Design and Development of Erbium Doped Fiber Amplifiers A Project Report submitted By Laiju P.Joy ( Reg. No: 95713004 ) in partial fulfillment of the requirements for the award of the degree of MASTER OF TECHNOLOGY INTERNATIONAL SCHOOL OF PHOTONICS COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY COCHIN-682022 June 2015
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Design and Development of Erbium Doped Fiber Amplifiers
A Project Report
submitted By
Laiju P.Joy
( Reg. No: 95713004 )
in partial fulfillment of the requirements
for the award of the degree of
MASTER OF TECHNOLOGY
INTERNATIONAL SCHOOL OF PHOTONICS
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
COCHIN-682022
June 2015
INTERNATIONAL SCHOOL OF PHOTONICS
COCHIN UNIVERSITY OF SCIENCE AND TECHNOLOGY
COCHIN-22
CERTIFICATE
This is to certify that the project work entitled ‘DESIGN & DEVELOPMENT OF
ERBIUM DOPED FIBER AMPLIFIERS‘ is a bonafide work done by Mr. LAIJU P.JOY
( Reg. No: 95713004 ) in partial fulfillment of the requirements for the award of the Degree
of Master of Technology in Opto Electronics & Laser Technology is carried out at
Laboratory for Electro-Optics Systems, ISRO, Bangalore during the academic year 2014-
2015.
Prof. P. Radhakrishnan Dr. M. Kailasnath
Professor Director
International School of Photonics International School of Photonics
CUSAT CUSAT
ABSTRACT
The fiber amplifier is a key enabling technology for high speed optical communication.
The development of EDFA provide tremendous growth in communication system capacity.
The EDFA characteristics are analyzed for two different EDF length 7m and 13m, and two
different wavelengths 1550nm and 1570nm. The gain and noise characteristics of amplifier is
observed and compared with simulations.
The basic EDFA mathematical model is developed with Giles parameter by Matlab. The
Gain Master software used to simulate the EDFA with different length, input power and pump
powers.
The Optimum length of EDFA is 7m and use forward pumping configuration for reduce
effect of noise. The EDFA in 1550nm has gain 34.47dB in 20µW input with 300mW pump power
and noise figure is 4.5 dB.
The 5W amplifier use MOPA configuration. It has a EYCDFA followed by EDFA pre –
amplifier. The optimum length selected for EYCDF is 6m with pump power of 18W.
ACKNOWLEDGEMENT
I would like to thank Mr. V.V. Laskshmi Pathi and Mrs. Lekshmi S. Rajan, Scientists,
Laboratory of Electro-Optics System (LEOS), they helped in reviewing the work progress,
results and offered valuable feedbacks in each and every stage of this project work.
I am thanking Dr. M. Kailasnath, Director, International School of Photonics, CUSAT for
giving all facilities that helped me to complete this mission.
I extend my sincere thanks to Dr. P. Radhakrishnan, Profeesor, International School of
Photonics, CUSAT for his motivation and support during my work
With deep sense of gratitude, I express my heartfelt thanks to Dr.V P N Nampoori, Emeritus
Professor, International school of photonics, CUSAT for the motivation, support in project work.
I thankful to my friends in International school of photonics, CUSAT for their support and help.
I sincerely thanks all the teachers who filled the knowledge and wisdom in me during each step
of my life. I also extend my sincere thanks to parents and God.
2.3.2 Super Luminescent Light Emitting Diode: SLED (Signal Source) 23
2.3.3 Isolator 24
2.3.4 Circulator 25
2.3.5 WDM Coupler 26
3. Design, Modeling & characterization of EDFA 27
3.1 MODELING OF EDFA 27
3.2 EDFA rate equations 27
3.2.1 Three level rate equations 27
3.2.2 Two level rate equation 30
3.3 Equations used for modeling 31
3.4 Amplifier Matlab Modeling with M-12 Generic Fiber using Giles Parameters 32
3.6 Simulations using Gain Master: Giles Model 38
3.7 Optimization of EDFA Parameters 39
3.7.1 EDF Length 39
3.7.2 Signal Power 40
3.7.3 Pump Configuration 41
3.8 EDFA Experiment 43
3.8.1 EDFA Experimental setup 44
3.8.2 Amplification 46
3.8.3 EDFA Gain Characteristics 48
3.8.5 Noise Figure 54
3.8.6 Noise Figure Characteristics 55
3.9 Experimental Data Analysis 56
4. Design of EYCDFA 57
4.1 Theory of EYCDFA 57
4.2 Experimental Setup for Simulation 58
4.3 Optimization of EYCDFA Parameters 59
4.3.1 Pump Power 59
4.3.2 Optimum Length 61
4.3.3 Signal Power 61
4.3.4 Wavelength 62
5. Conclusion 63
Future Plans 64
Bibliography 65
APPENDIX 66
ABSTRACT
The fiber amplifier is a key enabling technology for high speed optical communication.
