Download Light Sources - Mechanical and Construction Engineering

Optical Fibre Communication
Systems
Lecture 3: Light Sources
Professor Z Ghassemlooy
Northumbria Communications Laboratory
Faculty of Engineering and
Environment
The University of Northumbria
U.K.
http://soe.unn.ac.uk/ocr
Prof. Z Ghassemlooy
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Contents
 Properties
 Types of Light Source




 LED
 Laser
Types of Laser Diode
Comparison
Modulation
Modulation Bandwidth
Prof. Z Ghassemlooy
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Light Sources - Properties
In order for the light sources to function properly and find
practical use, the following requirements must be satisfied:
• Output wavelength: must coincide with the loss minima of the
fibre
• Output power: must be high, using lowest possible current and
less heat
• High output directionality: narrow spectral width
• Wide bandwidth
• Low distortion
Prof. Z Ghassemlooy
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Light Sources - Types
Every day light sources such as tungsten filament and arc lamps
are suitable, but there exists two types of devices, which are
widely used in optical fibre communication systems:

Light Emitting Diode (LED)

Semiconductor Laser Diode (SLD or LD).
In both types of device the light emitting region consists of a pn
junction constructed of a direct band gap III-V semiconductor,
which when forward biased, experiences injected minority carrier
recombination, resulting in the generation of photons.
Prof. Z Ghassemlooy
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LED - Structure
• pn-junction in forward bias,
• Injection of minority carriers across the junction gives rise to
efficient radiative recombination (electroluminescence) of
electrons (in CB) with holes (in VB)
n
p
Electron
hf  E g
--- Fermi levels
hf  E g
Hole
Homojunction LED
Prof. Z Ghassemlooy
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LED - Structure
•Spontaneous emission
•Optical power produced by the Junction:
int
hc
P0  I
hf  I
q
q
Pt
Fibre
Photons P0
n-type
p-type
Where
int = Internal quantum efficiency
q = Electron charge 1.602 x 10-19 C
P0
Narrowed
Depletion region
Electron (-)
I
+
Hole (+)
Prof. Z Ghassemlooy
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LED - External quantum efficiency ext
It considers the number of photons
actually leaving the LED structure
 ext 
Fn 2
4n x
2
Where
F = Transmission factor of the device-external interface
n = Light coupling medium refractive index
nx = Device material refractive index
Loss mechanisms that affect the external quantum efficiency:
(1) Absorption within LED
(2) Fresnel losses: part of the light gets reflected back,
reflection coefficient: R={(n2-n1)/(n2+n1)}
(3) Critical angle loss: all light gets reflected back if the incident angle
is greater than the critical angle.
Prof. Z Ghassemlooy
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LED - Power Efficiency
• Emitted optical power Pe 
External power efficiency
P0 Fn 2
4n x 2
ep 
• MMSF:
The coupling efficiency
• GMMF:
The optical coupling loss relative to Pe is :
Or the power coupled to the fibre:
pe
 100
P
%
c  NA2
NA2
c 
2
Lc  10 log 10
Pc
Pe
Pc (dBm)  Pe (dBm)  Lc (dB)
Prof. Z Ghassemlooy
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LED- Surface Emitting LED (SLED)
• Data rates less than 20 Mbps
• Short optical links with large NA fibres (poor coupling)
• Coupling lens used to increase efficiency
G Keiser 2000
Prof. Z Ghassemlooy
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LED- Edge Emitting LED (ELED)
• Higher data rate > 100 Mbps
• Multimode and single mode fibres
G Keiser 2000
Prof. Z Ghassemlooy
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LED - Spectral Profile
Intensity
1300-1550 nm
800-900
nm
65
45
15 0 15 45
65
Wavelength (nm)
Prof. Z Ghassemlooy
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LED - Power Vs. Current Characteristics
5
4
3
2
1
SELED
Temperature
Linear region
ELED
50
Current I (mA)
Since P  I, then LED can be intensity modulated by
modulating the I
Prof. Z Ghassemlooy
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LED - Characteristics
Wavelength
800-850 nm
1300 nm
• Spectral width (nm)
30-60
50-150
• Output power (mW)
0.4-5
0.4-1.0
• Coupled power (mW)
- 100 um core
- 50 um core
0.1-2 ELED
0.3-0.4 SLED
0.01-0.05 SLED
0.05-0.15
- Single mode
0.04-0.08
0.03-0.07
0.003-0.04
• Drive current (mA)
50-150
100-150
• Modulation bandwidth
(MHz)
80-150
100-300
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LED - Frequenct Response
