Transcript File

UNIT III
SOURCES AND DETECTORS
Direct Band Gap Semiconductors
Indirect Band Gap Semiconductors
E
E
E
CB
Direct Bandgap
Ec
Eg
Indirect Bandgap, Eg
CB
Photon
Ev
kcb
VB
–k
k
(a) GaAs
–k
VB kvb
(b) Si
Ec
CB
Er
Ev
k
–k
Ec
Phonon
Ev
VB
k
(c) Si with a recombination center
(a) In GaAs the minimum of the CB is directly above the maximum of the VB. GaAs is
therefore a direct bandgap semiconductor. (b) In Si, the minimum of the CB is displaced from
the maximum of the VB and Si is an indirect bandgap semiconductor. (c) Recombination of
an electron and a hole in Si involves a recombination center .
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
Light-Emitting Diodes (LEDs)
• For photonic communications requiring data rate 100-200 Mb/s
with multimode fiber with tens of microwatts, LEDs are usually
the best choice.
• LED configurations being used in photonic communications:
1- Surface Emitters (Front Emitters)
2- Edge Emitters
LED PRINCIPLE OF OPERATION
• In FOC LEDS must have
 high radiance o/p or brightness
 Fast emission response time
 High Quantum Efficiency
RADIANCE
Measure of optical power radiated into a unit
solid angle per unit area of the emitting
surface.(unit is Watts.)
• Emission Response Time
Time delay between application of a current
pulse and respective optical emission.
Quantum efficiency
Related to fraction of electron hole pairs
recombine radiatively.
LED structures
• For high radiance and quantum efficiency
The LED must have
Optical confinement
Achieve high level Radiative
Recombination in the active region of the
device-yield high quantum efficiency
Carrier confinement
Preventing absorption of the emitted
radiation by the material surrounding the PN
junction.
HETRO JUNCTION STRUCTURE
• Used to achieve Carrier and optical
confinement
• Consists of two adjoining semiconductor with
different band gap energies
• Band gap energy difference of adjacent
layers confines charge carriers.
• RI differences of adjoining layers confines
optical field to the central active layer.
Cross-section drawing of a typical
GaAlAs double heterostructure light
emitter. In this structure, x>y to provide
for both carrier confinement and optical
guiding.
b) Energy-band diagram showing the
active region, the electron & hole
barriers which confine the charge carriers
to the active layer.
c) Variations in the refractive index; the
lower refractive index of the material in
regions 1 and 5 creates an optical barrier
around the waveguide because of the higher
band-gap energy of this material.
 (m) 
1.240
Eg (eV)
[4-3]
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
LED CONFIGURATION
• Surface emitters LED • Homo structure
 Data rates above
1000bps
 Light emitting region
perpendicular to fiber
axis
• Edge emitters LED
 Emit more directional
light than SELED
 Use an etched well in
GaAs substrate in order
to prevent heavy
absorption.
• Double hetero
junction structure
 Giving increased
efficiency from optical
and electrical
confinement
Surface-Emitting LED
 High radiance etched well
is 0.8 to 0.9 um
 Due to large band gap in
confining layer- very low
internal absorption.
 GaAs is used in well- to
avoid heavy absorption of
emitted light.
 Circular active area of
surface emitter is 50 um
dia and 2.5um thick.
 Emission pattern is
isotropic with 120 half
power beam width.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
LAMBERTIAN PATTERN
Isotropic pattern from surface emitter.
Source equally bright in all directions-view
It decides coupling efficiency
Source equally bright but power dismisses
cosᶿ.
Power 50% down in its peak when ᶿ= 60.
So total half power beam width is 120
Coupled power
 The power coupled to MMSI is
PC =π(1-r)A RD(NA)2
R- fresenel coefficient of fiber surface
A- emission area of the source
RD - radiance of the source
 They allow more power to be coupled- more
difficult, expensive.
DH Edge Emitter LED
 Emit more directional light
 Reduce lose by absorption and more directional –light
collected from edge.
 Has transparent guiding layers with very thin active layer
of 50 um to 100 um – reducing self absorption.
 Guiding layer RI < surrounding material(core&cladding)>
outer surrounding material.
 Form wave guide channel that directs the optical
radiation towards fiber.
 To match fiber core diameter (50 to 100um) the contact
stripes for the edge emitters are 50-70um wide.
 In the plane parallel to the junction no waveguide effects.
 In the plane highly directional perpendicular to the
junction-when half power beam width is 25 to 35 by
proper choice of waveguide thickness.
Edge-Emitting LED
Schematic of an edge-emitting double heterojunction LED. The output beam is
lambertian in the plane of junction and highly directional perpendicular to pn junction.
They have high quantum efficiency & fast response.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Full Width Half Power Maximum
Width of spectral pattern and its half power
point.
Fundamental quantum relation E=hv
= h (c/λ)
V=c/λ
λ= hc/E
Peak emission wavelength
λ(um)=1.240/Eg(eV)
Light Source Material
 Most of the light sources contain III-V ternary & quaternary
compounds.
 Ga 1x Al x As by varying x it is possible to control the band-gap
energy and thereby the emission wavelength over the range of
800 nm to 900 nm. The spectral width is around 20 to 40 nm.
 In1 x Ga x As y P1 y By changing 0<x<0.47; y is approximately 2.2x,
the emission wavelength can be controlled over the range of
920 nm to 1600 nm. The spectral width varies from 70 nm to
180 nm when the wavelength changes from 1300 nm to 1600
nm. These materials are lattice matched.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Spectral width of LED types
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Quantum efficiency
Intro..
 Excess of electrons and holes in p and n type material
created in semiconductor by carrier injection at device
contacts.
 Excess carrier can be recombine radiatively or non
radiatively.
 Total carrier generated rate is sum of externally supplied
and thermally generated rates.
 Externally generated rate = j/qd
j – current density
q – electron charge