The development of EDFA provide tremendous growth in communication system capacity.
The EDFA characteristics are analyzed for two different EDF length 7m and 13m, and two
different wavelengths 1550nm and 1570nm. The gain and noise characteristics of amplifier is
observed and compared with simulations.
The basic EDFA mathematical model is developed with Giles parameter by Matlab. The
Gain Master software used to simulate the EDFA with different length, input power and pump
powers.
The Optimum length of EDFA is 7m and use forward pumping configuration for reduce
effect of noise. The EDFA in 1550nm has gain 34.47dB in 20µW input with 300mW pump power
and noise figure is 4.5 dB.
The 5W amplifier use MOPA configuration. It has a EYCDFA followed by EDFA pre –
amplifier. The optimum length selected for EYCDF is 6m with pump power of 18W.
List of Figures
1.1 A basic EDFA configuration 2
1.2 Erbium ion transition 3
1.3 Erbium ion transition in different energy levels 3
1.4 Overlap between erbium ion distribution and transverse intensity profile 6
1.5 Optimum length at maximum gain 7
2.1 Diagram of simple laser diode 12
2.2 Common structure of super luminescent diode 13
2.3 The symmetric and antisymmetric mode of the combined wave guides 15
2.4 Coupling of power in wave guides 15
2.5 Polarization dependent isolator with Faraday rotator, polarizer and analyzer 18
2.6 Polarization independent isolator 19
2.7 Behavior of an optical circulator 20
2.8 Configuration of a 3 port optical circulator from port 1 to port 2 transmission 20
2.9 Configuration of a 3 port optical circulator from port 2 to port 3 transmission 20
2.10 Circulator used to drop an optical channel from a WDM system using FBG 21
2.11 FBG structure, refractive index profile and spectral response 22
2.12 Pump LD characteristics 23
2.13 SLD characteristics 24
2.14 SLED output spectrum obtained from OSA 24
2.15 Experimental setup for characterize isolator 25
2.16 Experimental setup for characterize circulator 25
2.17 Experimental setup for characterize WDM 26
3.1 EDFA as a 3 level system 28
3.2 EDFA as a 2 level system 30
3.3 Variation of signal power along the length of the fiber 35
3.4 Variation of pump power along the length of the fiber 36
3.5 Signal gain in dB along the length of the fiber 36
3.6 Forward ASE spectrum along the length of the fiber 37
3.7 Backward ASE spectrum along the length of the fiber 37
3.8 Variation of Noise Figure with pump power 38
3.9 Giles parameters 39
3.10 Variation of output signal power at different length EDF for input signal of 10μW and pump power of 300Mw 40
3.11 Variation of output power with respect to input power 40
3.12 Different pumping configuration used in EDFA 41
3.13 Output power for different pumping configuration 42
3.14 Output ASE power for different pumping configuration 43
3.15 Block diagram of EDFA experimental setup 43
3.16 Input signal derived with FBG 44
3.17 EDFA experimental setup 45
3.18 Amplified output for 7m EDF for 1550nm 46
3.19 Amplified output for 13m EDF for 1550nm 47
3.20 Amplified output for 7m EDF for 1570nm 47
3.21 Amplified output for 13m EDF for 1570nm 48
3.22 Variation of gain for different input power in 7m EDF in 1550nm input 48
3.23 Variation of gain for different input power in 13m EDF in 1550nm input 49
3.24 Variation of gain for different input power in 7m EDF in 1570nm input 49
3.25 Variation of gain for different input power in 13m EDF in 1570nm input 50
3.26 Comparison of gain in 7m and 13m EDF for 1550nm input power of 10μW and pump power 300mW 50
3.27 Comparison of gain at 1550nm and 1570nm input signal for 7m EDF with 10μW input 51
3.28 Comparison of simulations and experiment for input 10μW with wavelength 1550nm 52
3.29 Forward ASE of EDF 7m without input 52
3.30 Backward ASE of EDF 7m without input 53
3.31 Forward ASE of EDF 7m with input 54
3.32 Noise figure for 1550nm signal 55
3.33 Noise figure for 1570nm signal 55
4.1 Erbium Ytterbium transitions 57
4.2 EYCDFA model configuration 58
4.3 Pump absorption of EYCDF 59
4.4 Signal absorption cross section of EYCDF 59
4.5 Variation of signal power with fiber length and pump power 60
4.6 Output Optical Power vs Length w.r.t different pump power 60
4.7 Signal output power vs length 61
4.8 Signal output power vs signal input 61
4.9 Variation of output power with wavelength 62
List of Tables