Magnitude (dB)
LED
LD
0
-3
1
10
100
1000
10,000
Frequency (MHz)
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Laser - Characteristics
• The term Laser stands for Light Amplification by Stimulated
Emission of Radiation.
•Is an optical oscillator - Comprises a resonant optical amplifier
whose output is fed back into its input with matching phase.
Any oscillator contains:
-
An amplifier with a gain-saturated mechanism
A feedback system
A frequency selection mechanism
An output coupling scheme
• Could be mono-chromatic (one colour), and is coherent in nature. (I.e. all
the wavelengths contained within the Laser light have the same phase). One the main advantage of
Laser over other light sources
• A pumping source providing power
• It had well defined threshold current beyond which lasing occurs
• At low operating current it behaves like LED
• Most operate in the near-infrared region
Prof. Z Ghassemlooy
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Laser - Basic Operation
Similar to LED, but based on stimulated light emission.
mirror 1
Mirrors used to
“re-cycle” phonons”
mirror 2
“LED”
coherent light
R = 0.90
R = 0.99
Reflectivity R = [(n-1)/(n+1)]2
Three steps required to generate a laser beam are:
• Absorption
• Spontaneous Emission
Current density:
• 104 A/cm2 down to 10 A/cm2
• Stimulated Emission
Prof. Z Ghassemlooy
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Absorption
When a photon with certain energy is incident on an electron in a semiconductor
at the ground state(lower energy level E1 the electron absorbs the energy and
shifts to the higher energy level E2.
The energy now acquired by the electron is Ee = hf = E2 - E1. Plank's law
E2
E1
E2
Incoming
photon
Ee = hf
Electron
E1
Initial state
E2
E1
Excited electron
final state
Prof. Z Ghassemlooy
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Spontaneous Emission
• E2 is unstable and the excited electron(s) will return back to the
lower energy level E1
• As they fall, they give up the energy acquired during absorption
in the form of radiation, which is known as the spontaneous
emission process.
E2
E1
E2
Photon
Ee = hf
E1
Initial state
Prof. Z Ghassemlooy
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Stimulated Emission
• But before the occurrence of this spontaneous emission process, if external
stimulation (photon) is used to strike the excited atom then, it will stimulate the
electron to return to the lower state level.
• By doing so it releases its energy as a new photon. The generated photon(s) is in
phase and have the same frequency as the incident photon.
• The result is generation of a coherent light composed of two or more
photons.
• In quantum mechanic – Two process: Absorption and Stimulated emission
E2
E1
E2
Ee = hf
Requirement:
Ee = hf
Ee = hf
E1
 <0
Coherent light
Ee = hf
Light amplification: I(x) = I0exp(-x)
Prof. Z Ghassemlooy
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Laser - Basic Operation
So we have a large number of electron inside a cavity, therefore need to talk about statistics.
Thus need to talk average rates of transition. I.e. what is the probability that a transition can
take place between two levels per unit time.
N2
The rate of absorption process is:
Transition probability from 1 to 2
[is a constant introduced by Einstein]
Occupation
probability of level 1
Photon density
In the cavity foe E21
Probability that
Lower level is empty
f1 and f2 are Fermi functions given as:
F1 and F2 are quasi Fermi levels (i.e., number of
electrons in the lower and upper levels,
respectively
Prof. Z Ghassemlooy
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Laser - Basic Operation
The rate of spontaneous emission process is:
Transition probability from 2 to 1
[is a constant introduced by Einstein]
Probability that
Lower level is empty
Occupation
probability of level 2
The rate of stimulated emission process is:
Photon density
In the cavity foe E21
Transition probability
from 2 to 1
The rate of total emission process is (upper level is depopulated):
Prof. Z Ghassemlooy
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Laser - Basic Operation
• At dynamic equilibrium
Absorption
=
emission
One need to solve this to determine
Prof. Z Ghassemlooy
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The Rate Equations
Rate of change of
photon numbers = stimulated emission + spontaneous emission + loss
dN
N
 CneN  Rsp 
dt
 ph
Rate of change of
electron numbers = Injection + spontaneous emission + stimulated spontaneous
dne J
n


