Dr. Umesh TiwariScientist

V-4 (PHOTONICS)E.mail: umeshtiwari@csio.res.in

CSIR-CSIO, SECTOR 30, CHANDIGARH - 160 030

Optical Amplification of OFC Signal

Outline

Optical fiber communication

Optical amplification

Erbium doped fiber amplifiers

Raman fiber amplifiers

Study and performance analysis of

Raman/EDFA hybrid amplifier

Summary

Attenuation Plot (in dB) with wavelength for optical Fiber

4

Introduction

•

Attenuation -

Optical amplification at regular intervals (~70-80 km)

•

Dispersion -

Dispersion compensating fiber

MUX: Multiplexer

DEMUX: Demultiplexer

DCF: Dispersion compensating fiberA: Amplifier

TX

: Transmission fiber

MUX DEMUX

LaserDiodes

Receivers

ATX DCF A

λ1 , λ2….. λnλ1 , λ2….. λn

A

Optical transmission

Information transmitted digitally in the form of optical pulses on various optical carrier frequencies

•Attenuation-Problem in detection at receiver (receiver sensitivity)Two major issues:

Dispersive medium

•Dispersion-Different frequencies components in a pulse travel with different group velocities-Overlap between pulses

Transmission loss

Overcoming attenuation

•

For

long distance transmission, loss needs to be overcome periodically

•

Electronic regenerators

•

Optical amplifiers

Electronic Regeneration

•

Regeneration with Retiming and Reshaping

•

Limited by speed of electronics

•

Single wavelength only

•

Capacity upgradation difficult and expensive3R

RxTx

Traditional Optical Communication System

Loss compensation: Repeaters at every 20-50 km

Optical Amplification

PLUS

•

No need of high speed electronics

•

Transparent to bit rate and format

•

Simultaneous amplification of multiple λ

MINUS

•

Adds noise

Optically Amplified Systems

EDFA = Erbium Doped Fibre Amplifier

Optical Fiber AmplifiersOptical Fiber Amplifiers

OpticalAmplifier

λn…….. λ1. λ2 λn…….. λ1. λ2

Desirable characteristics of an optical amplifier:

• Large gain bandwidth-wider range of signal wavelengths can be simultaneously amplified by the same amplifier

• Gain flatness-All the signal wavelengths get same amplified by the same amount

General Structure of an optical amplifier

Principal types of optical amplifiers

•

Erbium doped fiber amplifier (EDFA)•

For operation in the S-,C-

and L-bands

•

Raman fiber amplifier (RFA)•

All wavelength bands

•

Semiconductor optical amplifier (SOA)•

All wavelength bands

Different amplifier configuration

•

Booster amplifier

•

In-line amplifier

•

Preamplifier

Tx Rx

Boosteramplifier

In-line amplifier

Preamplifier

Pump Source

WDMErbium Doped

Fibre

IsolatorIsolator

= Fusion Splice

Input Output

980 nm signal

1550 nm data signal

Pow

er le

vel

980 nm signal

1550 nm data signal

Pow

er le

velPower

interchangebetween

pump and data

signals

Representation of the Principle of an EDFA

Source: Master 7_5

Erbium Doped Fiber Amplifier (EDFA)

• Erbium has broad emission around 1550 nm where silicaoptical fibers have minimum loss

• Using population inversion Er

doped silica fibers behaveas optical amplifiers

980 nmPump Laser

Doped fiber

Isolator

WDM CouplerSignal

17

Energy level diagram of Erbium ions

• Pump at 980 nm used to create population inversion between levels E2 and E1

• Stimulated emission between E2 and E1 leads to amplification

Absorption and Emission Cross Section*

E2

E3

E1

980 nm

1480 nm1520 - 1570 nm

*B Pedersen et al., J. Lightwave Tech., 9, 1105, 1991

Amplified spontaneous emission (ASE)

•

Erbium atoms in the excited state emit photons spontaneously

•

This emission is not coherent with the signal

•

Like signal this spontaneous emission also gets amplified

•

ASE appears in the both forward as well as backward direction

•

Wavelength range 1525 to 1570 nm

•

Power density 10 to 100 µw/nm

( ) BGhnP spASE Δ−= 12 ν

ASE noise in EDFA:

The output ASE noise is

Where212 /)( NNNnsp −=

Noise Figure: F = 2 nsp

ASE Spectrum

20

Erbium Doped Fiber Amplifiers: Analysis

22

22

0 ( ) 11 ln 11

p

p

bp pa p

b

dP N P z u w u edz w w

we

σ −Ω

−Ω

⎡ ⎤⎛ ⎞ ⎛ ⎞+⎛ ⎞⎢ ⎥⎜ ⎟= − − + −⎜ ⎟⎜ ⎟⎢ ⎥⎜ ⎟⎜ ⎟⎝ ⎠ ⎜ ⎟ ⎝ ⎠⎢ ⎥+⎝ ⎠⎣ ⎦

22

22

0 ( ) 11 ln 11

s

s

bs sa s

b

dP N P z v w v edz w w

we

σ −Ω

−Ω

⎡ ⎤⎛ ⎞ ⎛ ⎞+⎛ ⎞⎢ ⎥⎜ ⎟= − + + −⎜ ⎟⎜ ⎟ ⎜ ⎟⎢ ⎥⎜ ⎟⎝ ⎠ ⎝ ⎠⎢ ⎥+⎝ ⎠⎣ ⎦

Pump and signal power evolution equations are given as*

*A.Ghatak and K.Thyagarajan, “Introduction to Fiber Optics”, Cambridge University Press, (1999).

21

0

( )ps

p

P zv

Pη=

0

( )1

s s

s s

P zuP

ηη

=+

0 0

( ) ( )p s

p s

P z P zwP P

= +

( )0 (1 )s s

ssa sp s sa se sp

h hI

t tν ν

σ η σ σ= =

+ +

0p

pp a sp

hI

tν

σ=

ses

sa

ση

σ=

20 0s s sP Iπ= Ω2

0 0p p pP Iπ= Ω

10

0

( )( )( )

K WVaJ UU K W

Ω =

with:

22

Simulation Results

Variation of pump power along the fiber length

• Linear decrease for high pump powers• Exponential for lower pump powers

23

• For every input pump power there is an optimum length of the fiber

Variation of signal power along fiber length

24

• Threshold pump power for transparency, beyond whichgain increases with increasing pump power

Variation of gain with input pump power for different fiber lengths

25

Pump and signal power evolution equations are given as*

PaP

eP

p PdzdP

P12 )N-(N Γ= σσ

Contribution of forward and backward ASE is also taken into consideration

* P.C. Becker,N.A. Olsson, J.R. Simpson “Erbium Doped Fiber Amplifiers”

Academic Press, (1999)

sas

es

s PdzdP

s12 )N-(N Γ= σσ

NP

AhP

AhP

Ah

PAh

PAh

PAhN

PPP

eP

aP

jAs

ea

jsss

es

as

PPP

aP

jAs

a

jsss

as

j

jj

j

j

1)()()()(

)(

2

+Γ+

+Γ+

Σ+Γ+

Γ+ΓΣΓ=

νσστν

νσστ

νσστ

ντσν

ντσ

ντσ

ννν

νν

26

Noise Figure

Noise figure is ratio between SNR at the input and the SNR at the output

*Noise Figure related with the gain and forward ASE spectral density:

*B Pedersen et al., J. Lightwave Tech., 9, 1105, 1991

Gh

LS

F s

sASE⎟⎟⎠

⎞⎜⎜⎝

⎛+

=

+

1),(νν

For practical purpose noise figure calculated as;

GBhNGNF inout

ν−

=

27

Variation in gain of the amplifiers due to two factors:

PDL of optical components like isolators and directional couplers

PDG due to the polarization hole burning (PHB).