2.1 Experimental result of circulator characterization 25
2.2 Experimental result of WDM coupler characterization 26
3.1 M-12 Generic fiber parameters used for Matlab simulation 34
3.2 EDFA output power with respect to EDF length 39
Abbreviations
ASE Amplified Spontaneous Emission
EDF Erbium Doped Fiber
EDFA Erbium Doped Fiber Amplifier
ESA Excited State Absorption
EYCDF Erbium Ytterbium Co-Doped Fiber
EYCDFA Erbium Ytterbium Co-Doped Fiber Amplifier
FBG Fiber Bragg Grating
MOPA Master Oscillator Power Amplifier
NF Noise Figure
WDM Wavelength Division Multiplexer
Symbols
Aeff Effective erbium doped area in a fiber m2
τ Metastable life time of erbium ions s
h Plank’s constant Js
ν Frequency Hz
Δν Bandwidth Hz
g* Giles gain coefficient dB/m
α Giles loss coefficient dB/m
l Impurity and propagation losses in fiber dB/m
ζ Saturation Parameter m-3
( )aσ Absorption cross section m-2
( )eσ Emission cross section m-2
N Total population density of erbium ions/m3
N1 Ground state population density of erbium ions/m3
N2 Excited state population density of erbium ions/m3
λ Wavelength m
P Filed power W
I Intensity W/m2
1
Chapter 1
INTRODUCTION
Earlier long distance optical communication systems use electronic regenerators for
amplifying the optical signals. The attenuated optical signal is amplified electronically, first
convert optical signal into electrical domain and then conversion back to optical domain. Such
regenerators are designed to operate at one optical wavelength and specific bit rate. In a WDM
communication systems carrying multiple wavelength signals through one fiber, the electronic
regeneration would be very complex and expensive.
Optical amplifier can amplify the incoming optical signals in the optical domain itself
without any conversions to the electrical domain. It do not need high speed electronics circuitry
and they are transparent to bit rate, can amplify multiple optical signals at different wavelength
simultaneously. The development of EDFA provide tremendous growth in communication
system capacity using WDM, in which multiple wavelengths carrying independent signals are
propagated through same single mode fiber.
Optical amplifiers can be used at many points in communication link. A booster amplifier
is used to boost the power of the transmit signal before it launching into the fiber link. The pre
amplifier placed just before the receiver is used to increase the receiver sensitivity. In line
amplifiers are used at intermediate points in the fiber link to overcome losses.
Today, most of the optical fiber communication system use EDFAs, due to their
advantages in terms of bandwidth, high power output, and noise characteristics.
Erbium doped fiber amplifier is a optical amplifier, it amplifies weak input optical signals
directly without any conversions pumped with a laser diode. The main application of EDFA is to
amplify signals in optical domain.
The EDFA became a key enabling technology for optical communication networks, and
have since comprised the vast majority of all optical amplifiers deployed in the field. Erbium
Design and Development of Erbium Doped Fiber Amplifiers
2
doped fiber amplifier is most common optical amplifier, commercially available since the early
1990’s. It is a most stable optical amplifier with operating bands 1525 – 1565 nm wavelength
region. It works best in this range with gain upto 30 dB.
The main element in EDFA is Erbium doped fiber, which is developed by conventional
Silica fiber with rare earth element Erbium.
1.1 Basic Erbium Doped Fiber Amplifier
The basic EDFA configuration is shown in figure 1.1
It contains
Erbium Doped Fiber
Pump Source
Wavelength Division Multiplexer
Isolators
Figure 1.1 : a basic EDFA configuration
Erbium is generally preferred in fiber for amplification because of the inherent properties
associated with it. Erbium ions have quantum levels that can be stimulated to emit 1550nm band
with least power loss. Moreover the property of erbium is that its quantum levels allow it to get
excited by 980nm or 1480 nm pump signals. The amplification is achieved by stimulated
emission of photons from erbium ions in the doped fiber. The pump laser excites erbium ions
into a higher energy from ground level. The ions in the higher energy level will soon decay
spontaneously very fast to the metastable level, the life time of erbium ions in this level is 10ms.