 CneN
dt qd  sp
N is photons per unit volume (optical output power), J is the current density,
Rsp is the rate of spontaneous emission, ph photon life time, d depth of the active
Region, sp spontaneous recombination rate, C is the constant, ne injected electron per
unit volume
Prof. Z Ghassemlooy
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Laser Diodes (LD)
n
R1
I
R2
n0
Z=0
Z=L
Standing wave (modes) exists at
frequencies for which
L
L
i
2n
,
i = 1, 2, ..
Modes are separated by
f 
Optical confinement
layers
c
2nL
2nL 2nL 2nL
 


In terms of wavelength separation
i
i 1
i
(Longitudinal mode spacing)
2
2
 
 f
2nL c
Prof. Z Ghassemlooy
for i  1
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LD – Turn–on Delay
Ip
Input Current
Output Light Signal
d
For and applied current pulse of amplitude I p
the turn on delay

 d   th ln 

 Ip
Ip
 Ith
is given by:




with a bias current Ib applied:

 d   th ln 

 Ip
Ip
 Ib  Ith




To reduce the turn on delay:
• Use a low threshold laser and make Ip large
• Bias the laser at or above threshold
Turn on Delay (ns)
where  th is the delay at threshold (2ns Typ.)
Prof. Z Ghassemlooy
Ib=0
Ib=0.5Ith
Ib=0.9Ith
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LD - Spectral Profile

Intensity
Modes
Gaussian output
profile
5
3
1 0 1
3
5
Wavelength (nm)
Multi-mode
Prof. Z Ghassemlooy
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LD - Efficiencies
Internal quantum efficiency
int
number of photons generated in the cavity

number of injected electrons
External quantum efficiency
External power efficiency
ext 
Pe
IE g
Pe
ep 
P
Where P = IV
Power degradation over time
P  P0 e
 t / D
Lifetime decreases with current density and junction temperature
Prof. Z Ghassemlooy
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Power Vs. Current Characteristics
Temp.
5
4
3
2
1
LED
Stimulated
emission
(lasing)
Spontaneous emission
50
Current I (mA)
Threshold current
Ith
• Applying a bias current has the same effect as applying a pump laser; electrons are
promoted to conduction band. Fc and Fv get farther apart as well
• Increasing the temperature creates a population distribution rather than a sharp
cutoff near the Fermi levels
Prof. Z Ghassemlooy
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LD – Electrical Model
Package Lead
Inductance
Package Lead
Capacitance
Bond wire
Inductance
Laser contact
resistance
Laser Pad
Capacitance
Assume that the light output is
proportional to the current through
the laser junction
Simple large signal model
Use a large signal diode model for the laser
junction, this neglects the optical resonance
Laser
Junction
More exactly the laser rate equations can be
implemented in SPICE to give the correct
transient behavior under large signal
modulation
Small signal model
Prof. Z Ghassemlooy
(Hitachi)
29
LD - Single Mode
• Achieved by reducing the cavity length L from 250 m to 25 m
• But difficult to fabricate
• Low power
• Long distance applications
Types:
• Fabry-Perot (FP)
•Distributed Feedback (DFB)
• Distributed Bragg Reflector (DBR)
• Distributed Reflector (DR)
Prof. Z Ghassemlooy
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Laser - Fabry-Perot
 Strong optical feedback in the longitudinal direction
 Multiple longitudinal mode spectrum
Ppeak
 “Classic” semiconductor laser
– 1st fibre optic links (850 nm or 1300 nm)
– Short & medium range links
 Key characteristics
–
–
–
–
–
–
–