d – thickness of region
Themally generation rate = n/Ԏ
Ԏ- carrier life time,N – excess carrier
Rate equations, Quantum Efficiency & Power of
LEDs
• When there is no external carrier injection, the excess density
decays exponentially due to electron-hole recombination.
n(t )  n0 e t /
[4-4]
• n is the excess carrier density,
n0 : initial injected excess electron density
 : carrier lifetime.
• Bulk recombination rate R:
dn n
R

dt 
[4-5]
• Bulk recombination rate (R)=Radiative recombination rate +
nonradiative recombination rate
bulk recombinat ion rate ( R  1/τ ) 
radiative recombinat ion rate ( Rr  1/τ r )  nonradiati ve recombinat ion rate( Rnr  1/τ nr )
With an external supplied current density of J the rate equation for the electron-hole
recombination is:
dn(t ) J n
[4-6]


dt
qd 
q : charge of the electron; d : thickness of recombinat ion region
In equilibrium condition: dn/dt=0
J
n
qd
[4-7]
INTERNAL QUANTUM EFFICIENCY & OPTICAL POWER
 Fraction of electron and holes recombine radiatively
 Ratio of radiative recombination to total recombination
 nr
Rr

int 


Rr  Rnr  r   nr  r
[4-8]
int : internal quantum efficiency in the active region
Optical power generated internally in the active region in the LED is: [4-9]
I
hcI
Pint  int h  int
q
q
Pint : Internal optical power,
I : Injected current to active region
External Quantum Eficiency
ext 
Total no of photons emitted from LED
Total no of LED internally generated photons
[4-10]
 In order to calculate the external quantum efficiency, we need to
consider the reflection effects at the surface of the LED. If we
consider the LED structure as a simple 2D slab waveguide, only
light falling within a cone defined by critical angle will be emitted
from an LED.
ext
c
1

T ( )(2 sin  )d

4 0
4n1n2
T ( ) : Fresnel Transmissi on Coefficien t  T (0) 
(n1  n2 ) 2
If n2  1  ext 
1
n1 (n1  1) 2
Pint
LED emitted optical powr, P  ext Pint 
n1 (n1  1) 2
[4-11]
[4-12]
[4-13]
[4-14]
Modulation of LED
• 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
[4-15]
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
[4-16]
 P( ) 
 I ( ) 
Optical BW  10 log 
 10 log 