Polarization Dependent Gain (PDG)

⎟⎟⎠

⎞⎜⎜⎝

⎛∗=

Min

MaxdB P

PPDG log10

PDG defined as:

o u t i nE D F A

W W

N -G NN 1 1N F = + = +h ν G B G h ν G B G

Measurement of source spontaneous emission [2]

ASE = ASE

Spectra

Wavelength

Pn1

Pn2

eSourceNoisGainPP nn ×−+2

21

Ideally an EDFA should amplify the signal without producing any additional output. However, broadband emission takes place in the form of ASE as shown

Erbium doped fiber amplifiers

Input Monitor

Output Monitor

Co-

directional

Pump

Contra-

directional

Pump

Erbium Doped Fiber

WDM Coupler

WDM CouplerIsolator IsolatorTap

CouplerTap

Coupler

980 nm 980 nm

1525-1565 nm (C-band)

Steps in designing the amplifier

•

Characterized each component for their insertion loss, isolation and wavelength dependence

•

Studied the dependence of gain on the bend radius of the EDF

•

Chose an optimal radius of 5 cm for the coil •

Optimized the splice machine for splicing an EDF with SMF-28

•

The optimum length for a given pump power was found from the Optiwave

Software Package. In this

case it was around 13.5 mts

Experimental Results (Single Channel)

Noise Measurement Bandwidth = 0.5 nm

OA

Pin1

(λ) Pout1

(λ)

PASE

(λ)

EDFA Input Reference Plane

EDFA Output Reference Plane

MUX DEMUX

TX1

TX2

TX3

TXN

RX1

RX2

RX3

RXNPin2

(λ)

Pin3

(λ)

PinN

(λ)

Pout2

(λ)

Pout3

(λ)

PoutN

(λ)

Multi-Channel Characterization

( )0 0( ) 10 ( ) 10( ) 10 log 10 10 ( )wP PBBSG Pλ λλ λ= − −

PBBS(λ), P0w(λ), P0(λ) are measured in dBm

and G(λ) is measured in dB

Multi-Channel Testing –

Results

•

λ1

= 1530.33 nm•

λ2 = 1533.47 nm•

λ3 = 1536.61 nm•

λ4 = 1539.77 nm•

λ5 = 1542.94 nm•

λ6 = 1547.72 nm•

λ7 = 1552.52 nm•

λ8 = 1557.36 nm

Polarization Dependent Gain Spectra

Experimental Results (Electrical Characteristics)Eye Patterns (a) at the output of the transmitter, and (b) at the output of the EDFA

Presented at Photonics -2006

No significant degradation in the Eye pattern due to insertion of the EDFA is observedBit Error Rate - 1.3 x 10-9 (at received power level of -30.75 dBm and transmission rate of 2.5 Gbps)

Final Fabrication (In collaboration with Tejas

Networks, Bangalore and Optiwave

Photonics, Hyderabad)

Optical SpecificationsAmplifier Type

: BoosterOperational Wavelength

: C -

BandNo. of Channels

: OneInput Signal Power (Pin)

: -

25 dBm

to– 5 dBm

Total Output Power @Pin = -25dBm

: + 10 dBmTotal Output Power @Pin = -5dBm : + 15 dBmSmall Signal Gain

: + 34 dBNoise Figure

: ≤

5 dBPolarization Dependent Loss

: 0.20 dBPolarization Mode Dispersion

: 0.04 ps

Electrical SpecificationsPower Supply

: ±

12 V, 2 APower Consumption

:18 WMode of Operation

:Automatic Current Control & Power Control

DESIGN & DEVELOPMENT OF MIDDESIGN & DEVELOPMENT OF MID--STAGE STAGE ERBIUM DOPED FIBRE AMPLIFIERERBIUM DOPED FIBRE AMPLIFIER

Front view of fabricated MSA-EDFA

1530 1535 1540 1545 1550 1555 15602

4

6

8

10

12

14

16

18

20

22

24

26

28

24681012141618202224262830

Out

put P

ower

(dB

m)

Gai

n (d

B)

Wavelength (nm)

Output power Gain

1530 1533 1536 1539 1542 1545 1548 1551 1554 1557 15600

5

10

15

20

25

30

0

5

10

15

20

25

30

Noi

se F

igur

e (d

B)

Gai

n (d

B)

Wavelength (nm)

Multi Channel Gain Multi Channel Noise Figure

Need for an optical amplifier with provision of access to signals at mid-

stage for various functional elements such as :