Design and Development of Erbium Doped Fiber Amplifiers
3
The stimulated decay from metastable level to ground generate light amplification by stimulated
emission.
When the Erbium is illuminated with light energy at a suitable wavelength (either 980nm
or 1480nm) it is excited to a long lifetime intermediate state (see Figure 1.2), following which it
decays back to the ground state by emitting light within the 1525-1565 nm band.
Figure 1.2 : Erbium ion transitions
Figure 1.3 : Erbium ion transitions in different energy levels
Design and Development of Erbium Doped Fiber Amplifiers
4
When signal photon of wavelength equivalent to the band gap energy between the ground
state and metastable state is passing through the erbium doped fiber, two types of transitions
occur. First a small portion of the ions in the ground state absorb this signal photon and raise to
the metastable state known as stimulated absorption . The ions in the metastable band on
absorbing the energy from the signal photon can undergo stimulated emission and drop to the
ground level, thereby emitting a new photon of the same wavelength and same polarization that
of the input signal photon. Erbium ions can also be excited by a pump wavelength of 1480nm
but is not desirable because the pump and signal wavelengths are almost nearer and hence the
transitions between these wavelengths will lower the efficiency of the device and increase
amplifier noise. The 980nm pump source has a higher absorption cross section and hence will be
used where EDFA design demands low noise. Hence we have used a 980nm pump laser.
1.2 EDFA Models
The method of developing EDFA uses different modeling techniques. The Giles model
solves the steady state rate equation utilizing gain and absorption parameters which are
proportional to the cross sections. This model includes propagation equation which allow
modeling along the length of the fiber. Saleh-Jopson model provide analytical solutions to the
propagation equation. The higher erbium concentration model include the inhomogeneous
effects such as ion-ion interaction and ESA.
1.2.1 GILES MODEL
The simpler method of erbium doped fiber can be characterized by using amplifier
equation in terms of the erbium absorption coefficient α(λ), gain coefficient g*(γ), a fiber
saturation parameter ζ and excess loss in the fiber from scattering and impurity absorption l(λ).
These easily measured parameters allow the fiber performance evaluation in 980 nm or
1480 nm pumped optical amplifiers. Conventional fiber measurement techniques are used to
obtain these parameters, from which the amplifier performance can be calculated.
The rate equations for forward (+) and backward (-) propagating beams are
Design and Development of Erbium Doped Fiber Amplifiers
5
( ) ( )KKKKKKKKK lh
nngP
nng
dzdP
+−∆++=± ± ανα1
2*
1
2* 2 (1.1)
( )∑∑
++= ±
KKKKK
KKK
hgP
hP
nn
ζνα
ν
*1
2
1 (1.2)
where
( ) ( ) ( )Na λλσλα Γ=
( ) ( ) ( )Ng e λλσλ Γ=*
τζ NAeff=
1.2.2 Saleh –Jopson Model
This model developed for estimation of the pump and signal power along the length of
the EDF fiber. The Saleh-Jopson model valid for amplifiers with gain less than 20 dB, and gain
saturation by ASE can be neglected.
The pump signal absorption and signal gain in EDF can be obtained by solving
transcendental equation. When the pump and signal power propagate through fiber the change in
power in the Kth beam can be obtained by
( ) ( ) ( )( ) ( )tzPtzNNdz
tzdPK
aK
aK
eKKK
K ,,,2 σσσµ −+Γ= (1.3)
( )( )
+Γ−
Γ−=τσσ
σ aK
eKK
outinaKK
inK
outK
PPALNPP exp)exp( (1.4)
where inKP and out
KP are total input and output power.
1.2.3 Average Inversion Model
This model compute the gain and noise figure at other wavelengths from a computation of the average inversion from a measured reference spectrum and cross section ratio.
Design and Development of Erbium Doped Fiber Amplifiers
6
1.2.4 Higher Erbium Concentration Model
The higher +3rE ion concentration model require additional modeling terms which
account for concentration quenching or ion-ion interaction and ESA.
The concentration of +3rE ion increase in fiber leads to undesirable effect like cooperative
up-conversion and pair induced quenching. The ion- ion interaction effects limits the concentration of erbium ions in silica matrix.
Signal or pump excited state absorption is possible in EDF due to presence of other energy levels in erbium energy levels. These other energy levels in erbium can absorb signal or pump photon to higher energy level. This effect can be depletes the population inversion and also gain.