P
(
0
)
I
(
0
)




Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
[4-17]
LASER
(Light Amplification by the Stimulated Emission of Radiation)
• Laser is an optical oscillator. It comprises a resonant optical
amplifier whose output is fed back into its input with matching
phase. Any oscillator contains:
1- An amplifier with a gain-saturated mechanism
2- A feedback system
3- A frequency selection mechanism
4- An output coupling scheme
• In laser the amplifier is the pumped active medium, such as
biased semiconductor region, feedback can be obtained by
placing active medium in an optical resonator, such as FabryPerot structure, two mirrors separated by a prescribed distance.
Frequency selection is achieved by resonant amplifier and by
the resonators, which admits certain modes. Output coupling is
accomplished by making one of the resonator mirrors partially
transmitting.
LASER
Single wavelength- related to molecular
characteristics of material
Lasing medium – gas,liquid,insualting
crystal
Principle of operation
 Two energy levels E1(lower) and E2(higher)
 Three main process for laser action:
1- Photon absorption
photon with energy E2-E1 incident on the atom in E1,atom
excited into E2 through absorption of photon
2- Spontaneous emission
Atom return to E1 by random manner.
3- Stimulated emission
Photon have equal energy b/w two states(E1-E2) interact with
atoms causing it to lower state with creation of second photon. Give
coherent radiation.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Lasing in a pumped active medium
 In thermal equilibrium the stimulated emission is essentially
negligible, since the density of electrons in the excited state is
very small, and optical emission is mainly because of the
spontaneous emission.
 Stimulated emission will exceed absorption only if the population
of the excited states is greater than that of the ground state. This
condition is known as Population Inversion.
 Population inversion is achieved by various pumping
techniques.
 In a semiconductor laser, population inversion is accomplished
by injecting electrons into the material to fill the lower energy
states of the conduction band.
ILD ADVANTAGES
 Coherent light
 Less coupling loss
 High o/p power
 Used High bit rates
 Monochromatic light
 Good spatial coherence
DISADVANTAGES
 10 times expensive than LED
 Shorter life time due to high power operation
 Temperature dependent
SEMICONDUCTOR INJECTION LASER
BW Greater than 200Mhz.
Its have response time less than 1ns
HOptical BW 2nm or less
igh coupling efficiency
Multilayered
Smaller temp dependence
Fabry-Perot Resonator
M1
A
M2
m=1
Relative intensity
1
f
R ~ 0.8
R ~ 0.4
m=2
 m
B
L
(a)
m=8
(b)
m - 1
m
m + 1