Optical add-drop multiplexerDispersion Compensator VOA to achieve variable gain

EDF L1

GFFInput

ChannelsOutput

Channels

EDF L2

Gain Flattening Issues in MSA-EDFA

Wavelength (nm)Wavelength (nm)Wavelength (nm)

Gai

n (d

B)

Tra

nsm

issi

on (d

B)

Gai

n (d

B)

38

39

Raman Amplifier

Virtual Energy State

hνpump

hνSignal

ΔEphonon

Lower Energy State

SignalSignal

Pump

Description of Stimulated Raman Scattering

Raman gain curve*

*G.P. Agrawal, Non Linear Fiber Optics, Academic press, 3rd ed.,2001

41

The following equations for signal and pump evolution

ss s s p s

pp p p p s

d P P g P Pd zd P

P g P Pd z

α

α

= − +

± = − −

( )

eff

r p ss

gg

KAν ν−

=

pp s

s

g gνν

=

(m-1W-1) is the Raman gain coefficient of fiber normalized with respect to the effective area of the fiber i.e.

sg

Raman Amplifiers: Modeling Equations

•

Ps,p

= Power at signal and pump wavelength•

αp, αs

= Attenuation co-efficient at pump and signal•

K = Polarization factor•

= Effective Raman gain coefficient at signal and pump wavelength

•

Aeff

= Fiber effective area

sgs,p

Evolution of signal and pump powers for co-directional pumping in a SMF

Ps

(in) = -40 dBm

λs

= 1550 nm

Pp

(in) = 30 dBm

λp

= 1450 nmsignal

pump

Evolution of signal and pump powers for contra-directional pumping in a SMF

signal

pump

Ps

(in) = -40 dBm

λs

= 1550 nm

Pp

(in) = 30 dBm

λp

= 1450 nm

Experimental Results

WDMCoupler

WDMCoupler

Tunable Laser

OSA

1450 nmpump

Liquid Paraffin

Corning Vast®

DCF

Residual Pump

Isolator IsolatorTunable

LaserWDM

CouplerWDM

CouplerOSA

Experimental setup for the measurement of Raman gain

G = exp (gR

P0

Leff

/Aeff

)

Pump1455 nm

Pump

5.3 kmDCF

Index matching

Liquid

Signal

DCF based Raman amplifier

Gain profile of a multi-pumped RFA

Flattened gain profile with optimized pump powers

λ1 = 1420 nmλ2 = 1450 nmλ3 = 1480 nm

ΔG=1 dB

ΔG=2.1 dB

51

5.3 km

DCF

Residual pump

13 m

EDF1450 nm

Pump

5.3 km

DCF

1450 nm

Pump

13 m

EDF

Residual pump

5.3 km

DCF

1450 nm

Pump

13 m

EDF

Residual pump

Hybrid II Hybrid III

Hybrid I

J.H. Lee et al., Journal of Lightwave Technology 23, 3484 (2005).

Different Raman/EDFA Hybrid Amplifier Configurations

52

•

Hybrid amplifier offers several advantages

•

High Gain, low noise figure, High pump efficiency

•

Compensates both dispersion and loss

•

DCF has much higher Raman gain co- efficient

•

Amplification band expansion easily achieved within the transparency window of optical fiber

•

Wavelength Division Multiplexing (WDM) is the most suited methods for increasing the transmission capacity

•

Polarization dependent gain (PDG) modified the signal to noise ratio (SNR) and such an SNR modification leads to performance degradation

Need of Hybrid Amplifier

DCF

Residual pump

53Multi-channel gain and noise figure of hybrid I configuration

Results

5.3 km

DCF

Residual pump

13 m

EDF1450 nm

Pump

54Multi-channel gain and noise figure of hybrid II configuration

Results

5.3 km

DCF

1450 nm

Pump

13 m

EDF

Residual pump

55

Polarization Dependent Gain Spectra

Ref: Tiwari et al. Opt. Comm. 281 (2008) 1593FIO - 2007, San Jose, USA

PMD/PDL analyzer setup

•

Multi-channel performance and PDG measurements studied for first time

•

A trade off exists in the deployment of these amplifiers

•

Hybrid I has best PDG and BER

•

Hybrid II lowest noise figure

•

Hybrid III best pump efficiency

56

Multi-channel Gain and Noise figure Characterization of Raman/EDFA Hybrid Amplifiers