1.3 EDFA Characteristics
1.3.1 Overlap Factor of Amplifier
Consider one dimensional model of the fiber amplifier, the overlap factor is known as
overlap between transverse intensity profile of optical mode and transverse erbium ion
distribution profile. This overlap will stimulate absorption or emission from the transitions.
If we consider a single mode cylindrical geometry optical fiber with constant area and
erbium ion density, the overlapping is shown in figure 1.4
Figure 1.4 : Overlap between erbium ion distribution and transverse intensity profile
The Overlap factor is
Design and Development of Erbium Doped Fiber Amplifiers
7
−=Γ
−2
2
1 ωR
e (1.5)
where R is the erbium doped radius in fiber and is the spot size of the beam. The spot size
ω will vary with frequency, the overlap factor will be depends on frequency of the mode.
1.3.2 Optimum Length of EDF
The input signal is amplified along the length of the EDF at fixed pump power upto a
specific point, after that point the gain is negative, and so the fiber should be terminated at the
point. At this point the pump power is decreased to the threshold level. The length of the fiber at
that specific point determine optimum length of EDF for EDFA. The optimum length at
maximum gain is shown in figure 1.5
Figure 1.5 : Optimum length at maximum gain
1.3.3 Small Signal Gain
The signal field and pump field with corresponding intensities Is and Ip, are propagated
simultaneously in EDF and interact with ions in the fiber. Due to this interactions variations are
occurred in signal intensity and pump intensity, they are given by
Design and Development of Erbium Doped Fiber Amplifiers
8
NI
hI
hI
hI
dzdI
SS
P
PP
S
SS
P
PP
s σ
νσ
νσν
σ
++Γ
Γ−=
221
21
(1.6)
NI
hI
hIh
I
dzdI
PP
P
PP
S
SS
S
SS
P σ
νσ
νσ
νσ
++Γ
+Γ−=
221
21
(1.7)
If thP II > , the threshold condition for gain, for propagation of signal field.
2τσν
P
PthP
hII =≥ (1.8)
where Ith is the pump threshold intensity
The normalized intensities are given by
th
PP I
II ='
(1.9)
th
SS I
II ='
(1.10)
The quantity η and saturation intensity Isat(Z) as
P
S
S
P
hh
σσ
ννη =
(1.11)
( ) ( )
η21 ' ZIZI P
sat+
= (1.12)
The normalized variation of signal intensity and pump intensity as
( )( )
( )( ) ( )NzIzIzI
zIzIdzzdI
SSP
P
satS
S ''
'
'
'
11
)(11 σ
+−
+=
(1.13)
Design and Development of Erbium Doped Fiber Amplifiers
9
( ) ( )( ) ( ) ( )NzI
zIzIzI
dzzdI
PPPS
SP '''
'
211 ση
η++
+−=
(1.14)
The condition for small signal gain is satisfied, when satS II << , at this sate the signal propagation along the length of the fiber is
( ) )exp()0('' zIzI PSS α= (1.15)
where the gain coefficient defined as
NS
P
PP σα
11
'
'
+Ι−Ι
= (1.16)
We can see that the signal gain grows exponentially, with a coefficient proportional to the signal emission cross section and degree of population inversion.
1.3.4 Gain Saturation Region
In gain saturation region, the signal growth is linear. This occurs when the signal 'SI
grows to a large value comparable to satI . The signal growth is then damped by the saturation
factor,satS II '1
1+
The ratio of 'SI and satI becomes large compared to unity, 1
'
>>sat
S
II
The growth of signal in saturation region is determined by
N
III
dzdI
SP
Psat
S σ
+−
=11'
(1.17)
satI is linearly dependent with pump power, then signal saturation is varied with pump power.
The saturation output power is inversely proportional to emission cross section of fiber,
this causes the saturation power higher at 1550 nm than at 1530 nm.
The experimentally determined saturation output power is defined as the signal output
power at which the gain has been reduced by 3 dB.
Design and Development of Erbium Doped Fiber Amplifiers
10
1.3.5 Amplified Spontaneous Emission
It is a parasitic process, which can occur at any frequency within the fluorescence
spectrum of the amplifier transitions. The effect of ASE is to reduce the total amount of gain
available from the amplifier.
The excited ions can spontaneously relax from the upper state to ground state by emitting
a photon that is uncorrelated with the signal photons. The spontaneously emitted signal is
amplified, it travels along the fiber and it stimulates the emission of more photons from exicted
state. This process is known as ASE. The ASE power sometimes referred to as an equivalent
noise power.