(c)
Resonant modes : kL  m m  1,2,3,..
Schematic illustration of the Fabry-Perot optic al cavity and its properties. (a) Reflected
waves interfere. (b) Only standing EM waves ,modes, of certain wavelengths are allowed
in the cavity. (c) Intensity vs. frequency for various modes.R is mirror reflectance and
lower R means higher loss from the cavity.
© 1999 S.O. Kasap, Optoelectronics (Prentice Hall)
(1  R) 2
I trans  I inc
(1  R) 2  4R sin 2 (kL)
R: reflectance of the optical intensity, k: optical wavenumber
[4-18]
Hetero junctions
 Semiconductor lasers cleaved facets are used instead of
external mirrors
 Reflectivity Rm=(n-1/n+1)^2
 In DH structures-carrier and optical confinement reduces
threshold currents for lasing by a factor 100
 DH forward bias applied by +ve to p & -ve to n.
 Voltage correspond to band gap energy applied –large no
of es are injected into active layer lasing commences.
 Small amount of energy required for operation of laser.
 Realized only by laser pumped above threshold
 Threshold current- current needed to reach threshold
Fabry Perot resonator cavity for Laser Diode
•
Laser diode is an improved LED, in the sense that uses stimulated
emission in semiconductor from optical transitions between distribution
energy states of the valence and conduction bands with optical
resonator structure such as Fabry-Perot resonator with both optical
and carrier confinements.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Cavity much smaller
250 – 500 um long
5 – 15 um wide
0.1 – 0.2 um Thick
Laser Diode Characteristics
•
•
•
•
Nanosecond & even picosecond response time (GHz BW)
Spectral width of the order of nm or less
High output power (tens of mW)
Narrow beam (good coupling to single mode fibers)
• Laser diodes have three distinct radiation modes namely,
longitudinal, lateral and transverse modes.
• In laser diodes, end mirrors provide strong optical feedback in
longitudinal direction, so by roughening the edges and cleaving
the facets, the radiation can be achieved in longitudinal direction
rather than lateral direction.
Modes of cavity
The radiation with in the resonance cavity
of a laser diode set up a pattern of electric
and magnetic lines
Modes
TE modes
TM modes
Each modes described in lateral,
longitudinal and transverse.
Longitudinal modes
 Related to length L of the cavity.
 Much larger then lasing wavelength approximately
1um
Longitudinal modes
 Lin in the plane of p-n junction.
 Depends on side wall preparation, width of cavity.
Transverse modes
 Associated with electromagnetic field and beam profil
perpendicular to the plane of p-n junction.
Laser Operation & Lasing Condition
• To determine the lasing condition and resonant frequencies, we
should focus on the optical wave propagation along the
longitudinal direction, z-axis. The optical field intensity, I, can be
written as:
I ( z, t )  I ( z )e j (t  z )
[4-19]
• Lasing is the condition at which light amplification becomes
possible by virtue of population inversion. Then, stimulated
emission rate into a given EM mode is proportional to the
intensity of the optical radiation in that mode. In this case, the
loss and gain of the optical field in the optical path determine the
lasing condition. The radiation intensity of a photon at energy
h
varies exponentially with a distance z amplified by factor g, and
attenuated by factor
according to the following relationship:

LASING – Light amplification possible in laser
diode
I ( z)  I (0) expg (h )   (h )z
[4-20]
n1
R1
Z=0
R2
n2
Z=L
I (2L)  I (0) R1R2 expg (h )   (h )(2L)
[4-21]
 : Optical confinemen t factor, g : gain coefficien t
 n1  n2 

α : effective absorption coefficien t, R  
 n1  n2 
Lasing Conditions:
I ( 2 L )  I ( 0)
exp(  j 2 L)  1
2
[4-22]
Optical output vs. drive current
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Threshold gain & current density
1  1 

gth   
ln 
2 L  R1R2 
[4-23]
Laser starts to " lase" iff : g  gth
For laser structure with strong carrier confinement, the threshold current
Density for stimulated emission can be well approximated by:
gth  J th
 : constant depends on specific device constructi on
[4-24]
External quantum efficiency
• Number of photons emitted per radiative electron-hole pair
recombination above threshold, gives us the external quantum
efficiency.
ext 

• Note that:
i ( g th   )
g th
q dP
dP (mW )
 0.8065[ m]
E g dI
dI (mA )
i  60%  70%;
ext  15%  40%
[4-29]
Threshold current Density & excess electron density
•
At the threshold of lasing:
  0, d / dt  0, Rsp  0
from eq. [4 - 25]  Cn   /  ph  0  n 
•
1
C ph
 nth
[4-26]
The threshold current needed to maintain a steady state threshold
concentration of the excess electron, is found from electron rate
equation under steady state condition dn/dt=0 when the laser is just
about to lase:
J th nth
nth
0

 J th  qd
qd  sp
 sp
[4-27]
Semiconductor laser rate equations
•
Rate equations relate the optical output power, or # of photons per unit
volume,  , to the diode drive current or # of injected electrons per
unit volume, n. For active (carrier confinement) region of depth d, the
rate equations are:
d

 Cn  Rsp 
dt
 ph
Photonratestimulated emission spontaneous emission photon loss
[4-25]
dn
J
n