Wavelength: 850 or 1310 nm
Total output power: a few mw
Spectral width: 3 to 20 nm
Mode spacing: 0.7 to 2 nm
Highly polarized
Coherence length: 1 to 100 mm
Small NA ( good coupling into fiber)
Agilent Technology
Prof. Z Ghassemlooy
P
Threshold
I
250-500 um
Cleaved faces
5-15 um
31
Laser - Distributed Feedback (DFB)
No cleaved faces, uses Bragg Reflectors for lasing
Single longitudinal mode spectrum
High performance
– Costly
– Long-haul links & DWDM systems
Key characteristics
–
–
–
–
–
–
Corrugated feedback Bragg
Wavelength: around 1550 nm
Total power output: 3 to 50 mw
Spectral width: 10 to 100 MHz (0.08 to 0.8 pm)
Sidemode suppression ratio (SMSR): > 50 dB
Coherence length: 1 to 100 m
Small NA ( good coupling into fiber)
P peak
SMSR

Agilent Technology
Prof. Z Ghassemlooy
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Laser - Vertical Cavity Surface
Emitting Lasers (VCSEL)
 Distributed Bragg reflector mirrors
– Alternating layers of semiconductor material
– 40 to 60 layers, each  / 4 thick
– Beam matches optical acceptance needs of fibers more closely
 Key properties
–
–
–
–
–
Wavelength range: 780 to 980 nm (gigabit ethernet)
Spectral width: <1nm
Total output power: >-10 dBm
Coherence length:10 cm to10 m
Numerical aperture: 0.2 to 0.3
Laser output
p-DBR
active
n-DBR
Agilent Technology
Prof. Z Ghassemlooy
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Laser diode - Properties
Property
Multimode
Single Mode
• Spectral width (nm)
1-5
< 0.2
• Output power (mW)
1-10
10-100
0.1-5
1-40
1-40
25-60
• Drive current (mA)
50-150
100-250
• Modulation bandwidth
(MHz)
2000
6000-40,000
• Coupled power (W)
- Single mode
• External quantum efficiency
Prof. Z Ghassemlooy
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Comparison
LED
Laser Diode





















Low efficiency
Slow response time
Lower data transmission rate
Broad output spectrum
In-coherent beam
Low launch power
Higher distortion level at the
output
Suitable for shorter
transmission distances.
Higher dispersion
Less temperature dependent
Simple construction
Life time 107 hours




High efficiency
Fast response time
Higher data transmission rate
Narrow output spectrum
Coherent output beam
Higher bit rate
High launch power
Less distortion
Suitable for longer transmission
distances
Lower dispersion
More temperature dependent
Construction is complicated
Life time 107 hours
Prof. Z Ghassemlooy
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Modulation
The process transmitting information via light carrier
(or any carrier signal) is called modulation.
• Direct Intensity (current)
• Inexpensive (LED)
• In LD it suffers from chirp up to 1 nm (wavelength variation
due to variation in electron densities in the lasing area)
DC
RF modulating
signal
R
I
Intensity Modulated
optical carrier signal
• External Modulation
Prof. Z Ghassemlooy
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Direct Intensity Modulation- Analogue
LED
LD
Modulation Index
M = I/IB’
For LED IB’ = IB
For LD
IB’ = IB - Ith
Input signal
With no input signal m(t) the optical output P(t) = Pt[1 + M m(t),
G Keiser 2000
Prof. Z Ghassemlooy
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Direct Intensity Modulation- Digital
LD
Optical power
Optical power
LED
i
i
Time
t
Time
t
In a pulse modulated laser, if the laser is completely turned off
after each pulse, after onset of the current pulse, a time delay is
given by:


Ip
t d   ln 

I

(
I

I
)
 p
B
th 

Prof. Z Ghassemlooy
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 : carrier life time, I p : Current pulse amplitude, I B : Bias current
Direct Intensity Modulation- Digital
Laser
Monitor Photodiode
-
Data
Vref
+
-5V
Average number of 1s and 0s (the “Mark Density”) is linearly
related to the average power. If this duty cycle changes then
the bias point will shift
Prof. Z Ghassemlooy
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Direct Intensity Modulation- Limitations
 Turn on delay and resonance frequency are the two major factors that limit the
speed of digital laser modulation
–
the photon life time in the laser cavity:
1
 ph
–

c
1
1  c
 a  ln
  gth
ne  2L R1R2  ne
the relaxation oscillation frequency given by:
1
f 
2
1
 sp ph
 I


 1
 I th

1/ 2
 Saturation and clipping introduces nonlinear distortion with analog modulation
(especially in multi carrier systems)
 Nonlinear distortions introduce higher order inter modulation distortions
(IMD3, IMD5…)
 Chirp: Unwanted laser output wavelength drift with respect to modulating
current that result on widening of the laser output spectrum.
Prof. Z Ghassemlooy
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External Modulation
• For high frequencies 2.5 Gbps - 40 Gbps, and is more complex,
higher performance.
• AM sidebands (caused by modulation spectrum) dominate
linewidth of optical signal
DC
MOD
R
I
Modulated
optical
carrier signal
RF (modulating signal)
• Total relative phase difference between th e two interferin g signals :
Phase shift in the upper arm output is L  m
Phase shift in the lower arm output is L
If m is even - -  constructi ve interferen ce (inphase)
If m is odd - -  destructiv en interferen ce (opposite phase)
Light intensity modulation will result for all other valu es of m
Prof. Z Ghassemlooy
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LED - Modulation
 The frequency response of an LED depends on:
1- Doping level in the active region
2- Injected carrier lifetime in the recombination region,  i.
3- Parasitic capacitance of the LED
 If the drive current of an LED is modulated at a frequency of  the output optical
power of the device will vary as:
P ( ) 
P0
1  ( i ) 2
Electrical current is directly proportional to the optical power, thus we can define
electrical bandwidth and optical bandwidth, separately.
 p() 
 I() 
Electrical BW  10log 

20
log

 I (0) 
 p(0) 


p : electrical power, I : electrical current
Prof. Z Ghassemlooy
42
Modulation Bandwidth
Optical Bandwidth Bopt - Larger than Bele
 P( ) 
 I ( ) 
Optical BW  10 log 
 10 log 


P
(
0
)
I
(
0
)




Optical 3 dB point
G Keiser 2000
Prof. Z Ghassemlooy
43
Light Source - Nonlinearity
x(t)
Nonlinear function y=f(x)
y(t)
x(t )  A cos t
y (t )  A0  A1 cos t  A2 cos 2t  ...
Nth order harmonic distortion:
 An 
20 log  
 A1 
Prof. Z Ghassemlooy
44
Intermodulation Distortion
x(t )  A1 cos 1t  A2 cos  2 t 
y (t )   Bmn cos( m1  n 2 )t
m,n  0,1,2,...
m,n
Harmonics:
n1 , m 2
Intermodulated Terms:
1   2 ,21   2 ,1  2 2 ,...
Prof. Z Ghassemlooy
45
LD – Noise Sources
 Modal (speckel) Noise: Fluctuations in the distribution of energy
among various modes.
 Mode partition Noise: Intensity fluctuations in the longitudinal modes
of a laser diode, main source of noise in single mode fiber systems.
 Reflection Noise: Light output gets reflected back from the fiber joints
into the laser, couples with lasing modes, changing their phase, and
generate noise peaks. Isolators & index matching fluids can eliminate
these reflections.
Prof. Z Ghassemlooy
46
LD – Transmitter Package
Prof. Z Ghassemlooy
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