For a single transverse mode fiber with two independent polarizations for a given mode at
frequency ν, the noise power in a bandwidth Δν, corresponding to spontaneous emission, is equal
to
ν∆= hPASE 20 (1.18)
The total ASE power at a point z along the fiber is the sum of the ASE power from the
prevision sections of the fiber and added local noise power 0ASEP .This local noise power will
stimulate the emission of photons from excited erbium ions.
The propagation equation for the ASE power propagating in a given direction is thus
( ) ( )( ) ( )( )( ) ( ) ( ) ( )( )νσνννσνσν e
ASEASEaeASE NPPNN
dzdP
20
12 +−= (1.19)
1.4 High Power Amplifier
The gain of EDF is determined by erbium ions density in the fiber, then gain is increased
by adding extra erbium ions. But, after a particular level of doping the concentration quenching
effect is occure in EDF. This is due to reduce the gain of amplifier.
We want higher amplified power in range of Watts, then use Ytterbium Co-doped with
Erbium fiber. The Yb ions around Er ions prevent the concentration quenching. Then We will
Design and Development of Erbium Doped Fiber Amplifiers
11
get higher amplified output power. In EYCDFA the Yb ions are pumped at 800-1100nm
wavelengths. The Yb ions transfer energy to the Er ions and Er ions are excited to higher energy
state. The stimulated emission of Er ions fom metastable state give higher output power.
1.5 Motivations & Contributions
According to the previous literature survey, the Erbium doped fiber (EDF) is a reliable
gain media for lasers and amplifiers. The selection of appropriate pump wavelength sources are
very crucial.
Erbium ions exhibit a very narrow absorption bands, 10 nm at 980 nm for 3 level system
and 1480 nm for 2 level system. This is due to the selection of pump source for the amplifier is
limited to InGaAs laser diode and Titanium Sapphire (Ti:S) laser source which operate within
these wavelength region. The small absorption cross section of erbium ions is not suitable for
high power amplifications due to the limitation of ions concentrations inside fibers.
Throughout this work, the suitable parameters to operate the EDFA have been identified.
This leads to optimized output obtained by the amplifier.
Design and Development of Erbium Doped Fiber Amplifiers
12
Chapter 2
EDFA Components & Characterization
2.1 Components used in EDFA configuration
2.1.1 Pump Laser Diode
A laser diode, or LD, is an electrically pumped semiconductor laser in which the active
laser medium is formed by a p-n junction of a semiconductor diode similar to that found in a
light-emitting diode.
A laser diode is electrically a P-i-n diode. The active region of the laser diode is in the
intrinsic (I) region, and the carriers, electrons and holes, are pumped into it from the N and P
regions respectively. All modern lasers use the double-heterostructure implementation, where the
carriers and the photons are confined in order to maximize their chances for recombination and
light generation. The goal for a laser diode is that all carriers recombine in the I region, and
produce light. Thus, laser diodes are fabricated using direct bandgap semiconductors.
Diagram of a simple laser diode shown in figure 2.1
Figure 2.1 :Diagram of simple Laser Diode
Design and Development of Erbium Doped Fiber Amplifiers
13
The commonly used pump laser source for optical amplifiers are
980 nm – InGaAs
1480 nm – InGaAsP
2.1.2 Super Luminescent Light Emitting Diode : SLED (Signal Source)
A superluminescent diode (SLED or SLD) is an edge-emitting semiconductor light
source based on superluminescence. Its output is high power and brightness with the low
coherence, and emission bandwidth is 5–100 nm wide.
A superluminescent light emitting diode is, similar to a laser diode, based on an
electrically driven PN-junction that, when biased in forward direction becomes optically active
and generates amplified spontaneous emission over a wide range of wavelengths. The peak
wavelength and the intensity of the SLED depend on the active material composition and on the
injection current level. The basic structure of SLED is shown in figure 2.2 .
Figure 2.2 : Common structure of superluminescent diode
A SLED consists of a positive (p-doped) section and a negative (n-doped) section,
electrical current will flow from the p-section to the n-section and across the active region that is
sandwiched in between the p- and n-section. During this process, light is generated through
Design and Development of Erbium Doped Fiber Amplifiers
14
spontaneous and random recombination of positive (holes) and negative (electrons) electrical
carriers and then amplified when travelling along the waveguide of a SLED.
The PN-junction of the semiconductor material of a SLED is designed in such a way that
electrons and holes feature a multitude of possible states (energy bands) with different energies.
Therefore, the recombination of electron and holes generates light with a broad range of optical