57

New Raman/EDF Hybrid Amplifier with Enhanced Performance

1450 nmPump

5.3 kmDCF

Residual pump

13 mEDF

Schematic of New Raman/EDF Hybrid Amplifier

Simulation and experimental characterization of the new Raman/EDFA hybrid amplifier have been studied

The EDF is followed by a DCF section, and EDF section in the co-pump geometry

58

1520 1530 1540 1550 1560 1570

68

101214161820222426 Input signal power @ - 20 dBm

Input signal power @ - 5 dBm

Input signal power @ - 10 dBm

Simulated Measured Measured Measured Simulated Simulated

Gai

n (d

B)

Wavelength (nm)1520 1530 1540 1550 1560 15700

2

4

6

8

10

12

14

Noi

se F

igur

e (d

B)

Wavelength (nm)

Simulated: Input signal power @ - 5 dBm Simulated: Input signal power @ - 10 dBm Simulated: Input signal power @ - 20 dBm Measured: Input signal power @ - 5 dBm Measured: Input signal power @ - 10 dBm Measured: Input signal power @ - 20 dBm

Simulated and measured gain and noise figure plot

Results

59Photonics -2008, IIT Delhi & SPIE Best Paper AwardRef: Tiwari et al. Opt. Comm. 282 (2009) 1563

-16 -14 -12 -10 -8 -6 -40.0

0.3

0.6

0.9

1.2

1.5

1.8

2.1

2.4

PDG

(dB

)

Signal Power (dBm)

Hybrid II Hybrid III Hybrid I Novel Configuration

•

Best noise figure and lowest gain ripple•

Experimental and simulated results are in close agreement•

Polarization dependant gain (PDG) is smallest

Comparison of PDG of different configurations with signal power

Results

Highlights:

Multi-channel gain comparison between measured (solid line) and simulated (dotted line) gain

60

EDFA /SOA hybrid amplifier for broadband amplification

With increased capacity requirements, wide gain bandwidth amplifiers have become important In order to improve the performance of wide band amplifiers EDFA and SOA can be utilized in conjunctionBy combining the properties of EDFA and SOA, the amplification bandwidth can be enhanced by about 15-20 nmThe various parameters have been optimized to achieve the best performance of EDFA/SOA hybrid amplifierSince SOA suffers from significant polarization dependent gain (PDG), while the EDFA is known to have smaller PDG, and there is a need to study the PDG characteristics of EDFA/SOA in conjunction

n -

InP

InGaAsPp -

Inp

p+ InGaAs

Injection Current

Input signal

Amplified output

Metallization

Active Region

Essentially a Semiconductor Laser diode with Anti-reflection coating on facets

Semiconductor Optical amplifier

6262

SOA SOA --

FeaturesFeatures

••

Small size , electrical pumping Small size , electrical pumping ••

Broad gain spectrum ( 30 ~ 50 nm)Broad gain spectrum ( 30 ~ 50 nm)

••

Exploitable nonlinear characteristicsExploitable nonlinear characteristics••

Compatibility with photonic integrated circuitsCompatibility with photonic integrated circuits

••

Diverse range of applicationsDiverse range of applications––

optical switching, wavelength convertersoptical switching, wavelength converters

––

logic gates, multiplexers, fliplogic gates, multiplexers, flip--flopsflops

ASE spectra

In presence of signal- ASE gets suppressed - Peak shifts towards higher wavelengths

64

Schematic of experimental setup of EDFA/SOA hybrid amplifier studied*

EDFA/SOA Hybrid Amplifier

In the first configuration the EDFA section in the co-pump geometry is followed by an SOA. In other configuration SOA is followed by EDFA section in the co-pump geometry

The hybrid amplifier in the proposed configurations are experimentally characterized in terms of single channel gain, noise figure, multi channel gain, and PDG

SOA

10 mEDF

980 nmPump

10 mEDF

SOA

980 nmPump

Schematic of experimental setup of SOA/EDFA hybrid amplifier studied

*C. H. Yeh et al., Laser Phys. Lett.4, 433 (2007).

