlinearity

■■ Does not scale well beyond OC192Does not scale well beyond OC192

Introduction: TDM,WDM strategiesIntroduction: TDM,WDM strategies

■■ WDM strategyWDM strategy ■■ Multiple wavelengths in a single fiberMultiple wavelengths in a single fiber

■■ Better utilization of optical fiber transmission windowBetter utilization of optical fiber transmission window

■■ Lower Lower bitratebitrate & lower power per wavelength & lower power per wavelength

■■ From CWDM to DWDM: increase bandwidth efficiencyFrom CWDM to DWDM: increase bandwidth efficiency example:example:2.5Gb/s per-channel data rate and 50GHz channel spacing, the

bandwidth efficiency is 0.05 bit/Hz.

■■ Technical difficulties: Technical difficulties: lightwavelightwave sources, optical filters, sources, optical filters, dispersion management, dispersion management, nonliearnonliear crosstalkcrosstalk, …., ….

Introduction: SCM/WDM and OSSBIntroduction: SCM/WDM and OSSB

■■ SCM leverages mature microwave technologySCM leverages mature microwave technology

■■ Two-step modulation: RF modulation at sub-carriers, opticalTwo-step modulation: RF modulation at sub-carriers, optical modulation at wavelengths.modulation at wavelengths.

■■ OSSB is preferred: less dispersion penalty, higher bandwidthOSSB is preferred: less dispersion penalty, higher bandwidth efficiencyefficiency

fOf 3- f 2 - f 1 - f 4+f 3+f 2+f 1+f 4-

a. O ptical DSB SCM

b. O ptical SSB SCM

fO f 4+f 3+f 2+f 1+

op tic a l d e tec tion us ing filte r

Introduction: SCM/WDM and OSSBIntroduction: SCM/WDM and OSSB

■■ SCM/WDM combinationSCM/WDM combination ■■ Large number of channels, low channel speed, easy groomingLarge number of channels, low channel speed, easy grooming

and scaling, lower dispersion penaltyand scaling, lower dispersion penalty

■■ Advanced modulation format, phase modulation possibleAdvanced modulation format, phase modulation possible

■■ NonliearityNonliearity-introduced distortion-introduced distortion

b. W DM

a. W DM/SCM

f 4+f 3+f 2+f 1+

1λ 2λ

1λ 2λ

An exemplary setup of SCM/WDM systemAn exemplary setup of SCM/WDM system

λm

λ1

f1

Σ

f2 ch.2

fn ch.n

.

.

Receiver

ch.1

O pt

ic a

l W D

M M

U X

O pt

ic a

l W D

M D

M U

XAdd/drop

f1

Σ

f2 ch.2

fn ch.n

.

.

Receiver

ch.1

Transmitter f1

Σ

f2 ch.2

fn ch.n

.

.

λ1ch.1

Transmitter f1

Σ

f2 ch.2

fn ch.n

.

.

λm

ch.1

50GHz

λ1 λ2 λ3 λ4 λ

5GHz

Numerical model for SCM/WDM systemsNumerical model for SCM/WDM systems

■■ Goals and general requirementsGoals and general requirements ■■ To be able to handle different types ofTo be able to handle different types of multi multi-wavelength-wavelength

SCM/WDM systemsSCM/WDM systems

■■ Include both dispersion and fiber Include both dispersion and fiber nonlinearitynonlinearity

■■ To be able to handle a variety of modulation formats, OSSB,To be able to handle a variety of modulation formats, OSSB, ASK, BPSK, QPSK and binary IMDDASK, BPSK, QPSK and binary IMDD

■■ Combine Combine waveform waveform distortion and noise: distortion and noise: sensitivitysensitivity

Transmitter simulation model

C hanne l 4 da ta 2 .488G b it/s

C hanne l 4 R F osc illa to r

(17 .5G Hz)

C hanne l 1 da ta 2 .488G b it/s

C hanne l 1 R F osc illa to r (3 .6 G Hz)

channe l 3 (13G Hz)

channe l 2 (8 .3G Hz)

T ransmitter fiilte r

T ransmitter filte r

S C M T ra nsm itter B lock D ia gram

RF mu ltip lexer

M Z m odu la tor

90degree phase shift

B ias

LD Power

amplifie r W D M coup le r

f ro m o ther laser sourses

to f iber

■ 27-1 PRBS ■ Random relative delay between different sub-carriers

and wavelengths ■ 6-order Butterworth filter for band-limiting ■ 64 to 256 samples per bit depending on the required

simulation bandwidth

Baseband signal generation

Baseband signal generation

2 0 0 4 0 0 6 0 0 8 0 0 1 0 0 0 1 2 0 0 1 4 0 0 1 6 0 0 1 8 0 0

0

5 0 0

1 0 0 0

1 5 0 0

2 0 0 0

2 5 0 0

3 0 0 0

3 5 0 0

4 0 0 0 C h a n n e l N u m b e r = 1 C h a n n e l W a v e le n g t h = 1 5 5 0 n m N u m b e r o f s a m p l e s / b i t = 6 4

(a)eyediagram of baseband signal after the electrical filter

0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 4 . 5 5

x 1 0 4

- 2 0 0 0

- 1 0 0 0

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

I c h a n n e l T R A N S M I T F I L T E R O U T P U T P U L S E - - > I N T E N S I T Y a f t e r f i l t e r i n g

P u

ls e

in

t e

n s

it y

(

m w

)

T i m e ( p s )

C h a n n e l N u m b e r = 1 B i t R a t e = 2 . 4 8 G b / s C h a n n e l W a v e l e n g t h = 1 5 5 0 n m C a r r i e r f r e q u e n c y = 2 . 6 G H z

- 6 - 4 - 2 0 2 4 6 8 - 3 0

- 2 5

- 2 0

- 1 5

- 1 0

- 5

0

5

1 0

•Signal eyediagram

•Signal waveform

•Baseband Spectrum

)sin()()cos()()( tnTtfyAtnTtfxAts csnccsnc ωω ∑∑ ∞

∞−

∞

∞−

−−−=

cA peak signal value

nx and ny information bits (0, 1 or -1)

)(tf normalized baseband bit shape function

cω subcarrier frequency ASK: BPSK: QPSK:

0or 1=nx 1-or 1=nx 1-or 1=nx 0=ny 0=ny 1-or 1=ny

If f(t) represents a rectangular bit shape , the PSD of the complex envelope of MPSK is

) )sin(

()( 2

S

S Sc

fT

fT TAfP

π π

= , where ST is the symbol time.

The 3dB bandwidth of BPSK and QPSK are both 0.88R if they have the same symbol rate R. Each symbol in QPSK represents 2 bits, so the spectrum efficiency is doubled.

Sub-carrier generation

4-subcarrier BPSK composite signal4-subcarrier BPSK composite signal

0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 4 . 5 5

x 1 0 4

- 5 0 0 0

- 4 0 0 0

- 3 0 0 0

- 2 0 0 0

- 1 0 0 0

0

1 0 0 0

2 0 0 0

3 0 0 0

4 0 0 0

5 0 0 0

T R A N S M I T F IL T E R O U T P U T P U L S E - - > IN T E N S IT Y a f t e r c o m b i n e r

P u

ls e

in

t e

n s

it y