 Cn
dt qd  sp
electron rate  injection  spontaneous recombination  stimulated emission
C : Coefficien t expressing the intensityof the opticalemission & absorptionprocess
Rsp :rate of spontaneous emission into the lasingmode
 ph : photonlife time
J :Injectioncurrent density
DFB(Distributed FeedBack) Lasers
•
In DFB lasers, the optical resonator structure is due to the incorporation
of Bragg grating or periodic variations of the refractive index into
multilayer structure along the length of the diode.
Optical Fiber communications, 3rd ed.,G.Keiser,McGrawHill, 2000
Laser operation beyond the threshold
J  J th
• The solution of the rate equations [4-25] gives the steady state
photon density, resulting from stimulated emission and
spontaneous emission as follows:
s 
 ph
qd
( J  J th )   ph Rsp
[4-28]
Laser Resonant Frequencies
• Lasing condition, namely eq. [4-22]:
exp(  j 2 L)  1 
• Assuming
mode is:

mc
m 
2 Ln
2n

2 L  2m , m  1,2,3,...
the resonant frequency of the mth
m  1,2,3,...
c
2
  m  m1 
  
2 Ln
2 Ln
[4-30]
[4-31]
Spectrum from a laser Diode
 (  0 ) 
g ( )  g (0) exp 
 : spectral width
2

2 

[4-32]
Laser Diode Structure & Radiation Pattern
• Efficient operation of a laser diode requires reducing the # of
lateral modes, stabilizing the gain for lateral modes as well as
lowering the threshold current. These are met by structures that
confine the optical wave, carrier concentration and current flow
in the lateral direction. The important types of laser diodes are:
gain-induced, positive index guided, and negative index
guided.
(a) gain-induced guide
(b)positive-index waveguide
(c)negative-index waveguide
Laser Diode with buried heterostructure (BH)
Single Mode Laser
• Single mode laser is mostly based on the indexguided structure that supports only the fundamental
transverse mode and the fundamental longitudinal
mode. In order to make single mode laser we have
four options:
1- Reducing the length of the cavity to the point
where the frequency separation given in eq[4-31] of
the adjacent modes is larger than the laser transition
line width. This is hard to handle for fabrication and
results in low output power.
2- Vertical-Cavity Surface Emitting laser (VCSEL)
3- Structures with built-in frequency selective grating
4- tunable laser diodes
.
VCSEL
Frequency-Selective laser Diodes:
Distributed Feedback (DFB) laser
2ne 
B 
k
[4-33]
Frequency-Selective laser Diodes:
Distributed Feedback Reflector (DBR) laser
B 2
1
  B 
(m  )
2ne Le
2
[4-35]
Output spectrum symmetrically distributed around Bragg wavelength in an idealized DFB laser diode
Frequency-Selective laser Diodes:
Distributed Reflector (DR) laser
Modulation of Laser Diodes
• Internal Modulation: Simple but suffers from non-linear effects.
• External Modulation: for rates greater than 2 Gb/s, more
complex, higher performance.
• Most fundamental limit for the modulation rate is set by the
photon life time in the laser cavity:
1
 ph
c
1
1  c
  g th
   ln
n
2L R1 R2  n
[4-36]
• Another fundamental limit on modulation frequency is the
relaxation oscillation frequency given by:
1
f 
2
1
 sp ph
 I


 1
 I th

1/ 2
[4-37]
Relaxation oscillation peak
Pulse Modulated laser
• In a pulse modulated laser, if the laser is completely turned off
after each pulse, after onset of the current pulse, a time
t d delay,
given by:


Ip
t d   ln 

 I p  ( I B  I th ) 
 : carrier life time
I B : Bias current
I p : Current pulse amplitude
[4-38]
Temperature variation of the threshold
current
I th (T )  I z e
T / T0
Linearity of Laser
Information carrying
electrical signal s(t)
LED or Laser diode
modulator
Optical putput power:
P(t)=P[1+ms(t)]
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 
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 ,...
Laser Noise
• 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.