65

1510 1520 1530 1540 1550 1560 1570 1580 1590 16000

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Noi

se F

igur

e (d

B)

NF: - 25 dBm I/PNF: - 15 dBm I/P

NF: - 5 dBm I/P

Gain: - 5 dBm I/P

Gain: - 15 dBm I/P

Gain: - 25 dBm I/P

Gai

n (d

B)

Wavelength (nm)

1520 1540 1560 1580 1600 16200

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Noi

se F

igur

e (d

B)

Gai

n (d

B)

Wavelength (nm)

Gain: - 25 dBm I/P Gain: - 15 dBm I/P Gain: - 5 dBm I/P NF: - 5 dBm I/P NF: - 15 dBm I/P NF: - 25 dBm I/P

Results

Measured Gain (G) and Noise figure (NF) comparison for three different I/P power

levels EDFA/SOA Hybrid

Measured Gain (G) and Noise figure (NF) comparison for three different I/P power

levels SOA /EDFA Hybrid

Simulated Gain (G) and Noise figure (NF) comparison for three different I/P power

levels EDFA/SOA Hybrid

1520 1540 1560 1580 1600 16200

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Noi

se F

igur

e (d

B)

Gai

n (d

B)

Wavelength (nm)

Gain @- 5dBm Gain @- 15dBm Gain @- 25dBm NF @ - 5dBm NF @- 15dBm NF @- 25dBm

66

Measured Gain (G) and Noise figure (NF) comparison

1520 1540 1560 1580 1600

5

10

15

20

25

30

35

40

0

5

10

15

20

25

30

35

40

Gai

n (d

B)

Wavelength (nm)

Gain: EDFA Gain: EDFA/SOA Gain: SOA/EDFA Gain: SOA NF: EDFA NF: EDFA/SOA NF: SOA/EDFA NF: SOA

Noi

se F

igur

e (d

B)

P (980 nm) = 127 mW, 150 mA SOA current

67

Results

Comparison of measured PDG spectrum of the different amplifier configuration at -5

dBm signal input power

• Multi channel gain characteristics are quite different from single channel case, and gain tilt is observed due to spectral hole burning

• A better gain flatness was observed for -15 dBm and -5 dBm signal input power• Reported, for the first time, comparison of the PDG characteristics• Improvement in the PDG of the hybrid amplifier is demonstrated

Photonics-2012, IIT Madras

Highlights:

1530 1540 1550 1560 15700.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

PDG

(dB

)

Wavelength (nm)

EDFA EDFA_SOA SOA SOA_EDFA

Optical Amplifier: Comparison

Source: Internet

The measured PDG of fabricated EDFA was quite comparable with PDG of commercially available EDFA

The gain spectra for the multichannel case was significantly different form the single channel case.

Hybrid II amplifier has been demonstrated to have a low noise figure.

The new Raman/EDFA configuration presented in this analysis hasnot only the best noise figure performance but also has lowest gain ripple.

Experimental and simulated results are in close agreement.

Simulation and experimental characterization of single channel

gain and noise figure of a EDFA /SOA hybrid amplifier is presented.

We have reported, for the first time, comparison of the PDG characteristics.

CONCLUSION

References

1. A.Ghatak and K.Thyagarajan, “Introduction to Fiber Optics”, Cambridge University Press, 1999.

2. P.C. Becker,N.A. Olsson, J.R. Simpson “Erbium Doped Fiber Amplifiers” Academic Press, 1999.

3. G.P. Agrawal, Non Linear Fiber Optics, Academic press, 3rd ed.,2001.

4. C. Headly and G.P. Agrawal “ Raman amplification in Fiber Optical communication systems, Elsevier Academic Press,2005.

5. M.N. Islam, “Raman Amplifiers for Telecommunications 1”, Springer-Verlag New York, Inc., 2004.